An exceptionally long paleoseismic record of a slow-moving fault: The Alhama de Murcia fault (Eastern Betic shear zone, Spain)

Abstract

Most catastrophic earthquakes occur along fast-moving faults, although some of them are triggered by slow-moving ones. Long paleoseismic histories are infrequent in the latter faults. Here, an exceptionally long paleoseismic record (more than 300 k.y.) of a slow-moving structure is presented for the southern tip of the Alhama de Murcia fault (Eastern Betic shear zone), which is characterized by morphological expression of current tectonic activity and by a lack of historical seismicity. At its tip, the fault divides into a splay with two main faults bounding the Góñar fault system. At this area, the condensed sedimentation and the distribution of the deformation in several structures provided us with more opportunities to obtain a complete paleoseismic record than at other segments of the fault. The tectonic deformation of the system was studied by an integrated structural, geomorphological, and paleoseismological approach. Stratigraphic and tectonic features at six paleoseismic trenches indicate that old alluvial units have been repeatedly folded and thrusted over younger ones along the different traces of the structure. The correlation of the event timing inferred for each of these trenches and the application of an improved protocol for the infrared stimulated luminescence (IRSL) dating of K-feldspar allowed us to constrain a paleoseismic record as old as 325 ka. We identifi ed a minimum of six possible paleoearthquakes of M w = 6-7 and a maximum mean recurrence interval of 29 k.y. This provides compelling evidence for the underestimation of the seismic hazard in the region.

Combining surface exposure dating and burial dating from paired cosmogenic depth profiles. Example of El Límite alluvial fan in Huércal-Overa basin (SE Iberia)

“…Age results from El Límite 10 Be and 26 Al dataset (11-213 ka) are in good agreement with independent IRSL ages obtained in the same unit by Ortuño et al (2012) (Late Pleistocene). The CSEB model also suggest that sediments forming El Límite alluvial fan were exhumed from the source area in the Late Pleistocene, Middle Pleistocene or before (t b + t ∼ 11 − 220 ka; Fig.…”

“…Additionally, alluvial fans generated in the same phase are folded and thrusted by secondary faults of the Alhama de Murcia fault system (Ortuño et al, 2012). Thus, the dating of this alluvial phase is a clue in the determination of the fault slip-rates and the chronology of tectonic deformation.…”

“…Its western margin is particularly rectilinear. No faults have been observed along that margin, and the most probable cause of this geometry seems to be the draping of the fan onto the underlying alluvial fan surface, which is deformed by the activity of minor NE-SW faults oblique to the Alhama de Murcia fault system (Ortuño et al, 2012).…”

“…El Límite fan is geomorphologically correlative with neighbouring fans that yielded an age of 50-125 ka (Ortuño et al, 2012), based on an improved M A N U S C R I P T…”

“…As we cannot assume a continuous deep shielding of the sediments between exhumation and deposition in El Límite, we must consider t+t b as a minimum exhumation age of El Límite sediments. Therefore, according to Ortuño et al (2012) data and the CSEB model results, this alluvial fan is the result of the reworking of older alluvial deposits, revealing a complex transportation history from the source area (Pliocene conglomerate) to El Límite alluvial fan. Fig 1 shows that older and deformed alluvial sediments (Middle Pleistocene; Ortuño et al, 2012) are located at both sides of the South Alhama de Murcia fault, surrounding the apex of El Límite alluvial fan.…”