SCIENCE CHINA Earth Sciences RESEARCH PAPER September 2014 Vol.57 No.9: 2036 2044 doi: 10.1007/s11430-014-4827-2 A rupture blank zone in middle south part of Longmenshan Faults: Effect after Lushan M s 7.0 earthquake of 20 April 2013 in Sichuan, China GAO Yuan 1*, WANG Qiong 1,3, ZHAO Bo 2,3 & SHI YuTao 1,3 1 Institute of Earthquake Science, China Earthquake Administration, Beijing 100036, China; 2 China Earthquake Networks Center, China Earthquake Administration, Beijing 100045, China; 3 Institute of Geophysics, China Earthquake Administration, Beijing 100081, China Received July 19, 2013; accepted November 14, 2013; published online May 20, 2014 On April 20, 2013, the Lushan M s 7.0 earthquake struck at the southern part of the Longmenshan fault in the eastern Tibetan Plateau, China. The shear-wave splitting in the crust indicates a connection between the direction of the principal crustal compressive stress and the fault orientation in the Longmenshan fault zone. Our relocation analysis of the aftershocks of the Lushan earthquake shows a gap between the location of the rupture zone of the Lushan M s 7.0 earthquake and that of the rupture zone of the Wenchuan M s 8.0 earthquake. We believe that stress levels in the crust at the rupture gap and its vicinity should be monitored in the immediate future. We suggest using controlled source borehole measurements for this purpose. Lushan earthquake, Longmenshan Fault, rupture gap, crustal seismic anisotropy, double difference relocation, borehole measurements of stress change Citation: Gao Y, Wang Q, Zhao B, et al. 2014. A rupture blank zone in middle south part of Longmenshan Faults: Effect after Lushan M s 7.0 earthquake of 20 April 2013 in Sichuan, China. Science China: Earth Sciences, 57: 2036 2044, doi: 10.1007/s11430-014-4827-2 1 The Lushan M s 7.0 earthquake and the background of regional seismicity The M s 7.0 earthquake that hit Lushan County, Ya an, in the Sichuan province of China on April 20, 2013 at 08:02 (Beijing time) caused serious casualties in Lushan, Baoxing, and the surrounding area. According to the China Earthquake Network Center (CENC), the location of the epicenter was at 30.0 N, 103.0 E and the magnitude and depth of the hypocenter were M s 7.0 and 13 km, respectively. Here, we refer to this earthquake, called the 4 20 M s 7.0 Lushan earthquake in Sichuan, as the Lushan earthquake. Since Cenozoic times, the Indian plate has been pushing *Corresponding author (email: gaoyuan@seis.ac.cn) the Eurasian plate, causing the uplift of the Tibetan Plateau and a shortening and thickening of the crust in that region. The eastern part of the Tibetan Plateau, from north to south, extrudes northeastward, eastward, and southwestward, respectively. This large-scale continuous tectonic movement frequently brings about strong earthquakes in the interior of the Tibetan Plateau and its vicinity (Gao et al., 2000, 2001). The Lushan earthquake occurred in the southern part of the Longmenshan (LMS) fault which is located along the eastern margin of the Qinghai-Tibetan block (QTB). Because of the eastward extrusion of the QTB and the resistance of the rigid crust of the Sichuan basin, crustal material beneath the eastern margin of the QTB flows eastward, leading to an upward thrust of the ductile material in the lower crust which forms the thrust-type LMS fault (Zhang et al., 2013). The tectonic features and velocity structure of the LMS Science China Press and Springer-Verlag Berlin Heidelberg 2014 earth.scichina.com link.springer.com
