Edward H. Field

The theoretical response of sedimentary layers to ambient seismic noise

For over thirty years, attempts have been made to gain information about sediment amplification during earthquakes from observations of ambient seismic noise. While the results of several feasibility studies have been encouraging, theoretical support for the technique is scant. We present a model for the response of sedimentary layers to ambient seismic noise. The noise sources are modeled as a random distribution (in time and space) of point forces located on the Earths free surface. This model is applied to a site where observed noise spectral ratios, relative to a rock site, have previously been shown to reveal the fundamental resonant frequency of a soft clay layer. Approximating the sediment site as a single layer over a half-space, the horizontal noise spectrum predicted by our model reveals the fundamental resonance and first harmonic of the layer. We also examine an estimate of site response proposed by Nakamura (1989), which is formed by dividing the horizontal-component noise spectrum by that of the vertical component. Nakamuras estimate applied to both observed and predicted noise-spectra was also successful in identifying the fundamental resonance, with a slight (<10%) shift toward lower frequencies. Future work is needed to determine the generality of our results, and to elucidate the influence of the simplifying assumptions.

Nonlinear ground-motion amplification by sediments during the 1994 Northridge earthquake

It has been known since at least 1898 (ref. 1) that sediments can amplify earthquake ground motion relative to bedrock. For the weak ground motion accompanying small earthquakes, the amplification due to sediments is well understood in terms of linear elasticity (Hookes law), but there has been a long-standing debate regarding the amplification associated with the strong ground motion produced by large earthquakes. The view of geotechnical engineers, based largely on laboratory studies, is that Hookes law breaks down at larger strains causing a reduced (nonlinear) amplification. Seismologists, on the other hand, have tended to remain sceptical of this nonlinear effect, mainly because the relatively few strong-motion observations seemed to be consistent with linear elasticity. Although some recent earthquake studies have demonstrated nonlinear behaviour under certain circumstances,, the significance of nonlinearity for the type of stiff-soil sites found in the greater Los Angeles region remains unresolved. Here we report that ground-motion amplification due to sediments for the main shock of the 1994 Northridge earthquake was up to a factor of two less than the amplification observed for its aftershocks. These observations imply significant nonlinearity in such amplification, and bring into question the use of measurements of weak ground motion to predict the strong ground motion at sedimentary sites.

Accounting for Site Effects in Probabilistic Seismic Hazard Analyses of Southern California: Overview of the SCEC Phase III Report

This article presents an overview of the Southern California Earthquake Center (SCEC) Phase-III effort to determine the extent to which probabilistic seismic hazard analysis (PSHA) can be improved by accounting for site effects. The contri- butions made in this endeavor are represented in the various articles that compose this special issue of BSSA. Given the somewhat arbitrary nature of the site-effect distinction, it must be care- fully defined in any given context. With respect to PSHA, we define the site effect as the response, relative to an attenuation relationship, averaged over all damaging earthquakes in the region. A diligent effort has been made to identify any attributes that predispose a site to greater or lower levels of shaking. The most detailed maps of Quaternary geology are not found to be helpful; either they are overly detailed in terms of distinguishing different amplification factors or present southern California strong-motion observations are inadequate to reveal their superiority. A map based on the average shear-wave velocity in the upper 30 m, however, is found to delineate significantly different amplification factors. A correlation of amplification with basin depth is also found to be significant, implying up to a factor of two difference between the shallowest and deepest parts of the Los Angeles basin. In fact, for peak accel- eration the basin-depth correction is more influential than the 30-m shear-wave ve- locity. Questions remain, however, as to whether basin depth is a proxy for some other site attribute. In spite of these significant and important site effects, the standard deviation of an attenuation relationship (the prediction error) is not significantly reduced by making such corrections. That is, given the influence of basin-edge-induced waves, subsur- face focusing, and scattering in general, any model that attempts to predict ground motion with only a few parameters will have a substantial intrinsic variability. Our best hope for reducing such uncertainties is via waveform modeling based on first principals of physics. Finally, questions remain with respect to the overall reliability of attenuation re- lationships at large magnitudes and short distances. Current discrepancies between viable models produce up to a factor of 3 difference among predicted 10% in 50-yr exceedance levels, part of which results from the uncertain influence of sediment nonlinearity.

Managing Large-Scale Workflow Execution from Resource Provisioning to Provenance Tracking: The CyberShake Example

This paper discusses the process of building an environment where large-scale, complex, scientific analysis can be scheduled onto a heterogeneous collection of computational and storage resources. The example application is the Southern California Earthquake Center (SCEC) CyberShake project, an analysis designed to compute probabilistic seismic hazard curves for sites in the Los Angeles area. We explain which software tools were used to build to the system, describe their functionality and interactions. We show the results of running the CyberShake analysis that included over 250,000 jobs using resources available through SCEC and the TeraGrid.

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.

