Installation and Preliminary Results From a Small Aperture Seismic Array in Tripoli, Greece

Abstract

A small aperture seismic array was installed by the University of Athens, in the area of Tripoli, Greece, on July 16 th 2003, in order to test the performance of seismic array processing in the area of Greece and assess its contribution to earthquake location, especially in offshore areas not azimuthally covered by the existing, conventional seismological networks.The array consists of four three-component seismological stations, one of them in the middle of a small, almost equilateral triangle, formed by the deployment of the other three stations. Despite the fact that array siting is a compromise of array installation criteria, equipment safety and logis tics, the test character of the experiment can be served successfully. The array transfer function depicts good azimuthal coverage nonetheless the existence of side-lobes and a rather wide main lobe is characteristic of spatial aliasing and low resolution in the two-dimensional wavenumber do main. The resolvable wavenumber passband of the array permits the determination of most of the common seismic body wave phases (P n , P g , S n , S g , etc.) for local and regional events in the area of Greece.Location of recorded events was performed using slowness and backazimuth data, calculated by f-k analysis of the seismic waveforms. Preliminary results have been compared to epicentres calculated by the Geodynamic Institute of the National Observatory of Athens. Although some dif ferences are observed, these are not significant and location results as well as overall array performance can be improved by array calibration and travel-time, azimuth and slowness correction calculations.

Mislocation vectors for the Tripoli seismic array, Greece, and structural effect implications from backazimuth and slowness residual analysis

“…Automatic algorithm results are reviewed by an analyst. TRIS AR restricted aperture and small number of sensors result in poor array resolution in terms of slowness and backazimuth estimation, consistent with the rather wide main-lobe of the transfer function for frequencies lower than 15 Hz (Pirli et al 2004, Pirli 2005. Slowness distribution is characterised by very large scatter, introducing significant uncertainties in seismic phase discrimination.…”

“…1). A broadband station, serving as reference element, is situated in the middle of this deployment (Pirli et al 2004). All array elements are equipped with 3-component instruments and are installed on a plane area without elevation differences.…”

Regional and teleseismic event detection capability of the small-aperture Tripoli seismic array, Greece

“…The array installation site was decided after taking into consideration the smooth topography and uniform geology in the area (Alexopoulos 1988), as well as the fact that the geographic location of the area of Tripoli provides the possibility to monitor a variety of high seismicity regions. There are several sources of background noise in the area, most of them due to human activities, such as the military unit where the array is installed, the nearby town of Tripoli and its surroundings, as well as the highway crossing central Péloponnèse, which is located at a minimum distance of approximately 1 km from the array (Pirli et al 2004, Pirli 2005. TRISAR very small aperture and restricted number of sensors limits its performance to a rather small epicentral distance range.…”

“…The Tripoli Seismic Array (TRISAR) is a small-aperture array, installed and operated by the Seismological Laboratory of the University of Athens, in the vicinity of the town of Tripoli, in central Péloponnèse, Greece (Pirli et al, 2004). The array consists of four 3-component elements, three of which are equipped with short period sensors (CMG-40T-lHz) and are situated at the peaks of an almost equilateral triangle with a maximum side length of approximately 250 m. The fourth element, situated inside this deployment, carries a broadband sensor (CMG-40T-60s) and is appointed as reference site of the array (Fig.…”