USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Crustal Subsidence, Seismicity, and Structure
Near Medicine Lake Volcano, California
--
D. Dzurisin, J.M. Donnelly-Nolan, J.R. Evans, and S.R. Walter, 1991,
Crustal Subsidence, Seismicity, and Structure Near Medicine Lake Volcano,
California:
IN: Journal of Geophysical Research, v.96, no.B10.,
p.16,319-16,333, September 10, 1991
Abstract
The pattern of historical ground deformation, seismicity, and crustal structure
near Medicine Lake volcano illustrates a close relation between magmatism and
tectonism near the margin of the Cascade volcanic chain and the Basin and Range
tectonic province. Between leveling surveys in 1954 and 1989 the summit of
Medicine lake volcano subsided 389 +/- 43 mm with respect to a reference bench
mark 40 km to the southwest (average rate = 11.1 +/- mm/yr). A smaller survey
across the summit caldera in 1988 suggests that the subsidence rate was 15-28
mm/yr during 1988-1989. Swarms of shallow earthquakes
(M </= 4.6) occurred in the region during August 1978,
January-February 1981, and September 1988. Except for the 1988 swarm, which
occurred beneath Medicine Lake caldera, most historical earthquakes were located
at least 25 km from the summit. The spatial relation between subsidence and
seismicity indicates (1) radially symmetric downwarping of the volcano's summit
and flans centered near the caldera and (2) downfaulting of the entire edifice
along regional faults located 25-30 km from the summit. We propose that
contemporary subsidence, seismicity, and faulting are caused by (1) loading of
the crust by more than 600 cubic kilometers of erupted products plus a large
volume of mafic intrusives; (2) east-west extension in the western Basin and
Range province; and, to a lesser extent, (3) crystallization or withdrawal of
magma beneath the volcano. Thermal weakening of the subvolcanic crust by mafic
intrusions facilitates subsidence and influences the distribution of
earthquakes. Subsidence occurs mainly by aseismic creep within 25 km of the
summit, where the crust has been heated and weakened by intrusions, and by
normal faulting during episodic earthquake swarms in surrounding, cooler terrain.
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04/06/00, Lyn Topinka