Long-Term Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3)

The 2014 Working Group on California Earthquake Probabilities (WGCEP 2014) presents time-dependent earthquake probabilities for the third Uniform California Earthquake Rupture Forecast (UCERF3). Building on the UCERF3 time-in- dependent model published previously, renewal models are utilized to represent elastic- rebound-implied probabilities. A new methodology has been developed that solves applicability issues in the previous approach for unsegmented models. The new meth- odology also supports magnitude-dependent aperiodicity and accounts for the historic open interval on faults that lack a date-of-last-event constraint. Epistemic uncertainties are represented with a logic tree, producing 5760 different forecasts. Results for a variety of evaluation metrics are presented, including logic-tree sensitivity analyses and comparisons to the previous model (UCERF2). For 30 yr M ! 6:7 probabilities, the most significant changes from UCERF2 are a threefold increase on the Calaveras fault and a threefold decrease on the San Jacinto fault. Such changes are due mostly to differences in the time-independent models (e.g., fault-slip rates), with relaxation of segmentation and inclusion of multifault ruptures being particularly influential. In fact, some UCERF2 faults were simply too long to produce M 6.7 size events given the segmentation assumptions in that study. Probability model differences are also influential, with the implied gains (relative to a Poisson model) being generally higher in UCERF3. Accounting for the historic open interval is one reason. Another is an effective 27% increase in the total elastic-rebound-model weight. The exact factors influencing differences between UCERF2 and UCERF3, as well as the relative im- portance of logic-tree branches, vary throughout the region and depend on the evalu- ation metric of interest. For example, M ! 6:7 probabilities may not be a good proxy for other hazard or loss measures. This sensitivity, coupled with the approximate nature of the model and known limitations, means the applicability of UCERF3 should be evaluated on a case-by-case basis.

A Summary of Previous Working Groups on California Earthquake Probabilities

This article summarizes the 1988, 1990, 1995, and 2002 Working Groups on California Earthquake Probabilities (wgceps). Each of these studies used the best available science to make a time-dependent earthquake forecast. All involved applying elastic-rebound-theory–motivated recurrence models to estimate the probability of rupture on discrete fault segments. The focus was necessarily limited to those faults that had sufficient information to make a time-dependent analysis, although the later studies included probabilities for other events as well. This manuscript does not give a point-by-point critique or review of the previous wgceps, both because it would take considerably more text and because such discussions will necessarily be documented by future working groups as they justify changes. One opinion that is emphasized, however, is that future wgceps should resist overcomplexity in certain aspects of the model given basic assumptions that have been made and/or limitations in our understanding of the earthquake system. For example, one might argue that the level of sophistication applied by previous wgceps with respect to recurrence-interval variability and uncertainty was overkill given their basic assumptions that large earthquakes obey segment boundaries, ruptures never jump from one fault to another, and earthquake-clustering effects can be ignored.

Nonlinear sediment response during the 1994 Northridge earthquake: Observations and finite source simulations

We have addressed the long-standing question regarding nonlinear sediment response in the Los Angeles region by testing whether sediment amplification was similar between the Northridge earthquake and its aftershocks. Comparing the weak- and strong-motion site response at 15 sediment sites, we find that amplification factors were significantly less for the main shock implying systematic nonlinearity. The difference is largest between 2 and 4 Hz (a factor of 2), and is significant at the 99% confidence level between 0.8 and 5.5 Hz. The inference of nonlinearity is robust with respect to the removal of possibly anomalous sediment sites and how the reference-site motion is defined. Furthermore, theoretical ground-motion simulations show no evidence of any bias from finite source effects during the main shock. Nonlinearity is also suggested by the fact that the four sediment sites that contain a clear fundamental resonance for the weak motion exhibit a conspicuous absence of the peak in the strong motion. Although we have taken the first step of establishing the presence of nonlinearity, it remains to define the physics of nonlinear response and to test the methodologies presently applied routinely in engineering practice. The inference of nonlinearity implies that care must be exercised in using sediment site data to study large earthquakes or predict strong ground motion.

SCEC CyberShake Workflows—Automating Probabilistic Seismic Hazard Analysis Calculations

The Southern California Earthquake Center (SCEC) is a community of more than 400 scientists from over 54 research organizations that conducts geophysical research in order to develop a physics-based understanding of earthquake processes and to reduce the hazard from earthquakes in the Southern California region [377].

The variability of PSV response spectra across a dense array deployed during the Northridge aftershock sequence

This study addresses the variability of pseudo-velocity response spectra across an array deployed on stiff soil in the San Fernando Valley during the Northridge (M w 6.7) aftershock sequence. The separation between stations ranged from 0.5 to 5 km, and the aftershock magnitudes ranged from 2.3 to 4.0. We find that 95-percent of observed response spectra are within a factor of 1.9 to 2.6 of the network average. Statistically significant relative amplification factors were found for some of the sites, but the variability of observed response spectra is not significantly reduced by correcting for these effects. This implies that microzonation efforts on less than 5-km distance scales are not warranted at these types of sites. We also found a distance dependence for the response-spectral variability between neighboring sites. 95-percent are within a factor of ∼2.3 at 0.5 km, increasing to 95-percent within a factor of ∼4.2 at 5 km. No frequency dependence in these values could be resolved. Additional work is needed to examine the influence of other factors such as earthquake magnitude.