county of los angeles -...
TRANSCRIPT
THOMAS A. TIDEMANSON, Director
September 6, 1990
Mr. Earl w. Hart Senior Geologist
COUNTY OF LOS ANGELES DEPARTMENT OF PUBLIC WORKS
900 SOUTH FREMONT AVENUE ALHAMBRA, CALIFORNIA 91803-1331
Telephone; (818) 458-5100
Division of Mines and Geology Department of Conservation Bay Area Regional Office 380 Civic Drive, Suite 100 Pleasant Hill, CA 94523-1997
SUBMITTAL OF REPORT - TRACT 48457, PALMDALE
Dear Mr. Hart:
ADDRESS ALL CORRESPONDENCE TO: P.0.BOX 1460
ALHAMBRA, CALIFORNIA 91802-1460
IN REPLY PLEASE REFER TO FILE· L-4
The enclosed fault-geologic report is submitted in compliance with policies and criteria of the State Mining and Geology Board (Section 3603-f) for the Alquist-Priolo Special Studies Zones Act.
Very truly yours,
T. A. TIDEMANSON Director of Public Works
~~N~Z~t~~ Supervising Civil Enginee Land Development Division
DP:sh H:48457
C -~-,
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GEOLOGIC AND SEISMOLOGIC INVESTIGATION FOR A PROPOSED DEVELOPMENT OF SINGLE-FAMILY RESIDENCES NEAR AVENUE T-8 AND 50TH STREET EAST, PALMDALE AREA,
TRACT 48457, LOS ANGELES COUNTY, CALIFORNIA
Prepared For
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Northridge, California 913dilJ /J!)
Report No. 90-113
May 1990
~sM\<--
JUN 0 6 1990
PROCESSING CENTER LAND DEV. DIV.
SCHELL GEOLOGICAL CONSULTING. COMPANY
3454 Olive Avenue Long Beach, California 90807
Telephone: (213) 595-9100
-.wS JW.l111tw.------S_C_H_E_L_L_G_e_o_lo_g_i_ca_l_C_on_s_u_l_ti_·n_g_C_o_m_p_a_n_y
• '-
14 May 1990
Royal Investment Inc. P.O. Box 7355 Northridge, California 91327
Attention: Herb Yeung
Project Number 90-113
SUBJECT: GEOLOGIC AND SEISMOLOGIC INVESTIGATION FOR A PROPOSED DEVELOPMENT OF SINGLE-FAMILY RESIDENCES NEAR AVENUE T-8 AND 50TH STREET EAST, PALMDALE AREA, TRACT 48457, LOS ANGELES COUNTY, CALIFORNIA
• Dear Mr. Yeung,
•
Pursuant to your request, the accompanying report has been prepared for the purpose of providing information on surface faulting and engineering geology within the subject site. If you have any questions regarding information presented in this report, plaease call me.
It has been a pleasure working on this project and I hope to be of service to you on similar projects in the future.
'/?")~ {n~· . {/ Bruce A. Schell , RG 3524, EG 1434
3454 Olive Avenue, Long Beach, California 90807 Telephone: (213) 595-9100
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TABLE OF CONTENTS
page
LIST OF ILLUSTRATIONS .......... ,............................ 3 1. 0 INTRODUCTION . . . . . . . . • . . . . . . . . • . . . . . . . . . . . . . . . . . . . • . . . . 4
1 . 1 PURPOSE ....................•.... I • • • • • • • • • • • • • • • • 4 1 . 2 SCOPE OF WORK • . . • • • . • • . . • . . . • . . . . . . . • . • • . • . . . • . • . 4 1. 3 SITE DESCRIPTION ..................•. I • • • • • • • • • • • • 4 1.4 METHODS OF INVESTIGATION .......................... 7
2.0 GEOLOGY•••••••••••••••••••••••••••••••••••••••'"•····· 8 2. 1 REGIONAL GEOLOGY . . . • . . . . . . . . . . . . • . . . . . . . . . . . . . . . 8
2.1.1 Regional Geologic Setting ................. 8 2.1.2 Regional Geomorphology ................... 8 2.1.3 Regional Structure ....................... B 2.1.4 Regional Stratigraphy ..................... 10
2. 2 SITE GEOLOGY .... , . . . . . • . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2.1 Site Geomorphology ....................... 16 2.2.2 Site Stratigraphy ................•......... 17 2.2.3 Site Structure ........................... 17
3.0 SEISMOLOGY AND GROUND RUPTURE •••••••····••••••••••••• 21 3.1 SEISMICITY .. I ••••••••••••••••••••••••••••••••••• 21 3.2 GROUND RUPTURE •••••••••••••••••••••••••••••••··· 21 3. 3 GROUND MOTION .... I ..... I........................ 25
3.3.1 Uniform Building Code Seismic Criteria .... 25 3.3.2 Maximum Probable Ground Motion ............ 26 3.3.3 Maximum Credible Ground Motion ......•..... 26
3.4 EARTHQUAKE-INDUCED GROUND FAILURES .............. 28 3. 4 .1 Liquefaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4.2 Lurching .................................. 29 3.4.3 Tectonic Uplift and Subsidence ...•....... 30 3.4.4 Slope Failures (Landsliding) ........••...• 30
4.0 OTHER GEOTECHNICAL CONSIDERATIONS .................•.. 31 4.1 HYDROCOMPACTION •. ••••••• .......................•. 31 4.2 EXPANSIVE SOILS .................................• 31 4.3 RECOMPACTION OF TRENCH EXCAVATIONS ............... 31
5 . 0 HYDROLOGY .•... I •••••••••••••••••••••••••••••••••• I • • • 3 2 6.0 CONCLUSIONS AND RECOMMENDATIONS .•.................... 33
6 .1 GENERAL .............................. I........... 33 6 . 2 FOUNDATION s .... I ••••• I •••••• I • I •• I ••••••• I • • • • • • • 3 3 6. 3 GRADING ..................... I................... 33 6 . 4 FA ULT RUPTURE ••••••...........•..•..•••.••••.. I • • • 3 4 6. 5 GROUND MOTION . ..••.............•....•. I •• I • • • • • • • • 34
7.0 STATEMENT OF LIMITATIONS ...............•....•.•....... 35 8. 0 REFERENCES CITED •. I ••••••• I ••••••••••••••••••••••• I • • • 3 6
APPENDIX: GEOLOGIC TIME SCALE
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FIGURE NUMBER
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TABLE NUMBER
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90-113
LIST OF ILLUSTRATIONS
TITLE PAGE
Location Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Regional Topographic Map
Geological Provinces Map
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.................... 6
9
Alquist- Priolo Special Studies Zones Map ... 11
Regional Geological Map ........... ••••••••••• 12
Regional Geological Cross-Section........... 13
Site Geological Map ......................... Log of Trench 1 .............................
