Spatial Variations of Local Magnitude in Earthquakes Recorded in Northeastern Spain

Since 1984, a seismic surveillance of the Baix Ebre Region (NE of the Iberian Peninsula) and the La Cerdanya Valley (Eastern Pyrenees) has been performed by the Institut d’Estudis Catalans and the Universitat de Barcelona, by means of four digital stations. An investigation of the apparent magnitude has been carried out to explain the observed anomalous differences between the amplitude of the recorded events and their associated magnitude, as given by the different agencies. Calculating the local magnitude with independent attenuation parameters and comparing the obtained results using the different stations, we obtain differences up to 1.0. These differences are interpreted in terms of very local effects and point out the uselessness of some stations to assign magnitudes of local events. Results emphasize that the effectiveness of the trigger algorithms in digital stations rarely depends on the apparent (local) magnitude. As expected, the magnitude sensitivity is related to the values of the station correction terms, meaning that extra information is required for an optimum surveillance of a local area. A 2-D analysis of the distribution of the differences between the computed and reported magnitude shows no significant azimuthal dependence. This indicates that the obtention of a local magnitude scale with an appropriate station correction term related to epicentral distance, results in an important factor to provide magnitude to local events. The obtained results corroborate previous works done at the same area and emphasis is placed on further analysis directed to obtain a better characterization of the local site influence.