A precursorlike change in coda excitation before the Western Nagano Earthquake (Ms = 6.8) of 1984 in central Japan
A precursorlike change in coda excitation before the Western Nagano Earthquake (Ms = 6.8) of 1984 in central Japan
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1984 年日本中部长野西部地震 (Ms = 6.8) 之前尾波激励的前兆变化
DOI：
10.1029/jb092ib02p01356
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发表时间：
1987
影响因子：
--
通讯作者：
H. Sato
中科院分区：
文献类型：
--
作者：
H. Sato
本文研究了1984年9月14日发生在日本国立防灾研究中心关东-东海微震观测网西北角附近的一次地壳地震的尾波持续时间的时间变化。本文分析了1982年2月至1984年12月期间在靠近主震震中（Δ = 24 km）的格尔台记录的垂直地震图。在余震区中部小范围内发生的185次地震，其尾波持续时间与平均震级的关系呈类抛物线变化。平均震级是根据直达波的最大振幅计算的台站震级的算术平均值。在主震前16个月的时间内，相同平均震级地震的尾波持续时间明显长于主震前后的尾波持续时间，置信水平为99.9%。尾波持续时间在这一时期平均增加约3s。尾波持续时间的异常是显着的流逝时间短于30秒，这对应于一个旅行半径小于1050公里。虽然在格尔测得的台站震级与平均震级的残差中没有发现明显的时间变化，但由于数据的大分散性，不能排除在震前时期的衰减变化。然而，裂纹的形成，是最合理的机制，在应力积累阶段的焦点区域周围的散射强度的增加，是与其他附加观测。非均匀地球介质中的裂缝群可能大到足以散射高频地震波，即使每个单独的裂缝可能非常小。
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.