California Geological Research Articles

Roadside Geology of Northern and Central California, Second Edition

Book Review| August 03, 2022 Roadside Geology of Northern and Central California, Second Edition Robert Anderson Robert Anderson 2552 Humphrey Road, Loomis, CA 9565 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2022) 28 (3): 331. https://doi.org/10.2113/gseegeosci.28.3.331 Article history first online: 03 Aug 2022 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert Anderson; Roadside Geology of Northern and Central California, Second Edition. Environmental & Engineering Geoscience 2022;; 28 (3): 331. doi: https://doi.org/10.2113/gseegeosci.28.3.331 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Reading Roadside Geology of Northern and Central California, Second Edition (Alt and Hyndman, 2016), was like getting reacquainted with an old friend. A few things were different but nothing of consequence. For those looking for a short overview of local and regional geology along the alignments described, it is an easy and quick read supported by color images, easy-to-read cross sections, and simplified local geologic maps. At $26.00, the book is small enough to carry in a day pack or suitcase and travels well. The book is quite a bargain and would make a great gift for anyone... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Seismic response of the Eureka Channel Bridge-Foundation system

Recorded earthquake motions from the highly instrumented Eureka Channel bridge-foundation system have been compiled and made available by the California Geological Survey. During eight seismic events, more than 20 data channels have been documenting the seismic response of the bridge, abutments, and adjacent ground surface. Of special interest is the response of one of the bridge piers with records at the deck level, pile cap and within the underlying pile foundation. In this paper, records from the strongest to date 2010 Ferndale earthquake (PGA of about 0.25 g), along with other available low-amplitude events (2007–2014) are employed to evaluate the ground, pile foundation, and overall bridge seismic response. Spatial variation of the recorded motions is examined. Linear and nonlinear response of the ground and the bridge are assessed using system identification techniques. Finite Element (FE) modeling is employed along with the optimization software framework SNOPT, to derive salient response characteristics of the bridge system and its foundations. As such, the FE model is calibrated using the recorded data. On this basis, estimates of the column and foundation lateral stiffness during the low amplitude and the strong shaking phases of seismic response are derived. Results of this study, complement those reported earlier for the nearby Samoa Channel Bridge and geotechnical downhole array. In this regard, the Eureka Channel Bridge differs in geometry, foundation layout, and supporting soil profile.

Geologic Investigations for Compliance with the CARB Asbestos ATCM

ABSTRACT The California Air Resources Board Airborne Toxic Control Measure for Construction, Grading, Quarrying, and Surface Mining Operations (ATCM) provides requirements for the evaluation for naturally occurring asbestos (NOA) on a construction site. There are two compliance triggers: (1) a determination that the site is located within a geographic ultramafic rock unit, defined as a geographic area designated as an ultramafic rock on referenced maps, and (2) the presence of NOA, serpentinite, or ultramafic rock. The California Geological Survey requires that NOA evaluations be conducted by a licensed professional geologist. However, under the ATCM, a professional geologist is required only when a property owner wishes to demonstrate that a geographic ultramafic rock unit is not actually represented by ultramafic rocks. The professional geologist who must advise whether the ATCM applies at a construction site is therefore placed in a precarious position. Does a limited desktop review of geologic maps meet any standard of practice? If the ATCM is triggered by the presence of asbestos, is the geologist negligent if no evaluation is recommended or conducted? Could geologic units be pre-screened for asbestos potential? Using case studies and geologic data in the city of San Francisco and East Bay, this presentation reviews these issues and provides a context for the geologist to conduct the appropriate level of investigation for compliance with the ATCM.

Late Cenozoic uplift of the Sierra Nevada, California? A critical analysis of the geomorphic evidence

