DOCSLIB.ORG

The Enigmatic San Gorgonio Pass

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
The Enigmatic San Gorgonio Pass The enigmatic San Gorgonio Pass Doug Yule Department of Geological Sciences, California State University, Northridge, 18111 Nordhoff Street, Northridge, California 91330-8266, USA The largest discontinuity along the San P 117° 15′ 117° ip 116° 45′ W e s Andreas fault occurs in the San Gorgonio Pass C a t a (SGP) region of Southern California. Here, the n h n 1992 M 7.3 t a r u y w n A s t o a n S San Bernardino and Coachella Valley strands fa u l (from the northwest and southeast, respectively) 1992 Mw 6.2 t disaggregate into a family of irregular and discon- l t tinuous right-lateral, reverse, thrust, and oblique- M i San Gorgonio a u Yucca Valley l l C Mountain f r e a i n e k n t S f a n t o o u normal faults (Fig. 1) (Allen, 1957; Matti et al., an u l t P i M Be rna 1985; Matti and Morton, 1993; Yule and Sieh, rdin . o - .z S f an 2003; Langenheim et al., 2005). Seismological ills An i s s i n H dr M o n C to ea r f s e a f Burro Flats e k studies also show that crustal deformation here Cr au l f a t u l is diffuse and distributed primarily on strike-slip t 34° N 1986 M 5.9 L 1992 M 6.1 and thrust faults at depths of ~5–20 km (Jones w G Desert Hot Spgs S o a n r B C g anning fa o et al., 1986; Seeber and Armbruster, 1995; ult a o c h n e s ll i a s C a Magistrale and Sanders, 1996; Hauksson, 2000; o P f m oac V a l t s y e he a Banning u s t G a lla 1948 Mw 6.0 lle S r Va y Carena et al., 2004). Deformation along the San a n e t lle - n y - S J H B a a i an n l l ni A Andreas fault zone in the SGP region is therefore c ng nd S a i f a r n u fa ea C n ul s t l t f broadly distributed, in contrast to regions to the o o t au s J a c i n t o lt f a a north and south where deformation is restricted L u o l m t C o a Palm Springs a c to a much narrower zone. h e l l a V a l l e y f ot a H Sp N r V a l l e y Interpretations of the surface and subsur- u in San Jacinto Peak. l t g s face data have produced competing three- fa u l 33° 45′ dimensional (3-D) kinematic models for the 0 10 km t SGP region. All models include a combination of thrust and strike-slip faulting. Based on their Figure 1. Shaded-relief topographic map of San Gorgonio Pass (SGP) region. Traces of active faults shown in red, inactive faults shown in black. Mapping from Allen (1957), analysis of aligned nodal planes and slip vectors, Matti et al. (1985), and Yule and Sieh (2003). Beach-balls show epicenter and oblique slip dur- Seeber and Armbruster (1995) proposed that ing earthquakes on the north-dipping San Gorgonio Pass–Garnet Hill fault system at depth. a throughgoing, near-vertical strike-slip fault Stars show epicenters of other earthquakes on faults outside of SGP. exists at depth beneath the active SGP thrust. In this model, the thrust has “beheaded” the active San Andreas fault in the upper 5 km of the crust. Cooke (2009, p. 119 in this issue of Geology) 2005; Dair and Cooke, 2009; W. Behr, 2008, A second model envisions the San Andreas fault have taken advantage of an ever-increasing personal commun.). Even Dair and Cooke’s zone as a complex system of strike-slip and computing capacity and the relatively new vertical fault model reveals this trend. As they north-dipping thrust faults that transfer dextral- Southern California Earthquake Center Com- discuss, this decrease in slip can be explained oblique slip through the SGP region (Matti and munity Fault Model (CFM) (Plesch et al., 2007) by slip transfer to the west onto the San Jacinto Morton, 1993; Magistrale and Sanders, 1996; to compare vertical versus dipping fault mod- fault zone and to the east onto the Eastern Cali- Yule and Sieh, 2003; Carena et al., 2004). A els in the SGP. By applying a 3-D numerical fornia Shear Zone. However, the geologic slip third model is based on the analysis of gravity model to vertical and north-dipping scenarios, rates reported for SGP come from single fault and aeromagnetic data that identify a buried Dair and Cooke found that the dipping model strands, and therefore provide a minimum esti- magnetic source beneath the SGP. Langenheim provides a better match to the