On 5 September 2022, an Mw 6.6 strike-slip earthquake occurred in Luding, Sichuan, China. It illuminates the seismic gap on the southern end of Xianshuihe fault zone, that is Moxi segment, and is the most damaging earthquake in Chinese Mainland since 2014. In this article, we use multiple seismological methods to analyze the basic characteristics of the Luding earthquake, including its source parameters, rupture process, aftershock distribution, and further discuss its implications on preseismic fault behavior and impact on short-term seismic hazard. Using near-field stations, we first revisit the source parameters of the mainshock and obtain a much shallower hypocentral depth (∼9.3 km) than previously reported. Next, we jointly inverse the teleseismic P-wave data and near-field strong-motion data to constrain the mainshock rupture process. The coseismic slip consists of two subevents that spans for ∼20 km and shows an overall unilateral rupture to the south-southeast direction. The earthquake has 90% of the seismic moment released in its first 10 s. Third, we relocate the first 34 days of aftershocks with both manual phase picks and cross correlation, and finally obtain 5269 well-located events. The aftershocks reveal a near-vertical Xianshuihe fault and two sets of unmapped faults off the major Xianshuihe fault: one set of strike-slip faults on the north-northwest of rupture and the other one on the west of mainshock that generates the most intense seismicity with prevalent normal faulting. By examining the aftershock evolution and Coulomb stress change, we find that postseismic processes evolve both on and off the major fault, and the western off-fault system has highly variable fault orientations. Finally, we calculate the Coulomb stress change by the Luding earthquake on surrounding faults. Results show that the north Anninghe fault is significantly loaded by the Luding earthquake (>0.1 MPa), which poses high seismic hazard in the short term.

Abstract:On April 15, 2016, Kumamoto, Japan, was struck by a large earthquake sequence, leading to severe casualty and building damage. The stochastic finite-fault method based on a dynamic corner frequency has been applied to perform ground-motion simulations for the 2016 Kumamoto earthquake. There are 53 high-quality KiK-net stations available in the Kyushu region, and we employed records from all stations to determine region-specific source, path and site parameters. The calculated S-wave attenuation for the Kyushu region beneath the volcanic and non-volcanic areas can be expressed in the form of Q s = (85.5 ± 1.5)f 0.68±0.01 and Q s = (120 ± 5)f 0.64±0.05, respectively. The effects of lateral S-wave velocity and attenuation heterogeneities on the ground-motion simulations were investigated. Site amplifications were estimated using the corrected cross-spectral ratios technique. Zero-distance kappa filter was obtained to be the value of 0.0514 ± 0.0055 s, using the spectral decay method. The stress drop of the mainshock based on the USGS slip model was estimated optimally to have a value of 64 bars. Our finite-fault model with optimized parameters was validated through the good agreement of observations and simulations at all stations. The attenuation characteristics of the simulated peak ground accelerations were also successfully captured by the ground-motion prediction equations. Finally, the ground motions at two destructively damaged regions, Kumamoto Castle and Minami Aso village, were simulated. We conclude that the stochastic finite-fault method with well-determined parameters can reproduce the ground-motion characteristics of the 2016 Kumamoto earthquake in both the time and frequency domains. This work is necessary for seismic hazard assessment and mitigation.

An Ms 6.0 earthquake struck the Luxian County in southern Sichuan basin, China, on 15 September 2021, which was the largest local event thus far and caused heavy casualties and serious property losses. Using data of 107 temporary and six permanent stations from five months before to four and a half months after the time of the mainshock in the Rongchang-Luxian region, we utilize the machine learning-based workflow for phase picking, earthquake detection and relocation, and obtain precise hypocenter data for 22,806 events. We found that earthquakes increase significantly since July 2021, showing north-northeast-, northeast-, east-northeast- and east-southeast-directed seismic strips. Almost all events are located within the sedimentary layers at 1–7 km depth above the crystalline basement, and more than half of them occurred in Silurian and Ordovician formations. The Ms 6.0 Luxian earthquake sequence occurred in an area where no clear seismicity was observed before the mainshock. On map view, the aftershock zone shows a convex polygon with an overall strike of 112°. The focal depths of aftershocks southwest to the mainshock are distributed in a narrower range of 4–7 km and extend to a wider range of 2–7 km on the northeast side. It is speculated that the seismogenic fault is a blind thrust fault dipping to the southwest at 2–7 km depth. The focal mechanisms of 13 ML 3.0+ earthquakes show either pure reverse faulting or reverse faulting dominated mechanisms with a strike-slip component. The centroid depths fall in the range of 1.95–6.75 km. The maximum and middle principal stress axes are nearly horizontal; the azimuth of the maximum horizontal stress is about 120° and the stress shape ratio is 0.78. These results provide an important reference for reunderstanding the local seismic risk and for detailed studies on the seismological aspects about the unusual Luxian earthquake in the future.