The ETAS model introduced by Ogata (1988, 1989) extends the OmoriâUtsu law, assuming that each earthquake has a magnitude-dependent ability to trigger ...
Bulletin of the Seismological Society of America, Vol. 102, No. 5, pp. 2157–2164, October 2012, doi: 10.1785/0120110333
Stress Interactions within the Strong Earthquake Sequence from 2001 to 2010 in the Bayankala Block of Eastern Tibet by Ke Jia, Shiyong Zhou, and Rui Wang
Abstract
Four major earthquakes of M s > 7:0 occurred in the Tibetan Plateau during the last decade. They were the 2001 M s 8.1 Kunlun Mountains (Hoh Xil) earthquake in Qinghai, the 2008 Ms 7.3 Yutian earthquake in Xinjiang, the 2008 Ms 8.0 Wenchuan earthquake in Sichuan, and the 2010 M s 7.1 Yushu earthquake in Qinghai. All four occurred in relative proximity to the Bayankala block. By the integration of coseismic effects and viscoelastic relaxations, we studied the stress interactions among those four earthquakes. Then, the epidemic-type aftershock sequence (ETAS) model and stochastic declustering method were used to detect the local seismic signals of each succeeding major earthquake. This analysis considered increases or decreases with the Coulomb failure stress (CFS) variations induced by its preceding major earthquakes. The results showed that the stress interactions among these four major earthquakes are too weak to trigger each other and that the preceding major earthquakes did not obviously impact the local seismicity of the succeeding major earthquakes. It demonstrates that the occurrences of these four major earthquakes that clustered around the Bayankala block might not have been caused by the triggering effects. Instead these might have resulted from the southeastward movement of the Bayankala block, which related to the compression and penetration of the Indian–Australian Plate into the Eurasian Plate.
Introduction The 14 April 2010 M s 7.1 Yushu earthquake was the last in a series of four destructive earthquakes in Tibet over the last 10 years. The devastating effects of the quakes killed 2698 people and injured hundreds of thousands more. These four major earthquakes were the 2001 M s 8.1 Kunlun Mountains (Hoh Xil) earthquake in Qinghai, the 2008 Ms 7.3 Yutian earthquake in Xinjiang, the 2008 M s 8.0 Wenchuan earthquake in Sichuan, and the 2010 M s 7.1 Yushu earthquake in Qinghai. All of these events occurred around the boundaries of the Bayankala block (Fig. 1). It is important to investigate the stress interactions among the four strong earthquakes in order to understand the process as a whole and to identify the triggering effects, both of which are useful in assessing the seismic hazard of this region. In recent studies, it has been shown that the stress interactions within fault networks can play an important role in determining the features of seismicity, such as event triggering, clustering of events, and inhibition of events (Stein et al., 1992; Harris, 1998; Perfettini et al., 1999; King and Cocco, 2001; Robinson and Zhou, 2005; Zhou, 2008; Xiong et al., 2010; Jiang et al., 2011; Jiang and Wu, 2011). The major earthquake sequence around the Bayankala block (Fig. 1) shows the possibility for the time–space progres-
sion, which suggests some interactions among the earthquakes. Diao et al. (2010) found that the intensity and frequency of Yutian aftershocks had significantly increased after the occurrence of the Wenchuan M s 8.0 earthquake and inferred that there might be an internal relationship between these two major earthquakes. Therefore, it is possible that the occurrence of the four major earthquakes that clustered around the Bayankala block might be caused by triggering effects. On the other hand, the mechanisms of both the Hoh Xil earthquake and the Yushu earthquake, which occurred on the northern and southern boundaries of the block, respectively, demonstrate a left-lateral slip. While the Wenchuan earthquake, which occurred on the eastern boundary of the Bayankala block, is a reverse-faulting event, and the Yutian earthquake that occurred on the western boundary is a normal-faulting event. The kinematics of these events (see Fig. 1) suggest the southeastward movement of the Bayankala block, which is consistent with Global Positioning System observation (Zhang et al., 2004; Deng et al., 2010). Hence, the whole movement of the Bayankala block, which related to the compression and penetration of the Indian–Australian Plate into the Eurasian Plate, might be another possibility for the occurrence of these clustered earthquakes.
2157
2158
K. Jia, S. Zhou, and R. Wang
Figure 1.
