Paleoseismic indications along 'aseismic' fault segments in the Guadalentin Depression (SE Spain)

Abstract

The Guadalentín Depression, located in SE Spain (Murcia Region), is bounded by two of the main NE-SW master faults of the Eastern Betics Cordilleras: The Lorca-Alhama and the Palomares left-lateral strike-slip faults. Available earthquake data indicate that, in the last 600 years, some sectors of the Lorca-Alhama Fault and the entire sector of the Palomares Fault have not been associated with significant historical seismicity. However, they show a wide range of diagnostic features of earthquake surface displacements on late Pleistocene and Holocene alluvial and colluvial surfaces. Aside from the left-lateral offsets recorded along 045-050° master fault strands of the Lorca-Alhama Fault, major paleoseismic surface displacements show different kinematics in relation to the broad orientation of the fault strands: (1) vertical normal displacements along 010-020° trending faults mainly preserved as degraded fault scarps of 2.5-1.8 m high (Aljibejo site); and (2) vertical reverse displacements, with average offsets of 0.2-1 m, along 065-080° subsidiary faults. In this last group, the younger one (Carraclaca Baths site) remains as a fault scarp of 0.8 m height affecting a cascade tufa which was active until the Spanish Roman Period (2nd Century B.C. to 6th Century A.D.). In other cases, reverse offsets resulted in smaller displacements (0.26 m) of paleosols, but show a recurrent behaviour (La Escarihuela site). The strongest earthquakes recorded in the study area did not exceed more than Mb 4.5 or MSK Intensity VIII (historical) with no evidence of coseismic rupture. Therefore, the preliminary data presented here seem to indicate that the paleoseismic activity on both faults is capable of producing coseismic surface displacements, probably reaching magnitudes of at least 6.5. These data show that paleoseismic studies based on geomorphological analyses are a useful tool in the assessment of the relative degree of activity of apparently 'aseismic' fault traces.