Premonitory Phenomena of a Moderate Taiwan Earthquake

一個臺灣中規模地震之前兆現象

陳國誠(Kou-Cheng Chen) ； 王錦華(Jeen-Hwa Wang) ； 葉玉蓮(Yu-Lien Yeh)

《Terrestrial, Atmospheric and Oceanic Sciences》 1卷1期 (1990/03) Pp. 1-21

摘要

一九八三年五月十日台灣宜蘭太平山發生規模M(下標 D)=5.7之地震。在主震發生前之餘震區呈現較低的地震活動。餘震的震央分布和未露出地面的斷層帶有相當的關係。另外由每月地震發生頻率，可看出在餘震區及其鄰近地區存在異常的地震活動，而且此異常的地震活動是從震源區的外圍移至震源區。在主震發生約一年半前，b值增加而後遞減。在本文中同時分析遠距離測站和近距離側站的地震尾波總振動時間之比值，以為研究地震前兆之指標，此值我們稱之為總振動比值(duration ratio)。結果顯示此比值在主震發生後急速下降，原因是所分析的地震資料均發生在一小的時間間隔中，而高估了近距離測站的地震尾波之總振動時間。但在主震發生l小時後，振動比值上升，此現象乃由於主震發生後，地震規模M(下標 D)大於3的地震個數急速增加，形成較大的破碎密度所致。

並列摘要

An earthquake of M(subseript D)=5.7 occurred at Taipingshan, Ilan, Taiwan on May 10, 1983. A low seismicity zone appeared before the occurrence of the mainshock in the aftershock area. The epicentral pattern of aftershocks showed a significant correlation with an unexposed fault zone. The monthly frequency of events displayed abnormal seismicity both in the aftershock area and its neighborhood. This abnormal seismicity migrated somewhat from the outside area to the source area. About one and a half years before the mainshock, the b values increased markedly but then decreased before the major event. The duration ratio (DR) of the total coda waves duration time from a distant station compared to that at a closer station is also studied as an earthquake precursor. The DR values immediately decreased after the mainshock. An overestimation of total coda waves duration times for the close-in station might be due to a mixture of seismograms generated from the events which occurred in a small time interval and might result in this decrease. One hour after the mainshock, the DR values raised. This is interpreted to be a result of a large fracture density caused by the rapid increase of the number of M(subscript D)＞3 events immediately after the mainshock.