From the earliest days of California geology, the ramp-like profile of the northern half of the Sierra Nevada mountains and putative signs of recent incision have been interpreted as evidence that the range was formed by the tilting of a rigid block in the late Cenozoic. Over the years, various geomorphic analyses have been used to quantify the magnitude of uplift and to establish its timing, such as analyzing the gradients of ancient channels, examining the tilt of sedimentary beds, and reconstructing the incisional history of rivers. Most studies that have used these methods have supported substantial (>500 m) recent uplift of the Sierra. In contrast, investigations based on other sources of paleotopographic information, such as isotope records, thermochronology, and detrital zircon geochronology, have found that the Sierra have been at high elevations for much of the Cenozoic. This set of contradictory results motivates a re-examination of the geomorphic evidence for late Cenozoic uplift. A critical assessment of these geomorphic studies, based on new topographic analyses and field investigations, reveals that their conclusions are not well supported. For example, several studies based their results on reconstructions of ancient channels that would have flowed up and over bedrock ridges as high as 190 m, a physical impossibility. Other weaknesses include unjustified assumptions regarding the original tilt of fluvial deposits, misinterpretations of stratigraphic relationships, and inadequate recognition of the effect of lithology on channel profiles. The studies supporting recent tilting in the northern Sierra Nevada are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. In addition, analyses are presented to demonstrate that deep northern Sierran canyons thought to have been recently incised were, instead, cut as early as the Eocene-Oligocene. Two geomorphic studies from the southern Sierra are consistent with late Cenozoic tilting and uplift although ongoing tectonic activity may be insignificant. Finally, I present a conceptual model of the evolution of the Sierran landscape, applicable primarily to the northern half of the range, illustrating the development of three different drainage networks since the late Jurassic.

THE EXPLORATION AND FIRST ASCENT OF MOUNT WHITNEY, CALIFORNIA

ABSTRACT Mount Whitney, California, rises 14,505 feet in the Sierra Nevada of California. At the time of its discovery by members of the California Geological Survey in 1864, Mount Whitney was the highest peak in what then comprised the United States and remains the highest peak in the country south of Alaska. This paper examines early attempts to explore the mountain, measure its altitude and accurately plot its location

Geoethics in Hollywood: How Can We Put More Reality into Fault Hazard Zoning?

The 15-km-long Hollywood fault extends through some of the most densely developed and expensive areas in southern California. Given that billions of dollars of real estate could be impacted by an earthquake occurring on the fault, and that tens of thousands of people live near it, the seismic hazard posed by the Hollywood fault is concerning. But how much so? The City of West Hollywood has required geological fault investigations and building avoidance of at least 15 meters from the fault’s active trace since the mid-1990s, resulting in numerous site investigations. Geologic studies in support of the Los Angeles Metro subway and subsequent research found as-yet-unconfirmedequivocal evidence of an early Holocene-age (~8 ka) displacement event, and estimated strain rates of 0.3-0.9 mm/yr. In 2014, and following a “not-in-my-backyard” press campaign against the Hollywood Millennium project, the California Geological Survey zoned the eastern Hollywood fault as Holocene-active under their fault zoning program, requiring geological investigations and building setbacks from Holocene-age faults. In the years since the fault was zoned by the City of West Hollywood and the California Geological Survey, millions of dollars have been invested in geological studies of the fault for building projects. All of these dollars have been reluctantly spent by the private sector, and all of this work has been eagerly done by consulting firms. The results? With only four exceptions, two in West Hollywood, and two in the City of Los Angeles, and all having opportunities for alternative interpretations, no Holocene-age fault displacements have been found. Instead, other studies have found definitive evidence that the fault has not ruptured in 10s to 100s of thousands of years. So what is the ethical alternative? Should we ignore the hazard posed by the fault? Are we geo-professionals too enamored of the financial gains such regulatory zoning provides that we are unwilling to suggest changes? Could we move past the “one size fits all” zoning that requires that single-family homes are held to the same standard as 40-story buildings? In the following sections we present several case studies along the Hollywood fault and provide our suggestions for a more progressive program of fault hazard management.

Recorded seismic response of the Samoa Channel Bridge-foundation system and adjacent downhole array

A large set of earthquake records from the highly instrumented Samoa Channel bridge-ground system has been compiled and made available by the California Geological Survey. During six seismic events, more than 30 data channels have been documenting the seismic response of the bridge, abutments, and adjacent ground surface. Of special interest is the response of one of the bridge piers with records at the deck level, pile cap and within the underlying pile foundation. Response of this pile foundation is compared to that of the ground as documented by the nearby Eureka geotechnical downhole array. In this paper, records from the strongest to date 2010 Ferndale earthquake (PGA of about 0.16g), along with other available low-amplitude events (2007–2014) are employed to evaluate the ground, pile foundation, and overall bridge seismic response. Spatial variation of the recorded motions is examined. Linear and nonlinear response of the ground and the bridge are assessed using system identification techniques. During the strong shaking phase of the 2010 Ferndale Earthquake, a clear and significant stiffness reduction was observed in the column as well as in the foundation of the instrumented pier.

