2001年1月26日印度古杰拉特(Gujarat)M_s7.8地震时空破裂过程

SPATIAL AND TEMPORAL RUPTURE PROCESS OF THE JANUARY 26,2001, GUJARAT, INDIA, Ms7.8 EARTHQUAKE

从全球数字地震台网的长周期记录中,选择了震中距小于90°的27个台站的54个P波震相和44个S波震相资料.首先,用波形反演方法确定了2001年1月26日印度古杰拉特（Gujarat）Ms7.8地震的地震矩张量、震源机制、震源时间函数和时空破裂过程等震源参数.通过矩张量反演,并根据Kutch Mainland断层的走向、地震烈度的空间分布、余震震源的空间分布和震害的空间分布,确认2001年1月26日印度古杰拉特Ms7.8地震的发震断层的走向为92°、倾角为58°、滑动角为62°,即一走向近东-西向、断层面向南倾斜、以逆冲为主的左旋-逆断层.这次地震所释放的地震矩为3.5X1020N·m,矩震级Mw=7.6.然后,借助合成地震图,采用频率域求谱商的方法,得到了依赖于台站方位的27个P波震源时间函数、22个S波震源时间函数以及平均的P波震源时间函数和S波震源时间函数.对震源时间函数的分析表明,这次地震是一次连续的破裂事件,开始比较急途,但结束比较迟缓,总持续时间约19s.最后,以所提取的P波和S波震源时间函数为资料,采用时间域的反演技术得到了断层面上滑动的时空分布.滑动量在断层面上的静态分布表明,断层面上的最大滑动量约为7m.断层面上的最大应力降约为30MPa,平均应力降约为7MPa.滑动量大于0.5m的区域在走向方向长85km,在断层面倾斜方向宽?

The source parameters, such as moment tensor, focal mechanism, source time function (STF) and temporal-spatial rupture process, were obtained for the January 26, 2001, India, MS7.8 earthquake by inverting waveform data of 27 GDSN stations with epicentral distances less than 90? Firstly, combining the moment tensor inversion, the spatial distribution of intensity, disaster and aftershocks and the orientation of the fault where the earthquake lies, the strike, dip and rake of the seismogenic fault was determined to be 92? 58?and 62? respectively. That is, this earthquake was a mainly thrust faulting with the strike of near west-east and the dipping direction to south. The seismic moment released was 3. 5X 1020N.m, accordingly, the moment magnitude Mw was calculated to be 7.6. And then, 27 P-STFs, 22 S-STFs and the averaged STFs of them were determined respectively using the technique of spectra division in frequency domain and the synthetic seismogram as Green's functions. The analysis of the STFs suggested thatthe earthquake was a continuous event with the duration time of 19 s, starting rapidly and ending slowly. Finally, the temporal-spatial distribution of the slip on the fault plane was imaged from the obtained P-STFs and S-STFs using an inversion technique of time domain. The maximum slip amplitude on the fault plane was about 7 m. The maximum dress drop was 30 MPa, and the average one over the whole rupture area was 7 MPa. The rupture area was about 85 km long in the strike direction and about 60 km wide in the down-dip direction, which, equally, was 51 km deep in the depth direction. The rupture propagated 50 km eastwards and 35 km westwards. The main portion of the rupture area, which has the slip amplitude greater than 0. 5 m, was of the shape of an ellipse, its major axis oriented in the slip direction of the fault, which indicated that the rupture propagation direction was in accordance with the fault slip direction. The phenomenon is popular for strike-slip faulting, but rather rare for thrust faulting. The eastern portion of the rupture area above the initiation point was larger than the western portion below the initiation point, which was indicative of the asymmetrical rupture. In other words, the rupturing was kind of unilateral from west to east and from down to up. From the snapshots of the slip-rate variation with time and space, the slip rate reached the largest at the 4th second, that was 0. 2 m/s, and the rupture in this period occurred only around the initiation point. At the 6th second, the rupture around the initiation point stopped mostly, and started moving outwards. The velocity of the westward rupture was smaller than that of the eastward rupture. Such rupture behavior like a circle mostly stopped near the 15th second. After the 16th second, only some patches of rupture distributed in the outer region. From the snapshots of the slip variation with time and space, the rupture started at the initiation point and propagated outwards. The main rupture on the area with the slip amplitude greater than 5 m extended unilaterally from west to east and from down to up between the 6th and 10th seconds, and the western segment extended a bit westwards and downwards between the 11th and 13th seconds. The whole process lasted about 19 s. The rupture velocity over the whole rupture process was estimated to be 3. 3 km/s.