Evidence for prehistoric earthquakes on the Superstition Hills fault from offset geomorphic features

Offset geomorphic features along the Superstition Hills fault show evidence for at least one slip event prior to the 1987 surface rupture, and possibly as many as four to five earlier prehistoric earthquakes. We documented several geomorphic features that appeared offset by multiple events by making detailed topographic maps. Offset features were abundant along reaches of the fault with high topographic relief and large displacement. Slip distribution for the penultimate event, as recorded by offset rills, streams, and shrub-coppice dunes, is very similar to the slip distribution from the 1987 earthquake through April 1988. This similarity may prove to be fortuitous if afterslip from the 1987 event continues to increase the total slip for this earthquake. But if afterslip associated with the 1987 event ceases in the near future, then the past two earthquakes were nearly identical in slip, and the Superstition Hills fault may be expected to produce characteristic earthquakes of roughly magnitude 6½

Evidence for two surface ruptures in the past 500 years on the San Andreas fault at Frazier Mountain

Abstract We conducted paleoseismic studies in a closed depression along the San Andreas fault on the north flank of Frazier Mountain near Frazier Park, California. We recognized two earthquake ruptures in our trench exposure and interpreted the most recent rupture, event 1, to represent the historical 1857 earthquake. We also exposed evidence of an earlier surface rupture, event 2, along an older group of faults that did not rerupture during event 1. Radiocarbon dating of the stratigraphy above and below the earlier event constrains its probable age to between A.D. 1460 and 1600. Because we documented continuous, unfaulted stratigraphy between the earlier event horizon and the youngest event horizon in the portion of the fault zone exposed, we infer event 2 to be the penultimate event. We observed no direct evidence of an 1812 earthquake in our exposures. However, we cannot preclude the presence of this event at our site due to limited age control in the upper part of the section and the possibility of other fault strands beyond the limits of our exposures. Based on overlapping age ranges, event 2 at Frazier Mountain may correlate with event B at the Bidart fan site in the Carrizo Plain to the northwest and events V and W4 at Pallett Creek and Wrightwood, respectively, to the southeast. If the events recognized at these multiple sites resulted from the same surface rupture, then it appears that the San Andreas fault has repeatedly failed in large ruptures similar in extent to 1857

A 50,000-year record of lake-level variations and overflow from Owens Lake, eastern California, USA

A continuous lake-level curve was constructed for Owens Lake, eastern California by integrating lake-core data and shoreline geomorphology with new wind-wave and sediment entrainment modeling of lake-core sedimentology. This effort enabled refinement of the overflow history and development of a better understanding of the effects of regional and global climate variability on lake levels of the paleo-Owens River system during the last 50,000 years. The elevations of stratigraphic sites, plus lake bottom and spillway positions were corrected for vertical tectonic deformation using a differential fault-block model to estimate the absolute hydrologic change of the watershed-lake system. New results include 14C dating of mollusk shells in shoreline deposits, plus post-IR-IRSL dating of a suite of five beach ridges and OSL dating of spillway alluvial and deltaic deposits in deep boreholes. Geotechnical data show the overflow area is an entrenched channel that had erodible sills composed of unconsolidated fluvial-deltaic and alluvial sediment at elevations of ∼1113–1165 m above mean sea level. Owens Lake spilled most of the time at or near minimum sill levels, controlled by a bedrock sill at ∼1113 m. Nine major transgressions at ∼40.0, 38.7, 23.3, 19.3, 15.6, 13.8, 12.8, 11.6, and 10.6 ka reached levels ∼10–45 m above the bedrock sill. Several major regressions at or below the bedrock sill from 36.9 to 28.5 ka, and at ∼17.8, 12.9, and 10.4–8.8 ka indicate little to no overflow during these times. The latest period of overflow occurred ∼10–20 m above the bedrock sill from ∼8.4 to 6.4 ka that was followed by closed basin conditions after ∼6.4 ka. Previous lake core age-depth models were revised by accounting for sediment compaction and using no reservoir correction for open basin conditions, thereby reducing discrepancies between Owens Lake shoreline and lake-core proxy records. The integrated analysis provides a continuous 50 ka lake-level record of hydroclimate variability along the south-central Sierra Nevada that is consistent with other shoreline and speleothem records in the southwestern U.S

