Broadband seismic noise characteristics of the Arabian Shield

A total of nine portable broadband stations were deployed across the Arabia n Shield from November 1995 to March 1997. The stations consisted of STS-2 seismometers recorded continuously at 40 samples per second on RefTek datal oggers. Noise studies showed that most stations were exceptionally quiet with noise levels near the USGS low noise model for frequencies higher than 0.1 Hz. A t lower frequencies, the horizontal components showed high noise levels, po ssibly due to instrumental characteristics. High frequency (>1 Hz) noise va ried as much as 10 dB between day and night for some stations (RAYN and TAI F) while for more isolated stations (HALM) was constant. Seasonal noise lev els also varied, with April to June being the quietest months. Slight chang es in peak microseism frequency also occurred seasonally. The quietest stations were HALM, RAYN, AFIF, and UQSK, ail of which were lo cated in central Saudi Arabia and show noise levels near the low noise mode l for frequencies between 0.1 and 4 Hz. The optimal site for a new quiet st ation would be near HALM which showed very little diurnal variations of cul tural noise. These stations appear to be among the best sites in the world for the prope rties of detection thresholds and ground noise levels. Events with mb >3.5 could be detected at distances from 10 to 100 degrees.