Pere Santanach

The Alhama de Murcia fault (SE Spain), a seismogenic fault in a diffuse plate boundary: Seismotectonic implications for the Ibero-Magrebian region

Received 20 December 2002; revised 15 September 2003; accepted 26 September 2003; published 2 January 2004. [1] The shortening between the African and the Iberian plates is absorbed by a number of faults distributed over a very wide zone with very low slip rates and long periods of seismic loading. Thus a seismotectonic map based only on faults associated with seismicity or with expressive geomorphic features is incomplete. It is possible to characterize seismogenic faults using paleoseismology. First, paleoseismological results based on trenching analysis in the eastern Betics (Lorca-Totana segment of the Alhama de Murcia fault) are presented. The main paleoseismic parameters of this fault segment are (1) a minimum of two to three Mw 6.5‐7 earthquakes in the last 27 kyr (shortly before 1650 A.D., between 830 and 2130 B.C. and shortly before 16.7 ka, respectively), with a mean recurrence period of 14 kyr, and a very short elapsed time, and (2) a net slip rate of 0.07‐0.6 mm/yr during the last 30 kyr. These results were extrapolated to the rest of the known active faults in the eastern Betics and were added to the slip rates of the active faults at the African margin. The total slip rate of the transect, which crosses de Alhama de Murcia fault in Spain and reaches the Cheliff basin (Algeria), would represent 21‐82% of the total shortening between Africa and Eurasia estimated from plate motion models and seismic moment summation. A number of factors could account for this discrepancy: (1) hidden seismogenic faults in the emerged areas, (2) absence of correlation between current and late Pleistocene slip rates, (3) extensive small faults that are undetected and that absorb a significant amount of the deformation, and (4) possible overestimation of the convergence rates. INDEX TERMS: 7221 Seismology: Paleoseismology; 7230 Seismology: Seismicity and seismotectonics; 7215 Seismology: Earthquake parameters; 8150 Tectonophysics: Plate boundary— general (3040); KEYWORDS: eastern Betics, Alhama de Murcia fault, paleoseismology, trenching, seismotectonics, plate boundary

Basin formation at the end of a strike-slip fault: the Cerdanya Basin (eastern Pyrenees)

The Cerdanya basin was formed during dextral slip along the NE–SW La Tet fault (eastern Pyrenees) which formed a horsetail structure at its SW end. The resulting basin is clearly asymmetrical, formed by several basement blocks tilted towards the La Tet fault and limited by E–W faults. These faults absorbed the movement along the major strike-slip fault. The early basin evolution (Middle–Late Miocene) resulted from strike-slip, while the later evolution (Early Pliocene) involved a generally extensional regime. A variety of depositional environments developed in the basin during its early evolution: spatially restricted and widespread alluvial fan, distributary fluvial, marginal and open lacustrine. The structural features of the basin that developed during this evolutionary stage differ from those described from most other basins related to strike-slip faulting. Despite this, the structural and sedimentary features of the Cerdanya basin fit the general model of strike-slip related basins.

Palaeoenvironments of the Late Miocene Prüedo Basin: implications for the uplift of the Central Pyrenees

The nature, structure and extent of a palaeo-basin sedimentary infill exposed in the Aran valley (Central Pyrenees) was studied by combining stratigraphical and biostratigraphical analyses and an audio-magnetotelluric survey. The basin developed on top of a pre-existing peneplain and was formed by the North Maladeta Fault activity. The fluvio-palustrine sequence filling the basin was at least 100 m thick. Specimens of the taxon Hippuris cf. parvicarpa Nikitin were identified for the first time in a European palaeoflora. The palynological and carpological analyses allowed us to (1) constrain the age of the basin infill as Vallesian (11.1–8.7 Ma), (2) characterize the vegetation of the belt surrounding the basin as a mainly temperate to warm-temperate assemblage, and (3) estimate the palaeoaltitude of the site at between 700 and 1000 m, which leads to an altitude change of 900–1200 m for the North Maladeta Fault downthrown block and 1640–1640 m for the upthrown block. These data allowed us to estimate the regional uplift of the area at between 0.08 and 0.19 mm a−1. The different exhumation values obtained by other researchers for sites located at both sides of the North Maladeta Fault are in agreement with its activity as a normal fault since the Late Miocene. Supplementary materials Details of the data acquisition, processing and modelling for the Porèra profile are available at www.geolsoc.org.uk/SUP18551.

Integration and influence of paleoseismic and geologic data for the seismic hazard evaluation in the Catalan coastal ranges, Spain

Abstract In this study, the influence of paleoseismic and geologic data in the seismic hazard estimation for the Catalan coastal ranges is analysed. We computed the probabilistic seismic hazard using area seismic sources with a Poissonian assumption for the earthquake occurrence. For the computations, a previously published attenuation relationship based on European strong motion data was applied. The resulting hazard estimates show similarities to the previous assessments in the region. These results were then used as a reference for comparison with other new models. In order to analyse the influence of the paleoseismic data three different models were tested. Since the number of faults that are investigated in detail are few, the same area sources that were used in the Poissonian assumption were kept in all three new models. In addition, the new paleoseismic data with faults expressed as line sources were used. In this case, a cyclic earthquake occurrence was assumed. The three models were based on the paleoseismic data with different assumptions on the time elapsed since last event. The time elapsed was set to 0, 10 and 85% of the recurrence interval in each model. The results are presented as maps showing the difference between the three models and the reference model with the Poissonian assumption. The results are given in horizontal peak ground acceleration contour maps for different return periods, also taking into account large return periods as high as 25,000 years. This is done to demonstrate the effect of large recurrence intervals found for some of the active faults. In general, we observe that for short return periods ( 5000 years) the effects of the paleoseismic data become increasingly significant. In order to estimate the true seismic hazard potential of this apparently low seismicity area, long-term behaviour of the possible active faults in the region needs to be investigated systematically.

From small-scale faults to plate kinematics: palaeostress determinations in a fragmented arc complex (SE Livingston Island, S Shetlands, Antarctica)

Analysis of the polyphase fault population of southeastern Livingston Island led us to establish three brittle deformation phases characterized by homoaxial stress tensors. One of the horizontal axes trends NW-SE, parallel to the transform faults governing the relative movement between the Phoenix and Antarctic plates. On the basis of the principles of symmetry these tensors are interpreted as corresponding to the regional stress field, and the transition between the phases is seen as reflecting changes in the relative values of the principal axes of their corresponding stress tensors. Phases 1 and 2 correspond to strike slip regimes, the first having NW-SE-oriented (σl (maximum principal compressive stress), whereas σ1 of phase 2 has a NE-SW trend. Phases 2 and 3 show a NW-SE-oriented σ3 (minimum principal compressive stress). The decreasing magnitude of the NW-SE stress axis during the recorded history is interpreted as being related to the decreasing velocities of the interacting plates caused by the cessation of the accretion at the Antarctic-Phoenix Ridge. The kinematic evolution of the analysed fault population can be understood assuming that faults form according to the Anderson model, that extensional dykes and veins form perpendicular to u3, and that fault slip on pre-existing fractures occurs parallel to the maximum shear stress direction on those planes.