- © 2014 by the Seismological Society of America
Online Material: Earthquake hazard map; figures showing geodynamic and elastic block models; and table of Ms>6 earthquakes used in the study.
Following the devastating 2008 Wenchuan earthquake that ruptured the central–northern segments of the Longmenshan fault in Sichuan, China, many studies assessed its impact on other major faults in this region (e.g., Parsons et al., 2008; Toda et al., 2008). On 20 April 2013, the Mw 6.6 Lushan earthquake ruptured the southern segment of the Longmenshan fault, allowing these assessments to be tested.
The 2008 Wenchuan earthquake (Mw 7.9) came as a surprise, because the Longmenshan fault zone, which separates the growing Tibetan Plateau from the rigid Sichuan basin (Fig. 1), slips slowly (less than 3 mm/yr; Shen et al., 2009); with only moderate seismicity (Burchfiel et al., 2008; Wang et al., 2010). The earthquake‐hazard map by the Global Seismic Hazard Assessment Program shows the entire Longmenshan fault zone as being relatively safe, as shown in Figure S1 (available in the electronic supplement to this paper).
The same cannot be said for the Mw 6.6 Lushan earthquake, which occurred in the morning of 20 April 2013, killing ∼200 people and injuring ∼12,000. After the Wenchuan earthquake, the Longmenshan fault zone and the neighboring faults have been intensively studied in an attempt to assess earthquake hazard in this region. Are these assessments useful in forecasting the Lushan earthquake?
Most of the assessments were based on the change in the Coulomb failure stress on regional faults by the Wenchuan earthquake. The Coulomb failure stress (ΔσCFS) is given by: ΔσCFS=Δτ+μeΔσn, in which Δτ is the change in shear stress on the fault plane in the direction of fault slip, μe is the effective frictional coefficient, and Δσn is the change in normal stress. Increased shear stress and decreased normal stress (unclamping) tends to move the fault plane toward failure, and vice versa. Parsons et al. (2008) showed that the Wenchuan earthquake increased the Coulomb stress on the southern branches of the Longmenshan fault system, the Xianshuihe fault, and a number of other faults including the thrust systems within the Sichuan basin. Similar results were derived from different calculations. Luo and Liu (2010) found up to 0.3 MPa stress increase on the southern Longmenshan fault and 0.01–0.05 MPa increase on the Xianshuihe and Kunlun faults, consistent with the results of Parsons et al. (2008). Toda et al. (2008) results showed a stress increase of 0.02–0.05 MPa on the Xianshuihe and the Longmenshan fault just south of the Wenchuan rupture zone. Hence, stress changes due to the Wenchuan earthquake provided useful information for assessing earthquake hazard in its aftermath. In particular, all models predicted a stress increase in the southern Longmenshan fault segment that ruptured in the 2013 Lushan earthquake.
However, stress results alone could not have identified southern Longmenshan fault as the next fault to rupture after the Wenchuan earthquake. Stress increase was predicted in numerous other faults, especially the Xianshuihe fault (Toda et al., 2008), which was already assigned the highest risk in the hazard map (see Fig. S1 in supplement). A better forecast would be made by including previous large earthquakes in the stress analysis. Although the Xianshuihe fault slips much faster than other faults in the region, it had a sequence of M 6–7.6 earthquakes in the past century (Wen et al., 2008). Luo and Liu (2010) showed that these events could have lowered the failure stress on the Xianshuihe fault by as much as 0.5 MPa, easily overshadowing the stress increase (less than 0.05 MPa) by the Wenchuan earthquake.
BALANCE OF SEISMIC MOMENT
Another approach used to assess hazard was based on balancing seismic moments on the faults (Wang et al., 2010). Seismic activity is a process of energy accumulation and release. The rates of accumulation of strain energy, which can be measured by the scalar moment (the product of fault‐plane area, rigidity, and displacement) on fault segments, may be predicted from the fault slip rates and locking depths. Comparing the accumulated moment on a fault plane over a period with the seismic moment released during the same period, one may estimate the amount of moment (moment deficit) available for future earthquakes (Meade and Hager, 2005). Using this approach, Wang et al. (2010) found that “The largest moment deficit will be on the southwestern segment of the Longmenshan fault. If all the moment is released by a rupture of the whole segment, it could produce an earthquake as large as Mw 7.7 in the next 50 years.” It seems that nature was in a hurry.