zone display velocity variations that are strongly transverse, and heterogeneity in the crust (Wang et al., 2010; Zhang et al., 2009a, 2013; Lei et al., 2009; Zhang et al., 2011). Based on the crustal structure in the LMS zone (Lei et al., 2009), the hypocenter of the Lushan earthquake lies in the southern part of the LMS fault in the fast P-velocity variation zone; this is highly consistent with reports from recent studies of the southeast margin of the Qinghai-Tibetan Plateau (Gao et al., 2000). According to the global earthquake catalog of the Preliminary Determination of Epicenters (PDE), the distribution of earthquakes of M 5 after the year 2000 in the eastern margin of the QTB and its vicinity shows the following characteristics (Figure 1). (1) Before the Wenchuan Ms8.0 earthquake on May 12, 2008, earthquakes occurred mainly in the east and southeast margins of the QTB and extended into the Yunnan Province. Over a period of eight years and four months a total of 23 earthquakes of M 5 were recorded in the region, about 2.8 earthquakes per year. (2) In the period after the Wenchuan Ms8.0 earthquake and before the Lushan Ms7.0 earthquake, the distribution of earthquakes mainly in the east and southeast margin of the QTB was similar and extended to Yunnan. In the central and northern parts of the LMS fault, however, many strong aftershocks of the Wenchuan earthquake were recorded, as well as one earthquake in the Sichuan basin. In less than five years, 19 earthquakes of M 5 occurred in this region in addition to the Wenchuan earthquake and its 73 aftershocks of M 5, an average of about 3.9 earthquakes per year. (3) After the Lushan earthquake, only one earthquake occurred apart from the six Lushan earthquake aftershocks of M>5; this was the Mb5.3 earthquake on April 25, 2013. 2037 The preliminary report of the CENC shows that this earthquake was located at the junction of three counties: Changning, Hongxian, and Wenxing in Sichuan province and its magnitude was Ms4.8. Thus, we see an increase in the earthquake frequency in the period between the Wenchuan Ms8.0 earthquake and the Lushan Ms7.0 earthquake, compared with the period before the Wenchuan earthquake, apart from the Wenchuan earthquake and its aftershocks. 2 Direction of the principal compressive stress indicated by shear-wave splitting in the crust and its relationship to the LMS fault To study the crustal anisotropy around the LMS fault, records of the aftershocks of the Wenchuan Ms8.0 earthquake were analyzed. These were recorded by portable seismic stations around the LMS fault set up after the Wenchuan earthquake, and by the permanent Sichuan Seismic Networks (Zhang Y J et al., 2008). The shear-wave splitting results beneath the stations were obtained using the Systematic Analysis Method (SAM) of shear-wave splitting (Gao et al., 2008b). Shi et al. (2009) found that up to the boundary of Anxian County, the predominant direction of the polarizations of the fast shear-waves was NNE at stations located in the northeast part of the LMS fault (zone B). This is consistent with the strike of the fault. For stations located in the central and southern central part of the LMS fault (zone A), the predominant direction of the polarization of the fast shear-waves was NW, nearly perpendicular to the strike of the fault. From the area southwest of the epicenter of the Lushan earthquake in the southern part of the LMS fault, Figure 1 Seismicity of the Longmenshan fault zone and vicinity. Distribution of M 5 earthquakes (a) from January 1, 2000 to May 11, 2008; (b) from May 12, 2008 to April 19, 2013, yellow pentagram represents Wenchuan earthquake; (c) from April 20, 2013 to April 30, 2013, yellow pentagram represents the Lushan earthquake. Faults are depicted in blue lines. Earthquake data are from the PDE USA catalog. Red dots indicate Ms5.0 5.9 earthquakes and brown larger dots indicate Ms6.0 6.9 earthquakes. Yellow pentagrams represent the Wenchuan earthquake of May 12, 2008 and the Lushan eartqhake of April 20, 2013. The magnitudes of these two earthquakes are from China Earthquake Network Center (CENC).
