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HomeMy WebLinkAbout2001-10-23 - RESOLUTIONS - GP AMEND SAFETY ELEMENT (2)RESOLUTION NO. 01-133 A RESOLUTION OF THE CITY COUNCIL OF THE CITY OF SANTA CLARITA, CALIFORNIA, ADOPTING THE NEGATIVE DECLARATION AND APPROVING GENERAL PLAN AMENDMENT NO. 01-03 (MASTER CASE NO. 01-146) AMENDING THE SAFETY ELEMENT OF THE GENERAL PLAN THE CITY COUNCIL OF THE CITY OF SANTA CLARITA, CALIFORNIA, DOES HEREBY RESOLVE AS FOLLOWS: FINDINGS OF FACTS. The City Council of the City of Santa Clarita (hereafter "City") hereby makes the following findings of fact: A. The proposed General Plan Amendment 01-03 would amend the seismic section of the Safety Element, including the technical background report, map exhibits, tables, and planning goals & policies to be applied to existing and future planning projects. The proposed changes will be implemented throughout the City of Santa Clarita planning area. B. On June 25, 1991, the City Council adopted Resolution No. 91-88, approving and certifying the Environmental Impact Report for the General Plan and adopting the General Plan for the City of Santa Clarita. C. On September 12, 1995, the City of Santa Clarita City Council adopted Resolution No. 95-104 which designated authorized city officials to file a grant application to the State Office of Emergency Services in order to obtain Federal and State financial assistance to update the seismic portion of the Safety Element following the 1994 Northridge Earthquake. D. The California Office of Emergency Services approved the City's General Plan Safety Element seismic update grant request on March 10, 1998. E. On August 17, 1998, the State of California Department of Conservation Division of Mines and Geology developed maps identifying seismic hazards within the planning area addressing the areas of liquefaction and earthquake -induced landslides. The information on the state maps is reflected on the proposed seismic update to the Safety Element of the General Plan. F. On September 28, 2001, the State of California Department of Conservation Division of Mines and Geology submitted a letter to the City of Santa Clarita with suggested changes to the Safety Element. Those suggested changes have been incorporated into the Safety Element Amendment of the General Plan. G. On October 2, 2001, the Planning Commission of the City of Santa Clarita conducted a duly noticed public hearing on proposed General Plan Amendment No. 01-03. The Resolution Master Case 01-146 Page 2of5 public hearing was held at 7:00 p.m. at City Hall, Council Chambers, 23920 Valencia Boulevard, Santa Clarita: H. The Planning Commission fully considered the Draft Negative Declaration and Initial Study prepared for General Plan Amendment No. 01-03. I. The Planning Commission fully considered all testimony and evidence regarding proposed General Plan Amendment No. 01-03 updating the seismic section of the Safety Element, including the technical background report, map exhibits, tables, and planning goals & policies. J. The item was heard before the Planning Commission on October 2, 2001 and they recommended to the City Council to adopt the Negative Declaration for the proposed project and approve Master Case 01-146 consisting of General Plan Amendment 01- 03. K On October 23, 2001, the City Council of the City of Santa Clarita conducted a duly noticed public hearing on proposed General Plan Amendment No. 01-03. The public hearing was held at 6:00 p.m. at City Hall, Council Chambers, 23920 Valencia Boulevard, Santa Clarita. L. The City Council fully considered the Draft Negative Declaration and Initial Study prepared for General Plan Amendment No. 01-03. M. The City Council fully considered all testimony and evidence regarding proposed General Plan Amendment No. 01-03 updating the seismic section of the Safety Element, including the technical background report, map exhibits, tables, and planning goals & policies. N. The item was heard before the City Council on October 23, 2001 and they adopted the Negative Declaration for the proposed project and approved Master Case 01-146 consisting of General Plan Amendment 01-03. CEGA FINDINGS. Based upon the above findings of fact, and upon studies and investigations made on behalf of the City Council, the City Council further finds the following California Environmental Quality Act findings: A. A Draft Negative Declaration and Initial Study for this project have been prepared and circulated in compliance with the California Environmental Quality Act and adopted as required by that Act; and B. Said study found that no adverse impact to the existing and future environmental resources of the area would result from the proposal; and C. A Draft Negative Declaration and Initial Study have been circulated for review and Resolution Master Case 01-146 Page 3of5 17 comment by affected governmental agencies and the public, and all comments received, if any, have been considered. The public review period was from August 8, 2001 through October 2, 2001; and D. The proposed project would not have a significant adverse effect on the environment and a Draft Negative Declaration and Initial Study were posted and advertised on August 28, 2001; and E. The Draft Negative Declaration reflects the independent judgement of the City of Santa Clarita; and F. The City Council, based upon the findings set forth above, hereby finds that the Draft Negative Declaration for this project has been prepared in compliance with CEQtA. GENERAL PLAN FINDINGS. Based upon the above findings of fact, and upon studies and investigations made on behalf of the City Council, the City Council further finds the following General Plan findings: A. The proposed Safety Element General Plan Amendment is consistent with Government Code Section 65302 (g) which states that a Safety Element is a mandatory element of the General Plan and is included for the protection of the community from any unreasonable risks associated with the effects of seismically induced surface rupture, ground shaking, ground failure, tsunami, seiche, and dam failure, slope instability leading to mudslides and landslides, subsidence and other geologic hazards known to the legislative body, flooding, and wild land & urban fires. B. The proposed General Plan Amendment is necessary to update the seismic portion of the Safety Element in order to reflect the seismic hazard maps created by the State of California Department of Conservation Division of Mines and Geology which reflects hazards of liquefaction and earthquake -induced landslides. C. California State law requires that each City adopt a General Plan. The proposed Safety Element Amendment is consistent with the City of Santa Clarita General Plan and the existing elements contained within the document. D. The State of California Department of Conservation Division of Mines and Geology submitted a letter to the City of Santa Clarita with suggested changes to the Safety Element. Those suggested changes have been incorporated into the Safety Element Amendment of the General Plan. Resolution Master Case 01-146 Page 4 of 5 NOW, THEREFORE, BE. IT RESOLVED by the City Council of the City of Santa Clarita, California, as follows: SECTION 1. The Negative Declaration prepared for this project per the California Environmental Quality Act, as reference herein; and SECTION 2. The General Plan Safety Element Amendment (Master Case 01-146, General Plan Amendment 01-03) as set forth in Exhibit A, attached hereto