Quaternary slip rates from multi-site paleoseismic analysis of a complex deformation zone in the Alhama de Murcia Fault (SE Spain): improvements and challenges

Paleoseismology is a fundamental method to characterize the activity of faults in low to moderate strain regions such as SE Spain. Among the different parameters to characterize such activity, the slip rate is one of the most crucial for fault-based probabilistic seismic hazard assessments (PSHA) as it controls the rates of earthquake occurrence and ultimately the hazard levels likely to be exceeded in a given time period.

The Alhama de Murcia Fault (AMF) is the most active structure within the Eastern Betics Shear Zone (EBSZ), a transpressive fault system that accommodates the largest part of the Africa-Eurasia convergence in SE Iberia. The AMF has caused some of the most important earthquakes in the EBSZ since historical times, including the damaging 2011 Mw 5.2 Lorca event. In this setting, paleoseismic studies in the EBSZ have paid special attention to this fault, and particularly to its central segment (Lorca-Totana) as this is one of the most geomorphologically prominent.  Despite this, the segment comprises a wide deformation zone where the fault splays into five subparallel slip-partitioned branches, four of these still unstudied to date. We present a comprehensive paleoseismic study that integrates paleoseismic data from four out of the five branches that compose the segment. Our aim is to improve the representativeness of the geological slip rates by accounting for a nearly complete transect of the fault zone: we excavated eight new trenches across the four branches including seven fault-perpendicular and one parallel trench to measure vertical and lateral displacements, respectively. Fault slip analysis combined with OSL and radiocarbon dating allowed the calculation of slip rates for each branch and for the whole transect, as well as their variability over time.

A total net slip rate of 1.60 +0.16/-0.11 mm/yr for the past 18-15 ka is obtained, which is almost twice the previous estimations from a single fault branch (0.9±0.1 mm/yr). This points out the relevance of accounting for all structures of a fault zone for a more reliable characterization. The slip rate variability analysis depicts cyclic patterns of short slip rate accelerations followed by longer quiescence periods, some of which are interestingly similar to those identified in the neighboring Carrascoy Fault in previous studies. This may, for the first time, suggest potentially synchronous activity among faults in Iberia. The present study is therefore an important step to improve the representativeness of the slip rate estimations in the AMF, and ultimately for subsequent PSHA studies in the area. Despite this, two main challenges still need to be assessed; first, the intermittent deposition of alluvium in the area makes it difficult to have correlative time periods between sites to integrate slip rates. Second, the lack of data in one of the five fault branches and the lack of detailed 3D trenching in most branches suggests that the obtained slip rate values could be a minimum. In this sense, integrating data from new paleoseismic sites and refining the existing data would likely allow to refine the current estimations and potentially fill the present knowledge gaps.