关键词: 青海玛多M_S7.4地震, 随机有限断层, 地震动模拟, 地震应力降, 地震烈度

Abstract:

An M_S7.4 earthquake occurred in Maduo County, Qinghai Province on May 22, 2021, which is the largest magnitude event following the 2008 Wenchuan M_S8.0 earthquake in China. A total of 54 aftershocks occurred up until 3 June, 2021, including 15 M4.0~4.9 earthquakes and one M5.0 earthquake. These earthquakes primarily nucleated at a depth of 8~10km, and ruptured a NNW-SSE-trending fault with a length of~170km. Based on the post-earthquake field damage survey, the seismogenic fault information, aftershock spatial distribution, as well as the strong-motion and seismic-intensity observation recordings, the Ministry of Emergency Management of the People’s Republic of China officially released the macroseismic intensity map for this M_S7.4 earthquake. The maximum intensity reaches X(Chinese seismic intensity scale)in a narrow area close to the fault. Due to the strong earthquake-resistance and the low population density, the Maduo M_S7.4 earthquake causes few casualties and some injuries only.
The sparse strong-motion observation stations in this region, distributed mainly to the northeast of the epicenter, collected a few number of strong-motion recordings only at far field(over 150km to epicenter)during the M_S7.4 event. In order to evaluate the spatial distribution of ground motion intensity, the stochastic finite-fault method for simulating three-component(two horizontal and one vertical components)seismic ground motions were applied to reproduce the ground motions at 4 461 dummy observation positions during the Maduo M_S7.4 earthquake. The inverted source rupture process from USGS and Zhang Yong of Peking University, source slip distribution model from Institute of Geology of China Earthquake Administration, and the source kinematic rupture models stochastically generated were used separately to represent the source model of this earthquake in the simulations. The average stress drops for these source models are mainly in the range of 3~4MPa. The V_S30 scaling was considered to represent the amplification effects of the local site conditions. The peak ground accelerations(PGAs)and velocities(PGVs)for the simulated ground motions were first compared with those from the far-field observations and the predicted medians by the ground motion prediction equation applicable for Qinghai-Tibetan seismic zone. The PGAs and PGVs for the near-fault simulations are generally up to ~300cm/s² and ~30cm/s, respectively. The simulated PGAs and PGVs are in good agreement with those predicted medians, and well stand for the distance decay. However, the simulated peak values are, in general, smaller than those far-field observations. Moreover, the acceleration and velocity time histories, and the 5%-damped pseudospectra accelerations (PSAs)at some typical dummy observation positions, e.g., epicenter, Maduo County, urban district of Guoluo, were provided. It was found that the amplitudes of the time histories and the PSAs at vertical component are much lower than those at both horizontal components for the simulated recordings. The amplifications on the horizontal ground motions caused by the local site condition, and the much sharp source spectral decay rate for P-wave may by the possible reasons.
Finally, the simulated three-component ground motions were applied to calculate the seismic intensities. The spatial distribution of seismic intensity was then displayed. Results indicated that: 1)the maximum seismic intensity is up to Ⅸ or Ⅷ dependent on the specific source model; 2)the principal axis of seismic intensity is well consistent with the strike of the seismogenic fault; 3)the distribution of meizoseismal area is just concentrated in a narrow area along the ruptured fault; and 4)the isoseismal lines are irregular ellipses due to the considerations on the local site conditions. Compared with the officially released macroseismic intensity map, the seismic intensities based on the simulated ground motions are generally underestimated by about 1~2 degrees. We infer that the stress drop statistically averaged used for simulations bears the responsibility. Considering the uncertainty of the seismic stress drop, the uncertainty of the seismic intensity based on the simulated ground motions is about plus or minus 1.2. At the end, we re-simulated ground motions using the source rupture model of USGS, while the value of stress drop was set to mean plus one standard deviation(about 13.59MPa). In this case, the seismic intensity based on simulated ground motions show good agreements with the macroseismic intensity map, especially for those regions with intensity equal to or greater than Ⅶ, and the maximum intensity reaches Ⅹ.

Key words: Maduo M_S7.4 earthquake, stochastic finite-fault method, ground motion simulation, stress drop, seismic intensity