Abstract
Investigating relationships between temporally- and spatially-related continental earthquakes is important for a better understanding of the crustal deformation, the mechanism of earthquake nucleation and occurrence, and the triggering effect between earthquakes. Here we utilize Global Positioning System (GPS) velocities before and after the 2008 Mw 7.9 Wenchuan earthquake to invert the fault coupling of the Longmenshan Fault (LMSF) and investigate the impact of the 2008 Mw 7.9 Wenchuan earthquake on the 2013 Mw 6.6 Lushan earthquake. The results indicate that, before the 2008 Mw 7.9 Wenchuan earthquake, fault segments were strongly coupled and locked at a depth of ~18 km along the central and northern LMSF. The seismic gap between the two earthquake rupture zones was only locked at a depth < 5 km. The southern LMSF was coupled at a depth of ~10 km. However, regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were not coupled, with an average coupling coefficient ~0.3. After the 2008 Mw 7.9 Wenchuan earthquake, the central and northern LMSF, including part of the seismic gap, were decoupled, with an average coupling coefficient smaller than 0.2. The southern LMSF, however, was coupled to ~20 km depth. Regions around the hypocenter of the 2013 Mw 6.6 Lushan earthquake were also coupled. Moreover, by interpreting changes of the GPS velocities before and after the 2008 Mw 7.9 Wenchuan earthquake, we find that the upper crust of the eastern Tibet (i.e., the Bayan Har block), which was driven by the postseismic relaxation of the 2008 Mw 7.9 Wenchuan earthquake, thrust at an accelerating pace to the Sichuan block and result in enhanced compression and shear stress on the LMSF. Consequently, downdip coupling of the fault, together with the rapid accumulation of the elastic strain, lead to the occurrence of the 2013 Mw 6.6 Lushan earthquake. Finally, the quantity analysis on the seismic moment accumulated and released along the southern LMSF show that the 2013 Mw 6.6 Lushan earthquake should be defined as a “delayed” aftershock of the 2008 Mw 7.9 Wenchuan earthquake. The seismic risk is low along the seismic gap, but high on the unruptured southwesternmost area of the 2013 Mw 6.6 Lushan earthquake.
Highlights
IntroductionThe Longmenshan Fault (LMSF) zone, known as a major thrusting boundary structure, is located at the eastern margin of the Tibetan Plateau, which separates the plateau and the stable South China
The Longmenshan Fault (LMSF) zone, known as a major thrusting boundary structure, is located at the eastern margin of the Tibetan Plateau, which separates the plateau and the stable South ChinaRemote Sens. 2018, 10, 753; doi:10.3390/rs10050753 www.mdpi.com/journal/remotesensingRemote Sens. 2018, 10, 753 Remote Sens. 2018, 10, x FOR PEER REVIEW2 of 15 2 of 14 bblloocckk [[11,22]] ((FFiigguurree 11))
Our inversion results show that the hypocenter region of the 2013 Mw 6.6 Lushan earthquake was only loosely coupled, with a coupling coefficient of 0.3, before the occurrence of the 2008 Mw 7.9 Wenchuan earthquake, indicating that the southern LMSF was in an elastic strain accumulation deficit state or viscoelastic creeping state
Summary
The Longmenshan Fault (LMSF) zone, known as a major thrusting boundary structure, is located at the eastern margin of the Tibetan Plateau, which separates the plateau and the stable South China. Previous studies indicated that both the co-seismic rupture and postseismic relaxation due to the 2008 event may have increased the stress level on the southern LMSF, which triggered the 2013 event (e.g., [14,18,21]). Such studies have not considered the interseismic fault coupling characteristics [22]. In view of the aforementioned issues and debates, we use the horizontal GPS velocities to invert the fault coupling of the LMSF before and after the 2008 Mw 7.9 Wenchuan earthquake. Relationships between the 2008 Mw 7.9 Wenchuan earthquake and the 2013 Mw 6.6 Lushan earthquake are discussed, as well as the seismic hazard along the “seismic gap”
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