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

Dipartimento di Scienze Geologiche
Università di Catania
Corso Italia 55, 95129 Catania, Italy
gtusa{at}unict.it, gresta{at}unict.it
(G.T, S.G.)
Department of Earth and Space Sciences
Box 351310
University of Washington
Seattle, Washington 98195
steve{at}geophys.washington.edu
(S.D.M.)
Istituto Nazionale di Geofisica e Vulcanologia
Sezione di Catania
Piazza Roma 2, 95123 Catania, Italy
giampiccolo{at}ct.ingv.it, musumeci{at}ct.ingv.it
(E.G., C.M.)

Manuscript received 26 February 2003.

An increase of seismicity occurred at Mount St. Helens between May and July 1998, with more than 900 events (M_d <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 Q_p values are obtained for focal depths less than 5.5 km. Moreover, the spatial variations of Q_p 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 Q_p 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.

This article has been cited by other articles:

				E. Giampiccolo, S. D'Amico, D. Patane, and S. Gresta Attenuation and Source Parameters of Shallow Microearthquakes at Mt. Etna Volcano, Italy Bulletin of the Seismological Society of America, February 1, 2007; 97(1B): 184 - 197. [Abstract] [Full Text] [PDF]