1 Seismic Velocity Reduction and Accelerated Recovery Due to Earthquakes on the Longmenshan Fault

1 Seismic velocity reduction and accelerated recovery due to earthquakes on the Longmenshan fault 2 Shunping Pei1,2, Fenglin Niu3,4, Yehuda Ben-Zion5, Quan Sun2, Yanbing Liu2, Xiaotian Xue2, Jinrong Su6, 3 Zhigang Shao7 4 1CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences (CAS), 5 Beijing 100101, China 6 2 CAS Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, 7 Chinese Academy of Sciences(CAS), Beijing 100101, China 8 3State Key Laboratory of petroleum Resource and Prospecting, and Unconventional Gas Institute, China 9 University of Petroleum at Beijing, Beijing 102249, China 10 4Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, 11 Houston, TX 77005, USA 12 5Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA 13 6Earthquake Administration of Sichuan Province, Chengdu 610041, China 14 7Institute of Earthquake Science, China Earthquake Administration, Beijing 100029, China 15 Correspondence and requests for materials should be addressed to F.N. ([email protected]). 16 17 Various studies report on temporal changes of seismic velocities in the crust and attempt to relate 18 the observations to changes of stress and material properties around faults. While there is growing 19 number of observations on co-seismic velocity reductions, detailed observations of healing phases 20 are generally lacking. Here we report on pronounced co-seismic reduction of velocities around two 21 locked sections (asperities) of the Longmenshan fault with large slip during the 2008 Mw 7.9 22 Wenchuan earthquake, and subsequent healing of the velocities. The healing phase is accelerated 23 significantly at the southern asperity right after the nearby 2013 Mw 6.6 Lushan earthquake. The 24 results are obtained by joint inversions of travel time data at four different periods across the 25 Wenchuan and Lushan earthquakes. The rapid acceleration of healing in response to the Lushan 1 26 earthquake provides unique evidence of the high sensitivity of seismic velocities to stress changes. 27 We suggest that stress redistribution plays an important role in rebuilding fault strength. 28 29 Earthquakes are caused by the rapid conversion of stresses to inelastic strain (rock damage) along 30 faults (1-10). Recent studies show that fault failure can manifest as a small or large earthquake (11), as 31 aseismic slip (12), or as non-volcanic tremor (13-15). It is also found that fault interactions and other 32 processes can significantly affect the long-term stress build up by plate tectonics (16,17). Stress transfer 33 can be calculated in principle with elastic or viscoelastic modelling (16-21); however, estimating stress 34 changes from data is notoriously difficult, particularly at seismogenic depths. One promising approach is 35 to accurately monitor changes of subsurface seismic velocities (4,22), which are shown in laboratory 36 studies to be sensitive to the stress field (23-25) due to stress-induced changes in properties of cracks. 37 Indeed, there are an increasing number of observations on temporal changes of seismic velocities 38 associated with the occurrence of tectonic events, such as earthquakes (3-8, 26-28) and volcanic eruptions 39 (29,30). 40 The Longmenshan fault zone is located at a pronounced topographic boundary between the eastern 41 margin of the Tibetan plateau and the western Sichuan basin (Figure 1), where elevation changes from Fig. 1 42 ~5000 m to ~500 m within a distance of ~50 km. Geologically, the fault zone manifests itself as the thrust 43 front of the Himalayan orogen and consists of a series of long-angle transpressional faults that extend 44 from southwest to northeast for approximately 300 kilometers. Fault motion is dominated by thrust at the 45 southwestern section and gradually transitions to strike slip at the northeastern end. Over the last decade, 46 two major earthquakes, the 2008 Mw7.9 Wenchuan earthquake and the 2013 Mw6.6 Lushan earthquake, 47 ruptured the northeastern part and the southern end of the fault zone, respectively. The section between 48 with a length of ~60 km remained intact and is associated with seismic risk yet to be determined. The 49 area has been well instrumented before and after the earthquakes, providing unique opportunities to study 50 temporal variations of seismic properties and interaction among different segments of the fault zone. 2 51 The Longmenshan fault zone is in a seismically active region, which is closely monitored by the 52 regional seismic network operated by the Earthquake Administration of Sichuan Province (EASP). The 53 seismicity before the 2008 Mw7.9 earthquake was rather diffuse and spread widely across the entire 54 margin (black crosses in Figure 1b). It was replaced by a much more condensed aftershock seismicity 55 along the Longmenshan fault after the main shock (circles in Figure 1b). These small events are well 56 recorded and located by the EASP seismic network due to the good station coverage in both azimuth and 57 distance. 