Six strong earthquakes and their focal mechanisms that occurred around boundaries of the Bayankala block after the year 2000. Stars represent the Hoh Xil, Yutian, Wenchuan, and Yushu earthquakes; circles, two additional earthquakes; thick white lines edged with black, boundaries of the Bayankala block (modified from Diao et al., 2010); and dashed rectangles, study regions selected for seismicity analysis. Black lines are main faults: ATF, Altyn Tagh fault; KLF, Kunlun fault; XSF, Xianshuihe fault; LF, Longmenshan fault; JLF, Jiali fault; KRF, Karakorum fault. The color version of this figure is available only in the electronic edition.
In this study, we focus on the following questions: 1. How strong are the stress interactions among the four major earthquakes? 2. Did the stress interactions cause the local background seismicity to change? To answer these questions, we first calculate the Coulomb failure stress (CFS) variations on the faulting planes of the Yutian, Wenchuan, and Yushu earthquakes to see if they were induced by their preceding major earthquakes. Then epidemic-type aftershock sequence (ETAS) models and stochastic declustering methods are used to detect the local background seismicity variation.
Calculations of CFS Variations Stress interactions within fault networks can play an important role in determining a pattern of regional seismicity including event triggering, clustering of events, and inhibition of events. Usually such interactions are investigated by using the static changes in the CFS, which is calculated using dislocation theory (Okada, 1992). For the faults embedded in an elastic 3D half-space, the change in CFS (dCFS; Robinson and Zhou, 2005) is given by dCFS dτ shear μdτ normal β=3Σdτ ii ;
(1)
where dτ shear is the shear stress change (positive in the sense of motion of the relevant fault), μ is the (dry) coefficient of friction, dτ normal is the normal stress change, and β is Skempton’s coefficient. Note that this formula explicitly takes into account induced pore-pressure changes instead of using the more usual apparent coefficient of friction, which can cause inaccuracies (Beeler et al., 2000). However, in the immediate area where a large earthquake has occurred, it is not accurate
for both approaches. Preferred values for the friction coefficient and Skempton’s coefficient are 0.75 and 0.5, respectively. The preferred coefficient of friction is the dry value, based on both laboratory and field observations (Robinson and Zhou, 2005). Diverse controversial models of the Tibetan Plateau have been tested to understand the deformation and rheological property of the crust and mantle in Tibet (England and Molnar, 1997; Tapponnier et al., 2001; Meade, 2007; Thatcher, 2007; He and Chéry, 2008). An appropriate rheological model of the Tibetan Plateau, suggested by He and Chéry (2008), consists of a thin elastic or elastoplastic plate and a viscoelastic lithosphere below. In this study, we used a model of finite-faulting triggering sources embedded in a mixed elastic/inelastic layered half-space to calculate the evolution of dCFS considering viscoelastic relaxation of the crust, although the viscoelastic relaxation might be important over only tens of years (Pollitz et al., 2003). The Earth media structure consists of a layered elastic upper crust, a viscoelastic lower crust, and a viscoelastic upper mantle (see Xiong et al., 2010, for more details). The viscosities of the lower crust and upper mantle are set to be 1 × 1018 and 1 × 1020 Pa·s, respectively. The finite-faulting triggering sources of the Hoh Xil earthquake (Fig. 2a), Wenchuan earthquake (Fig. 2b), and Yutian earthquake (Fig. 2c) refer to the rupturing patterns inverted by Lasserre et al. (2005), Ji and Hayes (2008), and Zhang et al. (2011), respectively. Table 1 lists the fault-plane parameters of the four major earthquakes. The code PSGRN/PSCMP (Wang et al., 2006) was used in the calculations of the coseismic dCFS and the evolution of dCFS considering viscoelastic relaxation of the crust. We calculated the coseismic dCFS and the evolution of dCFS considering viscoelastic relaxation of the crust of each
Stress Interactions within the Strong Earthquake Sequence in the Bayankala Block of Eastern Tibet
2159
Figure 2
(a) Rupture pattern of the Hoh Xil earthquake adapted from Lasserre et al. (2005). (b) Rupture pattern of the Wenchuan earthquake adapted from Ji and Hayes (2008). The slip direction is indicated by white arrows; contours show the rupture initiation time in seconds. (c) Rupture pattern of the Yutian earthquake adapted from by Zhang et al. (2011).