Evidence for large earthquakes in metropolitan Los Angeles

The Sierra Madre fault, along the southern flank of the San Gabriel Mountains in the Los Angeles region, has failed in magnitude 7.2 to 7.6 events at least twice in the past 15,000 years. Restoration of slip on the fault indicated a minimum of about 4.0 meters of slip from the most recent earthquake and suggests a total cumulative slip of about 10.5 meters for the past two prehistoric earthquakes. Large surface displacements and strong ground motions resulting from greater than magnitude 7 earthquakes within the Los Angeles region are not yet considered in most seismic hazard and risk assessments

Central and Eastern United States (CEUS) Seismic Source Characterization (SSC) for Nuclear Facilities Project

This report describes a new seismic source characterization (SSC) model for the Central and Eastern United States (CEUS). It will replace the Seismic Hazard Methodology for the Central and Eastern United States, EPRI Report NP-4726 (July 1986) and the Seismic Hazard Characterization of 69 Nuclear Plant Sites East of the Rocky Mountains, Lawrence Livermore National Laboratory Model, (Bernreuter et al., 1989). The objective of the CEUS SSC Project is to develop a new seismic source model for the CEUS using a Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 assessment process. The goal of the SSHAC process is to represent the center, body, and range of technically defensible interpretations of the available data, models, and methods. Input to a probabilistic seismic hazard analysis (PSHA) consists of both seismic source characterization and ground motion characterization. These two components are used to calculate probabilistic hazard results (or seismic hazard curves) at a particular site. This report provides a new seismic source model. Results and Findings The product of this report is a regional CEUS SSC model. This model includes consideration of an updated database, full assessment and incorporation of uncertainties, and the range of diverse technical interpretations from the larger technical community. The SSC model will be widely applicable to the entire CEUS, so this project uses a ground motion model that includes generic variations to allow for a range of representative site conditions (deep soil, shallow soil, hard rock). Hazard and sensitivity calculations were conducted at seven test sites representative of different CEUS hazard environments. Challenges and Objectives The regional CEUS SSC model will be of value to readers who are involved in PSHA work, and who wish to use an updated SSC model. This model is based on a comprehensive and traceable process, in accordance with SSHAC guidelines in NUREG/CR-6372, Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts. The model will be used to assess the present-day composite distribution for seismic sources along with their characterization in the CEUS and uncertainty. In addition, this model is in a form suitable for use in PSHA evaluations for regulatory activities, such as Early Site Permit (ESPs) and Combined Operating License Applications (COLAs). Applications, Values, and Use Development of a regional CEUS seismic source model will provide value to those who (1) have submitted an ESP or COLA for Nuclear Regulatory Commission (NRC) review before 2011; (2) will submit an ESP or COLA for NRC review after 2011; (3) must respond to safety issues resulting from NRC Generic Issue 199 (GI-199) for existing plants and (4) will prepare PSHAs to meet design and periodic review requirements for current and future nuclear facilities. This work replaces a previous study performed approximately 25 years ago. Since that study was completed, substantial work has been done to improve the understanding of seismic sources and their characterization in the CEUS. Thus, a new regional SSC model provides a consistent, stable basis for computing PSHA for a future time span. Use of a new SSC model reduces the risk of delays in new plant licensing due to more conservative interpretations in the existing and future literature. Perspective The purpose of this study, jointly sponsored by EPRI, the U.S. Department of Energy (DOE), and the NRC was to develop a new CEUS SSC model. The team assembled to accomplish this purpose was composed of distinguished subject matter experts from industry, government, and academia. The resulting model is unique, and because this project has solicited input from the present-day larger technical community, it is not likely that there will be a need for significant revision for a number of years. See also Sponsors Perspective for more details. The goal of this project was to implement the CEUS SSC work plan for developing a regional CEUS SSC model. The work plan, formulated by the project manager and a technical integration team, consists of a series of tasks designed to meet the project objectives. This report was reviewed by a participatory peer review panel (PPRP), sponsor reviewers, the NRC, the U.S. Geological Survey, and other stakeholders. Comments from the PPRP and other reviewers were considered when preparing the report. The SSC model was completed at the end of 2011

