云南景谷地震震源深度:新生断裂脆韧性转换带深度探讨
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摘要
地震深度能够约束地壳流变结构,成熟断裂的脆韧性转换带深度为10km左右,而存在于古老稳定地块的新生断裂上,地壳脆韧性转换带深度在地壳浅部数千米处.但是构造活动区新生断裂的相关研究较少,2014年10月7日云南景谷M_w6.1地震为此提供了一个较为难得的案例.本文使用基于Pn/Pg相对定位方法和CAP(cutand paste)方法分别反演得到了景谷主震和2次5级以上余震的震源起始破裂深度和矩心深度.主震的起始深度为9.5km,矩心深度为5.0km,主震破裂于深部然后向浅部发展,而2次余震可能表现为圆盘式破裂,其震源矩心深度与起始深度较为接近.综合大地电磁测深结果和区域流变结构,发现3次地震的深度与电性高低阻分界面和岩石强度拐点深度接近,由此可以认为这一新生断裂的脆韧性转换深度约为10km.此次景谷地震研究结果表明,构造活动强烈区新生断裂的脆韧性转换带深度与成熟断裂类似,不同于古老稳定地块新生断裂的流变结构.
Since most continental earthquakes occur in the brittle layer instead of ductile layers, the focal depth of an earthquake provides an important constraint on the rheology structure of fault zone. According to the results of previous studies, the typical depth of brittle-ductile transition zone on mature faults is about 10 km. However, several moderate earthquakes occurred on young faults in stable cratons, and focal depths of those events are very shallow(e.g. the 1993 M_w6.1 India Killari earthquake and a series of shallow earthquakes in Australia). Those observations suggest that depth of the brittleductile zone on young faults in cratons is shallower than that on mature faults. In this study, we report a case on a young fault in a tectonically active region that has not been well studied and discuss implications for the rheology structure. On October 7, 2014, an M_w6.1 earthquake occurred in Jinggu, Yunnan(China), the southeastern part of the tectonically active Yunnan-Myanmar block, followed by two aftershocks of M>5 in December. According to historical seismicity and geologic studies in this region, it is suggested that the seimogenic fault of the Jinggu earthquake is a young fault. We use data from global and regional seismic networks, as well as aftershock data from the Lozhadu reservoir network and temporary stations, to study focal depths of the mainshock and aftershocks. The hypocenter of a reference event(Mw4.3) that was recorded with two close temporary stations is determined, and then the hypocenters of the mainshock and two moderate aftershocks are relocated with a relative method based on Pn/Pg arrival times. The relocated hypocenter of the mainshock is 9.5 km deep, whereas the hypocenters of the two aftershocks are about 10 km deep. The centroid depth is investigated with the CAP method based on waveform modeling. Both teleseismic and regional waveforms are used to invert for the centroid depth of the mainshock. The optimal waveform fit is obtained with a depth of 5 km. The two aftershocks are studied with regional waveforms only and the results are close to the hypocenter depths. Since the hypocenter indicates the initial point of rupture whereas the centroid location is close to the center of the main rupture patch, we propose the mainshock initiated at 10 km depth and expanded to the brittle layer at shallower depth. For two aftershocks, the consistency of hypocenter depth and centroid depth suggests circular rupture patterns. Therefore, we propose the bottom of the seismogenic layer and brittle-ductile transition zone is about 10 km deep. This hypothesis is also supported by other observations. Magnetotelluric study in this region reveals a low conductivity layer above 10 km that is regarded as brittle rock, whereas the high conductivity layer beneath 15 km is regarded as ductile rock. Furthermore, a regional rheology model, which is obtained with surface temperature, seismic velocity, and GPS data, also suggests the brittle-ductile transition zone is at about 9 km depth. In summary, the depth of the brittle-ductile transition zone on the young fault in the tectonic active region is similar to that of mature faults, which is useful information for lithosphere geodynamics studies.
