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Source parameter scaling for small earthquakes observed at the western Nagano 800-m-deep borehole, Central Japan

Research output: Contribution to journalArticle

  • Anna L. Stork
  • H. Ito
Original languageEnglish
Pages (from-to)1781-1794
Number of pages14
JournalBulletin of the Seismological Society of America
Issue number5
DatePublished - 1 Oct 2004


Source parameters of 68 small earthquakes (0.6 < M w < 3.0) in western Nagano, Japan, are investigated to determine the scaling of static stress drop (Δσ s) and apparent stress (σ a) with seismic moment (M o). 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) ω 2 model. A frequency-dependent attenuation model, Q(f), is calculated by spectra normalization. Static stress dropΔσ s, is self-similar for 10 10 < M o < 10 13 N m and values range between 0.01 and 10 MPa. An F distribution test shows there is a 10% probability that σ a calculated from Q has a constant mean value for 10 10 < M o < 10 13 N m. Furthermore, σ a calculated using Q(f) has a significant probability that there is a change in σ a values with M o. Using Q(f), 0.02 < σ a < 2 MPa for M o > 10 11 N m, and 0.002 < σ a < 0.2 MPa for M o < 10 11 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 σ a. Therefore, the constant scaling of Δσ s, with M o and the nonsimilarity and breakdown in σ a scaling could be true characteristics of small earthquakes (M w <1.3).



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