Seismological evidence for the brittle-ductile interaction hypothesis on earthquake loading

Abstract

We extended the analyses of temporal variation of coda Q-1 and seismicity by Jin and Aki (1989, 1993) for central and southern California to year 2003. We use the relative frequency N (Mc) of earthquakes in a certain magnitude range around Mc characteristic to a seismic region to quantify the seismicity. The cross-correlation function between coda Q-1 and N (Mc) is calculated using a 10-year moving time window. The correlation coefficient for the entire period of about 60 years is peaked at the zero-time shift with the value close to 0.8 for both regions. We found, however, the simultaneous correlation is disturbed before major earthquakes. The disturbance is, consistently, a delay in the change of coda Q-1 relative to that of N (Mc) before the occurrence of a major earthquake. We attribute the temporal change in coda Q-1 to fractures in the ductile part of the lithosphere and that in N (Mc) to the response of the brittle part to the ductile fracture. We believe that Mc characteristic to a seismic region is originated from a characteristic size of fractures in the ductile zone of the lithosphere. The observed delay of coda Q-1 change relative to N(Mc) before a major earthquake can be explained simply by the strain energy stored in the brittle part of lithosphere reaching a saturation limit and starting to flow back to the ductile part. © 2004, The Seismological Society of Japan, Society of Geomagnetism and Earth, Planetary and Space Sciences, The Volcanological Society of Japan, The Geodetic Society of Japan, The Japanese Society for Planetary Sciences. All rights reserved.