Walter J. Silva

Summary of the Abrahamson & Silva NGA Ground-Motion Relations

Empirical ground-motion models for the rotation-independent average horizontal component from shallow crustal earthquakes are derived using the PEER NGA database. The model is applicable to magnitudes 5–8.5, distances 0–200 km, and spectral periods of 0–10 sec. In place of generic site categories (soil and rock), the site is parameterized by average shear-wave velocity in the top 30 m (VS30) and the depth to engineering rock (depth to VS =1000 m/s). In addition to magnitude and style-of-faulting, the source term is also dependent on the depth to top-of-rupture: for the same magnitude and rupture distance, buried ruptures lead to larger short-period ground motions than surface ruptures. The hanging-wall effect is included with an improved model that varies smoothly as a function of the source properties (M, dip, depth), and the site location. The standard deviation is magnitude dependent with smaller magnitudes leading to larger standard deviations. The short-period standard deviation model for soil sites is also distant-dependent due to nonlinear site response, with smaller standard deviations at short distances.

Stochastic Modeling of California Ground Motions

Ground-motion relations are developed for California using a stochastic simulation method that exploits the equivalence between finite-fault models and a two-corner point-source model of the earthquake spectrum. First, stochastic simu- lations are generated for finite-fault ruptures, in order to define the average shape and amplitude level of the radiated spectrum at near-source distances as a function of earthquake size. The length and width of the fault plane are defined based on the moment magnitude of the earthquake and modeled with an array of subfaults. The radiation from each subfault is modeled as a Brune point source using the stochastic model approach; the subfault spectrum has a single-corner frequency. An earthquake rupture initiates at a randomly chosen subfault (hypocenter), and propagates in all directions along the fault plane. A subfault is triggered when rupture propagation reaches its center. Simulations are generated for an observation point by summing the subfault time series, appropriately lagged in time. Fourier spectra are computed for records simulated at many azimuths, placed at equidistant observation points around the fault. The mean Fourier spectrum for each magnitude, at a reference near- source distance, is used to define the shape and amplitude levels of an equivalent point-source spectrum that mimics the salient finite-fault effects. The functional form for the equivalent point-source spectrum contains two corner frequencies. Stochastic point-source simulations, using the derived two-corner source spectrum, are then performed to predict peak-ground-motion parameters and response spectra for a wide range of magnitudes and distances, for generic California sites. The sto- chastic ground-motion relations, given in the Appendix for rock and soil sites, are in good agreement with the empirical strong-motion database for California; the average ratio of observed to simulated amplitudes is near unity over all frequencies from 0.2 to 12 Hz. The stochastic relations agree well with empirical regression equations (e.g., Abrahamson and Silva, 1997; Boore et al., 1997; Sadigh et al., 1997) in the magnitude-distance ranges well represented by the data, but are better con- strained at large distances, due to the use of attenuation parameters based on regional seismographic data. The stochastic ground-motion relations provide a sound basis for estimation of ground motions for earthquakes of magnitude 4 through 8, at dis- tances from 1 to 200 km.

Comparisons of the NGA ground-motion relations

The data sets, model parameterizations, and results from the five NGA models for shallow crustal earthquakes in active tectonic regions are compared. A key difference in the data sets is the inclusion or exclusion of aftershocks. A comparison of the median spectral values for strike-slip earthquakes shows that they are within a factor of 1.5 for magnitudes between 6.0 and 7.0 for distances less than 100 km. The differences increase to a factor of 2 for M5 and M8 earthquakes, for buried ruptures, and for distances greater than 100 km. For soil sites, the differences in the modeling of soil/sediment depth effects increase the range in the median long-period spectral values for M7 strike-slip earthquakes to a factor of 3. The five models have similar standard deviations for M6.5-M7.5 earthquakes for rock sites and for soil sites at distances greater than 50 km. Differences in the standard deviations of up to 0.2 natural log units for moderate magnitudes at all distances and for large magnitudes at short distances result from the treatment of the magnitude dependence and the effects of nonlinear site response on the standard deviation.

Summary of the ASK14 Ground Motion Relation for Active Crustal Regions

Empirical ground motion models for the average horizontal component from shallow crustal earthquakes in active tectonic regions are derived using the PEER NGA-West2 database. The model is applicable to magnitudes 3.0–8.5, distances 0–300 km, and spectral periods of 0–10 s. The model input parameters are the same as those used by Abrahamson and Silva (2008), with the following exceptions: the loading level for nonlinear effects is based on the spectral acceleration at the period of interest rather than the PGA; and the distance scaling for hanging wall (HW) effects off the ends of the rupture includes a dependence on the source-to-site azimuth. Regional differences in large-distance attenuation and VS30 scaling between California, Japan, China, and Taiwan are included. The scaling for the HW effect is improved using constraints from numerical simulations. The standard deviation is magnitude-dependent, with smaller magnitudes leading to larger standard deviations at short periods, but smaller standard deviations at long periods. Directivity effects are not included through explicit parameters, but are captured through the variability of the empirical data.

