Temporal correlation between coda Q−1 and seismicity—evidence for a structural unit in the brittle-ductile transition zone

Abstract

Analysis of local earthquake coda waves recorded on Wood-Anderson (1940–1947) and Benioff (after 1947) short-period vertical seismographs at station Mt. Hamilton, Calif., revealed a large temporal variation in coda Q −1. A strong positive correlation was found between the temporal variation of coda Q −1 and the number of earthquakes with a magnitude of 4.0⩽ M⩽4.5 appearing in a series of 100 consecutive earthquakes with a magnitude M⩾3.0. A correlation coefficient of 0.84 was determined with a confidence level of 0.98. The amplitude and time constant of variation in coda Q −1 for central California are similar to those found earlier for southern California (Jin and Aki, 1989). The strong correlation between coda Q −1 and b-value reported by Jin and Aki (1989) was shown to be equivalent to a positive correlation between coda Q −1 and the fractional number of earthquakes with a magnitude 3.0⩽ M⩽3.5. A review of the literature reporting observed temporal correlation between coda Q −1 and b-value (both positive and negative) revealed that all the reported observations can be interpreted in terms of a positive correlation between the coda Q −1 and the fractional frequency of earthquakes in a magnitude range around a certain magnitude, M c, characteristic to each region. This positive correlation has been attributed to the aseismic creep in the brittle-ductile transition zone. Aseismic creep in the transition zone may occur over fractures with a characteristic size for each region. The existence of fractures with a characteristic size would necessarily violate the scale independent self-similarity of earthquake phenomena. This is consistent with the growing literature demonstrating a departure from self-similarity in various seismic phenomena.