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Yi G.-X.,China Earthquake Administration | Yi G.-X.,Sichuan Seistech Corporation Ltd. | Long F.,China Earthquake Administration | Zhao M.,China Earthquake Administration | And 3 more authors.
Dizhen Dizhi | Year: 2016

The Oct. 1, 2014 M5.0 Yuexi earthquake occurred on the Daliang Shan fault zone where only several historical moderate earthquakes were recorded. Based on the waveform data from Sichuan regional seismic network, we calculated the focal mechanism solution and centroid depth of the M5.0 Yuexi earthquake by CAP(Cut and Paste)waveform inversion method, and preliminarily analyzed the seismogenic structure. We also calculated the apparent stress values of the M5.0 earthquake and other 14 ML≥4.0 events along the Shimian-Qiaojia fault segment of the eastern boundary of the Sichuan-Yunnan block. The result indicates that the parameters of the focal mechanism solution are with a strike of 256°, dip of 62°, and slip of 167° for the nodal plane I, and strike of 352°, dip of 79°, and slip of 29° for the nodal plane II. The azimuth of the P axis is 121° with dip angle of 11°, the azimuth of T axis is 217° with dip angle of 28°, and the centroid depth is about 11km, and moment magnitude is MW5.1. According to the focal mechanism solution and the fault geometry near the epicenter, we infer that the seismogenic fault is a branch fault, i.e., the Puxiong Fault, along the central segment of the Daliang Shan fault zone. Thus, the nodal plane II was interpreted as the coseismic rupture plane. The M5.0 Yuexi earthquake is a strike-slip faulting event with an oblique component. The above findings reveal the M5.0 Yuexi earthquake resulted from the left-lateral strike-slip faulting of the NNW Dalang Shan fault zone under the nearly horizontal principal compressive stress regime in an NWW-SEE direction. The apparent stress value of the Yuexi earthquake is 0.99MPa, higher than those of the ML≥4.0 earthquakes along the eastern boundary of the Sichuan-Yunnan block since 2008 Wenchuan M8.0 earthquake, implying a relatively high stress level on the seismogenic area and greater potential for the moderate and strong earthquake occurrence. It may also reflect the current increasing stress level of the entire area along the eastern boundary, and therefore, posing the risk of strong earthquakes there. © 2016, Editorial Office of Seismology and Geology. All right reserved.


Wang M.,Institute of Geology | Wang M.,Sichuan Seistech Corporation | Zhou B.,Institute of Geology | Yang X.,Institute of Geology | And 2 more authors.
Acta Geologica Sinica | Year: 2013

Following the 2008 Wenchuan M8 earthquake, the seismic risk of the northeastern section of the Longmenshan fault zone and the adjacent Hanzhong basin has become an issue that receives much concern. It is facing, however, the problem of a lack of sufficient data because of little previous work in these regions. The northeastern section of the Longmenshan fault zone includes three major faults: the Qingchuan fault, Chaba-Lin'ansi fault, and Liangshan south margin fault, with the Hanzhong basin at the northern end. This paper presents investigations of the geometry, motion nature, and activity ages of these three faults, and reveals that they are strike slip with normal faulting, with latest activity in the Late Pleistocene. It implies that this section of the Longmenshan fault zone has been in an extensional setting, probably associated with the influence of the Hanzhong basin. Through analysis of the tectonic relationship between the Longmenshan fault zone and the Hanzhong basin, this work verifies that the Qingchuan fault played an important role in the evolution of the Hanzhong basin, and further studies the evolution model of this basin. Finally, with consideration of the tectonic setting of the Longmenshan fault zone and the Hanzhong basin as well as seismicity of surrounding areas, this work suggests that this region has no tectonic conditions for great earthquakes and only potential strong events in the future. © 2013 Geological Society of China.


Zhou R.-J.,Institute of Earthquake Engineering | Li Y.,Chengdu University of Technology | Su J.-R.,Institute of Earthquake Engineering | Wang S.-Y.,Institute of Earthquake Engineering | And 8 more authors.
Journal of Chengdu University of Technology (Science and Technology Edition) | Year: 2013

On April 20, 2013, the Lushan Mw 6. 6 earthquake occurred on the south segment of the Longmenshan fault zone and no significant coseismic ground rupture was observed. In this paper, the authors define the distribution of the intensity of this earthquake after redefining more than 400 macro-seismic damage survey points and referring to the near field ground acceleration (PGA) record of the Sichuan digital strong motion seismic network. The intensity of epicenter region reached K degree on Chinese seismic intensity scale, slightly showing flat oval shape with the long axis to NE, without apparent direction. Furthermore, after consolidating the results of relocating 3 323 early aftershocks, petroleum seismic exploration profiles and focal mechanism solutions, the authors think that the Dayi fault controlling the east piedmont of Mengshan is the main causative structure of the Lushan earthquake, resulting from the thrusting of the Dayi fault induced by shortening of the Longmenshan fault zone in NW-SE direction. The Xinkaidian fault also produces coseismic ruptures in the depth, causing the phenomena that the earthquake damage on the hanging wall is significantly higher than that on the footwall.