and incorporated herein by this reference. Resolution Master Case 01-146 Page 6 of 6 PASSED, APPROVED ANI; ATTEST: STATE OF CALIFORNIA COUNTY OF LOS ANGELES CITY OF SANTA CLARITA I, Sharon Dawson, CMC, City Clerk of the City of Santa Clarita, do hereby certify that the foregoing Resolution was duly adopted by the City Council of the City of Santa Clarita at a regular meeting thereof, held on the 23rd day of October, 2001, by the following vote of the City Council: AYES: COUNCILMEMBERS: Kellar, Ferry, Smyth, Darcy, Weste NOES: COUNCILMEMBERS: None ABSENT: COUNCILMEMBERS: None s:pbs \advance\ safetyel \ 0 1-146 \council\ reso ITA Safety Element Introduction The Safety Element addresses seismically -induced geologic hazards within the City's Planning Area. The following hazards were considered in this evaluation: • Faults • Seismically -induced ground shaking • Ground surface rupture • Liquefaction • Slope stability and landslides • Tsunami • Seiche The City is located within a geologically active region of Los Angeles County. The most significant geologic hazard identified in this geological evaluation is the presence of the active San Gabriel fault, which crosses the City. In addition, numerous other active faults are located in or near the City. _ Historically, the area within the City limits has been subject to seismically induced ground shaking. Future seismically -induced ground shaking will likely affect the area within City limits. Other geologic hazards that have historically affected the planning area or could affect the City in the future include: • Ground rupture from movement of active faults. • Liquefaction in areas of unconsolidated sediments and a shallow groundwater table. • Landslides and other slope movement in hilly areas. • Flooding in the event of dam failure to Bouquet Canyon or Castaic dam. Definitions Select technical terms used in the Safety Element are defined below: Active Fault- As defined by the State Mining and Geology Board, a fault which has had surface displacement within Holocene time (about the last 11,000 years). Attenuation- A decrease in ground acceleration as waves propagate away from the seismic source. Blind Thrust Fault- Low angle dip -slip or detachment faults that do not reach the ground surface. Crustal Shortening- The decrease in distance between two accurately, measured points as a result of seismic events. Critical Structures and Facilities- Structures and facilities which are subject to specified seismic safety standards because of their immediate and vital public need or because of the severe hazard presented by their structural failure. The type of structures vary but may include (1) structures such as nuclear power reactors or large dams whose failure might be catastrophic; (2) major communication, utility, and transportation systems; (3) involuntary- or high -occupancy buildings such as prisons or schools; and (4) emergency facilities such as hospitals, police and fire stations, and disaster -response centers. Dip -slip Fault- Chiefly inclined fractures along which rock masses have shifted vertically. Epicenter- The location on the Earth's surface vertically above the point (focus or hypocenter) where a seismic rupture initiates. Inactive Fault- A fault that has not had surface displacement within the last 1.6 million years. Fault- A fracture or zone of closely associated fractures along which rocks on one side have been displaced with respect to those on the other side. Fault Trace- Intersection of a fault with the ground surface; also, the line commonly plotted on geological maps to represent a fault. Ground Acceleration- The rate of ground movement due to a seismic -shock wave. Ground acceleration is commonly expressed in percentage of gravity. Holocene Time- The most recent geologic epoch of time; the past 11,000 years. Hypocenter- The point within the Earth where an earthquake rupture initiates. Intensity- A subjective numerical index describing the severity of an earthquake in terms of its effects on the Earth's surface and on humans and their structures. Landslides- Slope failure that occurs when the rocks or soil underlying an area can no longer maintain the load of material above it. Liquefaction- Process by which water -saturated sediment temporarily loses strength, usually because of strong shaking during a major earthquake, and behaves as a fluid. 2 Low Angle Detachment Fault- A fault that is oriented horizontally or near horizontal, with the fault plane located below the ground. Magnitude- A number that characterizes the size of an earthquake, based on measurement of the maximum motions recorded by a seismograph for earthquake waves of a particular frequency. The scale most commonly used is local magnitude commonly referred to as Richter magnitude. Modified Mercalli Scale- A measure of intensity of an earthquake's effects in a given locality based on observations of the earthquake damage. Values on this scale range from I to XII. Normal Fault- A dip -slip fault whereby the side above the fault plane has been displaced downward. Oblique Slip Faults- Faults that have significant components of both strike slip and dip slip displacement. Potentially Active Fault- A fault which shows evidence of surface displacement during Quaternary time (last 1.6 million years). Richter Magnitude- A method developed in 1932. by the late Dr. Charles F. Richter to measure earthquake size. This scale was originally designed to use the maximum trace amplitude registered on a seismogram from a standard instrument, called a Wood - Anderson torsion seismograph, Seismic Moment- A measurement of the energy that can be radiated by an earthquake. The seismic moment of an earthquake is determined by the strength of resistance of rocks to faulting (shear modulus) multiplied by the area (length times width) of the fault that ruptures and by the average displacement that occurs across the fault during the earthquake. Tectonic Uplift- Upward displacement of the Earth's crust due to seismic events. Reverse (or thrust) Fault- Fault movement of a dip -slip fault whereby the side above the fault is elevated. Seiche- Oscillation of the surface of an enclosed body of water as a result of earthquake shaking. Seismicity- The geographic and historical distribution of earthquakes. Settlement- The downward movement of a soil or of the structure which it supports, resulting from a reduction in the voids in the underlying strata. k t Slip Rate- The average rate of displacement at a point along a fault as determined from geodetic measurements, from offset manmade structures, or from offset geologic features whose age can be estimated. Strike -slip Faults- Chiefly vertical fractures along which rock masses have shifted horizontally. Subsidence- Downward settling of the Earth's surface with little or no horizontal motion. May be caused by natural geologic processes (such as sediment compaction or tectonic activity) or by human activity (such as mining or withdrawal of ground water or petroleum). Surface Faulting- Displacement that reaches the ground surface during slip along a fault. Commonly accompanies moderate and large earthquakes having focal depths to 20 km. Tsunami- A generated sea wave of local or distant origin that results from large-scale seafloor displacements associated with large earthquakes, major sea -floor landslides, or exploding volcanic islands. Geologic Setting Regional Geology The City of Santa Clarita is located in the Transverse Range Geomorphic Province