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Regional Seismicity Map...................... 22
Large Historical Earthquakes Map............. 23
TITLE
Stratigraphic Column .................. , ..... . 14
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1.0 INTRODUCTION
1.1 PURPOSE
The purpose of this investigation was to obtain information on geological and seismological conditions for evaluating the feasibility of developing a site for construction of singlefamily residences. The site is located about 2 miles southeast of the city of Palmdale in Los Angeles County, California (Figure 1). The investigation was conducted to satisfy the requirements of the Alquist-Priolo Act of 1972 and the county of Los Angeles building requirements.
1.2 SCOPE OF WORK
Work performed for this investigation included the following: 1) analysis of aerial photographs, 2) review of pertinent published geologic literature and
maps, and unpublished geologic reports, 3) excavation and logging of 1 trench 4) analysis of data collected, and 5) preparation of this report •
1.3 SITE DESCRIPTION
The site is a rectangular-shaped lot with a westerly extending "panhandle" in the northwest corner (Figure 2). The site is presently undeveloped but remnants of building foundations , irrigation lines, and a linear alignment of Tamarisk trees indicates that part of the site was occupied by a farm or orchard during this century. The easiest access to the site is via 47th Street East and Avenue T-8, both of which are paved roads.
The immediate area surrounding the site is undeveloped except to the south where single-family residences of Narcissa View Estates (TR 31579) are nearly complete. Another part of the Narcissa View Estates (TR 44400) is about half a mile to the west.
The site is about 1247 feet long in the north-south direction and about 449 feet in the east-west direction. The site is bounded on the south by Avenure T-8 which has just recently been paved. The eastern boundary is at 50th Street East which is an ungraded dirt trail as is Avenue T-4 at the northern site boundary.
The site has a primarily sandy surface with scattered pebbles and a few cobbles. Vegetation at the site consists of sparse grasses, brush, and the above-mentioned row of Tamarisk trees . Grasses occur as thin, low, ground cover and as scattered, widely spaced clumps. Brush consists mostly of Sage and Mormon Tea.
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1.4 METHODS OF INVESTIGATION
Prior to beginning field work, published geologic literature and unpublished reports on file at the county of Los Angeles were reviewed for pertinent information. The principal published source of geologic information was California Division of Mines and Geology (CDMG) Open-File Report 85-10 (Barrows, et al, 1985). The CDMG report provided preliminary information on geologic conditions in the site region.
Two pairs of U.S. Department of Agriculture, black and white, stereo, aerial photographs were analyzed. These were
AXJ-12K-166 and 167. dated 4-15-53, scale 1:20,000 AXJ-13K-21 and 22, dated 5-30-53, scale 1:20,000.
The use of older photographs is significant because they allow examination of geologic conditions prior to recent modification of natural conditions by man.
One long trench was dug by a 4-wheel-drive backhoe with an extendable boom capable of excavating to depths of about 15 to 16 feet. Trenches were excavated to a depth where stratigraphic conditions were sufficient to evaluat~ fauJ.ting, The depth of excavation varied from a maximum-o.f-a~out 14 fee} to a minimum of about 6 feet depending on the conditions encountered. The principal objective was to excavate into an older, reddish-brown geologic unit underlying the young surficial alluvial deposits (see Section 2.2.2) .
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2.0 GEOLOGY
2.1 REGIONAL GEOLOGY
2.1.1 Regional Geologic Setting
The site is situated near the boundary between the Western Transverse Ranges and the Mojave Desert geologic provinces (Figure 3). The boundary between these provinces in this region is the San Andreas fault zone which is the major structural discontinuity between the North American and Pacific tectonic plates.
2.1.2 Regional Geomorphology
The site area is near the southwestern edge of the Mojave Desert near the base of the San Gabriel Mountains. The Mojave Desert is a broad, flat, sandy region with scattered hills and isolated peaks. The San Gabriel Mountains, lying to the south of the property, are a rugged and lofty range of mountains that are amoung the highest in California. Throughout much of the winter the higher peaks are capped with snow. Runoff from rainfall and melting snow in the mountains provides water for the desert fringes below.
The site area is at the northern edge of low-relief hills between these two extremes, in an area of predominantly ancient alluvial fans which form a gently sloping apron of sediments extending from the mountains to the desert. These alluvial fans have been deeply eroded by streams descending from the mountains. This erosion is greatly affected by uplift of the mountains relative to the desert as the tectonic plates grind past each other. The alluvial fan surfaces occur at several different elevations and have differing degrees of erosional maturity indicating alternating periods of erosion and stability between tectonic pulses. Generally the oldest fans are the highest and theyounger fans are the lowest in elevation. Movement along the San Andreas fault system has modified the toes of the alluvial fan complex into a series of northwest-southeast trending ridges and troughs along the lower margins of the alluviai fans. The ridge just south of the site -is part of this ridge/trough complex.
2.1.3 Regional Structure
The San Andreas fault extends from the Salton Sea area of southern California to Cape Mendocino in northern California, a distance of more than 600 miles, As should be expected of a crustal break of such great length, the San Andreas is not a simple narrow discontinuity, but a rather complex zone of major
90-113 8
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FIGURE 3. GEOLOGICAL PROVTNCF8 MAP
• and subsidiary faults, in some places of great width. As shown on the Special Studies Zones Map of the Palmdale Quadrangle (Figure 4), two special-studies zones are delineated in the site region. These are the San Andreas fault zone which includes the Little Rock and the Nadeau faults, and the Cemetery fault zone. The site is about 2,000 feet north of the San Andreas Special Studies Zone and about 6,000 feet southeast of the mapped termination of the Cemetery fault Special Studies Zone. However, Barrows et al (1985) have mapped faults of late-Quaternary age southeast of, but aligned with, the Cemetery fault (Figure 5) suggesting that the Cemetery fault may extend southeasterly beyond the Special Studies Zone.
