Present-day Deformation Related to Large Earthquakes in the Past 100 Years in the Tibetan Plateau

Abstract:

Large earthquakes are often accompanied with significant postseismic deformation that can last several decades. In the past century, a few large earthquakes occurred in the Tibetan Plateau (Fig. 1), e.g., the 1920 M7.8 Haiyuan earthquake, 1951 M7.7 Beng Co earthquake, 2001 M7.8 Kokoxili earthquake, 2008 M7.9 Wenchuan earthquake (https://earthquake.usgs.gov/). Sentinel-1 satellite-borne SAR data, with unprecedented temporal resolution, provide us an opportunity to investigate the present-day deformation associated with those large quakes, which will help us better understand the faulting behavior and rheological structure in the Tibetan Plateau, and thus the associated seismic hazards.

Fig.1 Historical large earthquakes (M≥6.8) in the Tibetan Plateau.

We use the GMTSAR software (https://topex.ucsd.edu/gmtsar/) to process the SAR data, following the conventional steps of InSAR time series analysis. The images are first aligned using precise satellite orbit. Hundreds of interferograms are then formed, with temporal baselines longer than one year. The topographic and atmospheric noises are reduced by using the SRTM DEM and GACOS model (http://ceg-research.ncl.ac.uk/v2/gacos/). The average crustal motion rates are derived by the coherence-based SBAS method. Velocity profiles perpendicular to the fault traces are then constructed to investigate the slip rates and locking extents of faults.

The derived InSAR rate maps show that most shallowly locked fast-slipping segments of active faults in the Tibetan Plateau are related to historical large earthquakes, e.g., the East Kunlun Fault (the 2001 M7.8 Kokoxili earthquake), the Margai-Caka Fault (the 1997 M7.5 Mani earthquake), the Beng Co Fault and northern Yadong-Gulu Rift (the 1951 M7.7 and 1952 M7.4 earthquakes), suggesting the presence of postseismic signals even seven decades after the occurrence of the mainshock.

On one hand, those signals can provide clues on the rheological attributes of the lower crust and/or upper mantle of the Tibetan Plateau. In addition, the surface expression of the postseismic relaxation may help us to settle down the controversy about the hazard level of some well-known seismic gaps where destructive earthquakes are expected to occur but have not been observed, e.g., at the Tianzhu seismic gap of the Haiyuan Fault Zone. A 35-km-long stretch of the Laohushan fault within this seismic gap is creeping in the uppermost crust (e.g., Jolivet et al., 2012), which may have connections to the 1920 Haiyuan earthquake. With Sentinel-1 interferometry, we observed two similar phenomena in northwest Tibetan Plateau: 1) a 30-km-long section of the South Xor Kol fault, which lies to the west of the 2014 M6.9 Yutian earthquake surface rupture identified by Li et al. (2016), but probably within the range of the ruptured fault plane at depth; 2) the eastern segment of the ENE-striking sinistral Gozha Co fault, adjacent to the 2008 M7.2 Yutian earthquake (NS-striking normal faulting) but lies outside the zones of surface rupture and dense aftershocks. The difference is that, the former may be the surface expression of postseismic relaxation at depth; while the latter may be triggered by neighboring seismic event.

Our observations show that postseismic shallow creeps on fault sections adjacent to historical earthquakes in the Tibetan Plateau are more common than previously thought. By taking into account results from geological and paleoseismic investigations (e.g., Chen et al., 2018), it is preferred that the shallow creep at the Laohushan fault is probably transient postseismic creep resulted from the 1920 Haiyuan earthquake, or the 1990 M_w5.8 Jingtai earthquake, not persistent creep in geological time scale.

Reference

Chen T., J. Liu-Zeng, Y. Shao, et al. (2018), Geomorphic offsets along the creeping Laohu Shan section of the Haiyuan fault, northern Tibetan Plateau, Geosphere, 14 (3), 1165-1186.

Li H., J. Pan, A Lin, et al. (2016), Coseismic Surface Ruptures Associated with the 2014 Mw 6.9 Yutian Earthquake on the Altyn Tagh Fault, Tibetan Plateau, Bulletin of the Seismological Society of America,106 (2), 595–608. doi:10.1785/0120150136.

Jolivet, R., C. Lasserre, M.-P. Doin, et al. (2012), Shallow creep on the Haiyuan Fault (Gansu, China) revealed by SAR Interferometry, J. Geophys. Res., 117, B06401, doi:10.1029/2011JB008732.