关键词: 玛多M_S7.4地震, gPhone重力仪, 同震重力变化, 位错模型

Abstract:

The Bayan Kara block is a secondary block in the Qinghai-Tibet Plateau interior with the strongest seismic activity occurring in the recent years. Unlike the 8 earthquakes above M7.0 occurring on the block boundary in the past 20 years, the Maduo M_S7.4 earthquake occurred inside the block, thus providing a new research perspective for the composition of coseismic gravity change observations. At the same time, China Earthquake Administration set up 5 gPhone continuous gravity stations, which operated normally before and after the earthquake, about 800km away from the epicenter, near the northeast edge of the Qinghai Tibet Plateau and the adjacent areas. Among them, the Maqin gravity station is inside the block and the Songpan gravity station is on the block boundary. The location of this earthquake and the distribution characteristics of the gravity stations provide natural experimental sites and samples for studying the coseismic gravity change in the stations at different locations in and around the block. In order to check the dislocation theory based on surface deformation observation, accumulate coseismic gravity change data by strong earthquakes, and analyze the features of the coseismic gravity change by surface gravity observation from different perspectives, the gravity earth tide and barometric observation data measured by gPhone gravimeters and sampled at 1Hz in these 5 continuous gravity stations from May 10 to 25, 2021 are collected. In this paper, firstly, exponential and step function methods are used to extract the coseismic gravity change in these 5 gravity stations, so as to analyze whether the post-seismic gravity signals contain relaxation signal. The relaxation time observed by gravimeter is very short compared to the near-field results of Lushan M_S7.0 earthquake, only about 11 minutes. And the step method result is more consistent with the model one. Comparing with the coseismic dislocation theory by Okubo and Sun model, it is found that the difference between the observation results of the two methods and the extreme value simulated by the two models is nearly 1×10^-8m·s^-2. Moreover, the observed and simulated results have a good consistency in terms of direction. However, there is about an order of magnitude difference between the observation and simulation in the gravity station position. By discussing the calculation accuracy of barometric admittance, mean tidal factor and co-seismic gravity change of 7-days data before and after the earthquake at the 5 stations, analyzing the relationship between the coseismic gravity change and vertical displacement in the same site of GNSS and gravity station at Maqin and Songpan, comparing the observations and simulations of the coseismic gravity changes in the two gravity stations(Maqin and Songpan)located at the block boundary and inside the block induced by the Jiuzhaigou M_S7.0 and Maduo M_S7.4 earthquake which occurred also on the block boundary and inside the block, and calculating the coseismic gravity change at Zhongdian station by the Maduo M_S7.4 after removing the effect of Yangbi M_S6.4 earthquake, it is considered that, these gPhone gravimeters analyzed in the paper can capture more than 0.5×10^-8m·s^-2 coseismic gravity change, and the Maqin station, which is about 175km away from the epicenter, observed about [(2.9±0.70)~(4.0±0.70)]×10^-8m·s^-2 coseismic gravity change generated by Maduo M_S7.4 earthquake. Based on the daily solution of GNSS vertical displacement, the co-seismic vertical deformation at Maqin and Songpan stations is all about-(4±5)mm. Taking the average gravity gradient value, -320×10^-8m·s^-2/m, the displacement-induced gravity change by Maduo M_S7.4 earthquake is calculated, which is(1.2±1.5)×10^-8m·s^-2. It is proved that part of the contribution of the co-seismic positive gravity variation at Maqin and Songpan stations comes from the variation of vertical deformation in this area. By eliminating the co-seismic effect from the Yangbi M_S6.4, the co-seismic gravity change induced by Maduo M_S7.4 earthquake in Zhongdian station is about(1.09±0.76)×10^-8m·s^-2. Compared with the observed and simulated co-seismic gravity change induced by Jiuzhaigou M_S7.0 earthquake, the co-seismic gravity change of about(9.1±0.22)×10^-8m·s^-2 by Maduo M_S7.4 earthquake recorded at Songpan station should include the effect of other factors. This may be related to the seismic and tectonic background, as Songpan station is just at the east boundary of Bayan Kara block. The coseismic gravity change in Linzhi station is negative, which is consistent with the simulation results of dislocation theory. Based on the observation results of this paper, it is considered that the Maduo M_S7.4 earthquake can produce about(0.5~4)×10^-8m·s^-2 co-seismic gravity change in the far field range of 175~800km. The coseismic gravity variation signal observed by the gravity station is not only related to crustal deformation and epicentral distance, but also to the seismotectonic background of the block where the gravity station locates. The results can provide a reference for determining the coseismic gravity change caused by medium strong earthquakes in the future.

Key words: the Maduo M_S7.4 earthquake, gPhone gravimeters, co-seismic gravity change, dislocation model