Elsevier

Journal of Asian Earth Sciences

Volume 75, 5 October 2013, Pages 19-25
Journal of Asian Earth Sciences

Constraints on the seismogenic faults of the 2003–2004 Delingha earthquakes by InSAR and modeling

https://doi.org/10.1016/j.jseaes.2013.06.013Get rights and content

Highlights

  • Use the 3D finite element method to model the InSAR deformation interferograms.

  • Determine the seismogenic fault through comparing the model results with InSAR observations.

  • Analayze the shallow slip deficit of the co-seismic deformation of the 2003 Delingha earthquake.

Abstract

The 2003 Mw6.3 Delingha earthquake happened at the Dacaidan-Zongwulong fault system, which is an important active tectonic belt on the northeastern margin of Tibetan plateau. Until the end of 2004, 6 approximately Mw5.0 aftershocks occurred on almost the same seismogenic fault. The conventional viewpoint is that the seismic hazard near the continental seismogenic fault will be reduced in decades or centuries after rupture. Why did so many large earthquakes happen on almost the same seismogenic fault within a short temporal interval? The seismogenic fault information and the character of the co-seismic slip distribution are the keys to answer this problem. In this paper, the co-seismic slip distribution of the 2003 Mw6.3 Delingha earthquake was inverted using 18 teleseismic broadband P waveforms, 8 SH waveforms and 28 long period surface waveforms data. The peak slip concentrates at 12 km depth beneath the epicenter, and the slip decreases gradually from 6 km depth to surface and beneath 12 km depth. The shallow slip deficit of the co-seismic slip distribution may be caused by inelastic failures in the uppermost crust. Beneath 12 km depth, the strain energy may be accumulated by the velocity-strengthening frition behavior and may trigger aftershock. Three post-seismic InSAR interferograms spanning the most aftershocks were formed. Through analysis on the aftershock distribution and the focal mechanisms of Mw  4.9 aftershocks, two 3D finite element models containing topography were built and used to simulate the InSAR observations. Comparing the modeled results with the InSAR observations, we obtained the seismic source of the 2003–2004 Delingha earthqakes. It consists of two southwest-dipping seismogenic faults, named the west-fault and the east-fault. The east-fault extends beneath the west-fault. The 2003 Mw6.3 Delingha earthquake occurred on the west-fault, while 6 large aftershocks occurred on the east-fault. This shows that the complex spatial location relationship between the seismogenic faults may be an important factor to the large earthquake occurrence with high frequency at almost the same locality. Due to special tectonic setting and complex structure, the Dacaidan-Zongwulong fault system will be a high seismic risk zone in the future.

Introduction

On 17 April 2003, an Mw6.3 earthquake hit Delingha region, Qinghai province, China. Until the end of 2004, more than 180 Mw  2.0 aftershocks and 6 Mw  4.9 aftershocks occurred in this region (see Table 1 and Fig. 1). According to the focal mechanisms given by GCMT and Sun et al. (2012), these earthquakes occurred on the west part of the Dacaidan-Zongwulong fault system, which is an important active tectonic belt between the Qaidam basin and the Qilian Mountain on the northeastern margin of the Tibetan Plateau (see Fig. 2). Moreover, the epicenters of Mw  4.9 aftershocks are nearly located on the same seismogenic fault. Similarly, Elliott et al. (2011) studied the 2008 and the 2009 Mw6.3 Qaidam earthquakes and found that those earthquakes occurred on almost the same seismogenic fault in the different depths. Generally speaking, the seismic hazard of the seismogenic fault in the continental crust will be reduced in serval decades or centuries after the rupture releasing the strain energy. Why can so many earthquakes with similar magnitude happen on almost the same seismogenic fault within a short time interval?.

The seismogenic fault geometry and the character of the co-seismic slip distribution are the keys to answer the above problem. In addition, the seismogenic fault information is the base to study seismic source, seismic risk, regional geodynamics, and so on. Due to the sharp topography in the Delingha region, it is very difficult to study the seismogenic faults by the field geological investigation. According to the previous results, Jiang et al. (2006) inferred that the dip of the seismogenic faults is toward southwest (see Fig. 3 in Jiang’s paper). However, Sun et al. (2012) studied a part of aftershocks of the 2003 Mw6.3 Delingha earthquake and inferred that the seismogenic faults are dipping to northeast according to the aftershock distribution. Up to now, the seismogenic fault geometry of the 2003–2004 Delingha earthquakes is still unkown. In addition, Elliott et al. (2011) proposed that depth segmentation of the seismogenic continental crust allows a significant seismic hazard to remain after large earthquake. To look for the cause of the frequent occurrence of large earthquakes on almost the same seismogenic fault, this paper used the InSAR observations, the teleseismic waveform data and numerical simulation to constrain the seismogenic fault and analyzed the seismic risk of the Dacaidan-Zongwulong fault system.

Section snippets

Co-seismic slip inversion

Before the 2003 Mw6.3 Delingha earthquake, no SAR data covering the source region is available. Because the moment magnitude is up to 6.3, the main shock could be recorded by the global distributed seismic stations, whose epicenter distances are from 30° to 90°. The co-seismic slip distribution can be inverted using teleseismic broadband waveform data.

To invert the co-seismic slip of the 2003 Mw6.3 Delingha earthquake, we collected the focal mechanisms from GCMT and the GSN broadband waveforms

Post-seismic InSAR observations

After the main shock, the Advanced SAR sensor of the Envisat satellite acquired many SAR data sets covering the source region of the 2003 Mw6.3 Delingha earthquake. We collected 7 scenes SAR data sets from European Space Agency. In addition, we collected the meteorological data of the Delingha region. The meteorological conditions of the Delingha during 2003–2004 are very favorable to map the deforation by InSAR. In terms of less vegetation in this region, the deformation interferogram derived

The seismogenic fault

The seismogenic fault geometry is an important basic information to study the process of seismic source (Wang et al., 2012, Shan et al., 2011). According to the aftershock location and the focal mechanisms of the Mw  4.9 earthquakes from Sun et al. (2012) and GCMT, we inferred that there may exist two seismogenic faults, whose strikes are about 116° and 140° separately (see Table 1 and Fig. 5). In this paper, the seismogenic faults are named the west-fault and the east-fault. According to the

Discussion and conclusions

The seismogenic faults of the 2003–2004 Delingha earthquakes are a part of the Dacaidan-Zongwulong fault system, which is an important active tectonic belt between the Qaidam basin and the Qilian Mountain. In this study, the teleseismic waveform data of the 2003 Mw6.3 Delingha earthquake and three InSAR observations spanning all large aftershocks are analyzed. The co-seismic slip distribution from the seismic waveform inversion shows that the peak slip concentrates at 12 km in the depth

Acknowledgments

This work was supported by the Natural Science Foundation of China (40804006) and the Fundamental Research Funds for the Central Universities. All Envisat SAR data sets were provided by Eurimage under CAT-1 research category (ERC-176). We thank Dr. Zheng Jianqiu generously provided the meterorological data. We also thank Dr. Wong Huihui and Dr. Zhang Miao for their help. Thoughtful reviews by an anonymous reviewer, Professor Zhao Dapeng and the Editor greatly improved the manuscript.

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