Figure 6 shows a portion of a regional geologic cross-section from Barrows et al (1985) which was drawn through the area about 4,000 feet southeast of the site. The approximate location of the site is projected to the cross-section to show the approximate spatial relationships of the site to the regional geologic structures. (Note: The distance of the projection may be too far to provide an exact representation of the stratigaphic and structural relationships at the site; see section 2.2 for detailed site relationships).
• 2.1.4 Regional Stratigraphy
•
Rocks in the region surrounding the site generally consist of three major groups. These are 1) basement rocks (Mesozoic intrusive rocks and Precambrian metamorphic rocks), 2) middle Tertiary volcanic rocks, and 3) upper Tertiary sedimentary rocks. Although the•e rocks are commonly brought to the surface by tectonic activity along the faults, over much of the immediate site vicinity, they are covered by Quaternary-age sediments. The Quaternary sediments are composed of a variety of alluvial fan, slopewash, and stream deposits.
Figure 5 is a part of Barrows et al (1985) geological map showing the diversity of geologic formations in the site region. -Descriptions of the principal geologic formations identified by Barrows et al (1985) in the immediate site area are summarized below and on Table 1.
Younger Alluvium
Alluvium (Qal): Non indurated to weakly indurated, mostly undissected, stream gravel, sand, and silt.
Slope Wash (Qsw): Non indurated sand and rubble transported predominantly by mass wasting and runoff and deposited directly downslope from local sources .
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TABLE 1
STRATIGRAPHIC COLUMN
STRATIGRAPHIC UNIT APPROXIMATE AGE
YOUNGER ALLUVIUM Alluvium (Qal) Slope Wash (Qsw)
Holocene (<10,000 yrs) Holocene (<10,000 yrs)
OLDER ALLUVIUM with Pelona Schist Clasts (Qopl) with Pelona Schist & Syenite Clasts (Qops) with Volcanic and Syenite Clasts (Qovs)
Nadeau Gravel (Qn)
Harold Fm (Qh, Qhp
SEDIMENTARY ROCKS
Late Pleistocene Late Pleistocene Late Pleistocene
Late Pleistocene 600,000 yrs
Pleistocene 500,000 to 1.2 my
Anaverde Fm (Tar) Late Miocene to Early Pliocene 6 to 10 my
BASEMENT ROCKS Holcomb Quartz Monzonite and Granodiorite (Hqm)
NOTES:
Mesozoic
1) Abbreviations and symbols used on this table include: Fm = formation, my = million years, yrs =_years, < = less than.
2) Ages are from Barrows et al, 1985 .
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Older Alluvium
Older Alluvium with Pelona Schist Clasts (Qopl): Non indurated to weakly indurated, poorly stratified, moderately to deeply dissected, alluvial gravel, sand, and silt with subangular to rounded Pelona Schist clasts commonly mixed with varying proportions of angular to rounded granitic or gneissic clasts and clasts locally derived from underlying, older,sedimentary formations. North of the San Andreas fault zone,these deposits commonly cap ridges on older rocks and may locally overlie or be contemporaneous with Nadeau Gravel.
Older Alluvium with Pelona Schist and Syenite Clasts (Qops): Non indurated to moderately indurated, poorly sorted, poorly stratified, moderately dissected, stream gravel, sand, and silt containing subangular to rounded Pelona Schist clasts and angular to subangular clasts of ferruginous syenite. Angular to subangular granitic or gneissic fragments and granular debris may be present locally.
Older Alluvium with Volcanic and Syenite Clasts (Qovs): Non indurated to moderately indurated, poorly stratified to massive, poorly sorted, fluvial gravel, sand, and silt composed of a mixture of angular to subangular ferruginous syenite clasts and angular to subrounded fragments of volcanic rocks derived from the Vasquez formation. Angular to subrounded clasts derived from Pelona Schist or reworked from older rocks and mixed with granitic debris are present locally.
Nadeau Gravel (Qn): Coarse, poorly sorted, loose to weakly indurated, poorly stratified, stream, pebble to boulder gravel with a dark, reddish-brown, earthy matrix or, less commonly, a dark-gray sandy, matrix • Subangular to rounded disk- or blade-shaped, dark-bluish-gray, micaceous, Pelona Schist clasts predominate. Very common, angular, massive white quartz fragments scattered upon the reddish soif of the Nadeau Gravel make the unit readily recognizable. Locally contains clasts of ferruginous syenite, magnetitequartz rock, light-colored granitic rock, and volcanic rock from the Vasquez Formation. Remnants of ~adeau Gravel typically occur as ridge caps. Thickness commonly less than 25 feet.
Harold Formation, Undifferentiated (Qh): Light-brown, buff, light- to dark-gray, or reddish-brown, weakly to moderately indurated, massive to moderately well-stratified, silty, sandy, and gravelly alluvial fan and playa deposits that formed in a terrain of low relief and low stream gradient • Carbonate common as coating on clasts, crack-fillings, thin beds, and nodules. Resistant cemented layers occur locally.
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Harold Formation, Pelona Schist-Clast Member (Qhp): Wellbedded stream gravel with interbedded coarse sand. Locally poorly sorted. Silver and greenish-gray, subangular to subrounded micaceous Pelona Schist debris comprises at least 80 percent of the pebble- to cobble-size clasts. Also contains clasts of light-colored granitic rocks, maroonstained ferruginous syenite, and, rarely, Vasquez Formation volcanic debris. Although this member contains a clast assemblage similar to that of Nadeau Gravel, it lacks the distinctive red matrix, is generally finer grained, better stratified, and contains abundant carbonate veining.
Sedimentary Rocks
Anaverde Formation, Red Arkose Member (Tar): Pink to red, medium- to thick-bedded, locally massive, coarse, pebbly arkose and, near the bottom, arkosic conglomerate with very angular to subangular pebbles and cobbles of biotite hornbledne diorite. Contact wiith .underlying breccia member arbitrarily placed with the basal conglomerate where pervasive internal shearing resembles that in the breccia member.
