Highlights

High-resolution study of a microearthquake sequence in Southern Apennines (Italy)

European Seismological Commission, 32nd General Assembly, Sept. 6-10, 2010, Montpellier, France
T.A. Stabile, C. Satriano, A. Orefice, A. Zollo

Abstract

On May 25 2008, a microearthquake sequence started in a very limited region of the Irpinia fault system, in Southern Italy. The sequence lasted three days, with 14 events occurring within a volume smaller than 300 m per side. The moment magnitude Mw of the events ranges from 1.0 to 2.8, with the mainshock occurring at the middle of the sequence.
This sequence has been recorded by 21 stations of the Irpinia Seismic Network (ISNet) and 11 stations of the Italian National Institute of Geophysics and Volcanology (INGV). Such a dense instrumental deployment constitutes an excellent laboratory for studying the earthquake processes during the interseismic period. In particular, the observation of this peculiar sequences arises several questions about how are they related to larger-scale structures and how they contribute to the processes of fault loading and stress release.
We first located the events using a non-linear global approach (NonLinLoc) and after we refined locations considering a double-difference technique (HypoDD) and using both manual picks and cross-correlation data of P- and S-waves. The resulting cluster of 14 microearthquakes has horizontal and vertical errors less than 30 and 80 meters. Additional 5 events located around the cluster were rejected since they have errors of the same size of the cluster.
We studied the rupture process of the main event by first determining its focal mechanism, based on P-wave first motion polarities and S-wave polarizations, and, then, by performing a kinematic rupture modeling through the use of empirical Green’s functions, in order to retrieve the rupture length, orientation and velocity, and the average slip. Moreover we computed source parameters (seismic moment, corner frequency, source radius, …) for each event and mapped them in the volume surrounding the mainshock fault plane with the aim to investigate the space and time distribution and correlation of source properties for the events in the sequence.
The examination of this microearthquake sequence is of great interest since it reproduces at a reduced space and magnitude scale a classical foreshock-mainshock-aftershock series.