S U M M A R YRobust estimates of the magnetotelluric (MT) transfer function are found using an iterative reweighted method on time series data corrected for outliers and gaps. The MT transfer function, composed of several analytic functions smoothly varying in frequency, is used to represent the frequency-domain relationship between electric and magnetic time series. The smoothly varying transfer function facilitates identification and removal of electric and magnetic outliers (spikes), construction of the frequencyand time-domain weights used for obtaining robust smooth and band-averaged estimates, and separation of the time series into MT and correlated noise signals if a remote site exists that is free of the correlated noise. Errors in the transfer function are calculated using jackknife estimates of the solution covariance. The method is tested on: time series from a relatively clean MT site in central California; a test time series based on Tucson magnetic time series plus synthetic noise for a given transfer function; and time series from the Larderello geothermal region in central Italy where there are strong signals from d.c. electrified railways.

1 2In a geothermal area, a detailed knowledge of the three-dimensional velocity structures aids the 3 management of the field and the further development of the geothermal source. Here, we present a 4 high-resolution study of the three-dimensional S-wave velocity structures from microearthquake 5 travel times for the Larderello-Travale geothermal field, Italy. We have also deduced the Vp/Vs and 6 Vp*Vs parameters for this area to emphasize the deep variations in the physical rock properties due 7 to fluid content and porosity. Furthermore, effective porous medium modelling has been performed 8 for site-relevant lithologies, to improve our interpretation of the results in terms of rock physics 9signatures. This has allowed us to estimate the variation range of the seismological parameters 10 investigated, as well as their sensitivity for suitable rock under specific physical conditions. Low

No abstract

[1] With the aim of exploring the deep structure of the Larderello-Travale (LT) geothermal field, a high resolution 3-D tomographic inversion of microearthquake traveltimes has been performed. Results show that the deep part of the Larderello-Travale field is characterized by the presence of a structure having a velocity range of 6.0 -6.5 km/s and a convex shape deepening towards the northeastern and the southeastern sides of the field. Earthquakes are mostly concentrated on the top of the high velocity structure and below the 'K horizon' implying a transition of rheological properties at depth. The reported dependence on time of t s -t p observed at one station located above an earthquake cluster suggests that the variation in pore fluid pressure might be responsible for the transition of rheological properties along the contact. In such an area, changes in pore fluid pressure might be related to time-dependent hydraulic mechanisms that are very effective in crustal rocks at elevated temperatures.

We present active stress directions obtained from borehole breakout analysis performed on 15 geothermal wells located in the western coastal regions of Central Italy. The study area (a 200 km by 50 km NW‐elongated area bordering the Apennines) includes several Quaternary high‐K alkaline volcanoes active mainly after 0.6 Ma. We analyzed both paper logs and digital data to detect breakout directions, the two techniques yielding similar results. The breakout results show a predominant ENE direction of SHmin, with local deviations in one region (Sabatini volcano) where no seismicity is observed. The comparison of breakout data with stress directions inferred from inversion of microearthquake (M<4) focal mechanisms computed in three of the four volcanoes suggests that the whole area is presently undergoing NE to ENE extension.

No abstract

We computed the three‐dimensional P wave velocity structure of the upper crust beneath the adjacent Amiata and Vulsini Quaternary volcanoes (central Italy), inverting local earthquake arrival times. We used 676 selected earthquakes out of more than 3000 (in the M range 1.5–4.0) located between 1977 and 1992 by two contiguous microseismic networks operated by Ente Nazionale Energia Elettrica to monitor three geothermal areas (namely, Latera, Torre Alfina, and Mount Amiata). The resolving capability of the data set has been verified both with a priori synthetic tests and an a posteriori analysis of the resolution matrix. We find a 40×25 × 7 (h) km3 crustal volume where we can resolve details as small as 2 to 4 km. Our analysis is favored by numerous geological and geophysical data collected for geothermal exploration. These independent data give us specific constraints for both choosing a starting model and interpreting tomographic results. We find high‐velocity anomalies in the upper 3 km of the crust beneath Latera, Torre Alfina, and Mount Amiata geothermal fields, related to uplifted limestone units. The lateral extension of limestone units is defined by the three‐dimensional velocity pattern. A negative anomaly clearly identifies the Radicofani Graben, a Plio‐Quaternary depression filled with low velocity clayey sediments, between 1 and 3 km depth. A sharp gradient between high velocities (∼6.9 km/s) in the northern region and lower velocities in the southern one is recovered at 7 km depth. This feature may indicate the transition between the Tuscan province uplifted metamorphic crust and thickened carbonate series of the Latium province.

The area is characterized by numerous events with small magnitude; the duration magnitude, M d , of the events which occurred from 1977 to 1994 varies from − 0.5 to 3.5. The algorithm developed by SILENY et al. (1992) allows us to retrieve the full source moment tensor from waveform inversion. The factorization of the moment tensor rate functions into a source mechanism and a source time function, and the further decomposition into a double-couple, a compensated linear vector dipole and a volumetric component, allows us to determine if there are phenomena in the source that can be related to the circulation of fluids, a quite common event in geothermal areas.