Ground-Motion Simulations on Rough Faults in Complex 3D Media

Robert W. Graves

Published August 15, 2017, SCEC Contribution #7760, 2017 SCEC Annual Meeting Poster #247

Observations from past earthquakes show that the rupture process can exhibit significant spatial variations in rupture speed, slip and slip rate, as well as geometric complexity of the faulting surface. Incorporating these features within kinematic ground motion simulations is challenging due to uncertainty in the expected ranges and potential inter-correlations of the required parameters. Additionally, as the simulation bandwidth is pushed to higher frequencies, effects due to small-scale 3D variations in the seismic velocity structure need to be included. Here, we utilize a two-step process incorporating the previously described features to validate ground motion simulations over the 0-4 Hz bandwidth using recordings from the 1989 Loma Prieta earthquake. In the first step we run multiple realizations using the Graves and Pitarka (2010) hybrid method as implemented on the SCEC BB Platform and compare these with near-fault (R < 40 km) recorded motions. A total of 648 rupture scenarios are examined, with individual scenarios consisting of random realizations generated using the approach of Graves and Pitarka (2016). These realizations consider spatial variability in slip and fault roughness, as well as random samples of slip standard deviation, average rupture speed, down-dip fault width and moment magnitude. In addition to the standard RotD50 goodness-of-fit (GoF) criteria described by Goulet et al (2015), we also examine the fit to the individual fault-normal and fault-parallel components for very near-fault (<10 km) records. From these results, we then select the best fitting ruptures, which are then used in the second step of the validation. This second step consists of running 3D simulations using finely sampled (20 meter) seismic velocity meshes. The 3D structure is obtained from the USGS Bay Area velocity model, with modifications to include small-scale 3D stochastic perturbations. The minimum velocity is set at 400 m/s and the maximum frequency resolved by the simulations is about 4 Hz. The 3D results are compared with the recorded motions using the same GoF metrics described above and show very good agreement over the frequency band 0-4 Hz for distances out to 40 km. Additionally, the newly added features to the simulation process reduce the coherency of the radiated higher frequency (f > 1 Hz) ground motions, and homogenize radiation-pattern effects in this same bandwidth as demonstrated by the fit to the very near-fault records.

Key Words
strong ground motion, 3D simulations

Graves, R. W. (2017, 08). Ground-Motion Simulations on Rough Faults in Complex 3D Media. Poster Presentation at 2017 SCEC Annual Meeting.

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Ground Motions