available strike- mate. The broad zone of deformation defi ned by et al. (2005) interpreted this to be a block of San slip and uplift data for the San Andreas fault and the active faults in Figure 1 suggests that an as Gabriel and/or Mojave/San Bernardino Moun- southern San Bernardino Mountains. This inno- yet undetermined amount of dextral slip is parti- tains basement. In this model, a thrust-bounded vative result implies that, at least in the case of tioned onto a number of structures in the SGP. It wedge of Peninsular Ranges basement has been SGP, an oversimplifi ed fault model introduces is conceivable that an additional 50% of dextral inserted beneath the Banning and SGP thrust systemic errors and yields fl awed results. Given slip (3–5 mm/yr) could be carried by secondary faults and above the buried magnetic block. this result, it is intriguing to consider how model structures (folds and faults) in SGP, increas- Several researchers have used a relatively results may change in other regions where over- ing the total slip rate to 8–15 mm/yr. If correct, simple, vertical fault geometry to model the simplifi ed fault geometries have been used. this overlaps with slip rates obtained from both kinematics of the San Andreas fault in the SGP All of the available data indicate that San the vertical and the dipping model of Dair and (Meade and Hager, 2005; Olsen et al., 2006; Andreas fault slip decreases to a minimum of Cooke (2009). This point does not diminish their Smith and Sandwell, 2006). The choice to over- 5–10 mm/yr at the SGP, from 24 ± 3.5 mm/yr at fi ndings, but illustrates the need to continue this simplify the crustal structure at SGP relates to Cajon Pass on the northwest and 12–22 mm/yr work as well as geologic slip-rate studies. For the limits of computing capacity and the dif- at Indio on the southeast (Weldon and Sieh, example, it would be interesting to take Dair fi culty in using dipping fault grids. Dair and 1985; Yule and Sieh, 2003; Meade and Hager, and Cooke’s study a step further and analyze a © 2009 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, February February 2009; 2009 v. 37; no. 2; p. 191–192; doi: 10.1130/focus022009.1. 191 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/37/2/191/3941948/i0091-7613-37-2-191.pdf by guest on 29 September 2021 dipping fault model that incorporates all of the to consider using this method to evaluate sce- Matti, J.C., Morton, D.M., and Cox, B.F., 1985, active faults in the SGP region (Fig. 1). nario earthquakes. For example, by studying Distribution and geologic relations of fault sys- tems in the vicinity of the central Transverse One overarching objective of the research the available active fault maps (e.g., CFM), Ranges, Southern California: U.S. Geological in the SGP region is to answer a long-standing one could test whether or not scenario ruptures Survey Open-File Report 85–365, 23 p. question: is a throughgoing San Andreas rupture violate the 3-4 km limit set by historical data, Meade, B.J., and Hager, B.H., 2005, Block models possible? A defi nitive answer to this question either by rupturing across a >3-4 km step or by of crustal motion in Southern California con- has fundamental implications for forecast- stopping at a < 3-4 km step. At SGP, though dis- strained by GPS measurements: Journal of Geophysical Research, v. 110, B03403, doi: ing the earthquake hazard in the Los Angeles connected, the active faults shown in Figure 1 10.1029/2004JB003209. region. If “yes,” the region can expect relatively reveal no step larger than 3 km. Assuming that Olsen, K.B., Day, S.M., Minster, J.B., Cui, Y., Chourasia , infrequent but very large earthquakes (~Mw 7.8) slip occurs on all active structures in the SGP, A., Faerman, M., Moore, R., Maechling, P., and that rupture through SGP and simultaneously the Wesnousky (2008) model therefore supports Jordan, T., 2006, Strong shaking in Los Angeles expected from southern San Andreas earth- involve the Coachella Valley, San Bernardino, the Mw 7.8 ShakeOut scenario of a San Andreas quake: Geophysical Research Letters, v. 33, and the Mojave segments. If “no,” the region fault earthquake rupturing through the SGP. L07305, doi: 10.1029/2005GL025472.
Load more

Recommended publications
  • Introduction San Andreas Fault: an Overview
    Introduction This volume is a general geology field guide to the San Andreas Fault in the San Francisco Bay Area. The first section provides a brief overview of the San Andreas Fault in context to regional California geology, the Bay Area, and earthquake history with emphasis of the section of the fault that ruptured in the Great San Francisco Earthquake of 1906. This first section also contains information useful for discussion and making field observations associated with fault- related landforms, landslides and mass-wasting features, and the plant ecology in the study region. The second section contains field trips and recommended hikes on public lands in the Santa Cruz Mountains, along the San Mateo Coast, and at Point Reyes National Seashore. These trips provide access to the San Andreas Fault and associated faults, and to significant rock exposures and landforms in the vicinity. Note that more stops are provided in each of the sections than might be possible to visit in a day. The extra material is intended to provide optional choices to visit in a region with a wealth of natural resources, and to support discussions and provide information about additional field exploration in the Santa Cruz Mountains region. An early version of the guidebook was used in conjunction with the Pacific SEPM 2004 Fall Field Trip. Selected references provide a more technical and exhaustive overview of the fault system and geology in this field area; for instance, see USGS Professional Paper 1550-E (Wells, 2004). San Andreas Fault: An Overview The catastrophe caused by the 1906 earthquake in the San Francisco region started the study of earthquakes and California geology in earnest.
    [Show full text]
  • Springs of California
    DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY GEORGE OTIS SMITH, DIBECTOB WATER- SUPPLY PAPER 338 SPRINGS OF CALIFORNIA BY GEKALD A. WARING WASHINGTON GOVERNMENT PRINTING OFFICE 1915 CONTENTS. Page. lntroduction by W. C. Mendenhall ... .. ................................... 5 Physical features of California ...... ....... .. .. ... .. ....... .............. 7 Natural divisions ................... ... .. ........................... 7 Coast Ranges ..................................... ....•.......... _._._ 7 11 ~~:~~::!:: :~~e:_-_-_·.-.·.·: ~::::::::::::::::::::::::::::::::::: ::::: ::: 12 Sierra Nevada .................... .................................... 12 Southeastern desert ......................... ............. .. ..... ... 13 Faults ..... ....... ... ................ ·.. : ..... ................ ..... 14 Natural waters ................................ _.......................... 15 Use of terms "mineral water" and ''pure water" ............... : .·...... 15 ,,uneral analysis of water ................................ .. ... ........ 15 Source and amount of substances in water ................. ............. 17 Degree of concentration of natural waters ........................ ..· .... 21 Properties of mineral waters . ................... ...... _. _.. .. _... _....• 22 Temperature of natural waters ... : ....................... _.. _..... .... : . 24 Classification of mineral waters ............ .......... .. .. _. .. _......... _ 25 Therapeutic value of waters .................................... ... ... 26 Analyses
    [Show full text]
  • Nativeandintroducedearthworms
    United States Department of Native and Introduced Earthwormsfrom Agriculture Forest Service Selected Chaparral, Woodland, and Pacific Southwest Research Station Riparian Zones in Southern California General Technical Report PSW-GTR-142 Hulton B. Wood Samuel W. James Wood, Hulton B.; James, Samuel W. 1993. Native and introduced earthworms from selected chaparral, woodland, and riparian zones in southern California. Gen. Tech. Rep. PSW-GTR142. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture; 20 p. Relatively little is known about the earthworm fauna of southern California. Some 20 different species of earthworms were collected and identified in a survey of various southern California wildland habitats. The ecology and biology of earthworms are outlined, and the results of the survey are documented. Introduced species belonging to the Lumbricidae family were encountered most often; however, native species, primarily of genera Argilophilus and Diplocardia, are widely, distributed. Several of the natives collected are believed to be new species. Habitats for both the native and introduced species ranged from riparian zones to relatively dry chaparral sites. Preference of earthworms for certain types of plant communities began to emerge even in this somewhat limited survey: oak and grass being the most preferred, and conifers the least. Geographical separation of the two principal native genera occurs at about 34°N. Further research is needed relative to earthworm ecology, impacts on soils, ecosystem dynamics, and fire. An appendix includes all collection records by location, and vegetation and soil type. Retrieval terms: earthworms, Mediterranean ecosystems, southern California, new species, Lumbricidae, Megascolecidae, Acanthodrilinae, Ocnerodrilinae, Sparganophilidae, soil, habitat, chaparral, grassland, woodland, riparian, aquatic The Authors: Hulton B.