Unusual Downhole and Surface Free‐Field Records Near the Carquinez Strait Bridges during the 24 August 2014Mw 6.0 South Napa, California, Earthquake

This paper reports the results of Part A of a study of the recorded strong‐motion accelerations at the well‐instrumented network of the two side‐by‐side parallel bridges over the Carquinez Strait during the 24 August 2014 ( M w 6.0) South Napa, California, earthquake that occurred at 03:20:44 Pacific Daylight Time (PDT) with epicentral coordinates 38.22° N, 122.31° W (http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2014/20140824.php; last accessed October 2014). Both bridges and two boreholes were instrumented by the California Strong motion Instrumentation Program (CSMIP) of the California Geological Survey (CGS; Shakal et al. , 2014). A comprehensive comparison of several ground‐motion prediction equations as they relate to recorded ground motions of the earthquake is provided by Baltay and Boatright (2015). The moniker “odd couple” is appropriate for the pair of bridges across the Carquinez Strait, because each bridge is of a different design and a different age. The westbound suspension bridge (CGS station 68185), also known as the Alfred Zampa Memorial bridge, opened for service in 2003, when it replaced an older 1927‐era‐construction cantilever truss bridge that was deemed seismically unsafe. The eastbound cantilever truss bridge (CGS station 68184) opened for service in 1958. Figure 1 shows the layout of the two bridges and the locations of the associated geotechnical arrays. The inset in the figure indicates where the bridges are with respect to San Francisco, California. A photograph of both bridges is provided in Figure 2. The azimuths of the west‐ and east‐bound bridges are listed as 358° and 357°, respectively (Center of Engineering Stron Motion Data [CESMD] at http://www.strongmotioncenter.org/NCESMD/photos/CGS/lllayouts/ll68185.pdf and http://www.strongmotioncenter.org/NCESMD/photos/CGS/lllayouts/ll68184.pdf; last accessed April 2015). These parallel bridges are important lifelines on Interstate 80, connecting the California cities of Crockett and Vallejo, the San Francisco Bay area and the Sacramento regions, and the well‐known Napa Valley wine‐country and much of the remainder of northern …

The 2014 Mw 6.1 South Napa Earthquake: A Unilateral Rupture with Shallow Asperity and Rapid Afterslip

The Mw 6.1 South Napa earthquake occurred near Napa, California, on 24 August 2014 at 10:20:44.03 (UTC) and was the largest inland earthquake in northern California since the 1989 Mw 6.9 Loma Prieta earthquake. The first report of the earthquake from the Northern California Earthquake Data Center (NCEDC) indicates a hypocentral depth of 11.0 km with longitude and latitude of (122.3105° W, 38.217° N). Surface rupture was documented by field observations and Light Detection and Ranging (LiDAR) imaging (Brooks et al., 2014; Hudnut et al., 2014; Brocher et al., 2015), with about 12 km of continuous rupture starting near the epicenter and extending to the northwest. The southern part of the rupture is relatively straight, but the strike changes by about 15° at the northern end over a 6 km segment. The peak dextral offset was observed near the Buhman residence with right‐lateral motion of 46 cm, near the location where the strike of fault begins to rotate clockwise (Hudnut et al., 2014). The earthquake was well recorded by the strong‐motion network operated by the NCEDC, the California Geological Survey and the U.S. Geological Survey (USGS). There are about 12 sites within an epicentral distance of 15 km that had relatively good azimuthal coverage (Fig. 1). The largest peak ground velocity (PGV) of nearly 100 cm/s was observed on station 1765, which is the closest station to the rupture and lies about 3 km east of the northern segment (Fig. 1). The ground deformation associated with the earthquake was also well recorded by the high resolution COSMO–SkyMed (CSK) satellite and Sentinel-1A satellite, providing independent static observations.

