Subregional Attenuation of Ground Motion Amplitudes for Japan Megathrust Earthquakes

Date: 4/24/2019

Time: 11:45 AM

Room: Pine

As part of the NGA-Sub project, I am developing a ground motion model (GMM) for Japan megathrust earthquakes for PGA, PGV, and pseudo-absolute spectral accelerations with periods ranging from 0.01-10 s (GMIMs). During this development, I discovered that there are tectonic regions within Japan that exhibit significantly different attenuation properties, making it difficult to develop a single generic GMM that accurately predicts GMIMs within these regions. Regional differences are greatest for short-period GMIMs. The regions are defined by travel paths that are (1) offshore, (2) onshore but within the subduction forearc, and (3) onshore but within the subduction backarc. Discrepancies in GMIM amplitudes within these three regions can be reduced significantly if different anelastic attenuation coefficients are defined for each zone. I propose that the relatively low attenuation observed at coastal sites for events whose down-dip rupture extent is offshore is due to travel paths that are primarily within the slab. Once propagation of ground motion is onshore within the forearc region, these travel paths all contain a component that includes propagation upwards through a deepening continental crust, thus increasing the observed attenuation. Travel paths primarily within the backarc region are subject to additional attenuation as a result of traversing the volcanic line and propagating within higher-attenuating continental crust with relatively deep slab depths. Although these complex wave-propagation characteristics can be incorporated into the GMM, the use of such a GMM requires that the proportion of the travel path between the source and site within each of these regions be estimated and incorporated into a probabilistic seismic hazard analysis (PSHA). Since this might not be feasible, an alternative GMM is being developed that does not require these partitioned distances but with the penalty of larger within-event aleatory variability.

Presenting Author: Kenneth W. Campbell

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