Basement Rocks
Medium- to coarse-grained, buff-weathering, pinkish-white quartz monzonite to light-gray and black-and-white gneissose hornblende biotite granodiorite. Contains marble, skarn, amphibolite, and gabbro inclusions. Cut by aplite, alaskite, and pink-and-white pegmatite dikes and sills, and by many faults.
2.2 SITE GEOLOGY
2.2.1 Site Geomorphology
The site is essentially flat and gently slopes northerly from about 2840 feet elevation at its highest point near the southern property line to about elevation 2800 near the northern property line (Figure 2). A minor drainage channel extends northerly along and just west of the western property li~e.
Analysis of the 1953 aerial photographs suggest that the present topography is approximately natural. The aerial photographs indicate the surface of the site is mostly a slightly higher geomorphic surface with a smoother, more-mature microtopography than the drainage channel to the west. These characteristics indicate it is an older geomorphic surface than the.drainage channel to the west •
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2.2.2 Site Stratigraphy
Geologic formations in the site region are described in Section 2.1.4 and their areal distribution is shown on the regional geological map (Figure 5) and the site geological map (Figure 7). Investigations performed for this project such as trenching, aerial-photograph analysis, and geological mapping generally verified the presence of the geologic formations as mapped by Barrows et al (1985). Based on geomorphology, the site geologic map (Figure 7) differentiates two surface geologic units Qal 1 ,
and Qal 2 • Qal 2 corresponds to Unit 1 in Trench 1. Qal 1 is younger stream alluvium in the modern drainage channel along the western property margin.
Trenching revealed four stratigraphic units underlying the site (Figure 8). The upper three units are primarily sandy alluvial deposits of Holocene age, Unit 1 is a soil that has developed on sandy alluvial deposits of Units 2 and 3. Based on thickness, clay content, carbonate content, color, and soil structure, this soil is of Holocene age. Stratigraphic Unit 2 is a light brown, moderately well-bedded, sand and gravelly sand unit which appears to be a stream or distal alluvial fan deposit, probably originating from the hills just south of the site. Unit 3 is generally a less-well-bedded, finer grained, silty sand unit that appears to be a mixture of stream alluvium, slopewash, and windblown material. Units 2 and 3 are very distinct in the southern part of the trench, nearer the source area for Unit 2, but in the northern part of the trench, these units are interbedded.
The light-brown Holocene-age units overlie a reddish-brown clayey sand (Unit 4) which appears to be an ancient buried soil horizon that developed on Pleistocene alluvial deposits. The clear and abrupt erosional contact and color change indicate that Unit 4 is considerably older than the overlying units. Correlation to units mapped by Barrows et al (1985) is difficult because differentiation of their units was based primarily on weathered surficial characteristics whereas Unit 4 at the site is buried. Based on degree of soil development and the composition of pebbles and cobbles, Unit 4 is most likely Qopl, but could also be Qops or Qovs, or possibly even Qh. In any case, the unit is of Pleistocene age (Table 1).
Based on projection from regional data (Figures 5· and 6) the site is underlain at shallow depth by basement rocks of the Holcomb Quartz Monzonite (Hqm). However, site trenching did not encounter this unit within the depths excavated (14 feet) for this investigation.
2.2.3 Site Structure
The site is underlain by Holocene and Pleistocene alluvial
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GENERAL DESCRIPTION OF STRATIGRAPHIC UNITS (See graphic log for local variations)
UNIT 1: SOIL-SILTY SAND; with scattered pebbles and small cobbles, some pockets or lenses of loose sandy gravel. Light yellowish-brown (lOYR 6/4). Pockets of organic material, fine roots throuhout but mostly in upper 1 foot. Dry, hard, friable with light pressure. Sand is poorly sorted and ranges from fine grained to granular size. Laminar to moderately blocky soil structure. Diffuse lower contact. Parent material is Unit 2. Incipient to weakly developed carbonate horizon locally.
-------
UNIT 2: SAND; with beds, lenses, and scattered gravel. Color slightly lighter than Unit 1 (lOYR 6/3 to 6/4). Well-bedded in short lenticular beds (individual beds extend to about 10 feet). Generally loose but some beds are hard due to carbonate cementation. Clasts are pebble to cobble size with rare boulder size, well-rounded to subangular. Sharp to clear, planar to wavy lower contact, some interfingering locally and in northern part of trench.
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3: SILTY SAND; pale brown ( lOYR J'/3). Poorly bedded in thick to massive beds. Sand is ~ine grained; some scattered pebbles and small cobbles. More ipebble\cobble beds near base of unit. Dry to slightly moist, dense, slightly cemented (forms smooth wall), fr~able with light pressure. Pockets of cobbles along contact·' Clear to abrupt, wavy; lower contact. (
I 4: CLAYEY SAND; reddish yellow (:7.5YR 6/6) where dry to strong brown (7.5YR 4/6) and reddish brown (5YR 5/6) when moist. Soft in upper 1 to 2 feet', hard in lower part. Dry to slightly moist. Numerous den~ritic veinlets (root holes) filled with carbonate, effervesces violently. Larger holes are open but lined with 1 mm thidk calcic coatings. Some minor discontinuous vertical veiqs in harder lower part of unit (reminiscent of the Harold ~,ormation). In northern part of trench layers of gravelsloccur locally. Clasts consist of dioritics, light granitics, schist (Pelona and other), foliated granitics, dark!gray metamorphosed volcanics (?), gabbroics (very w~athered with thick weathering rind). Most clasts arle completely c.oated with calcium carbonate (about 1 mm thi'ck)
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deposits that dip gently northerly throughout the site and do not show any indication of faulting. The dip of the Holocene units is approximately parallel to the present ground surface. The dip of the older Pleistocene deposit (Unit 4) appears to be less than the overlying Holocene deposits (Figure 8) suggesting minor regional tilting or change in climatic conditions. Any tectonic causes must be from features or processes offsite such as tectonic activity on the San Andreas fault system to the south of the site •
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3.0 SEISMOLOGY AND GROUND RUPTURE
The site is in the tectonically active southern California region and lies near the San Andreas fault and its associated subsidiary faults. Two possible concerns associated with being close to such fault zones are the primary effects of fault rupture and earthquake shaking. In addition to these primary effects, some earthquake-induced soil-failure effects such as liquefaction, landsliding, settlement, and lurching may also need to be evaluated. The following text presents discussions on the potential for both the primary and the secondary effects. In summary, with respect to seismological concerns, the site is buildable providing recommendations contained in this report and sound engineering practices are adhered to.