Central and Eastern United States (CEUS) Seismic Source Characterization (SSC) for Nuclear Facilities Project

This report describes a new seismic source characterization (SSC) model for the Central and Eastern United States (CEUS). It will replace the Seismic Hazard Methodology for the Central and Eastern United States, EPRI Report NP-4726 (July 1986) and the Seismic Hazard Characterization of 69 Nuclear Plant Sites East of the Rocky Mountains, Lawrence Livermore National Laboratory Model, (Bernreuter et al., 1989). The objective of the CEUS SSC Project is to develop a new seismic source model for the CEUS using a Senior Seismic Hazard Analysis Committee (SSHAC) Level 3 assessment process. The goal of the SSHAC process is to represent the center, body, and range of technically defensible interpretations of the available data, models, and methods. Input to a probabilistic seismic hazard analysis (PSHA) consists of both seismic source characterization and ground motion characterization. These two components are used to calculate probabilistic hazard results (or seismic hazard curves) at a particular site. This report provides a new seismic source model. Results and Findings The product of this report is a regional CEUS SSC model. This model includes consideration of an updated database, full assessment and incorporation of uncertainties, and the range of diverse technical interpretations from the larger technical community. The SSC model will be widely applicable to the entire CEUS, so this project uses a ground motion model that includes generic variations to allow for a range of representative site conditions (deep soil, shallow soil, hard rock). Hazard and sensitivity calculations were conducted at seven test sites representative of different CEUS hazard environments. Challenges and Objectives The regional CEUS SSC model will be of value to readers who are involved in PSHA work, and who wish to use an updated SSC model. This model is based on a comprehensive and traceable process, in accordance with SSHAC guidelines in NUREG/CR-6372, Recommendations for Probabilistic Seismic Hazard Analysis: Guidance on Uncertainty and Use of Experts. The model will be used to assess the present-day composite distribution for seismic sources along with their characterization in the CEUS and uncertainty. In addition, this model is in a form suitable for use in PSHA evaluations for regulatory activities, such as Early Site Permit (ESPs) and Combined Operating License Applications (COLAs). Applications, Values, and Use Development of a regional CEUS seismic source model will provide value to those who (1) have submitted an ESP or COLA for Nuclear Regulatory Commission (NRC) review before 2011; (2) will submit an ESP or COLA for NRC review after 2011; (3) must respond to safety issues resulting from NRC Generic Issue 199 (GI-199) for existing plants and (4) will prepare PSHAs to meet design and periodic review requirements for current and future nuclear facilities. This work replaces a previous study performed approximately 25 years ago. Since that study was completed, substantial work has been done to improve the understanding of seismic sources and their characterization in the CEUS. Thus, a new regional SSC model provides a consistent, stable basis for computing PSHA for a future time span. Use of a new SSC model reduces the risk of delays in new plant licensing due to more conservative interpretations in the existing and future literature. Perspective The purpose of this study, jointly sponsored by EPRI, the U.S. Department of Energy (DOE), and the NRC was to develop a new CEUS SSC model. The team assembled to accomplish this purpose was composed of distinguished subject matter experts from industry, government, and academia. The resulting model is unique, and because this project has solicited input from the present-day larger technical community, it is not likely that there will be a need for significant revision for a number of years. See also Sponsors Perspective for more details. The goal of this project was to implement the CEUS SSC work plan for developing a regional CEUS SSC model. The work plan, formulated by the project manager and a technical integration team, consists of a series of tasks designed to meet the project objectives. This report was reviewed by a participatory peer review panel (PPRP), sponsor reviewers, the NRC, the U.S. Geological Survey, and other stakeholders. Comments from the PPRP and other reviewers were considered when preparing the report. The SSC model was completed at the end of 2011