- © 2014 by the Seismological Society of America
On 20 April 2013, nearly 5 years after the Wenchuan Ms 8.0 event of 12 May 2008, an earthquake of Ms 7.0 struck the Longmenshan fault zone in Lushan county of Sichuan province, China. Because the epicenters of the two events were only 90 km apart, and the aftershock areas were separated (Fig. 1), there has been great interest in both how the seismogenic structures of the two events interrelate, and whether events of Ms 7.0 or greater could recur in the southern segment of the Longmenshan fault zone. However, there are few data available on the late Quaternary activity of this section, and the degree of activity is disputed (Zhao et al., 1990; Yang et al., 1999; Densmore et al., 2007; Institute of Geology, China Earthquake Administration, 2009; Zhang and Li, 2010; Chen et al., 2013a). Since the 2008 Wenchuan earthquake, we have investigated the area repeatedly, acquiring evidence of fault activity. In particular, in the disaster area of the Lushan earthquake, we have undertaken 1:50,000 mapping, trenching, and shallow‐seismic exploration. Based on our observations, this paper discusses the seismogenic structure of the Lushan event and its relation to the 2008 Wenchuan earthquake.
Meizoseismal Area and Ground Damage
The maximum intensity of the Lushan event was level IX (The Chinese Seismic Intensity Scale) at the towns of Taiping, Shuangshi, Baosheng, and Longmen, in which the level of casualties and building damage were most serious (http://www.cea.gov.cn, last accessed November 2013). This meizoseismal area was oriented in the northeast direction and was approximately 24 km long and 11 km wide (Fig. 1b). Aftershocks also had a northeast‐trending distribution, largely consistent with the meizoseismal area (displaced slightly to the west by a distance of 3–5 km), covering an area with length of 35 km and width of 15 km (Du et al., 2013; Fang et al., 2013; Gao et al., 2013; Zhang and Lei, 2013).
Both emergency field observations (Xu, Wen, et al., 2013) during the period of 21 April to 1 May, and our 1:50,000 mapping, performed subsequently in the disaster area, indicated that no coseismic surface rupture presented. However, this earthquake produced a significant number of tensional ground fissures, landslides, bedrock collapses, and liquefaction, in addition to the damage to buildings. This ground damage was expressed along three subparallel northeast‐trending zones, distributed along the Dachuan–Shuangshi fault (DSF), the Longmen–Luyang synclinal valley, and Xinkaidian fault (XKDF).
Activity of the Major Faults in the Disaster Area
Similarly to the central segment of the Longmenshan fault zone at which the 2008 Wenchuan event took place, the southern segment comprises the Wenchuan–Maoxian fault (WMF), the Beichuan–Yingxiu fault (BYF), the Pengxian–Guanxian fault (PGF), and a series of subparallel secondary faults (see Fig. 1a). East of the PGF, there are wide and gently alternating linear anticlines and synclines of the Mesozoic and Cenozoic, as well as the anticline‐bounded range‐front buried fault (RFF). From Yingxiu to the southwest, the fault zone hosts additional secondary faults, and its width increases up to almost 90 km near Lushan.
In the disaster area, in which the seismic intensity reached level VIII, the major faults include the DSF, the Shiyang fault (SYF), and the XKDF (see Fig. 1b). Among these faults, it was only along the DSF that the offset of the late Quaternary strata was found. On a residual terrace about 30 m above the river, near the town of Taiping (102°59′48.54″ E, 30°19′30.17″ N), there was discovered a linear trough with relief of approximately 1.5 m. A trench across this trough revealed that the fault displaced a set of late Quaternary sediment‐fill beds of sag pond type and affected the near surface (Fig. 2). Three accelerator mass spectrometry radiocarbon dating samples, collected from unit U1 on the north wall were dated by Beta Analytic (United States) as 12,500±60, 10,180±50, and 10,080±40 yr B.P. Several other trenches at Longchigang, Shuangshi, and Daxi also showed disturbance in the late Quaternary strata. These excavations, together with the findings of previous works (Densmore et al., 2007; Institute of Geology, China Earthquake Administration, 2009; Chen et al., 2013a), suggest that the southern section of the PGF has been active since the Holocene, but with small offsets of single faulting events (<50 cm) and cumulative displacement.
Along the northeast‐striking Longmen–Luyang valley, the Lushan event left a linear zone of ground damage, such as ground fissures and tent‐like uplifting of cement‐covered pavement. A few researchers considered these as signs of coseismic surface rupture, and they suggested that the presumed blind Longmen–Luyang fault was the more likely seismogenic fault (Han et al., 2013). However, our excavation across this deformation zone found no evidence of a fault. A geological survey and shallow‐seismic exploration indicates that the Longmen–Luyang valley is a gentle syncline valley, in which there is no propagation of faulting into the Quaternary sediments.
Seismogenic Structure of the Lushan Earthquake
It has been noted that the coseismic rupture of the 2013 Lushan earthquake did not reach the surface. Furthermore, many fine relocation and source mechanism studies (e.g., Du et al., 2013; Fang et al., 2013; Liu, Yi, et al., 2013; Lue et al., 2013; Xie et al., 2013; Zeng et al., 2013; Zhang et al., 2013) have indicated that the epicenter of this event was located near the DSF and that the aftershocks were distributed on either side of this fault. Considering the dipping direction and angle of the source fault, as well as the focal depth, Xu, Wen, et al. (2013) and Xu, Chen, et al. (2013) suggested that the seismogenic structure of the Lushan Ms 7.0 event should be to the east of the DSF, as a typical blind folding fault.
