A precursorlike change in coda excitation before the Western Nagano Earthquake (Ms = 6.8) of 1984 in central Japan

Abstract

Temporal variations of coda duration were studied in relation to a crustal earthquake (September 14, 1984) which took place near the NW corner of the Kanto‐Tokai microearthquake observation network of the National Research Center for Disaster Prevention in Japan. Vertical seismograms recorded at station GER close to the main shock epicenter (Δ = 24 km) between February 1982 and December 1984 were analyzed. A precursorlike change was found in the relation between coda duration and average magnitude for 185 earthquakes which occurred in a small volume in the middle of the aftershock region. The average magnitude is the arithmetic mean of station magnitudes which are calculated from the maximum amplitudes of direct waves. Coda durations for earthquakes of the same average magnitude were significantly longer during the 16‐month period preceding the main shock than those before and after this period with a confidence level of 99.9%. The increase in coda durations was about 3 s on average in this period. The coda duration anomaly was significant for lapse time shorter than 30 s, which corresponds to a travel radius less than ∼50 km. Although no clear temporal change was found in the residuals of station magnitude measured at GER from average magnitude, an attenuation change in the preevent period can not be ruled out due to the large scatter of data. Crack formation, however, is the most plausible mechanism for an increase in scattering intensity around the focal region in the stress accumulation stage and is consistent with other precursory observations. Clusters of cracks in the heterogeneous earth medium might be big enough to scatter high‐frequency seismic waves, even though each individual crack may be very small.