Attenuation of Short-Period P Waves at Mount St. Helens

An increase of seismicity occurred at Mount St. Helens between May and July 1998, with more than 900 events (Md <2.2) recorded by the Pacific Northwest Seismograph Network. This article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before (January 1995-April 1998), during, and after (August-December 1998) the period of higher seismicity and recorded on three different one-component seismic stations. Epicentral distances of the studied events ranged between 0.5 and 15 km. We performed the analysis distinguishing the deep (depth > 5.5 km) from shallow (depth < 5.5 km) events. We used the frequency decay method to estimate the quality factor Q and station site correction S for P waves in the frequency bands 2-7 and 18-30 Hz. The results show that the attenuation varies from site to site systematically and decreases with depth. Lower Qp values are obtained for focal depths less than 5.5 km. Moreover, the spatial variations of Qp show minimum values (~30) in the crater area that are interpreted as due to a low-density mass distribution under the crater. We find that Qp was 30% higher before the period of high seismicity than after. This change may be attributed to an increase of pressure in the magma chamber producing new cracks and/or reopening of pre-existing cracks, which are the most viable mechanisms for increasing attenuation.