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Nature Geoscience: Seismic velocity reduction and accelerated recovery due to earthquakes on the Longmenshan fault

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Shunping Pei, Fenglin Niu, Yehuda Ben-Zion, Quan Sun, Yanbin Liu, Xiaotian Xue, Jinrong Su, and Zhigang Shao

Nature Geosci. 12 (2019) 387-392.

DOI: 10.1038/s41561-019-0347-1

Abstract

Various studies report on temporal changes of seismic velocities in the crust and attempt to relate the observations to changes of stress and material properties around faults. Although there are growing numbers of observations on coseismic velocity reductions, generally there is a lack of detailed observations of the healing phases. Here we report on a pronounced coseismic reduction of velocities around two locked sections (asperities) of the Longmenshan fault with a large slip during the 2008 Mw 7.9 Wenchuan earthquake and subsequent healing of the velocities. The healing phase accelerated significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The results were obtained by joint inversions of travel time data at four different periods across the Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan earthquake provides unique evidence for the high sensitivity of seismic velocities to stress changes. We suggest that stress redistribution plays an important role in rebuilding fault strength.

GRL: Precursory Stress Changes and Fault Dilation Lead to Fault Rupture: Insights From Discrete Element Simulations

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David G. Blank and Julia K. Morgan

Geophys. Res. Lett. 46 (2019) 3180-3188.

DOI: 10.1029/2018GL081007

Abstract

We use the discrete element method to create numerical analogs to subduction megathrusts with natural roughness and heterogeneous fault friction. Boundary conditions simulate tectonic loading, inducing fault slip. Intermittently, slip develops into complex rupture events that include foreshocks, mainshocks, and aftershocks. We probe the kinematics and stress evolution of the fault zone to gain insight into the physical processes that govern these phenomena. Prolonged, localized differential stress drops precede dynamic failure, a phenomenon we attribute to the gradual unlocking of contacts as the fault dilates prior to rupture. Slip stability in our system appears to be governed primarily by geometrical phenomena, which allow both slow and fast slip to take place at the same areas along the fault. Similarities in slip behavior between simulated faults and real subduction zones affirm that modeled physical processes are also at work in nature.

Plain Language Summary

Relatively little is known about how earthquakes start, what kind of behavior precedes them, and what physical processes control them. Earthquake precursors could serve as early warning signals and improve our ability to predict upcoming events. However, it is impossible to make direct observations on the earthquake initiation process in nature due to the fact that the earthquake source is buried many kilometers beneath the Earth’s surface. To overcome this limitation, we use computer models to simulate earthquakes and make direct observations on the rupture process. We find that slow slip and stress changes take place just prior to large earthquake rupture on our simulated fault. We argue that these processes are controlled by relatively simple geometrical phenomena.