Surficial Geologic Mapping and Associated GIS Databases for Identification of Alluvial Fans

The California Geological Survey (CGS) has completed a geographic information system (GIS)–based compilation of geologic maps of Quaternary age and older deposits in southern California. The compilation provides a consistent classification of surficial deposits within a 10 county area of interest to the Department of Water Resources (DWR) and the governor9s Alluvial Fan Task Force (AFTF). During the past two decades, rapid population growth in southern California has resulted in increased pressure for residential development on alluvial fans. Due to unique flood hazards associated with alluvial fans, the AFTF reviewed the state of knowledge regarding alluvial fan floodplains and developed recommendations specific to floodplain management. The CGS compilation was designed as a regional-scale planning tool to assist DWR, the AFTF, and local communities in evaluating hazards and locating future development on alluvial fans. The goal of the compilation was to represent southern California9s complex geology in a consistent user-friendly format that depicts areas of most recent alluvial fan deposition on a regional scale. The Derivative GIS Database merges new mapping and existing digital geologic data into a common seamless format that differentiates alluvial fan deposits, related Quaternary deposits, and various older deposits into 40 derivative units at a scale of 1: 100,000 for the entire mapped area. Original source data compiled at various scales and detail are retained in the Source GIS Database.

Late Cenozoic uplift of the Sierra Nevada, California? A critical analysis of the geomorphic evidence

From the earliest days of California geology, the ramp-like profile of the northern half of the Sierra Nevada mountains and putative signs of recent incision have been interpreted as evidence that the range was formed by the tilting of a rigid block in the late Cenozoic. Over the years, various geomorphic analyses have been used to quantify the magnitude of uplift and to establish its timing, such as analyzing the gradients of ancient channels, examining the tilt of sedimentary beds, and reconstructing the incisional history of rivers. Most studies that have used these methods have supported substantial (\>500 m) recent uplift of the Sierra. In contrast, investigations based on other sources of paleotopographic information, such as isotope records, thermochronology, and detrital zircon geochronology, have found that the Sierra have been at high elevations for much of the Cenozoic. This set of contradictory results motivates a re-examination of the geomorphic evidence for late Cenozoic uplift. A critical assessment of these geomorphic studies, based on new topographic analyses and field investigations, reveals that their conclusions are not well supported. For example, several studies based their results on reconstructions of ancient channels that would have flowed up and over bedrock ridges as high as 190 m, a physical impossibility. Other weaknesses include unjustified assumptions regarding the original tilt of fluvial deposits, misinterpretations of stratigraphic relationships, and inadequate recognition of the effect of lithology on channel profiles. The studies supporting recent tilting in the *northern* Sierra Nevada are inconclusive and rely on observations not unique to tectonic forcing. Indeed, much of the evidence based on the paleogradients of the Tertiary channels is consistent with an early trellis drainage network formed across alternating bands of resistant and weak lithologies. In addition, analyses are presented to demonstrate that deep northern Sierran canyons thought to have been recently incised were, instead, cut as early as the Eocene-Oligocene. Two geomorphic studies from the *southern* Sierra are consistent with late Cenozoic tilting and uplift although ongoing tectonic activity may be insignificant. Finally, I present a conceptual model of the evolution of the Sierran landscape, applicable primarily to the northern half of the range, illustrating the development of three different drainage networks since the late Jurassic.

Effects of California's Climate Policy in Facilitating CCUS

Effects of California's Climate Policy in Facilitating CCUS

The ShakeOut Scenario: A Hypothetical Mw7.8 Earthquake on the Southern San Andreas Fault

In 2008, an earthquake-planning scenario document was released by the U.S. Geological Survey (USGS) and California Geological Survey that hypothesizes the occurrence and effects of a Mw7.8 earthquake on the southern San Andreas Fault. It was created by more than 300 scientists and engineers. Fault offsets reach 13 m and up to 8 m at lifeline crossings. Physics-based modeling was used to generate maps of shaking intensity, with peak ground velocities of 3 m/sec near the fault and exceeding 0.5 m/sec over 10,000 km2. A custom HAZUS®MH analysis and 18 special studies were performed to characterize the effects of the earthquake on the built environment. The scenario posits 1,800 deaths and 53,000 injuries requiring emergency room care. Approximately 1,600 fires are ignited, resulting in the destruction of 200 million square feet of the building stock, the equivalent of 133,000 single-family homes. Fire contributes $87 billion in property and business interruption loss, out of the total $191 billion in economic loss, with most of the rest coming from shake-related building and content damage ($46 billion) and business interruption loss from water outages ($24 billion). Emergency response activities are depicted in detail, in an innovative grid showing activities versus time, a new format introduced in this study.