Since most continental earthquakes occur in the brittle layer instead of ductile layers, the focal depth of an earthquake provides an important constraint on the rheology structure of fault zone. According to the results of previous studies, the typical depth of brittle-ductile transition zone on mature faults is about 10 km. However, several moderate earthquakes occurred on young faults in stable cratons, and focal depths of those events are very shallow(e.g. the 1993 M_w6.1 India Killari earthquake and a series of shallow earthquakes in Australia). Those observations suggest that depth of the brittleductile zone on young faults in cratons is shallower than that on mature faults. In this study, we report a case on a young fault in a tectonically active region that has not been well studied and discuss implications for the rheology structure. On October 7, 2014, an M_w6.1 earthquake occurred in Jinggu, Yunnan(China), the southeastern part of the tectonically active Yunnan-Myanmar block, followed by two aftershocks of M>5 in December. According to historical seismicity and geologic studies in this region, it is suggested that the seimogenic fault of the Jinggu earthquake is a young fault. We use data from global and regional seismic networks, as well as aftershock data from the Lozhadu reservoir network and temporary stations, to study focal depths of the mainshock and aftershocks. The hypocenter of a reference event(Mw4.3) that was recorded with two close temporary stations is determined, and then the hypocenters of the mainshock and two moderate aftershocks are relocated with a relative method based on Pn/Pg arrival times. The relocated hypocenter of the mainshock is 9.5 km deep, whereas the hypocenters of the two aftershocks are about 10 km deep. The centroid depth is investigated with the CAP method based on waveform modeling. Both teleseismic and regional waveforms are used to invert for the centroid depth of the mainshock. The optimal waveform fit is obtained with a depth of 5 km. The two aftershocks are studied with regional waveforms only and the results are close to the hypocenter depths. Since the hypocenter indicates the initial point of rupture whereas the centroid location is close to the center of the main rupture patch, we propose the mainshock initiated at 10 km depth and expanded to the brittle layer at shallower depth. For two aftershocks, the consistency of hypocenter depth and centroid depth suggests circular rupture patterns. Therefore, we propose the bottom of the seismogenic layer and brittle-ductile transition zone is about 10 km deep. This hypothesis is also supported by other observations. Magnetotelluric study in this region reveals a low conductivity layer above 10 km that is regarded as brittle rock, whereas the high conductivity layer beneath 15 km is regarded as ductile rock. Furthermore, a regional rheology model, which is obtained with surface temperature, seismic velocity, and GPS data, also suggests the brittle-ductile transition zone is at about 9 km depth. In summary, the depth of the brittle-ductile transition zone on the young fault in the tectonic active region is similar to that of mature faults, which is useful information for lithosphere geodynamics studies.
引文
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1)魏星,王向腾,李志伟,等.基于Pn/Pg相对定位方法研究2017年8月8日九寨沟M7.0地震起始破裂深度.地球物理学报(已接受)
2 Shi Y L,Zhu S B.Contrast of rheology in the crust and mantle near moho revealed by depth variation of earthquake mechanism in continental China(in Chinese).Chin J Geophys,2003,46:359-365[石耀霖,朱守彪.中国大陆震源机制深度变化反映的地壳--地幔流变特征.地球物理学报,2003,46:359-365]
3 Ji S C,Zhong D L,Xu Z Q,et al.Rheology:A new departure in structural geology and geodynamics(in Chinese).Geotect Metal,2008,32:257-264[嵇少丞,钟大赉,许志琴,等.流变学:构造地质学和地球动力学的支柱学科.大地构造与成矿学,2008,32:257-264]