of California. This province is characterized by east -west trending mountains and faults. Mountain ranges within this province include the Santa Ynez, Santa Susana, Topatopa, San Gabriel, Sierra Pelona, and San Bernardino. The Transverse Range is comprised of rocks that are progressively older from west to east. East -west trending folds and faults predominate the region. Valleys, faults, and downwarps separate mountain ranges in this province. Sedimentary basins within the Transverse Range Geomorphic Province include the Ventura, Soledad, and Ridge Basins, and the San Fernando Valley. Geologic environments represented in the rocks found in the Transverse Range include periods of non -marine deposits (Saugus, Mint Canyon, Sespe formations), marine deposits (Pico, Repetto, Monterey, San Francisquito), volcanics (Conejo Volcanic series), and metamorphic or igneous rocks (Lowe Granodiorite, Pelona Schist, Mendenhall Gneiss). The faulting and seismicity of the Transverse Ranges is dominated by the intersection of the San Andreas fault and the Transverse Ranges fault systems. Seismic activity along the San Andreas fault is in response to differential movement between the Pacific geologic plate (west of the fault) and the North American geologic plate (east of the C! fault). Transverse Ranges faults generally reflect crustal shortening (reverse) faulting patterns. The Ventura, Soledad, and Ridge Basins are the result of the interplay of these two fault regimes (Yerkes, 1985). The highest rates of tectonic uplift within the Transverse Ranges have been measured along the coast west of Ventura, in an area of intense seismicity, active folding, and reverse faulting (Yerkes, 1985). Santa Clarita is located within the Soledad Basin of northern Los Angeles County. The basin is bounded on the north and east by the San Andreas, Bee Canyon and Clearwater faults, on the west and south by the San Gabriel fault and on the east and south by the San Gabriel Mountains. Terrain throughout the City varies from steep hills and canyons to flat areas along the valley bottoms. The principal drainage feature in the City is the Santa Clara River, a westward flowing river that bisects the City. Important Geologic Features within the City of Santa Clarita The City of Santa Clarita is located within the Santa Clarita Valley. This valley is bordered to the southeast by the San Gabriel Mountains, to the southwest by the Santa Susana Mountains, and to the north by Sierra Pelona. The Santa Clara River is the main drainage feature through the valley. Large tributary canyons to the Santa Clara River include Bouquet Canyon, San Francisquito Canyon, Sand Canyon, and Placenta Canyon. The majority of the land within City limits is comprised of the following geologic rock units: • Unconsolidated alluvium- found along the bottoms of canyons and in the area of the Santa Clara River. Unconsolidated landslide debris is found throughout the hills of the City. Saugus Formation- a non -marine fluvial (river -deposited) material that is comprised of pebble -cobble conglomerate, sandstone, and minor siltstone. The Saugus Formation is of Pliocene and Pleistocene age (11,000 to 5 million years old). Mint Canyon Formation- a non -marine material that is comprised of fine grained sandstone and interbedded siltstone and claystone. The Mint Canyon Formation is of Miocene age (5 to 22.5 million years old). The Santa Clarita area is cut by several geologic faults. The most important fault that is found within City limits is the San Gabriel fault, an active feature that trends northwestward across the City. Other important faults near the City include the San Andreas, Oak Ridge, Holser, San Fernando, and Santa Susana faults. Seismicity Definition of Fault Terms The California Division of Mines & Geology defines active faults as those that have had surface displacement within Holocene time (about the last 11,000 years). Surface displacement can be recognized by the existence of cliffs in alluvium, terraces, offset stream courses, fault troughs and saddles, the alignment of depressions, sag ponds, and the existence of steep mountain fronts. Potentially active faults (CDMG 1997) are ones that have had surface displacement during the last 1.6 million years. Inactive faults (CDMG 1997) -have not had surface displacement within the last 1.6 million years. Active faults that have been mapped on or near the City of Santa Clarita include the San Gabriel, San Andreas, Oak Ridge, Holser, San Fernando, and Santa Susana faults. The following discussion regarding magnitude and intensity is from the California Department of Mines and Geology Note 32, Soto Earthquakes are Measured, as listed on the Division of Mines and Geology web page. The strength of an earthquake is generally expressed in two ways: magnitude and intensity. The magnitude is a measure that depends on the seismic energy radiated by the earthquake as recorded on seismographs. An earthquake's magnitude is expressed in whole numbers and decimals (such as 6.8). The intensity at a specific location is a measure that depends on the effects of the earthquake on people or buildings. Intensity is expressed in Roman numerals or whole numbers (example: VI or 6). Although there is only one magnitude for a specific earthquake, there may be many values of intensity (damage) for that earthquake at different sites. Magnitude Scales Several magnitude scales have been developed by seismologists. The original is the Richter magnitude, developed in 1932 by the late Dr. Charles F. Richter who was a professor at the California Institute of Technology. The most commonly used scale today is the Moment magnitude (Mw) scale, jointly developed in 1978 by Dr. Thomas C. Hanks of the U.S. Geological Survey and Dr. Hiroo Kanamori, a professor at CalTech. Moment magnitude is related to the physical size of fault rupture and the movement (displacement) across the fault, and as such is a more uniform measure of the strength of an earthquake. Another measure of earthquake size is seismic moment. The seismic moment determines the energy that can be radiated by an earthquake and hence the seismogram recorded by a modem seismograph. The moment magnitude of an earthquake is defined relative to the seismic moment for that event. It is important to recognize that earthquake magnitude varies logarithmically with the wave amplitude or seismic moment recorded by a seismograph. Each whole number R step in magnitude represents an increase of ten times in the amplitude of the recorded seismic waves, and the energy release increases by a factor of about 31 times. The size of the fault rupture and the fault's displacement (movement) also increase logarithmically with magnitude. Magnitude scales have no fixed maximum or minimum. Observations have placed the largest recorded earthquake (off -shore from Chile in 1960) at Moment magnitude 9.6 and the smallest at -3. Earthquakes with magnitudes smaller than about 2 are called microearthquakes. Magnitudes are not used to directly estimate damage. An earthquake in a densely populated area, which results in many deaths and considerable damage, may have the same magnitude as an earthquake that occurs in a barren, remote area that does nothing more than frighten the wildlife. Earthquake Intensity The first scale to reflect earthquake intensities (damage) was developed by de Rossi of Italy and Forel of Switzerland in the 1880s and is known as the Rossi -Forel intensity scale. This scale, with values