Yi G.-X.,China Earthquake Administration | Yi G.-X.,Sichuan Seistech Corporation Ltd. | Long F.,China Earthquake Administration | Long F.,Sichuan Seistech Corporation Ltd. | And 4 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2016

The April 20th, 2013 Lushan M7.0 earthquake sequence occurred on the southern segment of the Longmen Shan fault zone, southwestern China. Utilizing the waveform data of M>3.0 earthquakes between 20 Apr. 2013 and 31 Dec. 2015 from Sichuan Regional Seismic Network, our present study determined the focal mechanism solutions, centroid depths and moment magnitudes for the Lushan mainshock and 87 M>3.0 aftershocks by CAP waveform inversion method. Along with analysis on the strain rosette and areal strain (As), we aimed to discuss the focal mechanism of the sequence and its implications to the tectonic deformation in the seismogenic area. The major findings are as follows: (1) The parameters of the ruptured plane are of strike 219°, dip 43° and slip 101° for the Lushan M7.0 mainshock with a moment magnitude of MW6.55 and centroid depth of 15 km. The 87 M>3.0 aftershocks are distributed asymmetrically near the mainshock in a region ~37 km long along the strike of the Longmen Shan fault zone, and ~16 km wide perpendicular to the fault zone. All the aftershocks are located within a depth range of 7~22 km. Most of the aftershocks are above the mainshock, and the average depth of the sequence is about 13 km. No aftershocks were found shallower than 7 km, indicating the seismogenic source was relatively deep. The estimated seismogenic source of the Lushan earthquake is 37 km×16 km×16 km. (2) The values of areal strain (As) show that reverse faulting is dominant for the sequence. The plots of the hypocenters of the sequence on the vertical cross-sections indicate that a NE-SW-striking thrust fault dipping about 45° to NW is the main seismogenic fault of the Lushan earthquake sequence, a portion of aftershocks occurred on the SE-dipping back thrust fault nearly perpendicular to the NW-dipping main fault, and the range-front fault of Longmen Shan fault zone may be responsible for some aftershocks. P-axis is nearly horizontal and orientated in NW-SE direction, coinciding with the regional tectonic stress field. This finding indicates that the seismogenic area is controlled by the stress field, and the Lushan earthquake sequence was resulted from the reverse faulting of the NE-SW-trending faults under a nearly horizontal principle stress with NW-SE orientation. Existence of non-thrusting type earthquakes and obvious variation of the plunge angle of P-axis surrounding the mainshock together indicate the stress disturbance in the local area was influenced by the Lushan mainshock. (3) The strain rosettes for the entire sequence and different classes of magnitudes all display NW-trending compressional white lobe, therefore, we infer that the geological structures for the entire seismogenic area are of thrust faulting under NW-oriented compressional deformation. The strain rosettes exhibit self-similarity in terms of orientation and shape for all classes of magnitudes, reflecting that the deformation pattern of the seismogenic faults is independent of magnitude. (4) The shape of the strain rosette at each depth is dominated by compressional white lobe in NW-NWW direction, indicating all the geological structures for the entire seismogenic area or individual segments within different depths are under NW-NWW-oriented compressional deformation. However, variation of both the shapes and orientations of the strain rosettes with depth is observed, indicating existence of segmentation of tectonic deformation in the vertical direction. (5) The dimension of the seismogenic source of the Lushan earthquake is relatively small, and the mainshock was not on the main faults along the southern segment of the Longmen Shan fault zone, and further we suggest that the cumulated strain energy was not released thoroughly, posing strong earthquake risk on the southern segment. © 2016, Science Press. All right reserved.


Wang M.-M.,China Earthquake Administration | Wang M.-M.,Sichuan Seistech Corporation | Zhou B.-G.,China Earthquake Administration | Yang X.-P.,China Earthquake Administration | Li J.-Y.,China Earthquake Administration
Dizhen Dizhi | Year: 2013

Since the Wenchuan earthquake, the seismic hazard of the northeastern segment of the Longmenshan Fault zone as well as the Hanzhong Basin has drawn more and more concerns. However, the essential data needed for further analysis on the seismic hazard in this region is scarce at present time, hence there is an urgent need for an in-depth study on the activities of faults around the basin. The faults around the Hanzhong Basin include five main faults, namely, the northern margin fault of Hanzhong Basin, the southern margin fault of Hanzhong Basin, the Qingchuan Fault, the Chaba-Lin'ansi Fault and the southern margin fault of Liangshan. Based on several detailed field investigations on the geometric distribution, movement nature and active ages of the five faults, and with consideration of previous work, our study shows that the late-Quaternary tectonic activity in the basin is relatively intense in west and weak in east. The west section of the north-margin fault of the Hanzhong Basin (east of Baohe) was active in early late Pleistocene, while its eastern section (west of Baohe) was active in middle Pleistocene. The south-margin fault of the basin was also active in middle Pleistocene. And the three faults in the southwest of the basin were all active in late Pleistocene. This activity pattern of high in the west and low in the east is also demonstrated by the difference in thickness of Quaternary system and the distribution of small earthquakes.


