Multi-Segment Ruptures of the 2023 Mw 6.0 Jishishan Earthquake, Tibetan Plateau: Implications for Seismogenic Mechanisms of Moderate Earthquakes

Abstract

Because moderate magnitude earthquakes generate significant local ground shaking, but are poorly preserved in the geologic record, they present a major challenge for characterizing seismic hazards along orogenic fronts. The Mw 6.0 Jishishan earthquake and most aftershocks were located to the east of the west-dipping main thrust fault of the Laji Shan, NE Tibetan Plateau, and the earthquake produced serious disaster and surface ruptures, which provides an opportunity to study the rupture patterns and seismogenic mechanisms of moderate earthquakes within the orogenic front. Here we found that it produced three oblique thrusting surface rupture zones with variable lateral components of slip through field investigations and photogrammetry, spanning a total length of ∼8.2 km. The maximum vertical and horizontal displacements of the earthquake were measured as ∼6 and ∼5 cm, respectively. The deep and shallow rupture geometry shows that the seismic faults dip to the east or northeast. Combined with the surveys of pre-existing structures and collected global earthquake cases, we believe that the earthquake is the first reported moderate event (M < 6.5) with multi-segment surface ruptures caused by antithetic thrusting at the front of an orogenic belt. Based upon the analysis of the rupture mechanisms of moderate magnitude earthquakes, we show that four seismogenic models can account for their occurrence along intracontinental orogenic belts, including the shallow thrust, backthrust, blind thrust, and antithetic thrust models. The small or secondary faults in orogenic fronts are worthy of attention in seismic hazard investigations and assessments.