3.1 SEISMICITY
Figure 9 is a map of earthquakes occurring in southern California during the years 1932 through 1986 with the 1987 Whittier earthquake and its major aftershock added. There has been no significant earthquake activity in the site region since these events so the map provides an accurate representation of the seismicity in the site area .
Figure 10 shows the location of large earthquakes known to have occurred in southern California since the area was inhabited by European settlers in the 18th century. Routine and systematic location of earthquakes by seismograph networks did not take place until 1932 when the Caltech network began operations, so the locations of older events shown on this figure are approximated from historical records.
The earthquake of most significance to the site area is the 1857 earthquake. The location shown on Figure 10 is in the Fort Tejon area·, about 50 miles northwest of the site, which was one of the few settlements in existence at the time of the event. Because of the sparseness of the population, the location of the epicenter is uncertain. Sieh (1984) has suggested that the epicenter was at the northern end of the surface rupture near Parkfield in central California, some 160 miles from the site. Even though the epicenter of the event may have been quite some distance from the site area, the event is still important for seismic design in the site area because the event was accompanied by a 180-mile-long surface rupture from the Carrizo Plain, through the site region, to near San Bernardino.
3.2 GROUND RUPTURE
As discussed above, the latest ground rupture event in the site region occurred on the San Apdreas fault in 1857. Lateral
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EARTHQUAKE MA.GNITUOE s ,.
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SEISMICITY MAP. EARTHQUAKES RECORDED FROM 1932 THROUGH 1986 WITH MAGNITUDES OF 4 AND GREATER,
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surface displacements along the northern part of the rupture were about 30 feet (Sieh and Jahns, 1984). In the site area, displacements were much smaller. Geomorphic features in the site region, such as offset stream channels, indicate that fault displacement was about 9 feet in a right-lateral sense during the 1857 event (Barrows et al, 1985), Studies by Sieh (1978, 1984) at Pallett Creek, about 10 miles southeast of the site area, documented 12 rupture events since about 260 AD. Strike-slip displacements during these events ranged from a few inches to about 16 feet.
The time intervals between events at Pallett Creek ranged from about 50 years to more than 300 years with an average of about 130 years. Because 132 years have elapsed since the last major earthquake, a large earthquake seems likely in the near future. Probability analyses by The Working Group on California Earthquake Probabilities (1988) indicate there is about a 30 percent probability that the Mojave segment of the San Andreas fault could rupture within the next 30 years. However, Sieh et al (1989) suggest that large earthquakes on the Mojave segment occur in clusters. Recurrence intervals within clusters are less than 100 years, but between clusters they are about 200 to 300 years. If the 1857 earthquake was the final event in a cluster, the probability of the next event occurring in the near future may be lower than previously thought.
Generally, geologists and seismologists think that the next rupture on the San Andreas fault system will occur primarily along the same trace as the 1857 event because it appears to be the only fault in the system that has had recurrent activity in late Holocene time. However, as worldwide historical events have shown, small secondary surface ruptures on subsidiary faults during a large earthquake on the main rupture cannot be precluded, especially on existing faults right next to the main rupture.
The ground rupture and earthquake potential of the Little Rock fault is unknown. The fault is an ancestral trace of the San Andreas system but hasn't acted in that capacity in lateQuaternary time. The latest displacements are primarily normal and reverse. The fault is considered potentially active on the Alquist-Priolo special studies zone map of the.Palmdale quadrangle. However, several trenches across the' feature by this consultant have yet to reveal any good evidence for Holocene activity. As described by Barrows et al (1985), the feature is not defined by any recent activty, but rather by features such as juxtaposed rock types and soil contrasts, These types of features show where the fault is, but they don't reveal much about the fault's activity. In fact, the great amount of scarp degradation along the fault trace suggests that a large portion of the fault has not ruptured the surface in Holocene time.
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The Cemetery fault is the least prominent of the major faults in the site region. As a continuous feature, the fault is about 2 miles long near the city of Palmdale. However, short faults occur in similar structural positions southeasterly to the Cheseboro Road area (Figure 5). If these segments are part of the Cemetery fault, the fault would have a total length of about 6 or 7 miles (Barrows et al, 1985). Quaterary activity along the Cemetery fault is indicated by juxtaposition of Quaternary stratigraphic units such as the Nadeau formation against younger boulder gravels. Holocene displacement is indicated by a low, north-facing scarp across holocene alluvium near 30th Street East (Barrows et al, 1985). Contrasts in rock type across the fault suggest that the fault is a right-lateral strike-slip fault with a north-side-down dip-slip component.
3.3 GROUND MOTION
The effects of ground rupture can be effectively miminized by not constructing buildings across the surface trace of active faults. However, the shaking which accompanies faulting cannot be avoided and will affect all structures within tens of miles of the earthquake epicenter. The strength of the ground motion experienced at a site essentially is a function of the magnitude of the earthquake, the distance and geologic conditions between the earthquake and the site, and the foundation conditions at the site. Generally, shaking is strongest close to an earthquake epicenter and weaker at farther distances.
The strength of the ground motion to be expected at a particular site can be estimated by several techniques and it is up to the designing engineer or regulating agencies to decide on the appropriate method for a site. For critical structures such as dams, hospitals, and power plants, sophisticated site-specific response spectra analyses are generally conducted. These methods are not discussed in this report. Ground-motion analyses for non critical structures are generally less rigorous and are based on published ground-motion attenuation relationships. To use these relationships, the maximum earthquake magnitude and the sourceto-site distance must be known. Most of these types of seismic design analyses use two levels of ground motion, a maximum probable ground motion and a maximum credible ground motion. Established methods for seismic design are sumarized below.