Yi G.-X.,China Earthquake Administration | Yi G.-X.,Sichuan Seistech Corporation Ltd. | Long F.,China Earthquake Administration | Wen X.-Z.,China Earthquake Administration | And 2 more authors.
Chinese Journal of Geophysics (Acta Geophysica Sinica) | Year: 2015

On 22 Nov. 2014, an M6.3 earthquake hit Kangding County in Sichuan Province, southwestern China. 3 days later, another M5.8 earthquake occurred in the same region, about 10 km southeastwards from the epicenter of the M6.3 mainshock. Both earthquakes were on the NW-striking Xianshuihe fault zone where no M≥6.0 earthquakes were reported after the 1982 M6.0 Ganzi event. From the relocated aftershock distribution and focal mechanism solutions, we aim to analyze the seismogenic structure of the 2014 M6.3 Kangding earthquake sequence. Along with the analysis on the characteristics of the strong earthquake ruptures and the spatial distribution of the relocated small earthquakes, we will further discuss the future strong-earthquake risk between Daofu and Kangding on the central segment of the Xianshuihe fault zone. Based on the digital waveform data from China National Seismic Network and Sichuan Regional Network, the Kangding earthquake sequence was relocated by a multi-step locating method developed by Long et al. The focal mechanism solutions and the centriod depths of the M6.3 and M5.8 earthquakes were inverted simultaneously by the gCAP (generalized Cut and Paste) moment tensor inversion method. The spatial-temporal distribution of the M≥6.5 strong earthquake ruptures since 1725 and the focal depth distribution of relocated small earthquakes from Jan. 2001 to Oct. 2014 along the central-southern segment of the Xianshuihe fault zone were used to identify the potential seismogenic region of the next strong earthquakes on the segment between Daofu and Kangding. The epicentral relocation of the M6.3 mainshock is at 101.69°E, 30.27°N, and its initial rupture depth is about 10 km, while the centroid depth is 9 km; the relocated M5.8 earthquake is at 101.73°E, 30.18°N with the initial rupture depth at about 11 km and the centroid depth of 9 km. The moment tensor solutions from gCAP method show that the two events are dominated by the double-couple component. The parameters of the best double-couple solutions are as follows, strike 143°, dip 82°, rake -9° for nodal plane I, and strike 234°, dip 81°, rake -172° for nodal plane II for the M6.3 earthquake. For the M5.8 earthquake, the parameters are listed as, strike 151°, dip 83°, rake -6° for the nodal plane I, and strike 242°, dip 84°, rake -173° for the nodal plane II. Most aftershocks during the first 3 days were distributed on the NW side of the M6.3 mainshock, and majority of the aftershocks after M5.8 earthquake were concentrated around the epicenter of the M5.8 event. The average focal depth of the 459 relocated earthquakes of the sequence is about 9 km. Focal depth distribution reveals that the sequence mainly concentrated in the depth range of 6~11 km and most of the aftershocks are shallower than the M6.3 and M5.8 earthquakes. The seismic source scale is estimated to be about 30 km in length and 4 km in width with 6 km in depth according to the aftershock distribution. On the space-time diagram of the historical M≥6.5 strong earthquake ruptures, we observe a gap on the Selaha fault of the central Xianshuihe fault zone, where no M≥6.5 earthquakes occurred since the 1748 M61/2 event. Aseismic gap below the depth 7 km between Kangding and Tagong and a low-seismicity region below the depth 2 km between Tagong and Songlinkou were identified on the vertical cross-section of the relocated small earthquakes since 2001 along the central-southern segment of the Xianshuihe fault zone. The nodel plane I of the focal mechanism solution was interpreted as the coseismic rupture plane for the M6.3 and M5.8 earthquakes based on the aftershock distribution and the fault strike. Both earthquakes are of left-lateral strike-slip faulting with some normal component. The relocated M6.3 earthquake and its aftershocks during the first three days are on the NW-striking Selaha fault with a near-vertical dip angle of 82°, while the M5.8 earthquake and its adjacent aftershocks are on the northern portion of the NW-striking Zheduotang fault with a dip angle of 83°, implying that the M5.8 earthquake on the Zheduotang fault may be induced by the M6.3 earthquake in the adjacent Selaha fault. The scarce aftershock region around the M6.3 mainshock may belong to a relatively large asperity where the accumulated energy was totally released as the M6.3 mainshock occurred. The 2014 M6.3 Kangding earthquake sequence occurred within the Selaha strong-earthquake rupture gap between Qianning and Kangding. Due to the duration of quiescence longer than the estimated average recurrence interval for the M7 earthquakes, we propose that the ruptures of the M6.3 and M5.8 earthquakes are too limited to fill up the gap, posing future M7 earthquake risk on the Selaha and its adjacent Qianning segments along the central segment of the Xianshuihe fault zone. Since most area of the previous seismic gap below the depth 7 km between Kangding and Tagong along the central-southern Xianshuihe fault zone was filled by the 2014 M6.3 Kangding earthquake sequence, and the most likely place of future strong earthquake occurrence will be below the segment between Tagong and Songlinkou where low seismicity is observed. ©, 2015, Science Press. All right reserved.

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