一九九六年九月五日蘭嶼地震震源破裂過程  吳典

[ 摘要 ]

本研究是利用遠震寬頻P波、SH波的位移資料,區域地動SH波位移資料以及聯合遠震與強震位移波形,藉由最小平方法的逆推方式來分析一九九六年九月五日蘭嶼地震(ML7.07)之震源破裂過程。本研究採用Kikuchi使用遠震長週期P波逆推所得之震源機制解,並依據餘震分布情形,決定此地震之破裂面為走向356.5。、傾角54.7。、滑移角70。,假設斷層面大小為沿走向長73公里,沿傾角寬35公里斷層面共分成240個子斷層面,每個子斷層面長3.03公里、寬3.5公里。對於每個子斷層面的格林函數計算,是以位於其中心點位置的點震源來表示。在強地動方面,其格林函數是利用廣義反射-透射係數矩陣以及離散波數方法計算,之後再組合震源時間函數,便可得到每個子斷層所提供的合成波波形;在遠震方面,則是利用泛化波線法計算。由聯合波形逆推結果可知,此地震主要的破裂方向為單一方向南破裂,斷層面上的錯動分布是不均勻的,可分為四個主要的錯動區,一為震源區附近,第二在距離震源以南10公里處,第三是在距震源以南15公里35公里之間(約於地震發生後6~12),第四則位於40公里左右。最大錯動約位於震源以南20公里,沿傾角深30公里處,其錯動量370公分此外,將三日內規模大於4.0以上的餘震投影到斷層面上,顯示餘震大部分皆發生在主要破裂的外圍,或是錯動梯度較大的地方,反映出在震源區主震發生後,應力有再分配的情形。若不考慮斷層邊緣格點的錯動分布,則斷層面上之平均錯動量為31.6公分,地震矩M01.8*1026 dyne-cm,利用M0換算出來的MW規模為6.8,圓形破裂模式所得應力降Δσ為5.34 bar,矩形破裂模式所得應力降Δσ為2.16 bar

[ 英文摘要 ]

We present a rupture model of the 1996 Lanyu earthquake (ML =7.07), determined from the least-square inversion of local strong motion records, P and SH teleseismic body waves, and combined the strong motion and teleseismic records, respectively. We adopt the focal mechanism using complex body waveform inversion. Based on the aftershock distribution, we determined the rupture plane of the earthquake is strike 356.5, dip 54.7, and rake 70. The fault has dimension of fault length of 73 km and the down-dip width of 35 km. The fault plane was discretized into a total of 240 subfaults, each has 3.03-km in length and 3.5-km in width. The subfault is representative by the response of a point source centered on the subault. The Green’s function on each subfault, for teleseismic P or SH body wave, is computed using the generalized ray method. The Green’s funciton for strong motions are computed for a layered velocity model with the discrete wavenumber method. We convolved the Green’s function with source time function to get synthetic seismogram produced by each subfault. From analysis of the combined waveform inversion results, we find a unilateral dislocation from north to south with four main asperities. First region is centered at the hypocenter, the second, third, and fourth is located at about 10 km, 15-35 km, and 40km southeast of the hypocenter, respectively. The aftershocks with M > 4.0 distribution were located outside the regions with large slip or with high slip gradients. This indicates that the redistribution of stress follows the primary failure on the fault plane. The average slip is about 31.6 cm and total seismic moment is about 1.8*1026 dyne-cm. The stress drops for circular and rectangular fault are 5.34 bar and 2.16 bar, respectively.