A coast to explore: coastal geology and ecology of Central California

A coast to explore: coastal geology and ecology of Central California

Seismic Hazard Mapping of California considering Site Effects

In this paper, we have combined the U.S. Geological Survey's National Seismic Hazard Maps model with the California geologic map showing 17 generalized geologic units that can be defined by their VS30. We regrouped these units into seven VS30values and calculated a probabilistic seismic hazard map for the entire state for each VS30value. By merging seismic hazard maps based on the seven different VS30values, a suite of seismic hazard maps was computed for 0.2 and 1.0 s spectral ordinates at 2% probability of exceedance (PE) in 50 years. The improved hazards maps explicitly incorporate the site effects and their spatial variability on ground motion estimates. The spectral acceleration (SA) at 1.0 s map of seismic shaking potential for California has now been published as California Geological Survey Map Sheet 48.

History of Modern Earthquake Hazard Mapping and Assessment in California Using a Deterministic or Scenario Approach

Modern earthquake ground motion hazard mapping in California began following the 1971 San Fernando earthquake in the Los Angeles metropolitan area of southern California. Earthquake hazard assessment followed a traditional approach, later called Deterministic Seismic Hazard Analysis (DSHA) in order to distinguish it from the newer Probabilistic Seismic Hazard Analysis (PSHA). In DSHA, seismic hazard in the event of the Maximum Credible Earthquake (MCE) magnitude from each of the known seismogenic faults within and near the state are assessed. The likely occurrence of the MCE has been assumed qualitatively by using late Quaternary and younger faults that are presumed to be seismogenic, but not when or within what time intervals MCE may occur. MCE is the largest or upper-bound potential earthquake in moment magnitude, and it supersedes and automatically considers all other possible earthquakes on that fault. That moment magnitude is used for estimating ground motions by applying it to empirical attenuation relationships, and for calculating ground motions as in neo-DSHA (Zuccoloet al., 2008). The first deterministic California earthquake hazard map was published in 1974 by the California Division of Mines and Geology (CDMG) which has been called the California Geological Survey (CGS) since 2002, using the best available fault information and ground motion attenuation relationships at that time. The California Department of Transportation (Caltrans) later assumed responsibility for printing the refined and updated peak acceleration contour maps which were heavily utilized by geologists, seismologists, and engineers for many years. Some engineers involved in the siting process of large important projects, for example, dams and nuclear power plants, continued to challenge the map(s). The second edition map was completed in 1985 incorporating more faults, improving MCE’s estimation method, and using new ground motion attenuation relationships from the latest published results at that time. CDMG eventually published the second edition map in 1992 following the Governor’s Board of Inquiry on the 1989 Loma Prieta earthquake and at the demand of Caltrans. The third edition map was published by Caltrans in 1996 utilizing GIS technology to manage data that includes a simplified three-dimension geometry of faults and to facilitate efficient corrections and revisions of data and the map. The spatial relationship of fault hazards with highways, bridges or any other attribute can be efficiently managed and analyzed now in GIS at Caltrans. There has been great confidence in using DSHA in bridge engineering and other applications in California, and it can be confidently applied in any other earthquake-prone region. Earthquake hazards defined by DSHA are: (1) transparent and stable with robust MCE moment magnitudes; (2) flexible in their application to design considerations; (3) can easily incorporate advances in ground motion simulations; and (4) economical. DSHA and neo-DSHA have the same approach and applicability. The accuracy of DSHA has proven to be quite reasonable for practical applications within engineering design and always done with professional judgment. In the final analysis, DSHA is a reality-check for public safety and PSHA results. Although PSHA has been acclaimed as a better approach for seismic hazard assessment, it is DSHA, not PSHA, that has actually been used in seismic hazard assessment for building and bridge engineering, particularly in California.

