COSEISMIC ELASTIC MODELS OF FOLDS ABOVE BLIND THRUSTS IN THE BETIC CORDILLERAS (SPAIN) AND EVALUATION OF SEISMIC HAZARD

Although it is generally considered that near-surface earthquakes resu lt from movements along faults that cut through the surface, several r ecent large earthquakes have been partly attributed to blind thrusts. Movements along blind thrusts lead to the formation of surface folds, which are highly dependent upon fault geometry at depth, and often not considered in seismic hazard evaluation. Several authors have studied the relationship between surface folding and thrusting for geological situations in which fault geometries are quite simple. However, activ e fault geometries can be quite complex, e.g., segmented thrust faults associated with strike-slip faults. The aim of this contribution is t o reconstruct the fault kinematics at depth for a relatively complex g eological structure located in the Eastern Betic Cordilleras (Orihuela -Guardamar-Torrevieja region) using the patterns of kilometre-scale fo lds observed in the field. In order to model surface deformation, the assumption is made that surface km-scale folds have been created by co seismic deformation associated with movement along blind thrusts. By m eans of a coseismic deformation model, movements at depth have been ca lculated for three possible hypotheses. Hypothesis 1 assumes that each superficial fold is created by an independent fault. Hypotheses 2 and 3 assume that a sequence of two superficial folds can be created by m ovement along a single fault displaying a flat and ramp geometry. In H ypothesis 2, the flat is a superficial decollement level between the s edimentary cover and the Betic basement; in Hypothesis 3, it is a deep er decollement level within the Betic basement. Knowing the approximat e age of surface deformation, rough estimates of fault slip-rates and recurrence periods for two possible earthquake magnitudes (7 Ms and 6. 7 Ms) have been made, from calculated dislocations at depth. Slip-rate s and recurrence periods for flat and ramp fault geometries are in the range of 0.75-1 mm/yr and 1000-2000 yr, respectively. These values ar e close to those calculated by direct methods in similar seismotectoni c contexts.