STRUCTURES OF SOURCE REGIONS OF THE 1979 MS 6.0 WUYUAN EARTHQUAKE AND THE 1996 MS 6.4 BAOTOU EARTHQUAKE IN INNER MONGOLIA, CHINA

doi:10.3969/j.issn.0253-4967.2014.03.004

Abstract

Abstract:

Study on structure of earthquake source regions is one aspect of earthquake geology, which has relatively small spatial-scale but needs to know more details about seismogenic structures of specific historical events and potential events. Most of shallow-focus and tectonio-genic earthquakes on continents usually nucleate at 10 to 30km below the earth's surface. Such a depth is not able to be reached from studies only using data and methods of geology, geomorphology, or only from analyses of remote sense imagery. It needs studies of coupling deep and shallow structures by combining data from geological investigations and seismological survey, as well as from inspections of multi-geophysical techniques. The study region of this article is the middle to western portion of the Hetao graben system in Inner Mongolia, China, which is an active boundary belt separating the Ordos and Yanshan blocks, and a large-scale seismogenic tectonic zone dominantly under tensional-stress in the northwestern North China. Along the system, at least two great earthquakes occurred in the historical time, and reportedly some lines of evidence of paleo-earthquakes have been found. Since the 20th century 4 strong events with magnitudes 6.0 to 6.4 have taken place in the Hetao graben system. The most recent two of the 4 are the M_S 6.0 Wuyuan earthquake on Aug. 25, 1979 and the M_S 6.4 Baotou earthquake on May 3, 1996, respectively. For seismogenic structures of the two, although relevant studies were made mainly based on analyzing the relation between aftershock's and seismic intensity's distributions and surface faults or "faults" inferred just from remote sensing images, queries still remain in the corresponding conclusions because information used in these studies are limited to the surface and far from the event's nucleation depths. Based on the previous studies, this study collects and combines more information available of active tectonics, petroleum seismic survey and relocated earthquake distribution, as well as seismic intensities and focal mechanism solutions of the mainshocks, further constructs and analyzes two seismo-tectonic profiles across the individual source regions of the two events, and then re-determines seismogenic faults of the two events. The author concludes that the M_S 6.0 Wuyuan earthquake of 1979 occurred as a result of normal faulting along the main one of the Sertengshan piedmont fault zone, which trends in near west-east and dips southward, and the M_S 6.4 Baotou earthquake of 1996 was produced by oblique-slip normal faulting along an unnamed blind fault that hides beneath the Wulashan horst, strikes west-northwest and dips south-southwest. The new conclusion is able to march and explain, to the maximum extent, the relevant information and phenomena in the individual source regions, including surface and subsurface active tectonics, aftershock, seismic intensity distributions and focal mechanism solutions of the mainshocks, and coseismic macro ground damages (as cracks yielded in bedrocks of the Wulashan horst during the 1996 mainshock). The only one phenomenon that cannot be completely explained is that about 2/5 of the area of the meizoseismal zone(with intensity Ⅷ) of the 1996 Baotou earthquake lies north(the foot wall)of the seismogenic fault determined in this study. In addition, case study of the seismogenic structure of the Baotou earthquake suggests that secondary active normal faults or oblique-slip normal faults may exist beneath horsts within large-scale active grabens, and they would have ability to produce strong earthquakes.

Key words: Inner Mongolia, Hetao graben system, Wuyuan earthquake of 1979, Baotou earthquake of 1996, structure of source regions, seismogenic faults

摘要：

在内蒙古河套断陷带中—西段，1979年8月和1996年5月分别发生了五原6.0级地震和包头6.4级地震。针对这2次地震的发震构造，在前人研究工作的基础上，进一步集成活动构造、石油物探、重新定位的地震分布、烈度分布以及震源机制解等信息进行分析，构建通过这2次地震震源区的地震构造剖面，并重新确定了相应的发震断层。结果表明：1979年五原6.0级地震更可能是走向近EW、倾向S的色尔腾山山前主断层发生正断层作用的结果；而1996年包头6.4级地震更可能是乌拉山凸起之下的一条走向NWW、倾向SSW的无名隐伏断层发生斜滑正断层作用的结果。对这2次地震发震构造的新认识，能够最大程度符合并解释各自震源区的地表活动构造、余震分布、主震的烈度分布与震源机制解以及主震时的地面宏观破坏等现象。惟一不能完全解释的是包头地震的极震区（Ⅷ度区）面积约有 2/5 位于所判定的发震断层北侧（下盘）。另外，包头地震发震构造的例子显示出在大型活动断陷带内部的相对凸起区之下，也可能存在具有发生强震能力的活动性正断层或斜滑正断层。

关键词: 河套断陷带, 1979年五原地震, 1996年包头地震, 发震断层, 内蒙古

CLC Number:

P315.2

WEN Xue-ze. STRUCTURES OF SOURCE REGIONS OF THE 1979 M_S 6.0 WUYUAN EARTHQUAKE AND THE 1996 M_S 6.4 BAOTOU EARTHQUAKE IN INNER MONGOLIA, CHINA[J]. SEISMOLOGY AND GEOLOGY, 2014, 36(3): 586-597.

闻学泽. 内蒙古1979年五原6.0级和1996年包头6.4级地震的震源区构造[J]. 地震地质, 2014, 36(3): 586-597.

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STRUCTURES OF SOURCE REGIONS OF THE 1979 M_S 6.0 WUYUAN EARTHQUAKE AND THE 1996 M_S 6.4 BAOTOU EARTHQUAKE IN INNER MONGOLIA, CHINA

内蒙古1979年五原6.0级和1996年包头6.4级地震的震源区构造

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