Improved Geological Slip Rate Estimations in the Complex Alhama de Murcia Fault Zone (SE Iberia) and Its Implications for Fault Behavior

Abstract

Fault slip rate is one of the most crucial parameters to characterize earthquake occurrence in fault-based seismic hazard assessments (SHA). Accordingly, paleoseismic studies have increasingly focused on constraining this parameter in active faults worldwide. We present a comprehensive paleoseismic study in the Alhama de Murcia Fault (AMF), one of the most active faults in SE Spain and source of destructing earthquakes such as the 2011 Mw 5.2 Lorca event. Contrasting with previous studies, we integrate paleoseismic data from four fault strands in the AMF and, based on trench slip analysis and numerical dates, we derive slip rate estimates of each strand over the whole transect and assess their time variability. The AMF has a minimum net slip rate between 1.35+0.16/−0.10 and 1.64+0.16/−0.11 mm/yr for the past 18 ± 1 to 15.2 ± 1.1 ka. These results prove the importance of accounting for the complete sections of a geological structure as they are almost twice the previous estimates for a single fault branch. Slip rate variability is identified in the AMF, with cyclic acceleration-quiescence patterns that could be related to stress field changes driven by fault interaction or synchronicity with neighboring faults (e.g., Carrascoy). We hope that the data presented here motivates their inclusion into forthcoming fault-based SHAs. In this regard, limitations related to the lack of paleoseismic data for one fault strand, along with poor characterization of the strike component of slip and insufficient age control of the units for another strand are highlighted and need to be accounted for by modelers.