Radiated Seismic Energy from Very Small and Very Large Earthquakes

Stanford University, Stanford CA

Investigators

Abstract

For short wavelengths, which carry the most information for very small earthquakes, the complexity of wave propagation in the Earth's crust makes it difficult to discern effects due to the earthquake source. Multiple empirical Green's function analysis, a method that uses extremely small earthquakes to correct for complicated wave propagation effects for somewhat larger earthquakes, when used for these very small events shows that unanticipated wave propagation effects are present even in deep borehole recordings. This may have led to incorrect conclusions about the energetics of very small (magnitude less than 2.0) earthquakes in past studies. This technique is being applied to an extensive data set in order to see if these conclusions generalize to earthquakes in diverse environments: viz. the San Andreas fault system in southern California and the deforming crust of Japan. This is a particularly timely study because propagation effects in borehole recordings may affect measurements obtained from ongoing and planned large-scale downhole experiments in Japan and in California. The technique developed by Satoshi Ide (University of Tokyo) is being used for large earthquakes, to determine the spatial distribution of radiated seismic energy from strong motion models of the earthquake. Studies of a set of three earthquakes, including the 1995 Kobe, Japan earthquake, shows that most of the seismic energy was radiated near the hypocenter, suggesting that the rest of the faulting process was largely dissipative. This method is being applied to a much larger available catalog of extended-source models in an attempt to improve understanding how the energy balance during faulting controls the size of an earthquake.