2038 extending almost to the Xianshuihe (XSH) fault (zone C), the polarization directions of the fast shear-waves appear very scattered; however, the average direction is almost E-W (Figure 2). Studies have shown that the predominant direction of fast shear-wave polarization is generally parallel to the direction of the in situ principal compressive stress (Gao et al., 2008c, 2011, 2012) whose characteristics are related to the regional tectonics and could indicate a hidden strike-slip fault (Gao et al., 2011). The varying polarization directions of the fast shear-waves of the Wenchuan earthquake aftershock sequence indicate a thrust in the southwest section of the LMS fault and clear strike-slip motion in the northeast section of the fault (Figure 2). Geological studies after the Wenchuan earthquake verified the existence of local strike-slip faults in the northeast section of the LMS fault. Comprehensive results at three stations (MDS, GZA, and L5503) near the intersection of the LMS fault, the XSH fault, and the An- ninghe (ANH) fault show that the average direction of the fast shear-wave polarization was close to E-W, although that of station L5503 was very scattered (Shi et al., 2009). This is related to the complicated stress distribution induced by the complex local tectonics around station L5503. After combining the records of the temporary and permanent seismic stations, additional observational information of crustal shear-wave splitting in the LMS fault zone was obtained from longer near-field records of small local earthquakes including aftershock sequences of the Wenchuan earthquake (Shi et al., 2013). Using records of small local earthquakes from January 2000 to April 2010, the central and western parts of the LMS fault were further divided into two sections, with their boundary at the location of the Lushan earthquake. Along the LMS fault we identified three subsections of predominant polarizations of fast shear-waves. The aftershock sequence of the Wenchuan earthquake also showed that the boundary line between the Figure 2 Equal-area projection rose diagram of polarizations of fast shear-waves in the crust from the Wenchuan aftershock sequence data.the Longmenshan Fault is divided into three parts: zones A, B, and C. The three equal-area rose diagrams of fast shear-wave polarizations (white circles) are the results of all the available data in zones A, B, and C. Black lines indicate faults and black thick arrows indicate the direction of the compressive stress; arrow with circle represents the principal compressive strain induced from GPS data. Yellow pentagram in the north represents the 2008 Wenchuan earthquake and that in the south is the 2013 Lushan earthquake. Blue triangles are regional seismic stations and white triangles are temporary stations. Straight lines at the station show the average polarization direction per station. Red lines represent effective data from more than 11 records and brown lines represent effective data from only one or two records. Grey small dots are Wenchuan aftershocks after relocation. QTB: Qinghai-Tibetan Plateau; LMS: Longmenshan Fault; XSH: Xianshuihe Fault; ANH: Anninghe Fault.
sections of different polarizations was in the same area, near Anxian County. Our analysis showed results similar to those of a previous study that was based only on the aftershock sequence of the Wenchuan earthquake (Shi et al., 2009): the predominant direction of the fast shear-wave polarizations in zone B is close to NNE, consistent with the strike of the fault. The predominant direction in zone A is close to NW, nearly perpendicular to the strike of the LMS faults but showing an obvious predominant direction consistent with the surface strike of the fault. In zone C, the polarizations of the fast shear-waves are scattered; however, the overall predominant polarization is nearly E-W (Figure 3). In the southern part of the LMS fault and in zone C where several faults intersect, the shear-wave splitting is related to the regional stress field influenced by the faults and the deep tectonics. At stations MDS and SMI, along the Sichuan basin, the direction of the predominant fast shear-wave polarization was close to E-W. This is related to 2039 the eastward push of the QTB that is obstructed by the Sichuan basin (Figure 3). We used local records (11 records from MDS and 19 from GZA (Shi et al., 2009)) of the Wenchuan earthquake aftershock sequences and some small earthquakes before and after the earthquake to obtain effective data of shear-wave splitting. The predominant fast shear-wave polarization at MDS had two directions; one was almost N-W and the other was close to ENE. The fast shear-wave polarization at GZA was scattered, without an obvious predominant direction, but the average direction was about E-W with a standard error of 32.8 (Figure 2). From seismic activity recorded between January 2000 to April 2010 at MDS and GZA we obtained effective shearwave splitting data (48 records from MDS and 52 from GZA) (Shi et al., 2013). The predominant fast shear-wave polarization direction at MDS is clearly ENE (63.5 ). The polarizations of the fast shear-waves at GZA is scattered, with no obvious predominant polarization, but the average direction is about in ENE, close to E-W, with a standard Figure 3 Equal-area projection rose diagram of polarizations of fast shear-waves in the crust in the study area. Data are from January 2000 to April 2010, and the temporary stations recorded only the Wenchuan aftershock sequences. The Longmenshan Fault is divided into three parts: zones A, B, and C. The three equal-area rose diagrams of fast shear-wave polarization (white circles) are the results of all the available data in zones A, B, and C. The diagram in zone C includes three permanent stations and a temporary station L5503 with only one record; red lines show the polarization. Blue and white triangles represent the regional permanent and temporary seismic stations, respectively. Yellow pentagrams represent the 2008 Wenchuan earthquake and the 2013 Lushan earthquake. Other notations are the same as in Figure 2.