earthquake as induced by the preceding major event along the slip direction at the initial rupture point of the receiver fault (Table 2). The main results are listed below: 1. The dCFS induced by coseismic and viscoelastic stress effects of the 2001 Hoh Xil earthquake on the active fault of the 2008 Yutian event 6.5 years after the preceding event is positive, which means the Hoh Xil earthquake may have promoted the Yutian earthquake. However, the CFS changes after the Hoh Xil earthquake are all much smaller than 0.01 MPa, which represents the minimum level of stress change correlating with seismicity change (Reasenberg and Simpson, 1992; Stein et al., 1992; Harris et al., 1995; Toda et al., 1998). Thus, we could infer that the promotion of the Yutian earthquake induced by the Hoh Xil earthquake is negligible. 2. The results of the calculations of the stress interaction between the Hoh Xil and Wenchuan earthquakes and of calculations of the triggering of the Wenchuan earthquake by the Hoh Xil earthquake are similar to the preceding result (result 1). Because the dCFS induced by the coseismic and viscoelastic effects of the Hoh Xil earthquake on the active fault of the Wenchuan event 6.5 years
after the preceding event is positive and is much smaller than 0.01 MPa, we infer that the Hoh Xil earthquake might have encouraged the Wenchuan earthquake, but its triggering effect was rather weak. 3. Because the dCFS induced by coseismic and viscoelastic effects of the 2001 Hoh Xil earthquake on the active fault of the 2010 Yushu event nine years after the preceding event is negative, we can infer that the Hoh Xil earthquake delayed the Yushu earthquake to some degree. 4. The 2008 Yutian earthquake produced negligible coseismic and viscoelastic stress change on the active fault of the 2010 Yushu earthquake; therefore, we infer that there is no obvious interaction between the Yutian and Yushu earthquakes. 5. The influence of the Wenchuan earthquake on the Yushu earthquake is similar to result 3. This demonstrates that the dCFS induced by coseismic and viscoelastic effects of the 2008 Wenchuan earthquake on the active fault of the 2010 Yushu event two years after the event is negative. Thus, we infer that the Wenchuan earthquake could not promote the Yushu earthquake.
Table 1 Focal Mechanisms of the Hoh Xil, Yutian, Wenchuan, and Yushu Earthquakes Used in This Study* Earthquake, Date (yyyy/mm/dd)
Hoh Xil, 2001/11/14 Yutian, 2008/03/21 Wenchuan, 2008/05/12 Yushu, 2010/04/14
Latitude (° N)
Longitude (° E)
Strike (°)
Dip (°)
Slip (°)
Depth (km)
35.80 35.43 31.44 33.05
92.91 81.37 104.10 96.79
94 168 229 119
61 60 33 83
−12 220 90 −4
15.0 12.0 12.8 15.7
*Focal mechanisms are from the Global Centroid Moment Tensor (Global CMT) catalog.
2160
K. Jia, S. Zhou, and R. Wang
Table 2 Calculated Coulomb Failure Stress Change (dCFS) of Each Earthquake Induced by the Preceding Major Event along the Slip Direction at the Initial Rupture Point of the Receiver Fault Hoh Xil, 2001/11/14 Coseismic
2 years
6.5 years
Yutian, 2008/03/21 9 years
Coseismic
2 years
Wenchuan, 2008/05/12 Coseismic
2 years
Yutian, 2008/03/21 7:001 × 10−5 7:999 × 10−5 1:302 × 10−4 Wenchuan, 2008/05/12 1:136 × 10−5 9:658 × 10−6 1:198 × 10−5 Yushu, 2010/04/14 −7:147 × 10−4 −8:554 × 10−4 −1:347 × 10−3 7:283 × 10−7 8:003 × 10−7 −1:324 × 10−4 −2:495 × 10−4 Calculated dCFS is in units of MPa. All dates are in the format yyyy/mm/dd.
6. The overlap of coseismic and viscoelastic stress changes caused by the 2001 Hoh Xil (nine years after the event), 2008 Yutian (two years after the event), and 2008 Wenchuan (two years after the event) earthquakes on the active fault of the 2010 Yushu event shows a negative value of dCFS. This indicates that those three earthquakes collectively could not promote the Yushu earthquake. It should be noted that the distance between the 2008 Yutian earthquake and the 2008 Wenchuan earthquake is very long (about 2500 km) and the time between the earthquakes was only 52 days, so the coseismic stress change and the viscoelastic stress change could be ignored (Pollitz et al., 2003). As a result, we did not consider the interaction between those two events in our analysis.
Local Background Seismicity Variations The ETAS model introduced by Ogata (1988, 1989) extends the Omori–Utsu law, assuming that each earthquake has a magnitude-dependent ability to trigger its own aftershocks (Ogata, 1988, 1992; Ogata and Katsura, 1993). According to this model and given the observation history in the past, the total seismicity rate is described by λt∖Ht μt Σti
