Schweizer, Jürg

The Larderello geothermal field is located ca. 100 km southeast of Florence, in the northern part of a volcanic zone that extends along the Italian Tyrrhenian coast. Geophysical investigations in the area suggest a crustal thinning and an injection of hot material from the upper mantle into the crust.

The technical concept of a future global seismic monitoring system includes 50 to 60 core stations, mostly arrays, which provide the primary detection and location capability. Due to the average station distance, these core (“alpha”) stations form a teleseismic network. Many of the proposed stations are to be newly installed and before the network can be regarded as fully operational, the stations have to be calibrated. As for traditional seismic networks, the station residuals - compared to a standard earth model - have to be determined. The standard earth model is defined in terms of travel-time tables and amplitude-distance curves. After recording a representative set of events, station residuals with respect to travel-time and magnitude can be calculated. In case of arrays, the determination of mislocation vectors (azimuth and slowness residuaIs) are of ulmost importance if array slowness vectors are used as starting solutions in a location procedure. Finally, in a monitoring context it is very important to estimate the station sensitivity for varying background noise conditíons and - in case of arrays - to know the frequency dependent improvement by beamforming. This paper uses the newly installed high-frequency GERESS array in Germany to demonstrate the calibration procedure.

In order to evaluate the detection and localisation improvement of a small aperture array in the Northern Apennines, we installed an irregularly spaced test configuration in the vicinity of Città di Castello (CDC) for a period of two weeks. The experimental array consisted of nine 3-component stations with inter-station distances between 150 m and 2200 m. Seismic data were digitised at 125 Hz and telemetered to a mobile acquisition, processing and storage centre. The data could only be recorded in trigger mode. The peculiarity of the test array installation was the exclusive use of 3-component sensors at all array sites, which also allowed beamforming for S-phases on the horizontal components. Since the altitudes of the single array sites differed considerably among each other, for f-k analysis and beamforming an elevation correction was included. During the two weeks of operation about 20 local earthquakes with magnitudes ML<2.6, 1 regional, and several teleseismic events were recorded. In addition to these events, the array occasionally triggered on coherent noise-signals generated by local industrial activity. The data analysis was performed by means of f-k analysis and beamforming, providing wavenumber characteristics of the incident plane wave. Typical apparent velocities were determined to be 4.8 km/s and 6 km/s for Pg-phases and ~10 km/s for Pn-phases. We observed local seismic events, which occurred just beneath the array. In these cases wavefronts with unusual high apparent velocities, similar to those found for the Pn-phase, were observed. Since no continuously recorded array data were available, we extrapolated the lower detection magnitude threshold as a result of the SNR improvement due to array beamforming. Compared to the actual detection threshold of MT ~1.6 reached by the national seismic network in this area, a nine element array would improve this value up to MT ~ 0.8.

In order to evaluate detection and localization capabilities of a future array in the Upper Tiber Valley (Northern Apennines – Italy), an irregularly configured test array was installed near Città di Castello (CDC) for a period of two weeks, consisting of nine sites with inter-sensor distances between 150 and 2200 m. This test-array installation is particular in its use of three component sensors at all array sites, which allows the application of all array specific analyses techniques for the full seismic wavefield, i.e., also for horizontal-component data. In this study we investigate the inter-sensor coherence of the seismic noise field for the test-array area. In addition to the “classic” noise analysis, where noise cross-correlation values are calculated at single vertical instruments without relative time shifts between the traces, we extend the study by a “dynamic” approach, which accounts for possible slowness characteristics of the noise field. Furthermore, we investigate how the noise characteristics are dependent on the chosen component of the seismic sensors, by analyzing the noise coherence not only between vertical components but also on the radial and the transverse components. The coherence found for noise observed by the different sensors of the test array shows strong azimuthal variations on all components, which are most pronounced for noise within the frequency passband of 1.5 - 4.0 Hz and an apparent velocity of 1.5 km/s (Rg waves). The calculated correlation lengths of noise observed for the CDC array are about half of the values found for the NORES array in Southern Norway. Therefore, a future permanent array installation should be planned for minimum inter-sensor distances between 150 and 200 m.

Abstract Rapid estimates of source parameters are needed for reasons of civil protection in regions where destructive events often occur. This information can prevent further damage and casualties. A relation between the first seconds of a P- wave onset and the local magnitude ML of the earthquake has been developed for the Italy region following results obtained in Japan and Southern California. The proposed dominant period estimate has been used in the present work and it gives reliable results from which to evaluate the size of the earthquake. The data set we evaluated consists of about 20,000 earthquakes that occurred in Italy and were well recorded by the stations of the MedNet Network. The proposed relationship will be one basis for developing and implementing an earthquake early warning system in Italy capable of delivering a rapid alert only a few seconds after the occurrence of a potentially destructive earthquake in the area. Recent extensive improvements of the Italian National Seismic Network, together with this new technique, will make pos- sible the release of a robust magnitude estimate no later than 10 sec after the occur- rence of the earthquake. However, no data are available for earthquakes with mag- nitudes ML

The merits of a seismic array for signal detection and event location are beyond question. The superior signal detection capability is obtained by beamforming and estimating backazimuth and slowness of the seismic wave field by f-k analysis or plane-wave fit gives the parameters for signal classification and event location.