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Bulletin of the Seismological Society of America; October 2004; v. 94; no. 5; p. 1781-1794; DOI: 10.1785/012002214
© 2004 Seismological Society of America
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Article

Source Parameter Scaling for Small Earthquakes Observed at the Western Nagano 800-m-Deep Borehole, Central Japan

A. L. Stork* and H. Ito

Geological Survey of Japan/AIST Tsukuba Central 7
Tsukuba, Ibaraki
305-8567 Japan
anna.stork{at}earth.ox.ac.uk

Manuscript received 22 October 2002.

Source parameters of 68 small earthquakes (0.6 < MW < 3.0) in western Nagano, Japan, are investigated to determine the scaling of static stress drop ({Delta}{sigma}s) and apparent stress ({sigma}a) with seismic moment (M0). Data from the 800-m-deep borehole in the area provides clean recordings containing a wide range of frequencies. Source parameters are determined by manipulation of P and S spectra in the frequency domain using individually determined time window lengths for arrivals on each seismogram. Frequency-independent quality factors, Q, corner frequencies, f c and the amplitude spectra levels are estimated with the best-fitting Brune (1970) {omega}2 model. A frequency-dependent attenuation model, Q(f), is calculated by spectra normalization.

Static stress drop {Delta}{sigma}s is self-similar for 1010 < M0 < 1013 N m and values range between 0.01 and 10 MPa. An F distribution test shows there is a 10% probability that {sigma}a calculated from Q has a constant mean value for 1010 < M0 < 1013 N m. Furthermore, {sigma}a calculated using Q(f) has a significant probability that there is a change in {sigma}a values with M0. Using Q(f), 0.02 < {sigma}a < 2 MPa for M0 > 1011 N m, and 0.002 < {sigma}a < 0.2 MPa for M0 < 1011 N m, a narrower range than Q analysis results.

Limits in recorded frequencies, variations in time window length, and source complexity are not found to significantly affect the calculation of {sigma}a. Therefore, the constant scaling of {Delta}{sigma}s with M0 and the nonsimilarity and breakdown in {sigma}a scaling could be true characteristics of small earthquakes (MW <1.3).




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S. Kinoshita
Deep-Borehole-Measured QP and QS Attenuation for Two Kanto Sediment Layer Sites
Bulletin of the Seismological Society of America, February 1, 2008; 98(1): 463 - 468.
[Abstract] [Full Text] [PDF]


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