Moderate Earthquake Ground-Motion Validation in the San Francisco Bay Area

Abstract We performed 3D ground-motion simulations for 10 recent small to moderate earthquakes ( M w 4.1–5.4) in the San Francisco Bay area to evaluate two versions of the USGS3D velocity model (Brocher, 2005; Jachens et al. , 2006; Brocher, 2008). Comparisons were made in terms of modeling phase arrival timing, peak ground-motion amplitudes, and the seismic waveforms. In the simulations we assumed the source parameters reported in the Berkeley Seismological Laboratory (BSL) Moment Tensor Catalog. Broadband seismic data from the Berkeley Digital Seismic Network (BDSN), and strong motion data from the USGS and the California Geological Survey (CGS) strong motion arrays were used in the analysis. The comparison of peak ground velocity (PGV) for both models reveals that both 3D models predict the observed PGV well over four orders of magnitude, and P - and S -wave timing and pseudospectral acceleration (PSA) are well modeled by the 3D structure. While the revised model (model 8.3.0) significantly improved the timing of the first arrival, and the waveform fit is generally good, there remain discrepancies in estimated amplitudes and durations that require improvements to the structure. Nevertheless, from our low-frequency (0.5 Hz) analysis we found that the 3D model is suitable for the simulation of PGV to assess the strong shaking hazard of future large earthquakes, because earthquakes larger than M 6 have PGV carried by waves of 1 to several seconds period.

California Geology, Second Edition

California Geology by Deborah Harden is a clearly written and abundantly illustrated college textbook. In the preface, she states “This book is intended to acquaint … students with the basic geologic history of California and to cover those topics that do interest non-geologists.” As a California Professional Geologist and consultant formerly practicing in the state, I think the author meets this goal and much more. The book is divided into three parts. Part I, chapters 1 through 4, consists of the basic principles of geology. Part II, “Geologic Highlights of California,” contains chapters 5 through 17 and is the heart of the book. Part III, “California Geology: An Integrated View,” consists of chapters 18 and 19, which provide a summary and treat geologic hazards and resources. In the following paragraphs of this review, select chapters will be highlighted. Chapter 8 is entitled “The Sierra Nevada: Granite, Gold, and Glaciers.” This chapter lives up to its name; it includes excellent explanations for the origin of the Sierra Nevada Batholith (in the context of plate tectonics) and its overlying roof pendants, a detailed discussion of the Sierran gold deposits, both lode and placer, and a presentation of glaciers with fine illustrations and photos of glacial landforms. Boxed essays in the chapter include, among others, “Caves of the Sierra Nevada,” “the Discovery of Gold by James Marshall in 1848,” “Earth's climatic cycles in relation to glaciers,” and “Lake Tahoe.” A concise summary provides the reasons for the location of the batholith: a subduction zone was present west of the current position of granitic rocks; the zone was oriented parallel to today's California coast; subduction was active along the length of California during Mesozoic time; and the subducted plate melted …

