The Bajo Segura Fault Zone: Active blind thrusting in the Eastern Betic Cordillera (SE Spain)

Abstract

The Bajo Segura Fault Zone, located at the NE end of the Eastern Betic Shear Zone, has been the site of some of the most intense seismic activity on the Iberian Peninsula in the historical and instrumental time periods. This structure is an active blind fault that does not show any surface rupture. It is characterised by a set of ENE-WSW trending blind thrust faults that offset the Triassic basement and cause active folding of the Upper Miocene-Quaternary sedimentary cover. The main active structures of this fault zone are two ENE-WSW striking reverse blind faults, the Torremendo and the Bajo Segura Faults, and several secondary NW-SE striking dextral faults (San Miguel de Salinas, Torrevieja and Guardamar Faults). These structures continue offshore to the east. From geological, geomorphological and geodetic data, we obtain fault slip rates between 0.2 and 0.4 mm/yr, whereas other authors have proposed higher values ranging between 0.75 and 1 mm/yr. The fault zone can generate earthquakes with maximum estimated magnitudes (Mw) from 6.6 to 7.1 and has approximate recurrence intervals between 4.500 and 21.500 years.

Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula – Mediterranean Sea)

“…Thus, the offshore part of the Bajo Segura fault has been active during the Upper Quaternary, with possible deformation events in the last 20 ka. The total length of the Bajo Segura fault onshore is 27 km, 10 km corresponding to the Hurchillo, 9 km to the Benejúzar and 8 km to the Guardamar segments (Alfaro et al, 2012). On the basis of the correlation between the different seismic profiles, the BSFZ can be traced for at least 27 km offshore.…”

“…In line with these observations and with the ages given to the different horizons, most of the structures were active until 135 ka and in some cases this activity postdates 20 ka. Considering the available slip rates of the main faults in the area, which vary between 0.12 and 0.40 mm yr −1 (Somoza, 1993;García-Mayordomo and Martíniez-Díaz, 2006;Giménez et al, 2009;Alfaro et al, 2012;García-Mayordomo et al, 2012), some authors have calculated the recurrence intervals of the large earthquakes, which range between 1700 and 30 000 yr (Alfaro et al, 2012;García-Mayordomo et al, 2012). Thus, reflectors above horizon H1 were probably deformed by very few earthquakes.…”

“…1), and the BSFZ (Montenat, 1977) to the south, both running approximately ENE-WSW. The BSFZ is characterized by several ENE-WSW blind thrusts that offset the Triassic basement of the Internal Zone and fold the Upper Miocene to Quaternary sedimentary cover (Montenat, 1977;Bousquet, 1979;López Casado et al, 1987;Somoza, 1993;Taboada et al, 1993;Alfaro, 1995;Alfaro et al, 2002aAlfaro et al, , 2012. The activity of these blind thrusts has produced an anticlinorium constituted by several north verging folds (Fig.…”

Terremotos de la Vega Baja del Segura

“…Se trata de una zona de falla ciega sin ruptura en superficie y su traza principal tiene varios segmentos casi para-lelos entre sí de dirección ENE-WSW (Alfaro et al 2012). Entre estos segmentos cabe destacar la falla del Bajo Segura y la falla de Torremendo, teniendo también asociadas varias fallas de dirección NW-SE (San Miguel de Salinas, Torrevieja y Guardamar).…”

“…Estas fallas tienen su continuidad hacia el Este en el mar Mediterráneo. Alfaro, et al (2012) han podido determinar tasas de desplazamiento que varían entre 0,2 y 0,4 mm/año. La actividad sísmica de la zona es moderada, si bien terremotos de carácter destructivo se han producido con altos intervalos de recurrencia.…”