摘要
2017年九寨沟M_s7.0地震位于巴颜喀喇地块东边界带附近的虎牙断裂NNW方向的延长线上,震源深度20km,震源机制解为左旋走滑型,矩震级为M_w6.5.震后6 d内共记录到3000多次余震,最大震级为M4.8,余震活动强度偏低但衰减正常.主震位于余震区中部,为双侧破裂.余震区呈NNW向线性分布,断层面近乎直立,长度超过30 km,破裂宽度约15 km.余震区中部、东南部余震的震源机制大多与主震震源机制一致,而西北部余震的震源机制均为逆冲型,可能与岷江断裂有关.余震区中部、东南部的应力也较西北部释放充分.九寨沟地震最大烈度为9度,记录的峰值加速度值NS向最大,EW向次之,垂直向最小,最近的强震台是九寨百河台,其加速度反应谱低于九寨沟地区的8度设防烈度.静态库仑应力研究表明,汶川地震对九寨沟地震具有较明显的触发作用.沿虎牙断裂发生的1973年松潘东北M_s6.5地震,1976年松潘M_s7.2地震和本次九寨沟地震的余震区沿NNW方向相连.结合主震破裂位移分布、余震的震源机制及应力降分析认为,九寨沟地震余震区中部、东南部发生强余震的危险性不大,但需注意余震区西北部及岷江断裂地震活动的发展.
The M_s7.0 Jiuzhaigou earthquake with the hypocenter depth of 20 km occurred at the NNW extension line of the Huya fault located next to the eastern boundary of the Bayan Har block. The focal mechanism solution of the mainshock based on the W-phase source inversion showed a sinistral slip and the moment magnitude was M_w6.5. Over 3000 aftershocks were recorded within six days, among which the largest was M4.8 and three others were larger than M4.0. The value of the aftershock frequency attenuation coefficient P was 1.03, and the b value of the magnitude-frequency relationship was 0.68. The aftershock activities were relatively inactive but the attenuation was normal. Relocation results of 603 aftershocks were obtained by using the double-difference earthquake location algorithm and the location errors of the EW, NS and the vertical directions were 0.81, 0.84 and 1.59 km respectively. The mainshock occurred in the center of the aftershock region and ruptured on both sides along the strike direction. The aftershocks were distributed linearly along a NNW direction, and the fault plane was nearly vertical with a length over 30 km and a width of about 15 km. Aftershocks were more concentrated in the southeast than in the northwest. The focal mechanism solutions of most aftershocks in the central and southeastern regions were consistent with the mainshock, while those in the northwest were thrust-types, which might be related to the Minjiang fault. Meanwhile, the regional stress was released more in the central and southeastern regions than in the northwest. The maximum intensity investigated in the damage zone was up to IX. The maximum intensity region was located in scenic areas, therefore few people were present when the earthquake occurred at night. The peak acceleration was highest in the NS records, followed by the EW and vertical records. The design spectra of intensity for Jiuzhaigou was Ⅷ, which is higher than the acceleration response spectra obtained from the Jiuzhaibaihe station, the nearest station which had strong motion recordings. These aforementioned factors may have contributed to the minor casualties resulting from this earthquake. The aftershock regions of the Jiuzhaigou earthquake, the 1973 M_s6.5 Songpan northeast earthquake, and the 1976 M_s7.2 Songpan earthquake are connected along the Huya fault which may extend to the northwest in the deep earth. Considering the rupture displacement distribution of the mainshock, the focal mechanism of aftershocks, and the stress drop, strong aftershocks are unlikely to occur in the central and southeastern Jiuzhaigou aftershock regions; however, the development of seismicity in the northwest and the Minjiang River fault should be noted. Static Coulomb stress triggering of other 4 earthquakes larger than M6.5 occurred near the Ganqingchuan border were calculated using the Coulomb 3.3 software. The results indicated that the Jiuzhaigou earthquake was triggered by the Wenchuan earthquake and related to the stress adjustment in the study area.
引文
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