Analysis of the September 2004 seismic crisis in the area of the 1428 Earthquake (I0=IX), Eastern Pyrenees (Spain)

Abstract

In the Eastern Pyrenees, at the same area of the 1428 destructive earthquake (I0=IX, EMS-98), an Ml=4.0 earthquake took place on September 21st 2004. It was widely felt in the region and produced slight damage at the epicentral zone. This area is characterized by a continued moderate seismicity, with a few events of magnitude close to 4 reported in the last decades. For this most recent event the hypocentral localisation from the permanent seismological regional network of the Institut Cartogràfic de Catalunya (ICC) is 42.34o N, 2.17o E and 4 km depth. In the following two days the permanent network registered about 120 aftershocks, three of which were felt again in the epicentral zone. A temporary network of 6 portable three-component stations were installed during 35 days in the epicentral area, the network operated in continuous mode and allowed detection and location of about 800 aftershocks with magnitudes lower than Ml=2.0. The earthquakes have been located first with the Hypo71 code. A revaluation of the hypocentral parameters with a probabilistic, non-linear global search earthquake location in 3D structures method have been conducted and the selected earthquakes have been relocated using the double difference location method HYPODD code considering ordinary absolute travel time measurements. The results obtained are discussed in this paper, their gives a better knowledge of the seismotectonic features for an area of major historical seismic activity.