## Abstract

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).

Original language | English |
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Pages (from-to) | 1781-1794 |

Number of pages | 14 |

Journal | Bulletin of the Seismological Society of America |

Volume | 94 |

Issue number | 5 |

DOIs | |

Publication status | Published - 1 Oct 2004 |