    [Show full text]
  • Palms to Pines Scenic Byway Corridor Management Plan
    Palms to Pines Scenic Byway Corridor Management Plan PALMS TO PINES STATE SCENIC HIGHWAY CALIFORNIA STATE ROUTES 243 AND 74 June 2012 This document was produced by USDA Forest Service Recreation Solutions Enterprise Team with support from the Federal Highway Administration and in partnership with the USDA Forest Service Pacific Southwest Region, the Bureau of Land Management, the California Department of Transportation, California State University, Chico Research Foundation and many local partners. The USDA, the BLM, FHWA and State of California are equal opportunity providers and employers. In accordance with Federal law, U.S. Department of Agriculture policy and U.S. Department of Interior policy, this institution is prohibited from discriminating on the basis of race, color, national origin, sex, age or disability. (Not all prohibited bases apply to all programs.) To file a complaint of discrimination, write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, DC 20250-9410 or call (202) 720- 5964 (voice and TDD). Table of Contents Chapter 1 – The Palms to Pines Scenic Byway .........................................................................1 Introduction ................................................................................................................................. 1 Benefits of National Scenic Byway Designation .......................................................................... 2 Corridor Management Planning .................................................................................................
    [Show full text]
  • Chapter IV. ENVIRONMENTAL RESOURCES
    Chapter IV. ENVIRONMENTAL RESOURCES City of Banning General Plan WATER RESOURCES ELEMENT PURPOSE The Water Resources Element addresses water quality, availability and conservation for the City’s current and future needs. The Element also discusses the importance of on-going coordination and cooperation between the City, Banning Heights Mutual Water Company, High Valley Water District, San Gorgonio Pass Water Agency and other agencies responsible for supplying water to the region. Topics include the ground water replenishment program, consumptive demand of City residents, and wastewater management and its increasingly important role in the protection of ground water resources. The goals, policies and programs set forth in this element direct staff and other City officials in the management of this essential resource. BACKGROUND The Water Resources Element is directly related to the Land Use Element, in considering the availability of water resources to meet the land use plan; and has a direct relationship to the Flooding and Hydrology Element, in its effort to protect and enhance groundwater recharge. Water issues are also integral components of the following elements: Police and Fire Protection, Economic Development, Emergency Preparedness, and Water, Wastewater and Utilities. The Water Resources Element addresses topics set forth in California Government Code Section 65302(d). Also, in accordance with the California Environmental Quality Act (CEQA), Section 21083.2(g), the City is empowered to require that adequate research and documentation be conducted when the potential for significant impacts to water and other important resources exists. Watersheds The westernmost part of the planning area is located at the summit of the San Gorgonio Pass, which divides two major watersheds: the San Jacinto River Watershed to the west and the Salton Sea watershed to the east.
    [Show full text]
  • Region of the San Andreas Fault, Western Transverse Ranges, California
    Thrust-Induced Collapse of Mountains— An Example from the “Big Bend” Region of the San Andreas Fault, Western Transverse Ranges, California By Karl S. Kellogg Scientific Investigations Report 2004–5206 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director U.S. Geological Survey, Reston, Virginia: 2004 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. iii Contents Abstract ……………………………………………………………………………………… 1 Introduction …………………………………………………………………………………… 1 Geology of the Mount Pinos and Frazier Mountain Region …………………………………… 3 Fracturing of Crystalline Rocks in the Hanging Wall of Thrusts ……………………………… 5 Worldwide Examples of Gravitational Collapse ……………………………………………… 6 A Spreading Model for Mount Pinos and Frazier Mountain ………………………………… 6 Conclusions …………………………………………………………………………………… 8 Acknowledgments …………………………………………………………………………… 8 References …………………………………………………………………………………… 8 Illustrations 1. Regional geologic map of the western Transverse Ranges of southern California …………………………………………………………………………… 2 2. Simplified geologic map of the Mount Pinos-Frazier Mountain region …………… 2 3. View looking southeast across the San Andreas rift valley toward Frazier Mountain …………………………………………………………………… 3 4. View to the northwest of Mount Pinos, the rift valley (Cuddy Valley) of the San Andreas fault, and the trace of the Lockwood Valley fault ……………… 3 5.
