Evolution and Distribution of the Early Aftershocks Following the 2008 Mw 7.9 Wenchuan Earthquake in Sichuan, China

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2017JB015136

Foreshocks, b Value Map, and Aftershock Triggering for the 2011 M w 5.7 Virginia Earthquake

Re-evaluating seismic hazard along the southern Longmen Shan, China: Insights from the 1970 Dayi and 2013 Lushan earthquakes


 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.

2022 Mw 6.6 Luding, China, Earthquake: A Strong Continental Event Illuminating the Moxi Seismic Gap

Early aftershocks contain important information about the physics of earthquake occurrence and postseismic relaxation processes. However, the standard catalogs of early aftershocks are usually incomplete. Many events can be missed in the main shock coda, some of which are strong enough due to the extremely high noise level. Under these conditions, the process of event identification becomes largely stochastic. Due to different network configurations and record processing methods, different agencies may register/miss different events, thus merging catalogs can improve the completeness of the aftershock sequence. When merging catalogs, the problem of identifying duplicates (records related to the same seismic event) arises. The main difficulty is discriminating aftershocks and duplicates, since both are events close in space and time. The problem is analogous to the problem of discriminating aftershocks and independent events. The solution methods are usually similar too. In this paper, we apply the nearest neighbor method modified for our problem. This method has become widespread in recent years in the problem of identifying aftershocks, and a probabilistic metric in the space of network errors in determining the epicenters and times of seismic events. It is applied for automatic identification of duplicates when merging catalogs of aftershocks for the Tohoku earthquake. An analysis of the space-time structure of duplicates and aftershocks shows their significant difference, which makes it possible to successfully solve the problem. In a sample from the global Advanced National Seismic System (ANSS) catalog ( M > 4 ), were found more than 700 events missed by the Japan Meteorological Agency (JMA) seismic network, which is one of the best in the world. Among the misses, there are several events with M > 6 in the first hours after the main shock. Duplicate identification reliability is >97%. The method can be used to improve the completeness of aftershock sequences. The reliable identification of duplicates allows, in addition, to study the correspondence of the magnitudes determined by different agencies. Therefore the present method is an effective tool for creating merged catalogs of earthquakes with a uniform magnitude.

/pdf/nearest-neighbor-method-for-discriminating-aftershocks-and-3jgb8zmp.pdf

Nearest Neighbor Method for Discriminating Aftershocks and Duplicates When Merging Earthquake Catalogs

We perform a comprehensive detection of early aftershocks following the 2013 Mw 6.6 Lushan earthquake, which occurred in the southern Longmenshan Fault Zone in Sichuan Province, China, about 5 years after the 2008 Mw 7.9 Wenchuan earthquake. We use events in both standard and relocated catalogs as templates to scan through continuous waveforms 2 days before and 3 days after the main shock. We successfully reduce the magnitude of completeness Mc by more than 1 order and obtain up to 6 times more events than listed in both catalogs. Aftershocks in the first hour mostly occur around the main shock slip region, and aftershocks at later times show systematic expansions in the along-strike, perpendicular-strike, and updip directions. Although postseismic deformation following the Lushan main shock has not been clearly identified, we suggest that early aftershock expansions are likely driven by afterslip of the Lushan main shock. This is consistent with the observations that most aftershocks were in the stress shadow of the Lushan main shock and that there was significant slip deficit in the top 10 km of the crust. We also find that seismicity on the back thrust fault was activated as soon as 20 min after the main shock, earlier than previously reported. We are unable to detect any clear foreshocks in the last 2 days before the Lushan main shock.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016JB013706

Spatial-temporal evolutions of early aftershocks following the 2013 Mw 6.6 Lushan earthquake in Sichuan, China

Several recent studies have shown that large crustal earthquakes are often preceded by microseismic activities that accelerate or migrate in space and time. However, it is not clear whether such patterns are universal for other large earthquakes that do not have obvious foreshocks. Here, we apply a matched‐filter technique to detect missing earthquakes around the rupture zone of the 2008 M w  7.9 Wenchuan earthquake from 1 January to 12 May 2008. Using continuous waveforms recorded by the Zipingpu reservoir seismic network, we detect ∼4 times more earthquakes than listed in a local catalog. The newly detected events did not show any systematic spatiotemporal migration before the Wenchuan earthquake. The seismicity rate is rather stable during the entire study period, except following an M L  3.7 earthquake on 14 February 2008 that likely occurred on the extension of the blind‐thrust fault beneath the Sichuan basin. We also inspect 2‐hr‐long seismograms right before the Wenchuan earthquake and confirm that there was no immediate foreshock. Finally, we do not find any correlation between the local seismicity and the water level of the Zipingpu reservoir in the last few months before the Wenchuan earthquake. Electronic Supplement: Figures of seismicity, satellite map, focal mechanisms with depth resolution, and static Coulomb stress changes.

Microseismic Activity in the Last Five Months before the Mw 7.9 Wenchuan Earthquake

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Spatial-temporal evolutions of early aftershocks following the 2013 Mw 6.6 Lushan earthquake in Sichuan, China

Microseismic Activity in the Last Five Months before the Mw 7.9 Wenchuan Earthquake