from I to X, was used for about two decades. A need for a more refined scale increased with the advancement'of the science of seismology. In 1902, the Italian seismologist, Mercalli, devised a new scale on a I to XII range. The Mercalli intensity scale was modified in 1931 by American seismologists Harry O. Wood and Frank Neumann to take into account modern structural features. Table 1, Modifted Mercalli Intensity Scale of 1931 lists the Modified Mercalli intensity scale. The Modified Mercalli intensity scale measures the intensity of an earthquake's effects in a given locality, and is perhaps much more meaningful to the layperson because it is based on observations of earthquake effects at specific places. It should be noted that because the data used for assigning intensities are obtained from direct accounts of the earthquake's effects at numerous towns, considerable time (weeks to months) is sometimes needed before an intensity map can be assembled for a particular earthquake. On the Modified Mercalli intensity scale, values range from I to XII. The most commonly used adaptation covers the range of intensities from the conditions of "I - not felt except by very few, favorably situated," to "XII - damage total, lines of sight disturbed, objects thrown into the air." While an earthquake has only one magnitude, it can have many intensities, which typically decrease with distance from the epicenter. It is difficult to compare magnitude and intensity because intensity is linked with the particular ground and structural conditions of a given area, as well as distance from the earthquake epicenter, while magnitude depends on the energy released by earthquake faulting. However, there is an approximate relation between magnitude and maximum expected intensity close to the epicenter. Table 2, Comparison of Richter 7 Magnitude and Modified Mercalli Intensity compares ground motion and intensity values. The areas shaken at or above a given intensity increase logarithmically with ground motion. Seismic Setting The City of Santa Clarita is within the Transverse Ranges Geomorphic Province. This province is extensively faulted with known active faults. Some of the large faults in or near the province include the San Andreas, Garlock, Oak Ridge, Newport -Inglewood, Santa Susana, Santa Ynez, Red Mountain, and Malibu Coast. There are many localized faults throughout the province. Recently active faults are depicted on Exhibit S-1, Recently Active Faults in the Southern California Region. Table 3, Active Faults Near the City of Santa Clarita lists faults near the Planning Area that have a high potential to impact the City of Santa Clarita. Active Faults near or within the City of Santa Clarita San Gabriel Fault. The San Gabriel fault is an active fault that crosses the City. This fault is a long break that extends from near Frasier Mountain, to near the Tejon Pass, near the city of San Bernardino. The fault has had right lateral strike -slip displacement. It is modeled as being capable of generating a maximum moment magnitude of 7.0. Holser Fault. The Holser fault is similar in orientation to the San Cayetano fault and might be considered as an extension of the same geologic feature. The Holser fault trends along the northern border of the Santa Clara River Valley. Based on a conversation with the Department of Conservation Division of Mines and Geology, the exact location in the City of Santa is concealed beneath alluvium therefore it has not been determined if this fault runs through the City of Santa Clarita. The fault is an east -west trending fault that dips to the north. This fault has a reverse sense of offset. It is modeled as being capable of generating a maximum moment magnitude of 6.5. Santa Susana Fault. The Santa Swans fault is an active fault located one mile south of the City. This fault is a reverse fault that extends from the northern edge of Simi Valley through the northern end of the San Fernando Valley. This fault has a length of about 16 miles and an estimated maximum moment magnitude of 6.6. San Andreas Fault. The San Andreas Fault Zone is the dominant active fault in California. It is located 16 miles northeast of the City. It is the primary surface boundary between the Pacific and the North American plate. There have been numerous historic earthquakes along the San Andreas fault. This fault is capable of producing a moment magnitude 8-8.5 earthquake. The fault has right lateral strike slip displacement. Oak Ridge Fault. The Oak Ridge fault is located seven miles west of the City. The fault is a steep south -dipping reverse fault that forms the boundary between Oak Ridge to the south and the Santa Clara River to the north. Activity along the Oak Ridge fault is known to have occurred during the Pliocene time (5.3 to 7.6 million years ago) and into the Pleistocene. The maximum credible earthquake is a moment magnitude of 6.9 for both the eastern and western parts of this fault. The magnitude 6.7 Northridge earthquake (in 1994) is thought to have occurred along the eastern end of the Oak Ridge fault (Yeates and Huftile, 1995). San Fernando Fault. The San Fernando fault is located six miles south of the City. This fault is part of the Sierra Madre -San Fernando fault system. The San Fernando fault was the source of the 1971 San Fernando (Syhnar) earthquake. The fault has reverse displacement. It is modeled as being capable of generating a maximum moment magnitude of 6.7. In addition to the previously identified active faults, there is the potential for ground shaking from blind thrust faults. Blind thrust faults are low angle detachment faults that do not reach the ground surface. Recent examples of blind thrust fault earthquakes include the 1994 Northridge (Magnitude 6.7), 1983 Coalinga (Magnitude 6.5), and 1987 Whittier Narrows (Magnitude 5.9) events. As described in Dolan et al (1995), much of the Los Angeles area is underlain by blind thrust faults. In their seismic model for Los Angeles, blind thrust faults are found at a depth of about 6 to 10 miles below ground surface and have the ability to produce magnitude 7.5 earthquakes. Faults generally produce damage in two ways: ground shaking and surface rupture. Seismically induced ground shaking covers a wide area and is greatly influenced by the distance of the site to the seismic source, soil conditions, and depth to groundwater. Surface rupture is limited to very near the fault. Other hazards associated with seismically induced ground shaking include earthquake -triggered landslides and tsunamis. The California Division of Mines and Geology (1996) classifies faults into two categories in their modeling of California's seismic risk. These categories are: • Type A faults- the faults have slip rates greater than 5 millimeters per year and well constrained paleoseismic data. The San Andreas fault is an example of a Type A fault. • Type B faults- all other faults not classified as Type A faults. Type B faults lack paleoseismic data necessary to constrain the recurrence interval of large events. The San Gabriel, Oak Ridge, Holser, and Santa Susana faults are Type B faults. Vj Historic Earthquakes Affecting the City of Santa Clarita Historically identified earthquakes have shaken the land that comprises the City of Santa Clarita. This historic record reaches back to about the mid 1800s. Prior to that date, the area was very sparsely populated, thus, the historic record is not complete. Table 4, Significant Historic Earthquakes Felt within