HR: 1330h
AN: S42C-0176    [PDF]
TI: Seismic Energy of Small Earthquakes Using Hi-net Data
AU: * Venkataraman, A
EM: anupamav@pangea.stanford.edu
AF: Stanford University, Department of Geophysics, 397, Panama Mall, Stanford, CA 94305-2215
AU: Beroza, G C
AF: Stanford University, Department of Geophysics, 397, Panama Mall, Stanford, CA 94305-2215
AU: Ide, S
AF: University of Tokyo, Dept. of Earth and Planetary Science, 7-3-1 Hongo,Bunkyo, Tokyo, 113-0033 Japan
AB: Seismic energy is a fundamental parameter of the earthquake source. The seismic energy of large earthquakes is well determined, but large earthquakes occur infrequently, so it is critical to develop accurate methods to study the more numerous smaller earthquakes as well. Additionally, a key question in earthquake source dynamics is whether there is something fundamentally different in the physics of the rupture process of small and large earthquakes. To answer this question, we need accurate estimates of the seismic energy for events of all sizes. The low signal to noise ratio of smaller earthquakes makes their seismic energy difficult to measure. Fortunately, there are high signal-to-noise ratio borehole recordings of small earthquakes available. Data from the high sensitivity borehole network in Japan (Hi-net) should be particularly useful for this purpose. Using these data and using techniques based on empirical Green's functions under the assumption of an omega-square source spectrum, we estimated seismic energy for 6 small earthquakes in the Ishikawa prefecture.Our results suggest that the scaling of seismic energy may break down with earthquake size; i.e., small earthquakes have smaller energy than is expected from a simple similarity scaling. The lower energies we observe appear to be explained by a change in the scaling of earthquake duration with size. In other words, we observed a change in corner frequencies, and hence inferred static stress drops, with earthquake magnitude. The ratio of apparent stress to stress drop yields an apparent radiation efficiency that differs from the true radiation efficiency depending on the degree of dynamic overshoot. While our observations of the apparent radiation efficiency do not vary with earthquake size, the results of other studies suggest that this quantity decreases for very small earthquakes. Under the assumption that rupture velocity is not different for such small earthquakes, we find that these small values may require unreasonably large overshoot.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2003 Fall Meeting