关键词: 玛多地震, 同震位移, 弥散变形, 走滑断裂, 滑动速率

Abstract:

The coseismic displacements are required to characterize the earthquake rupture and provide basic data for exploring the faulting mechanism and assessing seismic risk in the future. Detailed field investigation is still an important way to acquire reliable coseismic displacements comparing to geodetic measurements. Combining with previous research on other earthquakes, this study tries to discuss distributed deformation along the strike rupture and its implications. The M_W7.4 Madoi earthquake ruptured the southeast section of the Kunlun Shankou-Jiangcuo Fault on May 22, 2021, in Qinghai Province. It is a typical strike slip event, and its epicenter locates at~70km south of the East Kunlun Fault, which is the north boundary of the Bayan Har block. Field investigation results show that the surface rupture extends along the piedmont. The deformation features mainly include compression humps, extensional and shear fissures, and scarps. After the earthquake, we used the unmanned aerial system to survey the rupture zone by capturing a swath of images along the strike. The swath is larger than 1km in width. Then we processed the aerial images by commercial software to build the orthoimage and the digital elevation model(DEM)with high resolutions of 3~5cm. We mapped the surface rupture in detail based on drone images and DEM along the western section. Meanwhile, we also got the commercial satellite images captured before the earthquake, on 2nd January 2021. The images were processed with geometrical rectification before comparison. The spatial resolution of satellite images before earthquake is about 0.5m.
At the south of the Eling Hu(Lake), the clear offset tire tracks provide an excellent marker for displacement measurement. We located the positions of tracks precisely based on remote sensing images, and compared between the tracks lines after earthquake and the corresponding positions before earthquake, then extracted distance difference, which is defined as coseismic displacements. The results show that the total displacement is about 3.6m, which contains the distributed deformation of about 0.9m. The off-fault deformation is about 33% of the on-fault and about 25% of the total deformation. The ratios are similar to previous studies on earthquake worldwide. The fault zone width is probable about 200m. The total horizontal displacement measured by this study is similar to the slip in depth by InSAR inversion, which implies that there is no slip deficit at the west rupture section of the earthquake.
The results also present the asymmetry of distributed deformation that most distributed deformation occurs at the south of the surface rupture zone. Comparing with other earthquakes in the world, it is likely that the asymmetrically distributed deformation is common in strike-slip earthquakes and the asymmetric feature is not related to the property of the material. The characteristics of distributed deformation might be related to fault geometry at depth or local stress state. More work is needed to resolve this question in the future. This study implies that we probably underestimated the slip rates resulting from ignoring distributed deformation in the past. In order to avoid underestimation of slip rates, we can correct the previous results by the ratio of distributed deformation to total slip. It is also suggested that the study sites should be on the segment with narrow deformation and simple geometry.

Key words: Madoi earthquake, coseismic displacement, distributed deformation, strike-slip fault, slip rate