City Limits lists the principal historic earthquakes that have affected the area within City limits from 1850 to 2000. Exhibit S-3 Major Earthquakes within the Southern California Region depicts earthquake epicenters located within and near the City of Santa Clarita. The Northridge earthquake of January 17, 1994 caused over $650 million in damage to residential units, $41 million in damage to businesses, and over $20 million in damage to public buildings and roads. This earthquake had an epicenter located 13 miles southwest of the Santa Clarita Valley. Fortunately, no deaths were recorded in Santa Clarita due to the earthquake. The earthquake damaged the water distribution and filtration systems, natural gas service, electrical service, and roads and bridges. Due to possible water supply contamination, residents in the valley were told to boil drinking water. The boil water order was lifted on February 5, 1994. Other damage resulting from the earthquake included a crude oil release from a pipeline rupture and other hazardous materials spills. Ground Rupture Seismically induced ground rupture occura as the result of differential movement across a fault. An earthquake occurs when seismic stress builds to the point where rocks rupture. As the rocks rupture, one side of a fault block moves relative to the other side. The resulting shock wave is the earthquake. If this rupture plane reaches the ground surface, ground rupture occurs. The types of rupture are listed below: Strike -slip faults: when one side of the fault moves side to side relative to the other fault block. Strike slip faults are described as right lateral or left lateral faults. • Dip -slip faults: when one side of the fault moves up or down relative to the other fault block. Dip slip faults are described as normal or reverse faults. Ground rupture is potentially very damaging to any structure that straddles the fault trace. Structures cannot readily withstand the effect of differential movement of its foundation. Buildings typically collapse or suffer significant damage as a result of differential movement through a foundation. Alquist Priolo Earthquake Fault Zone i The Alquist Priolo Earthquake Fault Zone is an area within 500 feet from a known fault trace. Pursuant to the Alquist Priolo legislation, no structure for human 10 occupancy is permitted on the trace of an active fault. The term "Structure for human occupancy" is defined as any structure used or intended for supporting or sheltering any use or occupancy, which is expected to have a human occupancy rate of more than 2,000 person -hours per year. Unless proven otherwise, an area within 50 feet of an active fault is presumed to be underlain by active branches of the fault. In brief, the Alquist Priolo Earthquake Fault Maps are designed to identify areas that have a risk of seismically induced ground displacement. Certain seismic and engineering studies are required to be completed prior to development within these zones. The types of projects that are subject to the additional studies are defined in the legislation. The following statement is included in the California Division of Mines and Geology Special Publications 42 (1994, page 9): "Active faults may exist outside the Earthquake Fault Zones on any zone map. Therefore, fault investigations are recommended for all critical and important developments proposed outside the Earthquake Fault Zones." As stated in Section 3603, Specific Criteria (d): "Application for a development permit for any project within a delineated special studies zone shall be accompanied by a geologic report prepared by a geologist registered in the State of California, which is directed to the problem of potential surface fault displacement through the project site, unless such report is waived pursuant to Section 2623 of the Act. This required report shall be based on a geologic investigation designed to identify the location, age, and nature of faulting that may have affected the project site in the past and may affect the project site in the future. The report may be combined with other geologic or geotechnical reports". Alquist Priolo Earthquake Fault Maps have been prepared for portions of the City. The State of California Earthquake Fault Zones map for the Newhall Quadrangle depicts part of the San Gabriel fault within an Earthquake Fault Zone. Areas of ground rupture resulting from the 1971 San Fernando (Sylmar) earthquake, located near the southern end of the City of Santa Clarita, are also shown as Earthquake Zones on the Oat Mountain and San Fernando Quadrangle maps. Exhibit S-2, Faults in Planning Area depicts the Earthquake Fault Zones for the area within the City limits. Ground Acceleration Seismically -induced ground acceleration is the shaking motion that is produced by an earthquake. Probabilistic modeling is done to predict future ground accelerations. Probabilistic modeling generally considers two scenarios, design basis earthquake ground motion or upper -bound earthquake ground motion. Design basis earthquake ground motion calculations are typically applied for residential and commercial sites. 11 This ground motion is defined as a ground motion that has a 10 percent chance of exceedance in 50 years. Upper -bound earthquake ground motion calculations are applied to public schools, hospitals, skilled nursing facilities, and essential services buildings, such as police stations, fire stations, city hall, and emergency communication centers. Upper -bound earthquake ground motion is defined as the ground motion that has a 10 percent chance of exceedance in 100 years. Projects in Santa Clarita should consider design basis earthquake ground motion or upper bound earthquake ground motion in their design. Probabilistic Approach. The probabilistic approach attempts to model the probability of something occurring. In this approach, the models predict the possibility of a specified ground acceleration affecting a site within a specified timeframe. This is done by identifying faults that are active, determining the frequency of earthquake activity along modeled faults, the strength of the earthquakes, and attenuation relationships as described above. The California Division of Mines and Geology (CDMG, 1996 and 1999) developed a state-wide model that takes these variables into consideration. The model depicts peak accelerations that have a 10% probability of being exceeded in 50 years. To perform the probabilistic analysis using the CDMG 1996 model,, one can look up the ground acceleration on the maps included in that report. CDMG (1999) has updated the state model figure, providing more detail than what is shown in CDMG 1996. C Alternatively, site specific information (latitude.and longitude) can be input into the model. Soil types and other variables are input as defined in the California Building Code (1998). The model will then generate a. probabilistic ground acceleration. The accelerations are for peak horizontal ground acceleration in units of gravity. As shown in the Seismic Shaking Hazard Maps of California (California Division of Mines and Geology, 1999), the area within City limits has a 10% probability of experiencing 0.7-1.0 g peak horizontal ground acceleration within the next 50 years. Ground shaking that an area is subject to is primarily a function of the distance between an area and the seismic source, the type of material underlying a property, and the motion of fault displacement. In addition, the Northridge (1994) earthquake showed how peculiarities in basin effects can play a significant role in ground accelerations at particular areas. For instance, ground accelerations exceeding 1g were recorded at areas far from the epicenter of the Northridge earthquake. Because of the proximity to major active faults, such as the San Andreas and San Gabriel fault systems, it is reasonable that accelerations over 1g can occur anywhere within City limits. 