We do not attempt to refute this suggestion, but instead search for a model that is more reasonable. By analogy to the relationship between the mainshock, the aftershocks, and the seismogenic fault of the 2008 Wenchuan event, we speculate that the 2013 Lushan Ms 7.0 earthquake originated from the PGF–RFF system, and that the DSF (the southern segment of the PGF) is the major seismogenic fault (Fig. 3). On the one hand, from the focal parameters reported and the inversion of the rupturing process (http://neic.usgs.gov, last accessed November 2013; Liu, Zheng, et al., 2013; Wang et al., 2013; Zhang et al., 2013; Zhao et al., 2013), the source of this earthquake was a northeast‐trending thrust fault. However, the DSF was the only active fault that displaced Quaternary strata in the seismic area. On the other hand, the towns of Taiping, Shuangshi, and Daxi, which were destroyed in the event, stood on the hanging wall and abutted the fault. Coseismic ground damage along this fault was particularly pronounced; for example, there were ground fissures and eruptions of sand and water from the subsurface around Shuangshi and Taiping, and ground buckling in individual houses close to the fault near the Taiping trench (Fig. 2a). Additionally, although the meizoseismal area of this event encompassed other two towns (Baosheng and Longmen), 3–7 km east of the DSF, no surface fault was found in those locations. We infer that the seismic damage reported in these two towns was likely associated with the stress concentration along the axis of the syncline in this area.
Comparison between the Lushan and Wenchuan Earthquakes
Both the 2013 Lushan and 2008 Wenchuan earthquakes occurred in the Longmenshan thrust‐fault zone. However, their seismogenic structures were different. The Wenchuan shock is attributed to the BYF–PGF thrust system in the central segment of the Longmenshan fault zone, in which ruptures had a composite multiple‐seismogenic source (e.g., Liu‐Zeng et al., 2009; Xu et al., 2009; Zhang et al., 2009). Trench investigations reveal paleoearthquakes with displacements equivalent to the 2008 Wenchuan event on the BYF and PGF, of which the BYF exhibits much larger scale and intensity of thrusting (Ran et al., 2008, 2010, 2013; Chen et al., 2013b). However, the 2013 Lushan quake occurred in the PGF–RFF thrust system in the southern segment of the Longmenshan fault zone and showed no clear relation to the BYF. Compared with the BYF–PGF thrust system, the seismogenic structure of the Lushan event propagated in a piggyback fashion toward the Sichuan basin in the east. Moreover, field investigations indicate that all the single event and cumulative displacements of the PGF–RFF thrust system were much smaller than those of the BYF–PGF thrust system were.
These differences in structure and seismicity between the central and southern segments of the Longmenshan fault zone should be associated with fault structure, lithology, and deep media. First, magnetotelluric soundings revealed that the source was near the boundary of an area of low‐electric resistivity, within a body of high‐electric resistivity, deep beneath the central segment of the Longmenshan fault zone; no such structure was detected in the southern segment of the same fault zone (Zhao et al., 2012). Second, Zheng et al. (2013) found that both the Lushan and Wenchuan events occurred where crustal thickness and wave velocity are highly variable, but that the Wenchuan event, including the aftershocks, occurred in an area with a low Poisson’s ratio, whereas the Lushan event occurred in a region with a high Poisson’s ratio. Third, in the lithology exposed at the ground surface, there are two left‐step complexes from the central to the southern segment of the Longmenshan fault zone (see Fig. 1). Related to the Wenchuan event, the large‐scale Pengguan complex appears on the hanging wall of the BYF–PGF thrust system, and it is bounded by the BYF to the east. Meanwhile, the smaller northeast‐trending Baoxing complex, on the hanging wall of the PGF–RFF thrust system associated with the Lushan event, is bounded by the eastern DSF and several subparallel secondary faults that are the southern section of the PGF. Finally, south of the surface‐rupture zone of the 2008 Wenchuan event, the Longmenshan fault zone branches rapidly into a series of subparallel faults and further secondary faults. Near Baoxing, the northeast‐trending Longmenshan fault zone begins to intersect with the northwest‐striking faults and bends into an arc‐like shape, and its width increases from 30 km near the epicenter of the Wenchuan event in the north, to almost 90 km near Baoxing in the south. A combination of these factors may result in stress accumulation in the southern segment of the Longmenshan fault zone proceeding in a distributed manner and low resistivity. Therefore, the seismogenic potential or possible maximum magnitude of this segment should be much lower than that of the central segment of the Longmenshan thrust‐fault system.
This work suggests that the 2013 Lushan Ms 7.0 earthquake originated from the PGF–RFF thrust system in the southern segment of the Longmenshan fault zone, and that the DSF (southern segment of the PGF) is the major seismogenic fault. In comparison with the 2008 Wenchuan event, the seismogenic structure of the 2013 Lushan earthquake propagated in a piggyback fashion toward the Sichuan basin and it appeared as a thrust system with weaker activity and lower seismogenic potential. These differences between the two segments of the Longmenshan fault zone are probably associated with the fault structure, lithology, and physical properties at depth.
We are grateful to the Earthquake Prevention and Disaster Reduction Bureaus of Ya’an, Lushan, Yucheng, Tianquan, Yingjing, and Baoxing for their support of our field investigations. This work was supported by the Project of the China Earthquake Administration “Scientific Investigations on the 20 April 2013 Lushan, Sichuan Earthquake” and Special Projects for Basic Research Work of the Institute of Geology, China Earthquake Administration (IGCEA1004).