Investigation of the Foreshock Characteristics of the 2014 Ms 6.5 Ludian Earthquake

Abstract Foreshocks play a crucial role in providing insights into the earthquake nucleation process. Analyzing the foreshock characteristics of typical earthquake cases is essential for enhancing our understanding of the mechanisms behind earthquake nucleation. Although most case studies focus on earthquakes with significant foreshocks, studies of earthquakes with less obvious or initially undetected foreshocks can broaden our perspective on the earthquake nucleation process. The 2014 Ms 6.5 Ludian earthquake is a typical intraplate earthquake that was initially cataloged without obvious foreshocks. In this study, we first used the hypoDD method to accurately relocate earthquakes near the Ms 6.5 Ludian earthquake and then applied the match-and-locate method to detect and locate missing events from 65 days before to 29 days after the mainshock. The updated catalog included 4285 seismic events, nearly doubling the original catalog. The number of foreshocks increased from 19 to 155, with magnitudes ranging from ML−0.3 to 2.7. We found that the b value of the foreshock sequence is significantly higher than that of the aftershocks, which is likely due to differences in the fault segments associated with the foreshock and aftershock activity zones, as well as the incomplete release of stress along the fault during the mainshock. The spatiotemporal evolution of the foreshocks revealed a migration trend toward the mainshock, followed by a period of quiescence. Observations of migrating foreshocks and repeating earthquakes indicate that aseismic slip plays an important role in the initial nucleation process. Considering existing research on earthquake rupture processes and Global Positioning System observations, we suggest that cascading stress transfer also contributes to the nucleation process. Therefore, we propose that multiple mechanisms can work together, and the rate-dependent cascade-up model better explains the nucleation process of the Ludian earthquake.