3.3.1 Uniform Building Code Seismic Criteria
The most-widely used standard for earthquake resistant design is the Uniform Building Code (UBC), and this is the method most commonly applied to single-family residences. The UBC can be considered a minimum standard to safeguard against major failures and loss of life, The aim of the code is to provide structures
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that will 1) resist minor earthquakes without damage; 2) resist moderate earthquakes without structural damage,
but with some non-structural damage; 3) resist major earthquakes without collapse, but with some
structural and non-structural damage. The UBC method does not involve complex dynamic evaluations as the other methods might but, rather, uses equivalent static lateral forces. The basic formula used in the UBC is V=ZKCW. The Z factor represents the seismic zone in which the site is located and is the only factor in the equation governed by geology and seismology; the others are derived by engineers. The appropriate Z factor for the site area, obtained from the seismic risk map included in the UBC, is Zone 4.
3.3.2 Maximum Probable Ground Motion
The maximum probable ground motion is based on the maximum probable earthquake (MPE). The MPE is the maximum earthquake likely to occur during a specified time interval. The California Division of Mines and Geology guidelines (CDMG Note 43) specify a 100-year period. The MPE is based on a probalilistic analysis of earthquake data. Although a time period is stated, the MPE is not an assured event that will occur at that specific time; it is only an estimate of the ground motion likely to be experienced during the stated interval, based on what has happened in the past. Algermissen et al (1982) estimate a maximum probable ground motion of about 0.40g in the site area for a 100-year period. This ground motion has a 90 percent probability of not being exceeded in 10 years.
3.3.3 Maximum Credible Ground Motion
The maximum credible ground motion is based on a deterministic approach using the magnitude of the maximum credible earthquake (MCE). The MCE is described as the maximum earthquake capable of occurring under the presently known tectonic framework; it is a rational and believable event that is in accord with all known geologic and seismologic facts (California Div~sion of Mines and Geology, Note 43).
To determine the MCE, geologic data on fault-rupture lengths and displacements, as well as historical seismicity, must be analyzed. As discussed in the preceding section, the central segment of the San Andreas fault last ruptured in 1857. The magnitude of this event is unknown because it occurred before seismograph networks were in place. The amount of damage and size of the area over which the event was felt, suggest it was similar in size to the 1906 San Francisco earthquake which had a
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surface-wave magnitude (M 6 ) of about 8.25. More recently, a moment magnitude (Mwl of 7.9 has been estimated for the 1857 event from the size of the rupture area and amount of displacement (Sieh, 1978). An event of this size would cause larger ground motions at the subject site than a smaller event on closer faults therefore such an event is considered to be the MCE.
The segment of the San Andreas fault adjacent to the Mojave Desert has been remarkably aseismic since the 1857 event. As shown on Figure 9, there have been only a couple small earthquakes recorded in the vicinity of the San Andreas since a seismograph network was established in 1932. With the establishment of a more-sensitive network in recent years, more small events (about 10 events between 1978 and 1985) were detected, but these were all less than magnitude 4. Rather than signifying that the San Andreas is not active, the low level of seismic activity is generally interpreted by seismologists and geologists to mean that the fault is locked and storing strain energy for a large earthquake.
Determination of the maximum credible ground motion at the site from a great earthquake on the San Andreas is difficult because there are no empirical data on ground motions at a site directly adjacent to a magnitude 8.25 earthquake. The site area is about 4,500 feet from the trace of the 1857 rupture. Based on the attenuation relationship of Schnabel and Seed (1973), a magnitude 8.25 earthquake would generate accelerations of about 0.75g at a distance of 2 miles, the shortest distance considered in their analysis, However, their attenuation relationship is extrapolated well beyond the limits of empirical data and does not consider recent earthquakes which were more-completely instrumented than pre 1973 events. More-recent attenuation relationships may provide more-realistic ground motions.
A recent analysis by Joyner and Fumal (1985) indicates accelerations of about 0.70g for a magnitude 7.5 event at a distance of about half a mile. Their attenuation relationship does not predict accelerations for events larger than 7.5 or nearer than one-half mile.
Campbell's (1987) attenuation relationship suggests peak horizontal accelerations of about 1.0g from a magnitude 8 earthquake at about 2.5 miles distance, the closest distance used in his relationship.
Seed and Idriss (1883) indicate accelerations of about 0.70g for a magnitude 8.5 event at a distance of about 1 mile.
Krinitzky et al (1988) argued that attenuation relations such as Joyner and Fumal's and Campbell's, that do not consider accelerations in excess of 1.0g, are not conservative.
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Although the input parameters and derived values from the various relationships given above vary somewhat, determining a reasonable design maximum credible ground motion is still feasible. Estimating a maximum ground motion for events larger than M = 7.5 at close distances may not be as uncertain as it seems because the most attenuation relationships suggest that accelerations do not increase much above some saturation level. For example, the study by Joyner and Fumal (1985) shows that accelerations increase substantially as the distances d~crease to about 6 miles, but closer than that, the accelerations remain about the same.
Considering the above diecussion, peak ground motions at the site should be in the 0.75g to l.Og range. However, these acc~lerations are peak ground accelerations (PGA) which represent the highest peak or a single, short-duration, instantaneous pulse of ground motion. The use of PGA for seismic design represents a highly conservative or worst-case situation that may not be representative of the true damage potential. Damage during earthquakes is primarily a function of the duration of strong ground motion. For significant damage to occur, strong shaking must be sustained for a longer time than just that of the peak value. Ploessal and Slosson (1974) showed that 65 percent of PGA is a more realistic estimate of the strength of ground motion that should be used to determine design parameters for modern engineered structures. Applying the 65 percent criterion to the peak ground motions given above establishes maximum credible ground motions for the site from an M9 =8.25 (Mw=7.9) event to be in the range of about 0.50g to 0.65g.
3.4 EARTHQUAKE-INDUCED GROUND FAILURES
In addition to ground rupture and shaking, various other effects can result from earthquake shaking, Primarily, these are liquefaction, lurching, uplift, settlement, and landsliding. The potential for these are discussed in detail below.
3.4.1 Liquefaction
Liquefaction is the transformation of a granul~r material from a solid state into a liquefied state caused by increased pore-water pressure due to earthquake shaking (Youd and Perkins, 1978). Four types of ground failure commonly result from liquefaction: lateral spreading, flow failure, ground oscillation, and loss of bearing strength. Generally, before liquefaction can occur, four conditions must be present at the site:
1) the materials (soils) must be cohesionless, 2) the materials must have low density, 3) the ground-water table must be shallow, and 4) an earthquake must generate strong ground shaking for a
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sufficient duration.
