The SIL seismological data acquisition system —As operated in Iceland and in Sweden—

Abstract

The SIL (South Iceland Lowland) seismological data acquisition system is operated in Iceland with 43 digital short- and broad-band seismic stations and in Sweden with 38 digital broad-band seismic stations. The system is mainly designed for automatic evaluation of microearthquakes with minimum operational cost and has shown the capability of automatic evaluation of more then 1500 earthquakes per day or episodically several earthquakes per minute. The automatic, on-line, earthquake analysis performed by the SIL network can be divided into four categories: Singlestation analysis performed at the site stations includes a phase detector, which sends short messages with the characteristics of each detected phase to the centre, and a continuous ground motion monitor. Multi-station analysis performed at the centre using the phase reports from all detecting stations and producing information about all possible events including estimates of location, magnitude and fault plane solutions. Alert reporting is used in Iceland to notify the operators of the network in cases of a priori defined changes in parameters derived from the single- and multi-station analysis, with the aim of providing information for the authorities, media and public in case of large earthquakes or volcanic eruptions. Teleseismic data acquisition is performed based on electronic email messages from global seismological networks, no attempt is made to detect and locate teleseismic events locally. After the automatic analysis, further refinement is obtained through multi-event analysis, performed on groups of events within a limited hypocentral distance. This includes relative location of groups of similar events, seismicity pattern analysis using Spectral Amplitude Grouping (SAG) and stress tensor inversion of earthquake focal mechanisms.