Effect of complex fault geometry and slip style on near-fault strong motions and static displacement

Abstract

Although there are many studies that deal with complex slip distribution or rupture propagation on an earthquake fault, they usually regard a fault system as a fault of simple geometry. Actual fault systems have highly heterogeneous slip distribution and very complicated shapes, as is often observed through field surveys of surface breaks. In this study, we synthesize seismograms including static displacement near a fault using the discrete wavenumber method in order to estimate the effects of the above types of fault complexity in a quantitative manner. We introduce a complex slip distribution based on the Nojima Fault associated with the 1995 Hyogo-ken Nanbu earthquake. As a result, we show that strong motions at a frequency of lower than 1.0 Hz are strongly affected by the complexity of the fault geometry, at a scale of not more than several km, rather than the rupture propagation style. Distributions of static displacement fluctuate, depending on the fault geometry characterized by the length of each fault segment. Such small-scale variations in fault geometry (≤1 km) have been mostly ignored prior to this work. Our results also suggest that details of fault segmentation and bending can be determined by dense observations (e.g., GPS or geological surveys) of static displacement near a fault system, indicating the importance of simultaneous studies on static and dynamic near-fault motions.

Implications of the 26 December 2004 Sumatra–Andaman Earthquake on Tsunami Forecast and Assessment Models for Great Subduction-Zone Earthquakes

“…The combination of effects from physical heterogeneities with nonlinear dynamics of rupture results in mix of characteristic and noncharacteristic modes of slip distributions noted in both theoretical studies Rice, 1995, 1997;Zheng and Rice, 1998;Nielsen and Carlson, 2000;Perfettini et al, 2001;Shaw, 2004) and observational studies specific to subduction zone earthquakes (Thatcher, 1990;Mendoza, 1993;Boyd et al, 1995;Schwartz, 1999;Mazzotti et al, 2000;Hirata et al, 2004;Satake et al, 2006a). Recent studies have also indicated that spatial heterogeneity at scales smaller than typically resolved in seismic and geodetic inversions (Յ1 km) (cf., Perfettini et al, 2001) can have a significant effect on near-field static displacement (Honda and Yomogida, 2003;Fu and Sun, 2004). Small-scale spatial heterogeneity in slip translates to variations in local tsunami runup, though the effects are diminished with increasing propagation distance and source depth (Titov et al, 2001;Geist, 2002).…”

“…Tsunami generation models used for tsunami assessments such as presented here have been modified from models previously used for strong ground motion studies (Herrero and Bernard, 1994;Berge et al, 1998;Somerville et al, 1999;Hisada, 2000Hisada, , 2001Honda and Yomogida, 2003). At low wavenumbers, stochastic slip distributions are scaled relative to average slip or static stress drop.…”