Geology topics

E. H. Field

Publications and source records attributed to E. H. Field.

5 recordsLinked to original sources

USGS2012

2014 Update of the United States National Seismic Hazard Maps

The U.S. National Seismic Hazard Maps are revised every six years, corresponding with the update cycle of the International Building Code. These maps cover the conterminous U.S. and will be updated in 2014 using the best-available science that is obtained from colleagues at regional and topical workshops, which are convened in 2012-2013. Maps for Alaska and Hawaii will be updated shortly following this update. Alternative seismic hazard models discussed at the workshops will be implemented in a logic tree framework and will be used to develop the seismic hazard maps and associated products. In this paper we describe the plan to update the hazard maps, the issues raised in workshops up to March 2012, and topics that will be discussed at future workshops. An advisory panel will guide the development of the hazard maps and ensure that the maps are acceptable to a broad segment of the science and engineering communities. These updated maps will then be considered by end-users for inclusion in building codes, risk models, and public policy documents.

Conference Paper↗

USGS2009

Uniform California earthquake rupture forecast, version 2 (UCERF 2)

The 2007 Working Group on California Earthquake Probabilities (WGCEP, 2007) presents the Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2). This model comprises a time-independent (Poisson-process) earthquake rate model, developed jointly with the National Seismic Hazard Mapping Program and a time-dependent earthquake-probability model, based on recent earthquake rates and stress-renewal statistics conditioned on the date of last event. The models were developed from updated statewide earthquake catalogs and fault deformation databases using a uniform methodology across all regions and implemented in the modular, extensible Open Seismic Hazard Analysis framework. The rate model satisfies integrating measures of deformation across the plate-boundary zone and is consistent with historical seismicity data. An overprediction of earthquake rates found at intermediate magnitudes (6.5 ??? M ???7.0) in previous models has been reduced to within the 95% confidence bounds of the historical earthquake catalog. A logic tree with 480 branches represents the epistemic uncertainties of the full time-dependent model. The mean UCERF 2 time-dependent probability of one or more M ???6.7 earthquakes in the California region during the next 30 yr is 99.7%; this probability decreases to 46% for M ???7.5 and to 4.5% for M ???8.0. These probabilities do not include the Cascadia subduction zone, largely north of California, for which the estimated 30 yr, M ???8.0 time-dependent probability is 10%. The M ???6.7 probabilities on major strike-slip faults are consistent with the WGCEP (2003) study in the San Francisco Bay Area and the WGCEP (1995) study in southern California, except for significantly lower estimates along the San Jacinto and Elsinore faults, owing to provisions for larger multisegment ruptures. Important model limitations are discussed.

Bulletin of the Seismological Society of America↗

USGS2005

Loss estimates for a Puente Hills blind-thrust earthquake in Los Angeles, California

Based on OpenSHA and HAZUS-MH, we present loss estimates for an earthquake rupture on the recently identified Puente Hills blind-thrust fault beneath Los Angeles. Given a range of possible magnitudes and ground motion models, and presuming a full fault rupture, we estimate the total economic loss to be between $82 and $252 billion. This range is not only considerably higher than a previous estimate of $69 billion, but also implies the event would be the costliest disaster in U.S. history. The analysis has also provided the following predictions: 3,000-18,000 fatalities, 142,000-735,000 displaced households, 42,000-211,000 in need of short-term public shelter, and 30,000-99,000 tons of debris generated. Finally, we show that the choice of ground motion model can be more influential than the earthquake magnitude, and that reducing this epistemic uncertainty (e.g., via model improvement and/or rejection) could reduce the uncertainty of the loss estimates by up to a factor of two. We note that a full Puente Hills fault rupture is a rare event (once every ???3,000 years), and that other seismic sources pose significant risk as well. ?? 2005, Earthquake Engineering Research Institute.

Earthquake Spectra↗

USGS2000

A modified ground-motion attenuation relationship for southern California that accounts for detailed site classification and a basin-depth effect

The attenuation relationship presented by Boore et al. (1997) has been evaluated and customized with respect to southern California strong-motion data (for peak ground acceleration (PGA) and 0.3-, 1.0-, and 3.0-sec period spectral acceleration). This study was motivated by the recent availability of a new site-classification map by Wills et al. (2000), which distinguishes seven different site categories for California based on the 1994 NEHRP classification. With few exceptions, each of the five site types represented in the southern California strong-motion database exhibit distinct amplification factors, supporting use of the Wills et al. (2000) map for microzonation purposes. Following other studies, a basin-depth term was also found to be significant and therefore added to the relationship. Sites near the center of the LA Basin exhibit shaking levels up to a factor of 2 greater, on average, than otherwise equivalent sites near the edge. Relative to Boore et al. (1997), the other primary difference here is that PGA exhibits less variation among the Wills et al. (2000) site types. In fact, the PGA amplification implied by the basin-depth effect is greater than that implied by site classification. The model does not explicitly account for nonlinear sediment effects, which, if important, will most likely influence rock-site PGA predictions the most. Evidence for a magnitude-dependent variability, or prediction uncertainty, is also found and included as an option.

California↗

USGS1996

On the coherence of ground motion in the San Fernando valley

We present an analysis of the coherence of seismic ground motion recorded on alluvial sediments in the San Fernando Valley, California. Using aftershocks of the 17 January 1994 M w 6.7 earthquake recorded at a quasi-dense array of portable stations, we analyze the coherence of three well-recorded magnitude 3.7 to 4.0 events over the frequency range 0.5 to 15 Hz and a distance range of 0.5 to 5.3 km. All stations are located at sites with broadly similar near-site geology, characterized by medium to fine-grain Quaternary alluvial sediments. On average, relatively high values of coherence are observed for distances up to 3 to 4 km and frequencies up to 2 to 3 Hz; coherence drops sharply at frequencies near and above 3 Hz. Although average coherence functions are described reasonably well by a log-linear relationship with frequency, the curves at all distances exhibit a flattening at low frequencies that is not consistent with previous observations of coherence at hardrock sites. The distance decay of coherence is also markedly less strong, with high coherence values observed over station separations corresponding to multiple wavelengths. This may reflect fundamental differences in shallow-wave propagation in the two environments, with high-frequency scattering relatively more dominant in regions of hard-rock near-surface geology. Within a sedimentary basin or valley, the site response itself generally reflects a resonance phenomenon that may tend to give rise to more uniform ground motions. However, previous studies have demonstrated the existence of pathological focusing and amplification effects within complex sedimentary basin environments such as the greater Los Angeles region; our results undoubtedly do not quantify the full range of ground-motion variability at all sites, but rather represent the level of that variability that can be expected, and quantified, for typical source/receiver paths.

California↗