Coseismic slip gradient at the western terminus of the 1920 Haiyuan Mw 7.9 earthquake

Abstract

Stepovers in strike-slip fault systems play important roles in controlling the propagation of earthquake ruptures, depending partially on the competition between the stepover width and the coseismic slip gradient approaching the step. The 1920 Mw 7.9 Haiyuan earthquake is the most recent major earthquake that has occurred along the Haiyuan fault. The earthquake rupture broke through multiple stepovers along the fault and finally ended at the 4 km-wide Jingtai pull-apart basin (releasing stepover) at the western end. To understand the process of this termination of the Haiyuan earthquake rupture, we conducted detailed mapping of the surface rupture geometry, coseismic slip measurement and slip gradient calculation in the vicinity of the endpoint based on the 0.2 m-resolution SfM-derived DEM along the 20 km section east of the Jingtai pull-apart basin. Combining coseismic slip measurements from this study and published slip data from fieldwork in the 1980s, we calculated slip gradients of 84–160 cm/km as the rupture approached the Jingtai releasing stepover. These values are high for the 4 km-wide Jingtai releasing stepover compared to those from a dataset of worldwide historical earthquakes compiled by Elliott et al. (2009). The high values imply that the rupture could have a relatively high likelihood of breaching through the Jingtai releasing stepover. Alternatively, the slip gradient may be overestimated. Detailed mapping and field investigation, however, show that the rupture may extend ~1.86 km farther west of the end location indicated in previous studies. We acquired the new slip gradient with lower value of 50–82 cm/km, which is still considerably high compared to dataset of Elliott et al. (2009). We speculate that the slip gradient could be larger variation along the stepover boundary to stop fault slip in different geologic setting. Another factor control slip termination is related to fault properties. Creep along the southern boundary fault of the Jingtai stepover, a velocity-strengthening region, may also have played a role in stopping the rupture at this location. Our observations further indicate that the increasing LOS velocity of InSAR data within the Jingtai stepover is probably related to fault behavior rather than a nontectonic signal of subsidence.