12 Other Geologic Hazards Landslides Landslides occur when the underlying support can no longer maintain the load of material above it, causing a slope failure. The size of a landslide can vary from minor rock falls to large hillside slumps. The underlying bedrock bedding planes, degree of water saturation of a material, steepness of a slope and the general strength of the soil all contribute to the stability of a hillside. Basal erosion caused by water or human - induced modifications to the natural contour of a hill, including grading, have the potential to destabilize a hillside. Stability of a soil is influenced by many factors. Some of these factors include grain size, moisture content, organic matter content, degree of slope, and soil type. Unstable soils can produce landslides, debris flows, and rock falls. All of these phenomenon are manifestations of gravity driven flows of earth materials due to slope instability. Hill slopes naturally have a tendency to fail. Unless engineered properly, development in hillside areas tends to increase the potential for slope failures. Slope modifications by grading, changes in infiltration of surface water, and undercutting slopes can create unstable hill slopes, resulting in landslides or debris flows. Rock falls occur in virtually all types of rocks and especially on slopes steeper than 40 degrees where the rocks are weakly cemented, intensely fractured, or weathered. Landslides and rock falls are usually triggered by seismically induced ground shaking or by erosional destabilization of a hill slope, but can also be caused by undercutting of slopes during grading operations. The California Division of Mines and Geology has prepared Seismic Hazard Zone Maps of the Newhall, Mint Canyon, Oat Mountain, and San Fernando 7.6 -minute quadrangles. These four quadrangles comprise the City limits. These maps identify areas of liquefaction hazard and earthquake induced landslide hazard. In general, areas underlain by unconsolidated alluvium, such as along the Santa Clara River and tributary washes, are prone to liquefaction. Areas that are on topographic highlands, such as hill slopes, are subject to landslide. Exhibit S-4, Geologic Hazard Zones depicts the areas that are prone to earthquake induced landslide hazards. Geologic maps of the Santa Clarita area have been prepared by Thomas Dibblee, Jr. (Geologic Map of the Newhall Quadrangle, 1996; Geologic Map of the Mint Canyon Quadrangle, 1996; Geologic Map of the San Fernando and Van Nuys (North 1h) Quadrangles, 1991; Geologic Map of the Oat Mountain and Canoga Park (North 'A) Quadrangles, 1992). These four maps comprise the City of Santa Clarita area. Many landslides have been mapped within City limits. These landslides are depicted on the Newhall and Mint Canyon quadrangles. The majority of the landslides are mapped within the Saugus and Mint Canyon formations. 13 The seismic hazard maps differ from the geologic maps in the following way. The seismic hazard maps show areas that have the potential to be affected by liquefaction and landslides, whereas the geologic maps show existing landslides. Potential hazard areas are not shown on geologic maps. CDMG prepared Special Publication 117, Guidelines for Evaluating and Mitigating Seismic Hazards in California, 1997. This document provides recommendations to effectively reduce seismic hazards to acceptable levels, as defined in California Code of Regulations (CCR Title 14, Section 3721). For landslides, CDMG Special Publication 117 recommend that the following be performed: • A screening investigation to determine the possible presence of landslides. • If the screening investigation identifies the likely presence of landslides, then perform a quantitative evaluation of earthquake -induced landslide potential. This task includes field exploration, site sampling, and geotechnical testing. A slope stability analysis might also be appropriate here. • Evaluation of potential earthquake -included landslide hazards. • Mitigation of earthquake -induced landslide hazards. Some of the land surfaces within the City include areas that have steep slopes. In addition, some of these areas are in geological formations that are prone to failure. Because of these two factors, many areas within City limits are prone to landslides. Exhibit S-4, Geologic Hazard Zones depicts the areas that are prone to earthquake induced landslide hazards. Liquefaction Liquefaction is defined as the transformation of a granular material from a solid state to a liquefied state as a consequence of increased pore water pressure. During ground shaking, the alluvial grains are packed into a tighter configuration. Pore water is squeezed from between the grains, increasing the pore pressure. As the pore pressure increases, the load bearing strength of the material decreases. As a result, structures built on this material can sink into the alluvium, buried structures may rise to the surface or materials on sloped surfaces may run downhill. Other effects of liquefaction include lateral spread, flow failures, ground oscillations, and loss of bearing strength. Liquefaction is intrinsically linked with the depth of groundwater below the site and the types of sediments underlying an area. The following table lists the relationship between liquefaction hazard and groundwater depth. 14 Liquefaction Zone Criteria Geologic Unit I Depth to Groundwater Greater than 40 feet Less than 40 feet Qa Low High all other Low Low Source: CDMG, 1995. CDMG prepared Special Publication 117, Guidelines for Evaluating and Mitigating Seismic Hazards in California, 1997. This document provides recommendations to effectively reduce seismic hazards to acceptable levels. For liquefaction, CDMG Special Publication 117 recommend that the following be performed: • Screening investigations for liquefaction potential. • Qualitative evaluation of liquefaction potential. • Evaluation of potential liquefaction hazards. • Mitigation of liquefaction hazards. There are areas within the City that overlie unconsolidated alluvium with a high groundwater table. These areas are primarily found near the Santa Clarita River and tributaries to this river. Exhibit S-4, Geologic Hazard Zones depicts the areas that are prone to liquefaction. Tsunami Tsunamis are large ocean surges that are created as a result of a subsea earthquake or landslide. The waves created by the subsea earthquake or landslide travel across the ocean at high speeds (several hundreds of miles per hour). As the waves reach shore, their amplitudes increase. Once the waves reach land, they can cause widespread flooding. The areas susceptible to tsunamis are those near the ocean and along low- lying river channels. The City is over 20 miles from the Pacific Ocean. Thus, there is no risk of damage from a tsunami. Seiche A seiche is a wave or series of waves that are produced within an enclosed or partially enclosed body of water (such as a lake or bay). Most seiches are created as landslides fall into the body of water and displace the water. The' water then