4 Scholz C H.The brittle-plastic transition and the depth of seismic faulting.Geol Rundsch,1988,77:319-328
5 Scholz C H.Earthquakes and friction laws.Nature,1998,391:37-42
6 Scholz C H.The Mechanics of Earthquakes and Faulting.2nd ed.Cambridge:Cambridge University Press,2002
7 Zhang G M,Li L.Rheology of crustal media and a related seismogenic model(in Chinese).Seismol Geol,2003,25:1-10[张国民,李丽.地壳介质的流变性与孕震模型.地震地质,2003,25:1-10]
8 Panet I,Pollitz F,Mikhailov V,et al.Upper mantle rheology from GRACE and GPS postseismic deformation after the 2004 SumatraAndaman earthquake.Geochem Geophys Geosyst,2010,11:Q06008
9 Jackson I.The Earth’s Mantle:Composition,Structure,and Evolution.Cambridge:Cambridge University Press,1998
10 Wei S J,Ni S D,Chong J J,et al.The 16 August 2003 Chifeng earthquake:Is it a lower crust earthquake(in Chinese)?Chin J Geophys,2009,52:111-119[韦生吉,倪四道,崇加军,等.2003年8月16日赤峰地震:一个可能发生在下地壳的地震?地球物理学报,2009,52:111-119]
11 Luo Y,Ni S D,Zeng X F,et al.Seismological study of an aftershock sequence at the northeast end of the Wenchuan earthquake aftershock region(in Chinese).Sci China Earth Sci,2010,40:677-687[罗艳,倪四道,曾祥方,等.汶川地震余震区东北端一个余震序列的地震学研究.中国科学:地球科学,2010,40:677-687]
12 Dong Y,Ni S,Yuen D A,et al.Crustal rheology from focal depths in the North China Basin.Earth Planet Sci Lett,2018,497:123-138
13 Seeber L,Ekstr?m G,Jain S K,et al.The 1993 Killari earthquake in central India:A new fault in Mesozoic basalt flows?J Geophys Res:Solid Earth,1996,101:8543-8560
14 Wu W Z,Wei Z G,Chen W W,et al.Australian shallow earthquakes study progress and prospect(in Chinese).Prog Geophys,2016,31:371-379[吴为治,危自根,陈伟文,等.澳大利亚浅源地震研究进展和展望.地球物理学进展,2016,31:371-379]
15 Guo S M,Zhou R Q.Longling-Lancang fault zone in the southwestern Yunnan:A new rupture zone on the continental crust(in Chinese).Chin Sci Bull,1999,44:2118-2121[虢顺民,周瑞琦.滇西南龙陵-澜沧断裂带:大陆地壳上一条新生的破裂带.科学通报,1999,44:2118-2121]
16 Guo S M,Xu X W,Xiang H F,et al.Segmentation of earthquake rupture and earthquake prediction along the Longling-Lancang fault zone in the southwestern Yunnan province(in Chinese).Seismol Geol,2002,24:133-144[虢顺民,徐锡伟,向宏发,等.龙陵-澜沧新生断裂带地震破裂分段与地震预测研究.地震地质,2002,24:133-144]
17 Shao Y X,Yuan D Y,Liang M J.Seismic risk assessment of Longling-Lancang fault zone,southwestern Yunnan(in Chinese).Acta Seismol Sin,2015,(6):1011-1023[邵延秀,袁道阳,梁明剑.滇西南地区龙陵-澜沧断裂带地震危险性评价.地震学报,2015,(6):1011-1023]
18 Wang Q,Chu R,Yang H,et al.Complex Rupture of the 2014 Ms6.6 Jinggu earthquake sequence in Yunnan Province inferred from doubledifference relocation.Pure Appl Geophys,2018,doi:10.1007/s00024-018-1913-y
19 Xu X W,He R C.Research on Formation of New-generation Fault and Its Foreshock Activity(in Chinese).Research on Active Fault(5).Beijing:Seismological Press,1996.192-197[徐锡伟,何昌荣.新生断层的形成及其前震活动性研究.活动断裂研究(5).北京:地震出版社,1996.192-197]
20 Deng J M,Gao Q,Chen J,et al.About characteristics of seismicity anomalies before Jinggu M6.6 earthquake in Yunnan Province 2014(in Chinese).Seismol Geomagn Observ Res,2016,37:15-20[邓嘉美,高琼,陈佳,等.2014年云南景谷6.6级地震前地震活动异常特征研究.地震地磁观测与研究,2016,37:15-20]
21 Liu Y,Shao Z G.Analysis of seismicity changes prior to the 2014 Yunnan Jinggu Ms6.6 earthquake(in Chinese).Seismol Geol,2016,38:1070-1081[刘月,邵志刚.2014年云南景谷Ms6.6地震前地震活动变化分析.地震地质,2016,38:1070-1081]
22 Li Y H,Xu X M,Zhang E H,et al.Three-dimensional crust structure beneath SE Tibetan Plateau and its seismotectonic implications for the Ludian and Jinggu earthquakes(in Chinese).Seismol Geol,2014,36:1204-1216[李永华,徐小明,张恩会,等.青藏高原东南缘地壳结构及云南鲁甸,景谷地震深部孕震环境.地震地质,2014,36:1204-1216]
23 Xu X W,Cheng J,Xu C,et al.Discussion on block kinematic model and future themed areas for earthquake occurrence in the Tibetan Plateau:Inspiration from the Ludian and Jinggu earthquakes(in Chinese).Seismol Geol,2014,36:1116-1134[徐锡伟,程佳,许冲,等.青藏高原块体运动模型与地震活动主体地区讨论:鲁甸和景谷地震的启示.地震地质,2014,36:1116-1134]
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