EARTHQUAKE HAZARD ASSESSMENT IN THE IGLESIA VALLEY BY THE ANALYSIS OF QUATERNARY FAULTS

ROTHIS, LUIS M.; PERUCCA, LAURA P.; ALCACER SANCHEZ, JUAN M.; HARO, FEDERICO M.

SAN JUAN

Workshop; FIRST INTERNATIONAL ANDES-GEO WORKSHOP; 2018

In this work, the capability of the Quaternary faults in the Eastern piedmont of Cordillera Frontal and Western piedmont of Precordillera for producing surface ruptures has been demonstrated by the paleoseismic data. They include comprise evidences such as coseismic colluvial wedges, drainage anomalies, natural exposures showing fault kinematics and scarp-related deposits (Colangüil. Pismanta, Angualasto, El Tigre sites). The time-frame covered by the geological information in these area provides evidence on the occurrence of larger earthquakes than those recorded in seismic and historical catalog. These potential seismogenic faults have average recurrence intervals presumably much longer than the short coverage provided by the seismic record. In addition, the seismic record and historical events alone are not suitable for evaluating the seismogenic capability of these intraplate faults and there is a weak background as for neotectonic/paleoseismic, seismotectonic and geological data.The earthquake of 27 October 1894 with epicentre in the northern portion of the San Juan province is considered as the largest historical event in Argentina, causing approximately 100 deaths and severe damage in San Juan, La Rioja, Cordoba and Mendoza provinces.Great terrain and landscape changes characterized this earthquake, even at large distances from the epicentre. Perucca (1995) described liquefaction phenomena with fissures, sand volcanoes and spring water with 2 meters high observed by Bodenbender (1894) in saturated soils without cohesion located more than 200km far away from the epicentre in the northern San Juan region. La Union newspaper (1894) reported phenomena that occurred in Iglesia department where gullies changed their natural paths and formed little lakes. Rockfalls in cliffs, fissures surging water and springs appearing and disappearing were also described. Many fissures, deep channels and floods were registered along the Copiapo route (Rıo Blanco area). Soil was fissured in Albardon and northern Angaco departments (near San Juan city); with emerging water with greycoffee colour or yellowish water with sulphur smell. According to the El Debate newspaper, more than 50 wells, 40?45 cm wide and 3?4m deep containing black water appeared in Angaco. Morey (1938) described several cracks and fissures generated in a 50 km-long path in the Matagusanos Valley, and fissures 1m wide in Angaco.Despite the limitations stated above, several basic fault parameters have been estimated with the current information obtained from natural exposures and based on criteria derived from empiric data of other regions. This aims to provide basic data for an approach to the seismic hazard evaluation of this region, very close (~45 km) to the projected Agua Negra tunnel. The information obtained in at the moment, need in most cases to be complemented and improved by further detailed studies.The morphology of alluvial systems in both piedmont in the Iglesia valley has been modified by the tectonic activity related to the Colangüil, Pismanta, Angualasto and El Tigre Fault Systems that resulted in anomalies in the drainage network and a set of Quaternary favor-counter slope scarps affecting different alluvial levels. Natural exposures, which allowed knowing the kinematics of the faults and the affected deposits, were identified in all of these fault systems. The paleoseismological analysis suggests the occurrence of at least two seismic events in the region, implying that the last event have less than ~21,000 years. Finally, the occurrence of a third event during the Holocene may be not ruled out, (Alcacer y Perucca 2017).Besides, probable paleomagnitude for the Maximum Credible Earthquake were estimated for the different seismogenic sources according to common empirical relationships derived worldwide from parameters such as rupture length and coseismic slip. Estimated magnitudes for paleoearthquakes range from M 6.5 to M 7.0 for Cordillera Frontal piedmont fault systems and from 6.5 to 7.5 in the El Tigre Fault System (Siame et al., 1997). These authors interpreted 1-mm-dextral strike-slip displacement along the El Tigre Fault) a proposed three seismic event during the Holocene, with an average fault recurrence interval of approximately 5000 years.