CSRMT measurements in the frequency range of 1-250 kHz to map a normal fault in the Volvi basin, Greece

摘要

The Volvi basin is located 45 km northeast of the city of Thessaloniki in Greece. The basin is a 5-km wide graben of Neogene age. It has been chosen by European Union as a test site (EURO-SEISTEST) for studying Europe's most seismic active areas. This region has experienced several destructive earthquakes including the one occurring on 20 June 1978 (Ms = 6.5). From observations and analysis of fault-plane solutions it was concluded that the earthquake was due to displacement alongon a steeply dipping E-W trending fault at the surface. In order to gain a better understanding of the fault geometry,six CSRMT profiles were measured across the basin. The data were collected in two frequency ranges:a) 1-12.5 kHz using a remotely controlled double horizontal magnetic dipole transmitter (CSTMT measurements), and b) 15-250 kHz using the signal from distant radio transmitters (RMT measurements). For the CSTMT data acquisition five target frequencies were selected with one-octave resolution and for the RMT part, the signals from an average of twenty transmitters were used. The surface geology shows a predominantly 2D structure, and therefore we have used a 2D interpretation tool to model the data. 2D inversion of the measured tensor data shows a sharp change in the depth to the top of resistive gneiss-schist basement that is overlain by a less resistive overburden at both southern and north basin flanks. The change in depth to the bedrock is clearly seen in all 2D models along the measured profiles suggesting the existence of a normal fault with a strike of 67 degrees (SSW-NNE). An average of 2.5 RMS datafit with an error floor of 0.03 (on apparent resistivity) shows an acceptable data quality particularly in the CSTMT frequency range where use of a phase-lock technique provided a high S/N ratio.