    [Show full text]
  • Nehrp Final Technical Report
    NEHRP FINAL TECHNICAL REPORT Grant Number: G16AP00097 Term of Award: 9/2016-9/2017, extended to 12/2017 PI: Whitney Maria Behr1 Quaternary geologic slip rates along the Agua Blanca fault: implications for hazard to southern California and northern Baja California Abstract The Agua Blanca and San Miguel-Vallecitos Faults transfer ~14% of San Andreas-related Pacific-North American dextral plate motion across the Peninsular Ranges of Baja California. The Late Quaternary slip histories for the these faults are integral to mapping how strain is transferred by the southern San Andreas fault system from the Gulf of California to the western edge of the plate boundary, but have remained inadequately constrained. We present the first quantitative geologic slip rates for the Agua Blanca Fault, which of the two fault is characterized by the most prominent tectonic geomorphologic evidence of significant Late Quaternary dextral slip. Four slip rates from three sites measured using new airborne lidar and both cosmogenic 10Be exposure and optically stimulated luminescence geochronology suggest a steady along-strike rate of ~3 mm/a over 4 time frames. Specifically, the most probable Late Quaternary slip rates for the Agua Blanca Fault are 2.8 +0.8/-0.6 mm/a since ~65.1 ka, 3.0 +1.4/-0.8 mm/a since ~21.8 ka, 3.4 +0.8/-0.6 mm/a since ~11.8 ka, and 3.0 +3.0/-1.5 mm/a since ~1.6 ka, with all uncertainties reported at 95% confidence. These rates suggest that the Agua Blanca Fault accommodates at least half of plate boundary slip across northern Baja California.
    [Show full text]
  • A Pattern of Seismicity in Southern California: the Possibility of Earthquakes Triggered by Lunar and Solar Gravitational Tides
    A Pattern of Seismicity in Southern California: The Possibility of Earthquakes Triggered by Lunar and Solar Gravitational Tides David Nabhan Helen Keller wrote that the heresy of one age is the orthodoxy of the next, giving ample proof that although sightless, she possessed extraordinary vision. Moreover, it has been said that every new fact must pass through a crucible by which first it is ignored or ridiculed, then vigorously attacked, and finally accepted as though the truth had been apparent from the beginning. Earthquake prediction is no exception but for the anomaly that the first two stages have endured far longer than what the immense populations on the U.S. West Coast might hope to expect regarding a matter so important to their safety and welfare. Residents of Southern California have been left with a bewildering set of claims and counter-claims regarding this issue for many decades. While scientific data Seismic Gap Probabilities. Author's collection. can be presented any number of ways to say any number of things, there is another arbiter that, weighed alongside the conflicting findings, may allow fair-minded observers to come to their own cogent conclusions: history. There exists a long and verifiable record of events on the Pacific Coast and elsewhere David Nabhan, SDSU alumnus, is the author of Earthquake Prediction: Answers in Plain Sight (2012) and two other books on the subject. He is directing a public opinion campaign to impel the governor of California to convene the California Earthquake Prediction Evaluation Council for the purposes of determining the viability or fallibility of a seismic safety plan.
    [Show full text]
  • How Sensitive Are Spatial Estimates of Wilderness Recreation Values to Information About Hiking Destinations? José J
    Journal of Forest Economics, 2020, 35: 19–41 How Sensitive are Spatial Estimates of Wilderness Recreation Values to Information about Hiking Destinations? José J. Sánchez1 , Kenneth Baerenklau2 and Armando González-Cabán1* 1USDA Forest Service, Pacifc Southwest Research Station, 4955 Canyon Crest Drive, Riverside, CA 92507, USA 2School of Public Policy, University of California, Riverside, Riverside, CA 92521, USA ABSTRACT This study uses individual survey data to investigate the impact of information about hiking destinations on estimated wilderness values in a spatial context. The data is derived from a revealed preference survey of backcountry visitors who responded to ques- tions about their recreation behavior in the San Jacinto Wilderness of southern California. Two GIS data layers are developed showing spatial representations of non-market values derived from a Kuhn- Tucker demand model, with and without destination information. Each pixel in each data layer contains an estimate of the recreation value at that location. The destination data provides more detailed information on recreation behavior that can be used to more accu- rately allocate the landscape values. Results show that including destination information produces signifcantly greater heterogeneity in parcel value estimates for large areas of the wilderness. Keywords: GIS, Kuhn-Tucker demand system model, Nonmarket valuation, Web-based survey, Viewshed analysis *Correspondence author: José J. Sánchez, [email protected]. This work is based on the frst author’s PhD dissertation. Generous research funding was provided by a grant from the Western Wildland Environmental Threat Assessment Center for the years 2011– 2013. We thank Andy Smith and the sta˙ at San Bernardino National Forest San Jacinto Ranger District, and Bart Grant and sta˙ at Mount San Jacinto State Park & Wilderness for ISSN 1104-6899; DOI 10.1561/112.00000447 ©2020 J.
