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.