In general, the more recently a sediment has been deposited, the more likely it is to be susceptible to liquefaction, and certain types of deposits, such as stream-channel and flood-plain deposits, are more susceptible than gravelly, poorly sorted, alluvial fan or clayey deposits (Youd and Perkins,1978; Tinsley et al, 1985). Most of the site is underlain by late- to middleHolocene age, well sorted to poorly sorted, sandy, weakly to moderately indurated deposits. Some of these deposits may be susceptible to liquefaction but most of them should have a low potential. The southwestern and northwestern corners of the site may be underlain by Holocene-age stream deposits of the type susceptible to liquefaction.
Liquefaction susceptibility generally decreases as the depth to ground water increases. Liquefaction has occurred up to depths of about 100 feet, but such cases are rare. The studies by Tinsley et al (1985) have shown that when all other factors are favorable and equal, the relative susceptibility controlled by water depth is as follows:
Water Depth Liquefaction Susceptibility
shallower than 10 feet 10 to 30 feet 30 to 50 feet
deeper than 50 feet
very high high low
very low
The depth to groundwater at the site is not known but groundwater. does not occur in the Qal 2 Holocene deposits underlying the site.· The deeper deposits where groundwater may occur have a low susceptibility to liquefaction, therefore the liquefaction potential at the site is low. If structures are planned in the Qal 1 deposits of the southwestern or northwestern corners of the site (Figure 7), the project soils engineer should verify the potential for liquefaction in those areas.
3.4.2 Lurching
Lurching is a phenomenon associated with strong earthquakes whereby the ground is disturbed and cracked by earthquake oscillations. This phenomena is poorly understood and thus difficult to predict. Factors such as topography and ground water may play an important role. Although lurch effects should be expected near large earthquakes, these effects generally have not been known to cause catastrophic failures or structural collapse, except in areas of steep slopes. Typical effects of lurching are cracked walls and foundations and rupture of roads, decking, and irrigation systems .
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3.4.3 Tectonic Uplift and Subsidence
Areas in proximity to active faults can be uplifted or can subside if large earthquakes occur. The subsidence can result from either crustal depression or from compacting of granular, cohesionless sediments. Uplift or subsidence can change the existing slope angles or tilt ground that is now horizontal. Generally these changes occur over large areas and are minor with respect to a small site. Such tilts generally are not of a lifethreatening nature. A typical effect might be that drainage ditches no longer drain as well as originally intended. Other than avoidance of susceptible areas, there is little that can be done to prevent such effects and the usual procedure is to correct the damage after the event.
3.4.4 Slope Failures (Landsliding)
Severe shaking from earthquakes has been known to weaken natural slopes causing slope failure.
The site is essentially flat so there is little liklihood of slope failures •
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4.0 OTHER GEOTECHNICAL CONSIDERATIONS
4.1 HYDROCOMPACTION
Alluvial fan sediments deposited in arid environments are commonly susceptible to hydrocompaction. Hydrocompaction occurs when water is introduced into these sediments, such as by irrigation. This compaction removes foundation support to structures built on the compacting sediments.
The site is underlain by Holocene and Pleistocene alluvial sediments of possible alluvial fan origin and thus should be considered susceptible to hydrocompaction. New houses within the adjacent Narcissa View Estates development just west of the site are experiencing settlements suspected as being due to hydrocompaction. The project soils engineer should fully evaluate soils at the site for their hydrocornpaction potential.
4.2 EXPANSIVE SOILS
Clayey soils of certain mineralogical composition are subject to shrinking when they dry and swelling when they become wet; such soils are called expansive soils.
No deposits with high clay contents were revealed within the site area during this investigation.
4.3 RECOMPACTION OF TRENCH EXCAVATIONS
A north-south trending trench was excavated across the site area for this investigation (Figure 7). Although the trench was filled and compacted by driving a backhoe over the filled excavation, the backfill may be subject to settlement if water is introduced to the soils or when loads from surface structures are placed upon the backfill.
If plans call for construction of structures the area where the trench was excavated, the trench backfill material should be recompacted in accordance with standard engine~ring practice .
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5.0 HYDROLOGY
Presently, there are no major active surface drainages crossing the site. A minor surface drainage crosses the southwestern and northwestern corners of the site. There was no water in this drainage during the site investigation. Just south of the site, this drainage was being modified in conjunction with the development of the Narcissa View Estates. Similar measures may be required at the site. The project civil engineer should make recommendations regarding drainage at the site.
The depth to groundwater at the site is unknown. No water was encountered in site excavations which extended to a maximum depth of about 14 feet .
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6.0 CONCLUSIONS AND RECOMMENDATIONS
6.1 GENERAL
The conclusions and recommendations contained in this report are based on information provided to the consultant, information gathered from published sources, information gathered by onsite spot investigations by the consultant, and on the experience and professional judgement of the consultant. The recommendations contained in this report are consistent with standard industry practice. No other warranties are expressed or implied. Although some degree of risk is associated with any development, it is this consultants opinion that the site can be developed without adverse impact on adjoining properties, providing that regulatory agency requirements are adhered to, good construction and engineering practices are maintained, and the recommendations in this report are followed.
6.2 FOUNDATIONS
Preliminary plans call for single-family residences along the eastern side of the property, near 50th Street East. Specific plans as to the type of structures, their locations, areas to be cut and filled, etc. were not available to this consultant so no specific recommendations can be made.
The loose and weakly indurated sandy alluvial deposits at the site may be subject to hydrocompaction.
The exploratory trench fill should be recompacted if structures are to be founded in the area of the excavation.
6.3 GRADING
As stated above (Section 6.2), the proposed development had not proceeded to the stage where specific grading plans were prepared when this investigation was conducted, so no specific recommendations regarding grading can be made in this report. Some general observations and recommendations ~re as follows:
The site is underlain by 3.5 to 10 feet of weakly indurated sandy deposits which are easily excavatable using conventional grading equipment. The lower part of stratigraphic Unit 4 below depths of 6 to 12 feet is hard in places and may require ripping.
There were no features observed during this investigation that indicate foundation materials to be grossly unstable from a slope-stability viewpoint. The Holocene sandy deposits (stratigraphic Units 2 and 3; Figure 8) are weakly indurated and '
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generally will require shoring in temporary excavations and recompaction or retaining walls in permanent excavations. The project soils engineer should make recommendations regarding the appropriate treatment for permanent slopes.