sloshes out of the bay or lake, creating the seiche. If a seiche overtops a dam, the water can erode the dam face to the point where the dam can fail. Flooding occurs when water flows overwhelm an area's natural or manmade drainage system. Floods are generally described by their frequency of occurrence, such as a 100 year flood. The 100 year flood represents the flood magnitude with a 1% chance of 15 being equaled or exceeded in any given year. Flooding is generally a direct response to the amount, distribution, and intensity of precipitation. Man-made structures, such as levees and engineered stream channels, have been built to prevent floods in an area. In the event that a dam fails due to a seiche, there could be a large, relatively instantaneous release of water that would likely overwhelm the downstream drainage system. Such a release could have a catastrophic effect on the land downstream of the dam. Several manmade dams are located near the City. These include the Castaic Dam, which forms Castaic Lake, and the Bouquet Canyon Dam, which forms the Bouquet Reservoir. Both of these dams are outside of the City of Santa Clarita. However, failure of either of these dams would release water that would flow into the City. Seismically induced ground acceleration or seiche could jeopardize the integrity of the Castaic or the Bouquet Canyon dams. Floods that could result from such a dam failure are discussed in the Flooding section of this Safety Element. Subsidence Subsidence is the lowering of the ground surface. It often occurs as a result of withdrawal of fluids such as water, oil, and gas from the subsurface. When fluids are removed from the subsurface, the overburden weight, which the water had previously helped support through buoyant forces, is transferred to the soil structure. Subsidence typically occurs over a long period of time and results in a number of structural impacts. Facilities most impacted by subsidence are long, surface infrastructure facilities such as canals, sewers, and pipelines. The extraction of groundwater from an aquifer beneath an alluvial valley can result in subsidence or settlement of the alluvial soils. The factors that influence the potential occurrence and severity of alluvial soil settlement due to groundwater withdrawal include: degrees of groundwater confinement; thickness of aquifer systems; individual and total thickness of fine-grained beds; and compressibility of the fine-grained layers. As yet, no large-scale local subsidence has been reported in the City due to either groundwater or oil extraction. Furthermore, much of the City is located over consolidated sediments that are not very prone to subsidence. The subsidence potential associated with groundwater or oil removal within the City is low. There is some risk of changes in elevation as a result of seismic offset. The San Gabriel fault crosses the City. Movement of the San Gabriel fault is generally described as being strike -slip. With strike -slip motion, the principal movement direction is side to side, not up or down. However, it is common to have localized uplift or downdropping along strike -slip faults. Therefore, it is possible to have some localized seismically r induced subsidence within City limits. 16 References California Department of Conservation, Division of Mines and Geology, 1982, Areas Damaged by California Earthquakes, 1900-1949, California Department of Conservation Division of Mines and Geology, DMG Open File Report 82-17. California Department of Conservation, Division of Mines and Geology, 1987, Landslide Hazards in the East Half of the Newhall Quadrangle, Los Angeles County, California, DMG Open File Report 86-16. California Department of Conservation, Division of Mines and Geology, 1994, Fault - Rupture Hazard Zones in California, Special Publication 42. California Department of Conservation, Division of Mines and Geology, 1995, The Northridge, California, Earthquake of 17 January 1994, Special Publication 116. California Department of Conservation, Division of Mines and Geology, 1996, Probabilistic Seismic Hazard Assessment for the State of California, DMG Open -file Report 96-08. California Department of Conservation, Division of Mines and Geology, 1997, Guidelines for Evaluating and Mitigating Seismic Hazards in California, Special — Publication 117. Dibblee, T. W. Jr., 1991, Geologic Map of tlie'San Fernando and Van Nuys (North h) Quadrangles, Los Angeles County, California, Dibblee Geological Foundation. Dibblee, T. W. Jr., 1992, Geologic Map of the Oat Mountain and Canoga Park (North 2h) Quadrangles, Los Angeles County, California, Dibblee Geological Foundation. Dibblee, T. W. Jr., 1996, Geologic Map of the Newhall Quadrangle, Los Angeles County, California, Dibblee Geological Foundation. Dibblee, T. W. Jr., 1996, Geologic Map of the Mint Canyon Quadrangle, Los Angeles County, California, Dibblee Geological Foundation. Dolan J. F., Sieh, K, et al, 1995, Prospects for larger or more frequent earthquakes in the Los Angeles Metropolitan region; Science, V. 267, p. 199-205. Idriss, I. M., 1985, Evaluating Seismic Risk in Engineering Practice, in Proceedings of the eleventh International Conference on soil mechanics and foundation engineering, San Francisco, 12-16 August, Publications Committee of XI ICSMFE, editor A. A. Balkema, Rotterdam. 17 r Jennings, C. W., 1994, Fault Activity Map of California and Adjacent Areas, Map Number 6, California Department of Conservation, Division of Mines and Geology. Petersen, M., Beeby, D., Bryant, W., Cao, C., Cramer, C., Davis, J., Reichle, M., Saucedo, G., Tan, S., Taylor, G., Toppozada, T., Treiman, J., and Wills, C., 1999, Seismic Shaking Hazard Maps of California, CDMG Map Sheet 48. Sadigh, K., Chang, C.Y., Egan, J. A., Makdisi, F., and Youngs, R. R., 1997, Seismological Research Letters Volume 68, Number 1, January/February 1997, pages 180-189. Trinet Rapid Instrument Intensity Map for Hector Mine Earthquake, Trinet web page www.trinet.org, 2001 Toppozada, T.R., 1995, History of Damaging Earthquakes in Los Angeles and Surrounding Area, in Woods, M. C. and Seiple, W. R. editors, The Northridge, California, Earthquake of 17 January 1994; California Department of Conservation Division of Mines and Geology, Special Publication 116 p. 9-16. Unites States Geological Survey, 1994, The Magnitude 6.7 Northridge, California Earthquake of 17 January 1994, Science, Volume 266. United States Geological Survey National Seismic Hazard Mapping Project, January/February 1997, Seismological Research Letters. Volume 68, Number 1. Yeates, R. S. and Huftile, G. J., 1995, The Oak Ridge Fault System and the 1994 Northridge Earthquake, Nature, Volume 373, February 1995. Yerkes, R. F., 1985, Geologic and Seismologic Setting, in, Evaluating Earthquake Hazards in the Los Angeles Region- An Earth -Science Perspective, USGS Professional Paper 1360. Ziony, J. I., and Yerkes, R. F., 1985, Evaluating Earthquake and Surface Faulting Potential, in, Evaluating Earthquake Hazards in the Los Angeles Region- An Earth - Science Perspective, USGS Professional Paper 1360. N Goal 1 Minimize risks to life and property associated with fault rupture and seismically -induced groundshaking. Policies and Programs 1.1 Work with the California Division of Mines and Geology to review development proposals located within or adjacent to the Alquist-Priolo Special Studies Zone, along the San Gabriel Fault, and other potential active faults. 