58 Coseismic velocity reduction during the 2008 Wenchuan Earthquake 59 We selected the first P-wave arrivals in the distance range between 0.1° and 2.0°, which are known as 60 the Pg waves traveling through the upper crust, recorded in the period of 2000-2014. The Pg travel times 61 exhibit a linear relationship with epicentral distance (Supplementary Fig. S1), and the slope of the linear 62 trend corresponds to the average velocity of the upper crust sampled by the source-receiver raypaths. We 63 noticed a small yet systematic change in the slope of the traveltime curve. We organized the traveltime 64 data in a chronological order, and divided the15-year period into time intervals with roughly the same 65 amount of earthquakes. In particular, we used one-year and one-month intervals before and after the 66 Wenchuan earthquake, respectively, due to the large number of aftershocks. Using linear regressions of 67 the Pg traveltimes compiled in each period, we compute the corresponding average P-wave velocities and 68 observe significant variation across the 15-year period (Figure 2a). The estimated average P-wave Fig. 2 69 velocity remains more or less the same at ~6 km/s before the Wenchuan earthquake, and drops abruptly 70 nearly 4% to ~5.75 km/s right after the mainshock. It then rises gradually to ~5.85 km/s before the 2013 71 Mw6.6 Lushan earthquake, where a small coseismic drop is observed. The influence from the Lushan 72 earthquake, however, appears to have very short duration and the recovery of P-wave velocity seems to be 73 present one month after the Lushan earthquake and continues to grow nearly to the level prior to the 74 Wenchuan earthquake. 3 75 To further locate the lateral distribution of the observed average velocity changes, we developed a 76 tomography technique that jointly invert the travel times observed at different time periods for 2-D 77 subsurface velocity changes. We analyzed four time periods that sample right before and after the two 78 earthquakes, which are marked by black solid horizontal lines in Figure 2a. The length of each period is 79 chosen such that the four time periods have roughly the same amount of Pg traveltime data. Because of 80 the large numbers of aftershocks right after the Wenchuan earthquake, a very short time period P2 is 81 sufficient to accumulate the required amount of data, producing a ~3-year gap between P2 and P3. We 82 treated the Pg travel times in each period as independent observations and employed a 2-D traveltime 83 tomography method (31, 32) to jointly invert the data from two consecutive periods for the background 84 velocities of each lateral block, as well as their changes between the two periods (Methods). 85 The co-seismic velocity changes of the Wenchuan earthquake (Figure 3a) were obtained from the 86 joint inversion of the Pg data of the first two time periods, P1 and P2. Large velocity drops are clustered Fig. 3 87 at the Yingxiu town of the Wenchuan County and Beichuan County, which are hereafter referred to as the 88 Wenchuan asperity and Beichuan asperity, respectively. We note that they are spatially coincident with 89 the two areas of large coseismic slip (33-35) and surface deformation (36,37). Results from the joint 90 inversion of Pg data of the two time periods, P2 and P3, reveal postseismic velocity changes that occurred 91 in the four-year period following the mainshock and right before the 2013 Lushan earthquake (Figure 92 3b). During this period, most of the velocity recoveries are observed around the Beichuan asperity, 93 suggesting that fault healing during this period took place at this part of the fault. The Wenchuan 94 asperity, on the other hand, showed very little to no velocity change during this period. 95 Accelerated healing following the 2013 Lushan earthquake 96 The co-seismic velocity changes of the Lushan earthquake inverted from P3 and P4 (Figure 3c) 97 show a large velocity increase in the southern portion of the Wenchuan rupture zone, particularly in the 98 area near the Wenchuan asperity. Velocity changes in other regions including the Beichuan asperity area 99 are insignificant. We further computed the total amount of velocity recovery along the Longmenshan fault 4 100 over the six-year period after the Wenchuan earthquake by jointly inverting the P2 and P4 data (Figure 101 3d). Most of the co-seismic velocity drops of the Wenchuan earthquake that centered at the two asperity 102 zones appear to be nearly recovered during this period. 103 To examine how seismic velocity has evolved in the two asperity regions, we computed the average 104 velocities and their standard deviations using cells in the two black boxes marked in Figure 3.

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