6.4 FAULT-RUPTURE
No active faults were revealed at the site during this investigation.
6.5 GROUND-MOTION
The UBC seismic zone is zone 4.
Maximum probable ground motions are estimated to be about 0.40g.
Maximum credible peak horizontal accelerations in the range of 0.75g to 1.0g and repeatable accelerations of 0.50g to 0.65g could occur at the site if an M8 =8.25 (Mw=7.9) earthquake were to occur on the San Andreas fault in proximity to the site. Designers should recognize that these values are estimated from data extrapolated from events with large source-to-site distances relative to the situation at this site, therefore the possibility of somewhat higher accelerations cannot be ruled out.
The ground motions likely to be generated if a large earthquake occurs on the San Andreas fault could cause secondary ground deformations such as tilting and lurch cracking. Although complete elimination of such hazards cannot be guaranteed, incorporation of the recommendations of this report with standard engineering practices should result in minimization of adverse effects. Special design measures to strengthen structures and foundations would reduce possible effects further. For example, shear walls, extra steel reinforcement in floor pads or posttensioning of foundations may be appropriate. The project engineers and architects, in consultation with the engineering geologist, should make final recommendations on these matters .
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7.0 STATEMENT OF LIMITATIONS
The intent of this report is to advise the client on geologic matters involving the proposed development and to provide documentation for regulatory agencies responsible for issuing building permits. Because some verbal discussions between the client and consultant may not be included in this report, other use of this report can only be authorized by the client. Any transferring of information or other use directed by the client should be considered "advice by the client."
The services performed at the site and analyses given in this report were conducted in a manner consistent with the level of care and skill ordinarily exercised by members of the profession practicing in the same area under similar conditions. No other warranties are expressed or implied. Any errors or omissions noted by any party reviewing this report, and (or) any other geological aspect of the project, should be reported to this consultant in a timely fashion.
Consistent with industry practice, the interpretations and opinions expressed in this report are based on observations made at selected localities within and in proximity to the site and on information gathered by others. Results are based on the assumtion that conditions do not vary appreciably between observation points and that information from others is correct. No other warranties are expressed or implied. Although no significant variation is anticipated, it is important to recognize that variations commonly occur and could result in modification of results.
The conclusions and recommendations presented should be considered "advice". Other consultants could arrive at different conclusions and recommendations. Consistent with industry practice, minimum recommendations have been presented. Final decisions on acceptable risk are the responsibility of the client and (or) governing agencies. No warranties in any respect are made as to the performance of the project.
If any information presented in this report is not clearly understood or questions arise with respect to the information presented, this consultant should be notified._
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8.0 REFERENCES CITED
Algermissen, S.T., Perkins, D.M., Thenhaus, P.C., Hanson, S.L., and Bender, B.L., 1982. Probabilistic estimates of maximum acceleration and velocity in rock in the contiguous United States: U.S. Geological Survey Open-File Report 82-1033.
Barrows, A.G., Kahle, J.E., and Beeby, D.J., 1985. Earthquake hazards and tectonic history of the San Andreas fault zone, Los Angeles County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 85-10.
California Division of Mines and Geology, 1979. Special studies zone map, Palmdale quadrangle.
Campbell, K.W., 1987, Predicting strong gound motion in Utah, in Assessment of regional earthquake hazards and risk along the Wasatch Front, Utah: U.S. Geological Survey Open-file Report 87-585, p. Ll-L90.
Division of Mines and Geology, 1985. Fault rupture hazard zones in California .
Joyner, W.B. and Fumal, T.E., 1985. Predictive mapping of earthquake ground motion, in Evaluating earthquake hazards in the Los Angeles region: U.S. Geological Survey Professional Paper 1360, p. 203-220.
Ploessal, M.R., and Slosson, J.E., 1974. Repeatable high ground accelerations from earthquakes: California Geology, v. 27, p. 195-199.
Krinitzky, E.L., Chang, F.K., and Nuttli, O.W., 1988. Magnitude-related earthquake ground motions: Bulletin of the Association of Engineering Geologists, v. XXV, p. 399-423.
Schnabel, P.B., and Seed, H.B., 1973. earthquakes in the western United Seismological Society of America,
Accelerations in rock for States: Bulletin of the v. 63 p.501-516.
Seed, H.B., and Idriss, I.M., 1983. Ground motions and soil liquefaction during earthquakes: Earthquake Engineering Research Institute Monograph, Berkeley, California.
Sieh, K.E., 1978. Prehistoric large earthquakes produced by slip on the San Andreas fault at Pallett Creek, California. Journal of Geophysical Research, v. 83, p. 3907-3939 .
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Sieh, K.E., 1984. Lateral offsets and revised dates of large prehistoric earthquakes at Pallett Creek, Southern California: Journal of Geophysical Research, v. 89, p.7641-7670.
Sieh, K.E., and Jahns, R.H., 1984. Holocene activity of the San Andreas fault at Wallace creek: Geological Society of America Bulletin, v. 95, p. 883-89.
Sieh, K.E., Stuiver, M., and Brillinger, D., 1989. precise chronology of earthquakes produced by Andreas fault in southern California: Journal Geophysical Research, v. 94, p. 603-623.
A more the San of
The Working Group on California Earthquake Probabilities, 1988. Probabilities of large earthquakes occurring in California on the San Andreas fault: U.S. Geological Survey Open-File Report 88-398, 62 p.
Tinsley, J.C., Youd, T.L., Perkins, D.M., and Chen, A.T.F., 1985. Evaluating liquefaction potential, in Evaluating earthquake hazards in the Los Angeles region-an earth science perspective: U.S. Geological Survey Professional Paper 1360, p.263-315.
Uniform Building Code, International Conference of Building Officials, Whittier, Ca.
Youd, T.L., and Perkins, D.M., 1978. Mapping liquefactioninduced ground failure potential: Journal of the Geotechnical Engineering Division, American Society of Civil Engineers, v.104:GT4, p. 433-446 .
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' • DECADE OF NORTH AMERICAN GEOLOGY
.... 1983 GEOLOGIC TIME SCALE CEOLOOICAL SOCl!TY OF AMl!lllTCA
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