1.2 Require all structures to meet or exceed state required design standards pertaining to earthquake resistance. 1.3 Provide setbacks, as determined to be necessary, for any proposed development located on or near an active or potentially active fault. Appropriate setback distances will be determined through an appropriate geologic investigation. 1.4 Review the use of seismic design criteria and standards for linear system facilities, including transmission lines, water and sewage systems, and highways to ensure that they are adequate in protecting the public. Actual weaknesses or limitations within the system should also be determined and mitigated where feasible. 1.5 As necessary to avoid geologic hazards, require project modifications, including but not limited to hazard mitigation, project redesign, elimination of building sites and the delineation of building envelopes, building setbacks and foundation requirements. Goal 2 Minimize risks to life and property associated with geologic hazards, including but not limited to, landsliding, liquefaction, debris flows, mudslides, rockfalls, and expansive soils. Policies and Proerams 2.1 Continue to require that all construction be in accordance with the most current version of the Uniform Building Code and California Building Code. 2.2 Continue to require site-specific geotechnical studies for new development proposals in zones of required investigation as defined in the Seismic Hazards Mapping Act and elsewhere as appropriate. 2.3 Enforce and update, as necessary, the Ridgeline Preservation and Hillside Development Ordinance and standards, and encourage the use of cluster and planned unit developments for projects in or near geologically hazardous areas. 2.4 Continue to assist developers in obtaining necessary technical and policy information regarding seismic hazards and maintain a list of qualified geotechnical consultants. 2.5 Evaluate and review the potential for inundation from dam or levee failure from Castaic and Bouquet Reservoirs in the event of a major earthquake. J i r r • ��pSL 44 tel • Table 1 Modified Mercalll Intensity Scale Of 1831 Modified Mercalll Description of Effect Intensity Scale Number Not felt except by a very few under especially favorable circumstances. II Felt only be a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may 'swing. III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motorcars may rock slightly. Vibration like passing of truck. Duration estimated. IV During the day felt indoors by many, outdoors by few. At night some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motorcars rocked noticeably. V Felt by nearly everyone, many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbances of trees, poles, and other tall objects sometimes noticed. Pendulum clocks may stop. VI Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight. Vil Everybody runs outdoors. Damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable In poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motorcars. Viii Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fail of chimneys, factory stacks, columns, monuments, and walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Persons driving motorcars disturbed. IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken. X Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks. XI Few, If any, (masonry) structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipelines completely out of service. Earth slumps and land slips in soft ground. Rails bend greatly. XII Damage total. Practically all works of construction are damaged greatly or destroyed. Waves seen on ground surface. Lines of sight and level are distorted. Objects are thrown upward into the air. New Table 2 Relationships Between Peak Ground Acceleration, Peak Ground Velocity, and Modified Mercalll Intensity in California Perceived Shaking Not Felt Weak Light Moderate trop Very trop Severe Violent Extreme Damage Potential None None None V_yr Light i ht Moderate Moderate Heavy VVer Heavy to He Peak.00 Acceleration (a = aravity) <0.00170 017a 0.014a '201 9.03go- 0.0920 .Ort 4.18g4 0.34x 4.34.gt 0-650 0.65a - 1.24a A.24 0.0390 0.180 Peak elo Vci cm/sec 101 to 1_1 1_ 1 to 3.4 .4 to 8.1 to 16 to � ,� 62 0 to m Modified Mercalll Intensi ! 11_111 IV V VI VII VII IX X Table 3 Active Faults Near the City of Santa Clarita A Source: Calilomis Division of Mines and Geology, Probabilistic Seismic Hazanf Assessment for the State of Caliromia, 1998 B Source: Dolan at al, 1995 Approximate Estimated Fault distance from City Moment Limits miles Ma nitude" San Gabriel Crosses City 7.0 Holser 1 mile west of City 6.5 Santa Susana 1 mile south of City 6.6 Blind Thrust B Below City, depth of 7.5 6 B miles Sierra Madre -San 6 miles south of City 6.7 Fernando fault zone Oak Ride 7 miles west of City 6.9 San Andreas 16 miles northeast of 8.5 — city A Source: Calilomis Division of Mines and Geology, Probabilistic Seismic Hazanf Assessment for the State of Caliromia, 1998 B Source: Dolan at al, 1995 Table 4 Significant Historic Earthquakes Felt within City Limits Sources: Areas Damaged by California Earthquakes, 1900-1949, California Department of Conservation Division of Mines and Geology, DMG Open File Report 82-17,1982. Toppozada, T.R., 1995, History of Damaging Earthquakes in Los Angeles and Surrounding Area, in Woods, M. C. and Seiple, W. R. editors, The Northridge, California, Earthquake of 17 January 1994; California Department of Conservation Division of Mines and Geology, Special Publication 116 p. 9- 16. Trinet Rapid Instrument Intensity Map for Hector Mine Earthquake, Trinet web page www.trinet.org, 2001 Earthquake Affect Date Earthquake Location and County Richter within City Magnitude Limits July 10 1855 Los Angeles, Los Angeles County About 6 Unknown January 9, 1857 Fort Tejon, Los Angeles County 7.8 Significant Structural dama e September 5, 1883 Ventura -Kern County border About 6 Unknown April 4, 1893 San Fernando Valley, Los Angeles 5.5-5.9 House County destroyed October 23 1916 Near Lebec Kern County 5.2 MMI- 4 April 21 1918 Near'San Jacinto Riverside County 6.8 MMI- 6 June 29, 1925 Santa Barbara Channel, Santa 6.3 MMI- 5 Barbara County March 11, 1933 Huntington Beach (Long Beach 6.3 MMI- 5 -earthquake), Orange Count July 1, 1941 Santa Barbara Channel, Santa 5.9 MMI- 5 Barbara County March 15, 1946 Northeastern Kern County 6.3 MMI- 5 April 10 1947 Central San Bernardino County 6.2 MMI- 5 December 4, 1948 Near Desert Hot Springs, Riverside 6.5 MMI- 5 County July 21 1952 White Wolf Fault Kern County 7.5 MMI- 7 February 2,1971 San Fernando (Sylmar), Los Angeles 6.7 MMI- 8 County December 3 1988 Pasadena, Los Angeles County 5.0 MMI- 5 October 1 1987 Whittier Narrows Los Angeles County 5.9 MMI- 5 June 28 1991 Sierra Madre Los Angeles County 5.8 MMI- 6 January 17, 1994 Northridge, Los Angeles County 6.7 MMI- 9, Roadway collapse, Li uefaction October 16 1999 Hector Mine San Bernardino County 7.1 MMI- 4 Sources: Areas Damaged by California Earthquakes, 1900-1949, California Department of Conservation Division of Mines and Geology, DMG Open File Report 82-17,1982. Toppozada, T.R., 1995, History of Damaging Earthquakes in Los Angeles and Surrounding Area, in Woods, M. C. and Seiple, W. R. editors, The Northridge, California, Earthquake of 17 January 1994; California Department of Conservation Division of Mines and Geology, Special Publication 116 p. 9- 16. Trinet Rapid Instrument Intensity Map for Hector Mine Earthquake, Trinet web page www.trinet.org, 2001