Two new student* authored papers from the Olsen lab.

A Frequency-Dependent Ground-Motion Spatial Correlation Model of Within-Event Residuals for Fourier Amplitude Spectra

Nan Wang*, Kim Olsen, and Steve Day in Earthquake Spectra

Ground motion time series recorded at stations separated by up to about 50 kilometers show a frequency-dependent spatial coherency structure, and the corresponding ground motion intensity measures are found to be correlated. As omitting this correlation can result in underestimation of seismic losses in risk analysis, it is critical to quantify the spatial correlation structure for ground motion Fourier spectra estimated at different sites during a single event within a region. Toward this goal, we have developed an empirical frequency-dependent spatial correlation model for the within-event residuals of effective Fourier amplitude spectra from the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation (NGA) West2 database. The correlation model shows slower decrease of the spatial correlation with distance at lower frequencies as compared to higher frequencies, in agreement with the underlying ground motion data, and no significant dependence on the magnitude of the earthquakes is observed. We use this empirical model to incorporate frequency-dependent spatial correlation into a hybrid deterministic-stochastic broadband ground motion generation module, which successfully generates synthetic time series for 7 western U.S. earthquakes with frequency-dependent spatial correlation that closely mimics that of the empirical model. Furthermore, the method also significantly improves the correlation for spectral accelerations, cumulative absolute velocities, and Arias intensities, as compared with that derived from the original broadband module. 

Kinematic Rupture Generator Based on 3D Spontaneous Rupture Simulations along Geometrically Rough Faults

Bill Savran, and Kim Olsen in Journal of Geophysical Research

Spontaneous rupture simulations along geometrically rough faults have been shown to produce realistic far‐field spectra and comparable fits with ground motion metrics such as spectral accelerations and peak motions from Ground Motion Prediction Equations (GMPEs), but they are too computationally demanding for use with physics‐based probabilistic seismic hazard analysis efforts. Here, we present our implementation of a kinematic rupture generator that matches the characteristics of, at least in a statistical sense, rough‐fault spontaneous rupture models. To this end, we analyze ~100 dynamic rupture simulations on strike‐slip faults with Mw ranging from 6.4 to 7.2. We find that our dynamic simulations follow empirical scaling relationships for strike‐slip events and provide source spectra comparable to a source model with ω−2 decay. To define our kinematic source model, we use a regularized Yoffe function parameterized in terms of slip, peak‐time, rise‐time, and rupture initiation time.

These parameters are defined through empirical relationships with random fields whose one‐ and two‐point statistics are derived from the dynamic rupture simulations. Our rupture generator reproduces Next Generation Attenuation (NGA) West2 GMPE medians and intraevent standard deviations of spectral accelerations with periods as short as 0.2 s for ensembles of ground motion simulations. Our rupture generator produces kinematic source models for M6.4–7.2 strike‐slip scenarios that can be used in broadband physics‐based probabilistic seismic hazard efforts or to supplement data in areas of limited observations for the development of future GMPEs.