2040 error of 35.1 (Figure 3). Thus, we found that the predominant polarization of the fast shear-waves at station MDS changed from two predominant polarizations to one predominant polarization. This suggests a change of the stress field in the crust under station MDS. The predominant fast shear-wave polarization in the ENE direction indicates an in situ horizontal principal compressive stress in the ENE direction, which leads to a NE-striking rupture of the thrust fault. This inference needs further evidence. In addition, the predominant polarization of the fast shear-waves at SMI is clearly in the WNW direction, close to E-W (95.8 ). The horizontal principal compressive stress in this direction is almost perpendicular to the ANH fault nearby, which results in some locking of the ANH fault. 3 Relocation analysis of the Lushan earthquake and its aftershocks To monitor the aftershocks of the Lushan earthquake, temporary seismic networks were deployed immediately after the earthquake by the research institutes of the China Earthquake Administration and the Earthquake Administration of Sichuan Province. Combining the data recorded by the permanent and temporary seismic networks, we performed relocation analysis of the aftershock sequence of the Lushan earthquake. The double difference relocation algorithm (Waldhauser et al., 2000), as a relative location technique, has been applied most widely to relocation problems of earthquakes in recent years. It utilizes travel-time differences for pairs of earthquakes at each seismic station to reduce the dependence of the velocity model. The algorithm does not need to define the main shock and does not rely on the initial location of the earthquake, thus avoiding inaccuracy due to location error of the main shock (Zhao et al., 2013). It was used for the relocation of the aftershock sequence of the 2010 Yushu earthquake and the results showed that the aftershocks were well distributed along the strike of the Yushu fault, confirming the reliability of this method (Zhao et al., 2012). In this study we applied the double difference relocation algorithm to the aftershock data of the Lushan earthquake. Using observations from the temporary seismic network set up after the Lushan earthquake by the earthquake emergency management system of the China Earthquake Administration, the aftershock sequence of the Lushan earthquake from April 20 to May 2, 2013 (Figure 4) was used for the relocation. We collected records from a total of 3813 earthquakes and obtained relocation estimates of 3708 of these earthquakes using the double difference relocation algorithm. We used records from 75 seismic stations, which included 60 stations of the regional permanent seismic networks and 15 stations of the temporary seismic networks Figure 4 Relocation of the April 20, 2013 Lushan earthquake, Sichuan and its aftershock sequences. Aftershock sequence was recorded from April 20 to May 2, 2013. Yellow circle is the mainshock of the Lushan Ms7.0 earthquake and small red circles are the aftershocks after relocation. Triangles are seismic stations (blue: permanent stations, white: temporary stations). Other notations are the same as in Figure 2.
2041 (Figure 5). Comparing the histograms before and after the relocation, the aftershocks of the Lushan earthquake occurred mainly at a depth range of 10 20 km. The initial rupture depths of the hypocenters of the Lushan earthquake and the five strong aftershocks of magnitude M 5 range from 15 to 20 km. The initial rupture depth of the hypocenter of the main M s 7.0 shock is about 18 km (Table 1). In this depth at the southern part of the LMS fault, the hypocenter of the Lushan earthquake is at the transition zone between the high velocity and low velocity zones and is almost within the high velocity zone, which is the structure that promotes generation of large earthquakes (Gao et al., 2000). The parameters of the focal locations of the Lushan earthquake and its strong aftershocks of M 5 (Table 1) show a significant change in the focal depth parameters. It should be noted that the focal depth is a seismic parameter that is difficult to determine since it involves measurements and definitions which vary. For a strong earthquake with a large range of rupture, different depth definitions will result in different depth values (Gao et al., 1997). 4 The no rupture zone in the central Longmenshan fault Based on the relocation of the Wenchuan earthquake aftershock sequence (Zhao et al., 2011), the Lushan earthquake and its aftershock sequence occurred in the southern part of the LMS fault, southwest of the Wenchuan earthquake. From the earthquake location map (Figure 6) we see no activity in the middle and southern regions of the central part of the LMS fault, between the location of the Lushan earthquake including its aftershocks and that of the Wenchuan earthquake including its aftershocks; this region is called the no rupture zone. Based on the results of the emergency analysis of the focal source of the Lushan earthquake by the Institute of Earthquake Science 1), China Earthquake Administration (Gao et al., 2001), and the rapid results from other research institutions around the world, the focal mechanism of the main shock of the Lushan earthquake shows a reverse-type motion of high dip angle. Combining this with the distribution of faults and aftershocks, we conclude that the fault Figure 5 Histograms before and after relocation of aftershock sequences of the April 20, 2013 Lushan M s 7.0 earthquake, Sichuan. (a) Before relocation; (b) After relocation. Aftershock sequence is recorded from April 20 to May 2, 2013. Table 1 Relocation results of M 5 aftershocks of the Lushan M s 7.0 earthquake a) Time Before relocation After relocation Latitude (N) Longitude (E) Depth (km) Latitude (N) Longitude (E) Depth (km) Magnitude M s 2013-04-20 08:02:46 30.3 103.0 13 30.29 102.97 17.8 7.0 2013-04-20 08:07:29 30.3 102.9 10 30.35 102.95 18.5 5.1 2013-04-20 11:34:14 30.1 102.9 11 30.18 102.88 15.1 5.3 2013-04-21 04:53:44 30.3 103.0 17 30.35 103.04 19.8 5.0 2013-04-21 17:05:22 30.3 103.0 17 30.33 103.03 16.4 5.0 a) Hypocenter parameters before relocation; magnitudes are from the rapid official report of CENC 1) Institute of Earthquake Science, China Earthquake Administration. 2013. Working information, Nos. 31, 33, 34.
