HR: 15:10h
AN: S13E-07    [Abstracts]
TI: Energy Budget of Small Earthquakes from a Dense Seismic Array: Results from Western Nagano, Japan
AU: * Venkataraman, A
EM: anupamav@pangea.stanford.edu
AF: Stanford University, Department of Geophysics 397 Panama Mall, Stanford, CA 94305 United States
AU: Beroza, G C
EM: beroza@pangea.stanford.edu
AF: Stanford University, Department of Geophysics 397 Panama Mall, Stanford, CA 94305 United States
AU: Ide, S
EM: ide@eps.s.u-tokyo.ac.jp
AF: University of Tokyo, Dept. of Earth and Planetary Science, 7-3-1 Hongo, Bunkyo, Tokyo, 113-0033 Japan
AU: Imanishi, K
EM: imani@ni.aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Tsukuba Central 7 1-1, Higashi 1-Chome, Tsukuba, 305-8567 Japan
AU: Ito, H
EM: hisao.itou@aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Tsukuba Central 7 1-1, Higashi 1-Chome, Tsukuba, 305-8567 Japan
AU: Iio, Y
EM: iio@rcep.dpri.kyoto-u.ac.jp
AF: Kyoto University, Research Center for Earthquake Prediction, Disater Prevention Research Institute, Gokasho Uji, Kyoto, 611-0011 Japan
AB: We analyze micro-earthquake recordings from a dense array of 45 surface and 3 borehole seismometers located in an area of 16x12 $km^2$ in the Western Nagano region of central Japan. The borehole stations have high signal-to-noise ratios up to frequencies of 200-300Hz; and most of the surface stations also have good signal-to-noise ratios up to extremely high frequencies (150-200Hz). The high redundancy provided by the exceptional instrumental coverage in this region provides an opportunity to determine robust measurements of the radiated seismic energy and to explore sources of uncertainty in its measurement. We calculate the seismic energy for 24 micro-earthquakes ($M_{w}$ 0.9-3.8) using the spectral ratio approach (i.e., based on the EGF method). For a given event, the variation in energy estimates between stations is less than a factor of 3 and most of this variation can be explained by variation in corner frequency and spectral shape (roughness of spectra) between stations. For the dataset we examined, the energy to moment ratios vary between 1x$10^{-5}$ and 2x$10^{-4}$, similar to that observed for much larger events, and we do not observe any change in the scaling of this ratio with earthquake size in our data set. Stress drops determined from corner frequencies indicate that the radiation efficiency for our events are always larger than 0.5, similar to values found for other earthquakes of widely varying size.
DE: 1734 Seismology
DE: 1744 Tectonophysics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting