The Mediterranean broad band seismographic network

Abstract

The Mediterrean Network (MedNet) presently comprises 22 operating broadband seismic stations installed and maintained in cooperation with 13 geophysical institutions in Italy and in most of the countries adjacent to Mediterranean Sea. The number of stations may vary as stations are opened or sometimes closed due to different reasons like political, technical, etc., but usually temporarily. All the stations are equipped with Quanterra digitizers and Streckeisen sensors, mostly STS2 with a few STS1. Aim of the network is to contribute to monitoring of one of the most active seismic regions of the World in terms of providing high quality real-time broadband data to the seismological community. Operations started with off-line field data collection and dial-up capabilities were later added at selected sites. At present these have been replaced with more efficient TCP connections that provide for real-time data collection over the whole network. This important technological upgrade allows a prompt contribution to the seismic monitoring of Italy and of most countries bordering the Mediterranean Sea, since data are exchanged in real-time with other seismological observatories. SeedLink protocol has been adopted for transmission. As for data archiving and distribution, a fast system for retrieving data has been developed. Continuous data streams, collected both from field data tapes and from real time transfer, are stored at the MedNet Data Center and are directly available at users’ request by the standard AutoDRM and NetDC protocols (in GSE and SEED formats respectively). Station metadata and continuous waveforms are archived in a MySQL database on RAID systems and backed up on DLT tapes. Presently, fully automatic network functions include: daily monitoring of state of health; triggered retrieval of event waveforms (with magnitude- and region- specific selection criteria), local and surface wave magnitude determination, and update of web pages (http://mednet.ingv.it) for events and station information. Rapid semiautomatic moment tensor solutions are calculated by means of a modified Harvard technique, which lowers the Mw threshold down to 4.5 for regional events in those areas with proper station coverage. For smaller earthquakes in Italy a new approach to moment tensor estimation, based on higher signal frequencies, is now being developed. Preliminary tests on earthquake recordings (not only MedNet stations) from the 2002 Molise, South Italy sequence have proved very successful.