    [Show full text]
  • Geologic, Climatic, and Vegetation History of California
    GEOLOGIC, CLIMATIC, AND VEGETATION HISTORY OF CALIFORNIA Constance I. Millar I ntroduction The dawning of the “Anthropocene,” the era of human-induced climate change, exposes what paleoscientists have documented for decades: earth’s environment—land, sea, air, and the organisms that inhabit these—is in a state of continual flux. Change is part of global reality, as is the relatively new and disruptive role humans superimpose on environmental and climatic flux. Historic dynamism is central to understanding how plant lineages exist in the present—their journey through time illuminates plant ecology and diversity, niche preferences, range distributions, and life-history characteristics, and is essential grounding for successful conservation planning. The editors of the current Manual recognize that the geologic, climatic, and vegetation history of California belong together as a single story, reflecting their interweaving nature. Advances in the sci- ences of geology, climatology, and paleobotany have shaken earlier interpretations of earth’s history and promoted integrated understanding of the origins of land, climate, and biota of western North America. In unraveling mysteries about the “what, where, and when” of California history, the respec- tive sciences have also clarified the “how” of processes responsible for geologic, climatic, and vegeta- tion change. This narrative of California’s prehistory emphasizes process and scale while also portraying pic- tures of the past. The goal is to foster a deeper understanding of landscape dynamics of California that will help toward preparing for changes coming in the future. This in turn will inform meaningful and effective conservation decisions to protect the remarkable diversity of rock, sky, and life that is our California heritage.
    [Show full text]
  • 216166963.Pdf
    SEISMOLOGICAL RESEARCH ISSUES IN THE SAN DIEGO REGION Thomas H. Heaton and Lucile M. Jones U.S. Geological Survey 525 S. Wilson Ave. Pasadena, CA 911 04 June 1989 INTRODUCTION What is the nature of earthquake ground motions that can be expected in San Diego's foreseeable future? Although this is the most basic of questions underlying the adequate design of structures to resist earthquakes, answers to this question are disturbingly uncertain. A reasonable assumption is that future earthquake ground motions will be similar to those that have occurred in the past. When compared with San Francisco or Los Angeles, San Diego has historically experienced relatively mild earthquake shaking. Unfortunately, San Diego's written history is very short compared to the time scales of earthquake repetition. Are there sources of earthquakes that may cause damage in San Diego and what is their frequency? Mapping of geologic structures and the study of patterns of small earthquakes are the primary tools for recognizing potentially active faults. There are features in both the geologic structure and the seismicity that are suggestive of major active faults that could pose a serious hazard to San Diego. Furthermore, there is evidence that the rate of occurrence of small earthquakes has increased within the last 5 years when compared with the previous 50 years. However, these features are not well studied or understood. Even if the potential sources of earthquakes were well understood, the problem of anticipating the range of future ground motions is difficult. The nature of shaking from earthquakes is strongly affected by the nature of seismic wave propagation through complex geologic structures (path effects).
    [Show full text]
  • 1. NEOGENE TECTONICS of SOUTHERN CALIFORNIA . the Focus of This Research Project Is to Investigate the Timing of Rotation of T
    1. NEOGENE TECTONICS OF SOUTHERN CALIFORNIA. The focus of this research project is to investigate the timing of rotation of the Transverse Ranges and the evolution of the 3-D architecture of the Los Angeles basin. Objectives are to understand better the seismicity of the region and the relationships between petroleum accumulations and the structure and stratigraphic evolution of the basin. Figure 1 shows the main physiographic and structural features of the Los Angeles basin region, the epicenter of recent significant earthquakes and the our initial study area in the northeastern Los Angeles basin. Los Angeles basin tectonic model: Most tectonic models attribute the opening of the Los Angeles basin to lithospheric extension produced by breakaway of the Western Transverse Ranges from the Peninsular Ranges and 90 degrees or more of clockwise rotation from ca. 18 Ma to the present. Evidence of this extension includes crustal thinning on tomographic profiles between the Santa Ana Mountains and the Santa Monica Mountains and the presence in the Los Angeles basin of Middle Miocene volcanic rocks and proto-normal faults. Detailed evidence of the 3-D architecture of the rift created by the breakaway and the timing of the rift phase has remained elusive. The closing of the Los Angeles basin in response to N-S contraction began at ca. 8 Ma and continues today (Bjorklund, et al., 2002). A system of active faults has developed that pose significant seismic hazards for the greater Los Angeles region. Crustal heterogeneities that developed during the extension phase of basin development may have strongly influenced the location of these faults.
    [Show full text]