关键词: 巴基斯坦地震, 同震形变场, 断层滑动分布, 发震构造, 喜马拉雅主前缘逆冲断裂, 次级断裂

Abstract: In the global scale, ten destructive earthquakes with magnitude larger than 7 happen on average each year. Yet the number of small earthquakes with limited or even no damage but recordable by seismographs(magnitude between 2.5 and 4.5)is over one million per year. In between, there are hundreds to thousands of earthquakes with moderate to strong magnitude(magnitude between 5.5 and 6.5)with notable destructiveness. The massive moderate to strong earthquakes are often less noticed or even overlooked, with only very few exceptions which caused human casualties and/or structure damages due to the very shallow focal depths. For medium earthquakes, the traditional seismology means can obtain the source mechanism solution of earthquake, but because of the inherent fuzziness of the source mechanism, it cannot distinguish the fault plane from the auxiliary nodal planes, because earthquakes of this magnitude usually do not produce surface rupture, and the result error is large, so it is not suitable for the study of medium and small earthquakes. It is of fundamental significance to further study the source fault of the moderate earthquakes, and more independent methods other than traditional seismology, such as satellite geodesy are needed. As one of the most applied satellite geodesy technique, interferometry of SAR(InSAR)satellite images are commonly used to obtain coseismic deformation related to earthquakes. InSAR has very high spatial sampling, though the temporal sampling is very low, which is several days to over a month depending on the satellite revisit span. The precision of coseismic deformation by InSAR can reach 2~3cm, which is good enough to obtain the surface deformation caused by a moderate earthquake. It is noted that InSAR coseismic measurements can detect 1-dimensional(1D)deformation along Line-of-Sight(LOS)direction. With multiple observing modes including descending and ascending, the InSAR deformation data is very useful for identifying surface ruptures, and for source fault plane discrimination. As a new geodetic observation technology, InSAR uses the elastic dislocation model to obtain source parameters, and the inversion results of fault parameters and slip distribution are more reliable. On September 24th, 2019, an M_W6.0 earthquake hit New Mirpur, Pakistan. The nearest known fault to the epicenter is the Main Frontal Thrust on its south side. We used the Sentinel-1A SAR imagery(TOPS-model)to reconstruct the InSAR coseismic deformation fields generated by the 2019 M_W6.0 Pakistan earthquake along the ascending and descending tracks. The ascending and descending deformation fields indicate that coseismic deformation is asymmetric by a trend of NW-SE in the south secondary fault of the Himalayan frontal thrust fault, with a maximum LOS displacement of~0.1m. The structures of ascending and descending deformation are highly consistent with each other, but the LOS displacement of southern side is obviously larger than the northern side. The continuous change of interference fringes between uplift and subsidence areas shows that there is no coherent phenomenon caused by excessively large deformation gradient or surface rupture, which indicates that the seismic fault rupture did not reach to the ground surface. Two initial fault models constrained by InSAR deformation, with a southwest-dipping and northeast-dipping fault, were utilized in the inversion. We finally determined the northeast-dipping fault as the seismogenic fault by joint inversion of ascending and descending observations, combined with tectonic setting. Our fault model suggests that an obvious slip concentrated area is located in the depth of 2~4km, with a peak slip of~0.64m and a mean rake angle of~125°. The north-dipping thrust motion with a small amount of strike-slip component dominated the faulting. The earthquake occurred in the low-dipping subduction zone between the Indian and Eurasian plates. The dip angle of the fault plane is relatively low. When the fault is ruptured, the upper wall thrust southwards and the north wall subducted northwards. Due to the compressional nappe structure, the front end of the upper wall was uplifted and the back end was stretched to become the subsidence area. Seismogenic fault is the south secondary fault of the Himalayan frontal thrust fault inferred from our coseismic fault model and rupture kinematic features. Active faults on the land have caused many large destructive earthquakes, resulting in surface faults and promoting the development of tectonic landforms. The detailed observation of coseismic surface rupture not only provides basic information for understanding the earthquake itself and estimating the earthquake recurrence period, but also helps to interpret the tectonic and geomorphic features in other areas. Since the M_W6.0 earthquake in Pakistan in 2019, no studies have been reported yet on this earthquake using InSAR technology, so the study of this earthquake provides a rare opportunity to assess the seismic risk of active thrust faults and to study the seismicity of northern Pakistan.

Key words: the 2019 M_W6.0 Pakistan earthquake, coseismic deformation, fault slip distribution, seismogenic structure, secondary fault of the Main Frontal Thrust