Intracontinental active normal faulting and paleoseismicity in the eastern Weihe Graben, central China
Abstract
During the past decades, tectonic deformation and seismogenic behavior of active strike-slip and thrust faults have been well investigated, due to the high-frequent occurrence of large-magnitude strike-slip and thrust-type earthquakes. In contrast, normal-faulting earthquakes of M≥7 scarcely occurred, and the rupture process and deformation features of seismogenic normal-faults are still not clear. The intracontinental graben systems around the stable Ordos Block, central China, experienced extension over the past ~50 Ma, which are ideal places to study the extensional tectonic deformation. As well, these regions with high historical seismicity including 3 large earthquakes of M≥8, provide a good chance to learn the rupture mechanism of large intracontinental normal-faulting earthquakes. Based on the 3D analysis of high-resolution remote-sensing images (0.5-m WorldView and 1-m IKONOS images) and field investigations, active normal faults are mainly distributed along the margin zones of the uplifted mountainous blocks (e.g., Weinan Loess Tableland and Huashan Mountains), characterized by the distributed fault scarps. Striations and scratch steps observed on the main fault planes, reveal a normal slip-sense of active faults in study area. In combination with the 14C age dating, the vertical offset amount of ~30 m during the past 14,050-16,270 years was observed, yielding an average vertical displacement-rate of ~1.8-2.1 mm/a, which is consistent with previous estimation in the Weihe Graben. According to the field observations of fault outcrops and the exposed trench walls, the offset strata, scarp-derived colluvial deposits and in-filled fissures generally can be observed, indicating the occurrence of paleoearthquakes. Together with the 14C ages, the late Pleistocene-Holocene activity of normal faults was demonstrated. Especially, it is concluded that at least 3 strong earthquakes associated with surface-faulting in the past 2600 years, including the most recent 1556 M 8.5 Huaxian earthquake, with an average recurrence interval of ~900 years. This result is in contrast with previous estimated 2000-3000 years. Therefore, it is necessary to reevaluate the potential seismic hazard for the densely-populated region in the study area. The geophysical data reveal the crustal thinning in the graben relative to the neighboring regions, which suggest the variations in lithospheric structures probably control the extensional deformation since Eocene, resulting in thick graben basin sediments (>7000 m) and uplifted mountainous blocks. The results of our study may help to better understand the structural features of active normal faults, as well as the rupture mechanism of large intracontinental normal-faulting earthquakes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T53A2694R
- Keywords:
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- 7209 SEISMOLOGY / Earthquake dynamics;
- 8036 STRUCTURAL GEOLOGY / Paleoseismology;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting