Publication Date:
2017-04-04
Description:
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.
Description:
Published
Description:
1441–1455
Description:
3.6. Fisica del vulcanismo
Description:
JCR Journal
Description:
reserved
Keywords:
seismicity
;
attenuation
;
04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
