Gravity changes in the Aquila area before and after earthquake events

Abstract

In 1989 a joint work was developed in co-operation with the University"Sapienza"of Rome and Servizio Geologico d'Italia (SGI) (today ISPRA). A gravity research started in the Aquila area aimed to realize a gravity network for high precise gravity measurements to detect gravity changes due to mass redistribution in the underground inferred by the geodynamic activity. The gravity network has been designed so as to incorporate 17 stations, settled in 1989 and periodically reoccupied in 1990, 1992, 1995, 2005 and 2011. The local gravity network coincides with some benchmarkes of the local leveling networks (L126, L197, L124 and L9) [Giuliani & Mattone, 2010] in order to remove the effect of the vertical ground movements and taking into account the spatial distribution of seismic events. High precision gravity combined with levelling benchmarks offers increasing potentialities for the study of time variations of the earth gravity field and crustal deformations related with geodynamic or tectonic processes. Presently, the network consists of 17 relative stations which are not destroyed during the earthquake. Gravity readings were made using two LaCoste & Romberg D model gravimeters (D-60 and D-138) at the same time. Presently, and similar to previous studies the new gravity data set has been referred to the Absolute gravity station established in 1977 at the Osservatorio Astronomico di Monte Porzio Catone [Cannizzo et al., 1978]. The gravity readings, for both gravimeters, are corrected for tidal effects, atmospheric loading and instrument drift. Later, as regard the gravity survey, the corrected and weighted gravity differences between each pair of stations are organized on joined loops and then adjusted according to the least square method; finally the gravity value at any station is computed relative to the local reference station (Fonte Cerreto-Assergi (4) station) and linked to the absolute gravity station. During the time period covered by this study, six precise relative gravity surveys were also carried out at all stations of the network. Long-term spatial distribution of gravity changes performed before and repeated after earthquake is represented. Gravity changes have been observed on the whole area, before and after the earthquake period. The gravity distribution shows different patterns in the eastern, central and western part of the seismic area. The gravity variation field is smoother in the central part of L'Aquila city and it shows a gravity decrease from the stations located in the northen part of the city. Qualitative gravity results are not totally in agreement with vertical changes measured by local leveling networks. It is possible to suppose that the observed gravity changes should indicate a mass variation in the underground, not in the shallow part but in depth. Moreover, the results obtained from high precision gravity measurements show that, the most significant long-term observed gravity changes on Aquila area, were recorded not in the area corrisponding to the seismogenic belt but in a largest zone. However gravity changes were observed in the same magnitude before and after the earthquake.