HAZARD ASSESSMENTS BASED ON BOTH STRESS AND MOMENT CHANGES
Clearly, neither Coulomb stress changes nor moment balancing can tell where and when an earthquake will occur. Nonetheless, combining both pieces of information provides a better hazard assessment than using either of them alone. The stress change tells which fault has moved toward failure, and the moment deficit gives an estimate of the maximum size of the potential earthquake. Following this strategy, we recalculated the Coulomb stress changes using a geodynamic model for the region (Luo and Liu, 2010; see Fig. S2, in the supplement). Figure 2a,b shows the coseismic Coulomb stress changes due to the Wenchuan and Lushan earthquakes, respectively. The Wenchuan earthquake increased the Coulomb stress on the southern segment of the Longmenshan fault by 0.03–0.3 MPa. The Lushan earthquake ruptured a fault segment about 20 km long along the southern Longmenshan fault, elevating stress on both sides of the rupture zone. However, it reduced stress on the southernmost end of the Longmenshan fault (Fig. 2b). Both earthquakes also increased stress on the Xianshuihe fault and decreased stress on the Anninghe fault, but the effects of the Lushan earthquake are about an order of magnitude smaller than those of the Wenchuan earthquake. On the other hand, the stress changes on the Xianshuihe fault caused by both the Wenchuan and Lushan earthquakes are much smaller than those due to previous earthquakes on this fault (Fig. 3c). In this case we calculated the coseismic and postseismic stress changes due to ruptures of six segments of the Xianshuihe fault from 1893 to 1973. The geometry of the faults used in the model are simplified and will affect the resulted stress, but the general patterns of the predicted Coulomb stress changes are not altered by detailed variations of fault geometry (Luo and Liu, 2010).
To balance seismic moments on these faults (Fig. 3), we first used the Global Positioning System (GPS) site velocities (Gan et al., 2007) with an elastic block model to derive the slip rates and optimal locking depths on the simplified fault systems in this region (see Fig. S3, in supplement), then used the slip rates and locking depths to calculate the rates of moment accumulation on the faults (Wang et al., 2010). The Xianshuihe fault, with its high slip rates (10–20 mm/yr), has the highest rate of moment accumulation in the region, but has also had frequent large earthquakes (Wen et al., 2008). Hence, the moment deficit on the Xianshuihe fault is relatively low (Fig. 3a). In contrast, the Longmenshan fault zone, which has been accumulating moment slowly but without having M>7.0 events in the past millennium or longer (Densmore et al., 2007; Shen et al., 2009), had accumulated enough moment by 2008 for the Wenchuan earthquake (Fig. 3a). After the Wenchuan earthquake, the southern Longmenshan fault became the one with the highest moment deficit in the region (Fig. 3b). The Lushan earthquake released ∼1.0×1019 N·m (U.S. Geological Survey), or about 1/3 of the estimated moment deficit on the southern Longmenshan fault. On either side of the Lushan rupture zone, the stored moment could produce an M∼7 earthquake (Fig. 3c). The ∼60 km long segment of the Longmenshan fault northeast of the Lushan rupture zone, which had the highest stress increase (∼1.0 MPa) by both the Wenchuan and Lushan earthquakes (Fig. 2), is most likely to produce the next big earthquake in this region.
The 2013 Lushan earthquake illustrates once again the difficulties of earthquake prediction; it occurred without warning, whereas the entire Longmenshan fault zone has been under intensive study following the 2008 Wenchuan earthquake. Nonetheless, we find that studies of stress changes and seismic‐moment releases in the aftermath of the Wenchuan earthquake are useful for earthquake‐hazard assessment. When used together and with historic events included in the analysis, stress changes and moment deficits could provide useful insights for hazard mitigation and planning.
We thank Huajian Yao, the guest editor, and Seth Stein for helpful comments. M. L.’s research is supported by NSF/OISE grant 0730154 and the Chinese Academy of Sciences. H. W. acknowledges “Scientific Investigation of April 20, 2013 M 7.0 Lushan, Sichuan Earthquake” from China Earthquake Administration (CEA) and National Natural Science Foundation of China (Grants Number 41104058 and 41104057). G. L. acknowledges support from the Opening Fund of State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (Chengdu University of Technology; SKLGP2012K030).