PALEOSEISMIC INDICATIONS ALONG ASEISMIC FAULT SEGMENTS IN THE GUADALENTIN DEPRESSION (SE SPAIN)

The Guadalentin Depression, located in SE Spain (Murcia Region), is bo unded by two of the main NE-SW master faults of the Eastern Betics Cor dilleras: 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 s eismicity. However, they show a wide range of diagnostic features of e arthquake surface displacements on late Pleistocene and Holocene alluv ial and colluvial surfaces. Aside from the left-lateral offsets record ed along 045-050 degrees master fault strands of the Lorca-Alhama Faul t, major paleoseismic surface displacements show different kinematics in relation to the broad orientation of the fault strands: (1) vertica l normal displacements along 010-020 degrees trending faults mainly pr eserved as degraded fault scarps of 2.5-1.8 m high (Aljibejo site); an d (2) vertical reverse displacements, with average offsets of 0.2-1 m, along 065-080 degrees subsidiary faults. In this last group, the youn ger one (Carraclaca Baths site) remains as a fault scarp of 0.8 m heig ht affecting a cascade tufa which was active until the Spanish Roman P eriod (2nd Century B.C. to 6th Century A.D.). In other cases, reverse offsets resulted in smaller displacements (0.26 m) of paleosols, but s how a, recurrent behaviour (La Escarihuela site). The strongest earthq uakes recorded in the study area did not exceed more than Mb 4.5 or MS K Intensity VIII (historical) with no evidence of coseismic rupture. T herefore, the preliminary data presented here seem to indicate that th e paleoseismic activity on both faults is capable of producing coseism ic 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. (C) 1997 Elsevier Sci ence Ltd.