NGA-West2 Database

The NGA-West2 project database expands on its predecessor to include worldwide ground motion data recorded from shallow crustal earthquakes in active tectonic regimes post-2000 and a set of small-to-moderate-magnitude earthquakes in California between 1998 and 2011. The database includes 21,336 (mostly) three-component records from 599 events. The parameter space covered by the database is M 3.0 to M 7.9, closest distance of 0.05 to 1,533 km, and site time-averaged shear-wave velocity in the top 30 m of VS30 = 94 m/s to 2,100 m/s (although data becomes sparse for distances >400 km and VS30 > 1,200 m/s or <150 m/s). The database includes uniformly processed time series and response spectral ordinates for 111 periods ranging from 0.01 s to 20 s at 11 damping ratios. Ground motions and metadata for source, path, and site conditions were subject to quality checks by ground motion prediction equation developers and topical working groups.

NGA-West2 Research Project

The NGA-West2 project is a large multidisciplinary, multi-year research program on the Next Generation Attenuation (NGA) models for shallow crustal earthquakes in active tectonic regions. The research project has been coordinated by the Pacific Earthquake Engineering Research Center (PEER), with extensive technical interactions among many individuals and organizations. NGA-West2 addresses several key issues in ground-motion seismic hazard, including updating the NGA database for a magnitude range of 3.0–7.9; updating NGA ground-motion prediction equations (GMPEs) for the “average” horizontal component; scaling response spectra for damping values other than 5%; quantifying the effects of directivity and directionality for horizontal ground motion; resolving discrepancies between the NGA and the National Earthquake Hazards Reduction Program (NEHRP) site amplification factors; analysis of epistemic uncertainty for NGA GMPEs; and developing GMPEs for vertical ground motion. This paper presents an overview of the NGA-West2 research program and its subprojects.

SHEAR-WAVE VELOCITY CHARACTERISTICS OF GEOLOGIC UNITS IN CALIFORNIA

Site conditions can be classified by the average shear-wave velocity to 30 meters (Vs30) and used for estimating site effects in seismic hazard calculations. Large scale seismic hazard maps, which include site effects, may be produced, providing Vs30 can be well correlated with geologic units. Vs30 values for several geologic units can be easily classified into soil profile types of the UBC (ICBO 1997). Most geologic units have wide variations in Vs30 and some extensive geologic units, such as older alluvium, the Franciscan Complex or the Puente Formation cannot be easily classified.

Ground Motion Model for the 1989 M 6.9 Loma Prieta Earthquake Including Effects of Source, Path, and Site

The objective of this study is to assess the effects of source finiteness, crustal wave propagation, and site response upon recorded strong ground motions from the 1989 Loma Prieta earthquake. Our analysis uses band limited white noise (BLWN) with random vibration theory (RVT) to produce site-specific estimates of peak acceleration and response spectral ordinates for both a point-source and finite-source model. Effects of nonlinear soil response are modeled through an equivalent-linear approach. The point-source model additionally accommodates crustal propagation effects in terms of direct-plus-postcritical reflections.

Nonlinear Site Amplification Factors for Constraining the NGA Models

Amplification factors computed from the equivalent-linear method using the program RASCALS are used to develop constraints on the nonlinear soil response for possible use by the NGA ground-motion model developers. The site response computations covered site conditions with average VS30 values ranging from 160 to 900 m/s, soil depths from 15 to 914 m, and peak accelerations of the input rock motion (VS30=1100 m/s) between 0.01 g and 1.5 g. Four sets of nonlinear properties of the soils are used: EPRI, Peninsular Range, Imperial Valley, and Bay Mud. The first two soil models are used for VS30≥270 m/s and the later two are used for VS30≤190 m/s. Simple parametric models of the nonlinear amplification factors that are functions of the PGA on rock and VS30 are developed for the EPRI and Peninsula models.

Ground-Motion Attenuation Relationships for Cascadia Subduction Zone Megathrust Earthquakes Based on a Stochastic Finite-Fault Model

The number of strong ground motion recordings available for regression analysis in developing empirical attenuation relationships has rapidly grown in the last 10 years. However, the dearth of strong-motion data from the Cascadia subduc- tion zone has limited this development of relationships for the Cascadia subduction zone megathrust, which can be used in the calculation of design spectra for engi- neered structures. A stochastic finite-fault ground-motion model has been used to simulate ground motions for moment magnitude (M) 8.0, 8.5, and 9.0 megathrust earthquakes along the Cascadia subduction zone for both rock-and soil-site condi- tions. The stochastic finite-fault model was validated against the 1985 M 8.0 Mi- choacan, Mexico, and the 1985 M 8.0 Valpariso, Chile, earthquakes. These two subduction zone megathrust earthquakes were recorded at several rock sites located near the fault rupture. For the Cascadia megathrust earthquakes, three different rup- ture geometries were used to model the M 8.0, 8.5, and 9.0 events. The geometries only differ in their respective fault lengths. A fault dip of 9 to the east with a rupture width of 90 km was selected to represent average properties of the Cascadia sub- duction zone geometry. A regional crustal damping and velocity model was used with the stochastic finite-fault model simulations. Ground motions were computed for 16 site locations. The parametric uncertainties associated with the variation in source, path, and site effects were included in the development of the ground motions. A functional form was fit to the ground-motion model simulations to develop region- specific attenuation relationships for the Cascadia megathrust rupture zone for both rock and soil site conditions. The total uncertainty was based on a combination of the modeling and parametric uncertainties (sigmas). These newly developed atten- uation relationships for Cascadia subduction zone megathrust earthquakes can be used in both the probabilistic and deterministic seismic-hazard studies for engineer- ing design for the Pacific Northwest.