Key words paleoseismicity – slow active faults – fan morphology – Alhama de Murcia Fault – Betic Cordillera 1. Introduction In this work, we present a study of the El Saltador alluvial fan, which is situated in the Southeastern Betic Cordillera (fig. 1a-c), and is associated with an active mountain front creat- ed by the Lorca-Totana section of the Alhama de Murcia Fault (fig. 2a,b). The distal area of this fan is affected by a strand of the fault that shows Quaternary activity. The Southeastern Betic Cordillera is locat- ed in the southeast of the Iberian Peninsula close to the area where the African and the Eurasian plates converge. The convergence direction is NNW-SSE with a relative velocity of 4 to 5 mm/yr (Argus et al., 1989). Since this low velocity is distributed in a broad area (Meghraoui et al., 1996), the velocity of the ac- tive faults in this area is lower than 0.2 mm/yr (Martínez-Díaz, 1998). The instrumental seis- micity is scattered over the region (Sanz de Galdeano et al., 1995) and has low magnitude (fig. 1a-c). However, several historical earth- quakes with magnitude greater than 6.5 occurred in the Central and Southeastern Betic Cordillera (Buforn et al., 1995). 775 ANNALS OF GEOPHYSICS, VOL. 46, N. 5, October 2003 Effects of repeated paleoearthquakes on the Alhama de Murcia Fault (Betic Cordillera, Spain) on the Quaternary evolution of an alluvial fan system José J. Martínez-Díaz ( 1 ), Eulalia Masana ( 2 ), José L. Hernández-Enrile ( 1 ) and Pere Santanach ( 2 ) ( 1 ) Departamento de Geodinámica, Universidad Complutense de Madrid, Spain ( 2 ) Departamento de Geologia Dinàmica i Geofísica, Universitat de Barcelona, Spain Abstract In this work we present a study of an alluvial fan system, which is affected by the Quaternary activity of the left- lateral, reverse Alhama de Murcia Fault (Betic Cordillera). Paleoseismic studies in this area yield data that can be compared and correlated with the morphologic and tectono-sedimentary evolution of the alluvial fan. The spatial arrangement of the sedimentary alluvial fan units near the fault zone, shown in trenches, is controlled by the recurrent reverse, left-lateral coseismic events. We analysed the morphology of the drainage network using a 1:5000 scale orthoimage to identify and measure horizontal deflections along the fault. The channel pattern analysis allowed us to estimate the average horizontal slip rate of the SAMF for the last 130 ka. This value is 0.21 mm/a, which is slightly higher than the range of values obtained by trenching analysis for the last 30 ka, (0.06 to 0.15 mm/yr). The interpretation of the stratigraphic sequence exposed along the trench walls constrai- ned the occurrence of at least two surface faulting earthquakes during the last 30000 years. The most recent event happened after the El Saltador Creek dissected the alluvial fan. The penultimate event occurred while the allu- vial fan was still active. Mailing address: Dr. José J. Martínez-Díaz, Departa- mento de Geodinámica, Universidad Complutense de Madrid, Ciudad Universitaria, 28040 Madrid, Spain; e-mail: jmdiaz@geo.ucm.es