A Proposed Mid-Holocene Age Definition for Hazardous Faults in California

ROY J. SHLEMONR. J. Shlemon & Assoc., Inc., P.O. Box 3066, Newport Beach, CA 92659-0620Key Terms: Hazardous Fault, Alquist-Priolo Act,Surface-Fault Rupture, Geologic Hazards, StructuralMitigation, CaliforniaABSTRACTA California Technical Advisory Committee is nowreviewing the relative hazard of potential surface-faultrupture for single-family structures. Passage of theAlquist-Priolo (AP) Act in 1972 initially defined anactive fault as one with surface or near-surface rupturewithin approximately the past 10 ka. This deterministicnumber evolved to ,11 ka (Pleistocene-Holoceneboundary) and now, locally, to ,13 ka when expressedin calibrated radiocarbon years. Based on lessonslearned since 1972 and on the recurrence consideredacceptable for mitigation of other geologic processes, thepotential for surface-fault rupture should stem fromscience and the professional judgment of qualifiedgeological practitioners. Ideally, no specific determinis-tic number should define a hazardous fault, which may bepractically mitigated based on professional judgment,cost, available technology, and societal constraints. Butthis concept is generally not presently acceptable tomany California geological consultants and regulators.More pragmatic, therefore, is changing the existingactive fault definition from ,11 ka to a mid-Holocene(,4-ka to 6-ka) range. This range is non-rigid andshould be periodically revised based on technologicaladvances in the geologic and engineering sciences. Theproposed ‘‘mid-Holocene’’ age recognizes uncertaintyand closes the gap between the present 11-ka active-faultcriterion and the ,100–500-year recurrence of othernatural hazards typically mitigated by engineeringdesign. Change in regulation does not come easily andmay be detrimental to the economic livelihood of somegeologists. Nevertheless, to sensibly ensure public health,safety, and welfare, we must learn from new science andexperience and then adjust accordingly.INTRODUCTIONThe California State Mining and Geology Board(SMGB) established a Geological Hazards Commit-tee and an associated Technical Advisory Committee(TAC) to review and recommend improvements formitigation of surface-fault rupture potentially affect-ing habitable structures. The TAC discussions havebeen extensive and sometimes volatile. They have,however, produced many ideas for improving inter-pretation and application of regulations stemmingfrom ‘‘lessons learned’’ since passage of the Alquist-Priolo (AP) Act in 1972, a legislative response to the1971 Sylmar (San Fernando) earthquake in LosAngeles (Reitherman and Leeds, 1991). Currentagency policies and interpretations for the AP includedefining an active fault as being Holocene in age,mitigation of surface-fault rupture solely by avoid-ance, and presumption of fault activity within 50 ft(,15 m) unless demonstrably proven otherwise(Bryant and Hart, 2007). In light of new technicaldata, the TAC will now likely recommend that theterm ‘‘active fault’’ be replaced by the terms‘‘hazardous’’ and ‘‘non-hazardous’’ faults, respective-ly. Although not yet definitive, this change recognizesthat geotechnical and structural design can realisti-cally mitigate non-hazardous faults (Bray, 2001),namely those with cumulative ground displacementsof less than ,10 cm vertically and ,30 cm horizon-tally over an ,300-cm distance. In contrast, ahazardous fault cannot be technically or economicallymitigated except by avoidance. In this sense, mitiga-tion implies adequate engineering design to precludecatastrophic failure and loss of life, rather thanprevention of any structural damage, a concept thatis likely not well understood by many homeowners.Ostensibly, the hazardous-fault setback width (bufferzone) will vary and is mainly dependent on site-specific and regional fault characteristics (style andrecurrence) and on inherent uncertainties in exposure,mapping, and surface projection.Still problematic is the age (time of last displace-ment) of a hazardous fault. Based on new technicaldata and on lessons learned since 1972, the presentHolocene (,11 ka) definition for hazardous surface-rupture is an artifact that needs updating, especiallywhen compared with the much greater risks to lifesafety stemming from catastrophic failures outsidefault rupture zones. These failures typically resultfrom high seismic accelerations and ground shaking

History of the Alquist-Priolo Earthquake Fault Zoning Act, California, USA

The Alquist-Priolo Earthquake Fault Zoning (AP) Act was passed in California in 1972 following the destructive 1971 Mw 6.6 San Fernando earthquake. Surface-fault rupture hazard is addressed by prohibiting most structures for human occupancy from being placed over the trace of an active fault. Principal responsibilities under the AP Act are assigned to the following: 1) State Mining and Geology Board (SMGB), 2) State Geologist (California Geological Survey), and 3) lead agencies. The SMGB establishes specific regulations to guide lead agencies in implementing the law. The AP Act requires the State Geologist to issue maps delineating regulatory zones encompassing potentially hazardous faults that are sufficiently active (active in approximately the last 11 ka) and well defined. The first maps were issued in 1974—currently there are 547 maps affecting 36 counties and 104 cities. Lead agencies affected by the zones must regulate development “projects” in which structures for human occupancy are planned within the Earthquake Fault Zones (EFZs). Significant events in the history of the AP Act include A) the establishment of the Fault Evaluation and Zoning Program in 1976 (which also initiated the change from zoning faults with Quaternary displacement to those with Holocene displacement); B) the publication of the Reitherman-Leeds study in 1991, which evaluated the effectiveness of the AP Act; C) earthquakes associated with surface-fault rupture since the AP Act was passed, especially the 1992 Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine events; D) release of digital versions of EFZ maps, Fault Evaluation Reports, and site investigation reports in 2000–2003; and E) the appeal to SMGB by the City of Camarillo, resulting in the establishment of the SMGB's Technical Advisory Committee.