VARIATION IN SEISMIC NOISE AT SEISMIC STATIONS WITH IMPLICATIONS FOR EVENT DETECTION

A network of 23 analog seismic stations was established in Pakistan to monitor seismic activity. The instrumentation of seismic stations includes short period seismometers with natural period at 1 Hz. The seismic noise conditions of five seismic stations are studied to determine the detection capability in the frequency range from 1.0 Hz to 10.0 Hz regarding monitoring of local and regional seismicity. The Power spectral density of noise level of the records was computed by taking day and night samples during winter and summer seasons. The seismic noise levels at these stations range from –120 db to –157 db between 1.0 to 10 Hz, which is well within the limits defined by the Peterson’s new low and high noise model. The observed noise variations recorded during day and night range from 5 db to 17 db, depending on the site of the station. The detect-ability estimates of the selected five seismic stations installed at Fort Munroe, Cherat, Thammi Wali, Dhulian and Sargodha assume a conservative signal-to-noise ratio of 3. These stations showed detection capability 3.1, 3.3, 3.4, 3.5 and 3.8 magnitudes at 1000 km distance respectively.