2042 Figure 6 Aftershock sequences of the 2013 Lushan Ms7.0 earthquake and 2008 Wenchuan Ms8.0 earthquake. Large and small yellow circles represent the 2008 Wenchuan earthquake and Lushan earthquake, respectively. White circles represent strong aftershocks of 5.0 M<5.9. Aftershock sequences of the Lushan earthquake (blue dots) and Wenchuan earthquake (red dots) are the results after relocation. A no earthquake zone (white rectangle) is formed in the southern central section of the LMS faults. The grey thick line is the boundary between the first-order blocks and the black thin line is the fault. Other notations are the same as in Figure 2. plane dipping westward is the rupture plane of the Lushan earthquake. Considering the focal mechanism and rupture process of the main Wenchuan shock (Zhang et al., 2009; Wang et al., 2008), the main rupture of the Wenchuan earthquake started as a mechanism of pure thrust and ended as a mechanism of strike-slip. Therefore, the no rupture zone between the locations of the Lushan main shock and the Wenchuan main shock, was still controlled by the southeastward push and the obstruction in the northwest direction. If the stress in the mid-southern part of the LMS fault and its neighboring zone continues to increase gradually, the no rupture zone may break again. It could then propagate through the rupture zones of the Lushan and the Wenchuan earthquakes. The length of the no rupture zone along the LMS fault suggests that earthquakes generated from this zone in the near future will not be of large magnitude. If the rupture happens soon, the source mechanism is likely to have a dominant thrust component. Based on the shear-wave splitting data, the predominant direction of the fast shear-wave polarization indicates in situ and regional principal compressive stress orientation (Figures 2, 3). The rupture zone of the Wenchuan earthquake from the initial to the middle is in the central part of the LMS fault (zone A in Figure 3) and shows an almost pure thrust motion due to the eastward and southeastward movement of the Songpan-Garze block in the Tibetan Plateau. In the Lushan hypocentral area in the north of the southern part of the LMS fault (including the southern middle part) and at the intersection of the faults to the southwest (zone C in Figure 3), the faults gradually change from pure thrust to both thrust and strike-slip, influenced by the multiple faults. In addition, the motion is obstructed by the Sichuan-Yunnan Block in the southern part of the XSH Faults. Since zones A and C are in different tectonic locations, it shows clearly that the orientation of the principal compressive stress changes in zone C from NW in the north to nearly E-W at the intersection of faults LMS, XSH, and ANH.
