A more accurate relocation of the 2013 M s7.0 Lushan, Sichuan, China, earthquake sequence, and the seismogenic structure analysis

Abstract

A more accurate relocation of the 2013 M s 7.0 Lushan, Sichuan, China, earthquake sequence, and the seismogenic structure analysis Abstract We use a combined earthquake location technique to relocate the M s 7.0 Lushan, Sichuan, China, earthquake sequence of April 20, 2013. A stepwise approach, employing three existing location methods (the HYPOINVERSE method, the Minimum 1-D model, and the Double Difference method), is used to improve location precision by iteratively revising the velocity model station corrections, and hypocenter relocations throughout the process. Our stepwise approach has significantly improved the location precision of the Lushan earthquake sequence, yielding hypocenter locations with final errors of 359, 309, and 605 m in the E-W, N-S, and vertical directions, respectively, with average travel time residuals of 0.12 s. Furthermore, we analyzed the seismogenic structure surrounding the Lushan earthquake sequence by combining the results of the relocated hypocenter distribution with new focal mechanism solutions and information from regional geological and geophysical investigations. From our analysis, we conclude that the vast majority of the aftershocks of the Lushan earthquake sequence occurred at depths of 6-9 km, near the front of the southwestern segment of the NE-trending Longmenshan fault zone. Densely aligned hypocenters clearly suggest that the seismogenic structure of the mainshock consists of a set of basal thrust faults dipping to the NW at 40-50°, at a ramp of the deep basal décollement-thrust system at depths of 7-18 km. Focal mechanism solutions suggest that the seismogenic faults have produced almost pure thrusting. At least one SEdipping back-thrust is also observed within the basement, as indicated by the hypocenter relocations, which points to either a secondary rupture plane during the mainshock or a plane of aftershock slips. A small number of minor events in the Lushan sequence are located at depths of 0-6 km, with a distribution suggesting that the three NEtrending faults with surface traces running through or passing close to the aftershock area are confined to the upper Mesozoic sedimentary cover, making them independent of the deeper thrust faults that ruptured during the mainshock. Therefore, the 2013 M s 7.0 Lushan earthquake was a blind thrust fault generated on active thrust faults within the basement of the southwestern Longmenshan fault zone, with an upper limit estimation of the rupture length, average down-dip width, and rupture area of 40, 16, and 640 km 2 , respectively.Keywords Lushan earthquake sequence . Highprecision relocation . Combined solution of earthquake location . Error analysis . Seismogenic structure IntroductionEarthquake location techniques provide an effective way to study the 3-D attitudes of seismogenic structures

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

“…In order to reveal how seismicity evolved in the strike‐perpendicular direction, especially the activation of a back thrust fault [ Han et al , ; Fang et al , ; Long et al , ], we project all detected events along the strike‐perpendicular direction of 140°clockwise relative to north (Figure ). Similar to the along‐strike profile, we observe an expansion of aftershocks in the normal‐to‐fault direction for both standard (Figure b) and relocated catalogs (Figure d).…”

“…To further examine spatiotemporal evolutions of early aftershocks, we project all detections along the LMS fault strike (50° clockwise from north) and center around the main shock epicenter (102.9824°N, 30.2821°E, depth 14.8 km) as listed in the relocated catalog [ Long et al , ]. As shown in Figure b for the standard catalog, seismicity expands from −15 to 5 km along‐strike relative to the main shock in the first 1000 s to −30 to 20 km along‐strike at the end of detection period.…”

“…The first is from the standard catalog published by SSN (Table S1 in the supporting information), which includes 4498 events from 1 to 30 April 2013 (hereafter standard templates ). The second has 1809 hypoDD‐relocated events (Table S2) from 20 April to 9 June 2013 ([ Long et al , ] hereafter relocated templates ). Standard templates contain more events in a larger area, while relocated templates provide more accurate hypocenters (Figure S3).…”

Aftershock Observation and Analysis of the 2013Ms 7.0 Lushan Earthquake

“…However, the fine-scale fault geometry was not well imaged in the previous studies. For example, the y-shaped fault system and sharp fault planes have been shown vaguely in Han et al (2014) and Long et al (2015) and are not revealed in Lü et al (2013) and Zhang and Lei (2013). This may be due to the different datasets and earthquake location methods they used, because most of them used only several days' aftershock data recorded by the permanent seismic stations.…”

“…Some researchers conducted earthquake relocation, shearwave splitting, and earthquake tomography after the Lushan earthquake (Fang, Wu, Wang, Lü, et al, 2013;Jiang et al, 2013;Zhang and Lei, 2013;Zeng et al, 2013;Lei et al, 2014;Pei et al, 2014;Long et al, 2015). These results, together with the focal mechanism Zeng et al, 2013;Han et al, 2014) and rupture process of the mainshock (C. L. Wang et al, 2013;C.…”