ALBERT

Description: On 8 June 2008 a strong (Mw 6.4) earthquake occurred in the area of northwest Peloponnese, western Greece. The event originated in the lower crust and was caused by the rupture of a previously unknown fault probably inherited from past tectonic phases. In this study we perform a stress inversion of all available focal mechanisms in this area in order to obtain an estimate of the regional stress field. It is shown that the maximum principal stress axis has an azimuth of N273{degrees}E and forms an angle of 63{degrees} with the fault's strike, which is well-constrained by seismological observations. This implies that the fault was severely misoriented with respect to the prevailing stress field assuming friction coefficients in the range 0.65-0.85. Calculation of pore-fluid factors for a variety of input parameters seems to confirm the presence of elevated fluid pressure near the hypocenter because they reach superhydrostatic to lithostatic values (0.80-1.0). The source of these fluids is probably of deep origin and may have to do with upper mantle degassing. This suggestion is supported by the presence of mantle helium in spring waters close to the epicenter and by low Pn velocities consistent with partially molten mantle beneath northwest Peloponnese.

Description: We investigate the characteristics of ambient noise across Greece as recorded at the Hellenic Unified Seismic Network (HUSN). Power spectral densities (PSDs) and their corresponding probability density functions (PDFs) have been estimated for 110 broadband seismic stations using the continuous waveform data for a four-year period from 2007 to 2010. Using PDFs we monitor and show network performance in terms of overall station quality and the level of noise at each site. At high frequencies (〉7 Hz), the main source of noise is cultural with strong diurnal variations. Stations with constantly increased noise levels across this band indicate poor vault construction or poor site selection and provide an indication to the network operators for possible structural improvements or even station relocations. The microseismic noise levels show a clear seasonal variation at all stations. Specifically, for the double-frequency (DF; period range 4–8 s) band, the average noise level differs between stations on the mainland and those located on the islands, reaching a value as high as 10 dB. Furthermore, the DF noise peak is observed at all of the HUSN stations, and it is correlated well with local sea wave height measurements at buoys deployed in the Aegean and the Ionian seas. This indicates that the HUSN seismic network also monitors local sea–weather conditions within a range of a few hundred kilometers. The longer period single-frequency (SF) band is affected by sea–weather conditions at much longer distances in the North Atlantic. Finally, we calculate the HUSN mode noise model (HMLNM) that represents the highest probability ambient noise level in Greece. This model is a realistic noise threshold for future seismic station installations. Online Material: Table of seismic sensor description, and tables, figures, and movie describing microseismic noise measurements.