Continuing inflation at Three Sisters volcanic center,central Oregon Cascade Range,USA, from GPS,leveling, and InSAR observations |
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| Authors: | Daniel Dzurisin Michael Lisowski Charles W Wicks |
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| Institution: | (1) David A. Johnston Cascades Volcano Observatory, U.S. Geological Survey, 1300 S.E. Cardinal Court, Building 10, Suite 100, Vancouver, WA 98683-9589, USA;(2) U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, CA 94025, USA |
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| Abstract: | Uplift of a broad area centered ~6 km west of the summit of South Sister volcano started in September 1997 (onset estimated
from model discussed in this paper) and was continuing when surveyed in August 2006. Surface displacements were measured whenever
possible since August 1992 with satellite radar interferometry (InSAR), annually since August 2001 with GPS and leveling surveys,
and with continuous GPS since May 2001. The average maximum displacement rate from InSAR decreased from 3–5 cm/yr during 1998–2001
to ~1.4 cm/yr during 2004–2006. The other datasets show a similar pattern, i.e., surface uplift and extension rates decreased
over time but deformation continued through August 2006. Our best-fit model to the deformation data is a vertical, prolate,
spheroidal point-pressure source located 4.9–5.4 km below the surface. The source inflation rate decreased exponentially during
2001–2006 with a 1/e decay time of 5.3 ± 1.1 years. The net increase in source volume from September 1997 to August 2006 was 36.5–41.9 x 106 m3. A swarm of ~300 small (M
max = 1.9) earthquakes occurred beneath the deforming area in March 2004; no other unusual seismicity has been noted. Similar
deformation episodes in the past probably would have gone unnoticed if, as we suspect, most are small intrusions that do not
culminate in eruptions. |
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