Paleoseismic transect across the Alhama de Murcia Fault and implications of a fault-based seismic hazard assessment for the Eastern Betics

Abstract

The Eastern Betics Shear Zone (EBSZ) is one of the most active fault systems in SE Spain. It has caused damaging earthquakes in historical times, including the 2011 Lorca earthquake, which evidenced that the regional hazard estimations needed revision. In this context, the available fault data is usually too local and, for some faults, reliable and representative parameters are still lacking. Such heterogeneity has usually hampered seismic hazard models based on geological data. This thesis aims to be a comprehensive study allowing to: i) complete the paleoseismic record and parameters of one of the most active faults in the EBSZ, the Alhama de Murcia Fault (AMF), and ii) include fault data in a probabilistic seismic hazard assessment (PSHA). The study is thus divided in two main parts. (A) a local study presenting a structural characterization and integral paleoseismic study in four sites across a complete transect of the AMF. (B) a regional fault-based PSHA of the EBSZ with the main faults as inputs to define complex fault rupture scenarios using the SHERIFS code. A) The structural characterization has been carried out in the two central segments of the AMF, Lorca-Totana and Totana-Alhama. In the former, five fault branches are identified with partitioning of the deformation. From N to S: N1-AMF (lateral), N2a and b-AMF (dip slip), S-AMF (lateral) and F-AMF (dip slip). The Totana-Alhama segment shows more diffuse deformation, where the most prominent structure is the Amarguillo Fault (AF), a N-S transtensional ramification. The paleoseismic survey has been carried out in eight paleoseismic trenches across four fault branches of the Lorca-Totana segment. We obtained one of the completest paleoearthquake records in Iberia with seventeen events for the last ~100 ka in S-AMF. The recurrence varies from 5.7±1.7 to 3.1±1.4 kyr for the last 73-18 ka in S-AMF and F-AMF. The time compatibility of the last five events in these branches suggests that they could rupture synchronously. The total net slip rate of the segment since 18 ka is 1.55 +0.14/-0.18 mm/yr, considerably higher than previous estimations. The slip rate evolution shows fluctuations over time

(super-cycles), some of which coincide with patterns identified in nearby faults, inferring possible activity synchronicities. B) The fault-based PSHA suggests that multi-fault ruptures involving lengths of single to several whole faults of the EBSZ are feasible, contrary to the rupture of the whole system (~400 km). The hazard shows a clear control of the EBSZ faults, increasing the accelerations close to their traces with respect to area source PSHAs. The seismic hazard is dependent on the slip rates, as lower slip rate faults (Palomares fault; PF, or NE end of AMF) have negligible contribution. Our results for the 475-year return period are also more consistent with the accelerations reached in the 2011 Lorca event than the building code or national hazard map. For some faults, the lack of detailed paleoseismic studies can compromise the reliability of the hazard. This is a key discussion in the present study, marking the need for better constrained and reliable slip rates in the EBSZ. The integral paleoseismic study of the AMF has revealed to be a crucial step towards a more representative characterization of its paleoseismic parameters (slip rates, recurrence), and thus, of the EBSZ. Acquiring refined and reviewed paleoseismic data is key to improve seismic hazard evaluations. Hereby, further research should focus on poorly researched faults and on performing integrative studies in other EBSZ faults. The comprehensive approach followed, from paleoseismic data collection to PSHA, contributes to perform more critical interpretations of the seismic hazard, and aims to serve as a case example for other low-to-moderate seismicity regions.

La Zona de Cisalla de les Bètiques Orientals (ZCBO) és un dels sistemes de falles més actius del SE d’Espanya. Malgrat això, en aquesta zona les estimacions de perillositat sísmica rarament utilitzen dades de falles degut a que aquestes son sovint massa locals i poc acotades. En aquest context, la present tesi pretén ser un estudi exhaustiu que A) completi el registre paleosísmic i la representativitat dels paràmetres sísmics de la falla d’Alhama de Murcia (FAM), una de les més actives de la ZCBO, i B) porti a terme una avaluació probabilista de la perillositat sísmica (PSHA) de la ZCBO basada en dades geològiques de falles. A) La caracterització de la FAM ha permès obtenir dades paleosísmiques en quatre de cinc branques de falla que conformen un transsecte gairebé complet del segment central (Lorca- Totana). S’ha identificat un dels registres paleosísmics més complets de la Península Ibèrica amb disset paleoterratrèmols pels últims 100 ka. La recurrència d’aquests varia de 5.7±1.7 a 3.1±1.4 ka pels últims 73-18 ka i la seva compatibilitat temporal entre branques suggereix que les ruptures conjuntes son factibles. La velocitat de desplaçament neta total és de 1.55 +0.14/-0.18 mm/any pels últims 18 ka, malgrat aquesta ha presentat variacions cícliques almenys des del Pleistocè Superior també observades en d’altres falles de la ZCBO. B) El PSHA suggereix que les ruptures multi-falla que impliquen longituds d’una o varies falles completes son factibles a la ZCBO. En base a això, l’anàlisi portat a terme indica que les falles de la ZCBO dominen la perillositat a les àrees properes, incrementant considerablement els valors d’acceleració respecte a estudis previs que no consideren falles. Tot i això, les acceleracions son altament dependents de la velocitat de desplaçament de les falles, i en alguns casos, la manca d’estudis paleosísmics detallats por comprometre la fiabilitat dels models de perillositat. Cal doncs, focalitzar la futura recerca en aquestes falles menys estudiades.

L’enfocament integral que s’ha seguit aqui, des de la recopilació de dades paleosísmiques fins al PSHA, contribueix a realitzar interpretacions més crítiques de la perillositat sísmica i pretén servir d’exemple per d’altres regions de sismicitat baixa-moderada.