2043 Several faults intersect in the southern end of the LMS fault. Unlike the thrust motion in the central part of the LMS fault, the tectonics in the southern end of the LMS fault varies because of the intersection of several faults; therefore, the stress is also dissimilar. However, the Lushan earthquake itself was a thrust event, as was the Wenchuan earthquake. This highlights the existence of an unbroken zone between the location of the Lushan earthquake and that of the Wenchuan earthquake. The energy released by the Wenchuan earthquake originated from the slow accumulation of long-term large-scale crustal strain in the LMS fault in the western margin of Sichuan Basin. This strain is caused by the Bayan Har block pushing against the LMS fault from the west (i.e. the Songpan-Garze block in Figure 6) (Jiang et al., 2009). The seismic profile across the LMS fault reveals a discontinuity in the Moho and a sharp increase in the velocity ratio Vp/Vs (Zhang et al., 2009b, 2010). The velocity structure also shows a velocity variation under the LMS fault (Lü et al., 2013), indicating deep structure. Based on the tectonic characteristics of the region and on past and current earthquake activity, we note that for at least 1100 years before 2008 there were no earthquakes of magnitude M s 7.0 in the LMS fault. Along the LMS fault, a zone formed with very low seismicity compared with the seismic activity of the surrounding faults, and in 2008 the Wenchuan M s 8.0 earthquake occurred in this blank zone (Wen et al., 2009). After the Wenchuan earthquake, because of the continuous squeezing eastward of the Songpan-Garze block in the eastern Tibetan Plateau, studies suggested that a risk of rupture existed in the unbroken south segment of the LMS faults. The Lushan earthquake released some of this accumulated strain, thus reducing the possibility of an earthquake larger than M s 7.0 occurring in the near future in the southern segment of the LMS faults. However, we still need to keep a watchful eye on this no rupture zone to monitor the risk of it rupturing again. 5 Discussion and conclusions The focal locations (Zhao et al., 2011; Zhang R Q et al., 2008) and stress background (Zhang Y J et al., 2008; Shi et al., 2009, 2013; Ding et al., 2008) of the Wenchuan and Lushan earthquakes on the LMS fault were obtained by analyzing the activity of aftershocks and studying shearwave splitting data. In this study we examined the orientation of the regional principal compressive stress in the zone that extends southwest from the epicenter of the Lushan earthquake along the LMS fault to the intersection of the LMS, XSH, and ANH faults and the adjacent region. We found that the orientation changes from NW in the northern part of this zone to almost E-W further south. The spatial variation of the stress indicates deep tectonic movement which will require further investigation. The Lushan earthquake occurred in the southern part of the LMS fault. A no rupture zone appears after the Lushan earthquake, which is of about 60 km long between the rupture zone of Lushan earthquake and the rupture zone of Wenchuan earthquake. Within this no-rupture zone, an earthquake of magnitude M s 6.2 occurred in 1970; however, more than 40 years have passed since then. Our relocation study of the aftershocks of the Lushan earthquake indicates that the Lushan earthquake did not cut through itself and the rupture zone of the Wenchuan earthquake. It is important for scientific research and for the safety of the population in the region to pay close attention to any changes in the no rupture zone between the locations of the Lushan and Wenchuan earthquakes, by enhancing seismic monitoring in the southern part of the LMS faults. Close monitoring of stress variation in the crust can detect signs of tectonic activity. Studies have shown that crustal shear-wave splitting is one of the valid methods for monitoring stress change caused by earthquake activity (Gao et al., 2004, 2008a; Crampin et al., 2008). However, this technique requires a large number of small earthquake events in the vicinity of the seismic stations since the data sources are seismic waves of small earthquakes. More effective methods are the cross-well techniques using artificial borehole sources and borehole records, e.g. stressmonitoring sites (Gao et al., 2008a; Crampin et al., 2008). A study of the San Andreas Fault, California continuously monitored artificial borehole signals and measured velocity changes of seismic waves (Niu et al., 2008). This is another proven effective borehole method to measure the change of stress by a controllable source. Both borehole measurement methods require significant expenditure; therefore, their use is limited to measuring changes of stress and wave velocity in key zones only. Earthquakes occur frequently in the eastern and southeastern areas of the Tibetan Plateau. Monitoring very small changes in stress and velocity throughout this region will deepen our understanding of the seismic structure and seismic sources affecting the tectonic activity in the region. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41174042, 41040034) and the China National Special Fund for Earthquake Scientific Research in Public Interest (Grant No. 201008001). The relocation data of the aftershocks of the Lushan earthquake were obtained from the China Earthquake Network Center (CENC) and several research institutions of the China Earthquake Administration. We would like to thanks the data sharing program of earthquake emergency. We thank reviewers for their comments and suggestions to improve this manuscript. Crampin S, Gao Y, Peacock S. 2008. Stress-forecasting (not predicting) earthquakes: A paradigm shift? 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