Surface Rupturing and Buried Dynamic-Rupture Models Calibrated with Statistical Observations of Past Earthquakes

Abstract

In the context of the slip-weakening friction model and simplified asper- ity models for stress state, we calibrate dynamic rupture models for buried and surface-rupturing earthquakes constrained with statistical observations of past earth- quakes. These observations are the kinematic source models derived from source in- versions of ground-motion and empirical source models of seismic moment and rupture area. The calibrated parameters are the stress-drop distribution on the fault and average stress drop. We develop a set of dynamic rupture models that consist of asperities and surrounding background areas. The distribution of dynamic stress drop outside the asperity is characterized by a fraction of the stress drop on the as- perity. From this set of models, we identify dynamic fault models with defined stress- drop characteristics that satisfy the observations. The selected dynamic fault models show that surface-rupturing earthquakes are characterized by a large area of negative stress-drop surrounding the asperities, while buried earthquakes present positive or zero stress drop. In addition, the calibrated fault models that match the observations show that the average stress drop is independent of earthquake size for buried earth- quakes, but scale dependent for surface-rupturing earthquakes. This suggests that, in the context of our parameterization, buried earthquakes follow self-similarity scaling, and surface-rupturing earthquakes break this self-similarity. We apply the calibrated dynamic models to simulate near-source ground motion consistent with observations that suggest that buried earthquakes generate stronger ground motion than surface- rupturing earthquakes at high frequency. We propose possible mechanisms that satisfy this observation, as follows: buried rupture has a hypocenter location below the as- perity; this can produce strong directivity of the slip velocity function toward the free surface. That effect, in addition to a reduced fault area and low fracture energy during rupture, may be significant in enhancing high-frequency ground motion. On the other hand, surface-rupturing earthquakes have a shallow hypocenter, large fracture energy on the asperities, and enhanced energy absorption due to large areas of negative stress drop in the background area. These characteristics of large earthquakes inhibit severe directivity effects on the slip velocity function directly toward the free surface, redu- cing the high-frequency ground motion.