Active high‐resolution seismic tomography of compressional wave velocity and attenuation structure at Medicine Lake Volcano, Northern California Cascade Range

Abstract

We determine compressional wave velocity and attenuation structures for the upper crust beneath Medicine Lake volcano in northeast California using a high‐resolution active source seismic tomography method. Medicine Lake volcano is a basalt through rhyolite shield volcano of the Cascade Range, lying east of the range axis. The Pg wave from eight explosive sources which has traveled upward through the target volume to a dense array of 140 seismographs provides 1‐ to 2‐km resolution in the upper 5 to 7 km of the crust beneath the volcano. The experiment tests the hypothesis that Cascade Range volcanoes of this type are underlain only by small silicic magma chambers. We image a low‐velocity low‐Q region not larger than a few tens of cubic kilometers in volume beneath the summit caldera, supporting the hypothesis. A shallower high‐velocity high‐density feature, previously known to be present, is imaged for the first time in full plan view; it is east‐west elongate, paralleling a topographic lineament between Medicine Lake volcano and Mount Shasta. This lineament is interpreted to be the result of an old crustal weakness now affecting the emplacement of magma, both on direct ascent from the lower crust and mantle and in migration from the shallow silicic chamber to summit vents. Differences between this high‐velocity feature and the equivalent feature at Newbeny volcano, a volcano in central Oregon resembling Medicine Lake volcano, may partly explain the scarcity of surface hydrothermal features at Medicine Lake volcano. A major low‐velocity low‐Q feature beneath the southeast flank of the volcano, in an area with no Holocene vents, is interpreted as tephra, flows, and sediments from the volcano deeply ponded on the downthrown side of the Gillem fault, a normal fault mapped at the surface north of the volcano. A high‐Q normal‐velocity feature beneath the north rim of the summit caldera may be a small, possibly hot, subsolidus intrusion. A high‐velocity low‐Q region beneath the eastern caldera may be an area of boiling water between the magma chamber and the ponded east flank material. These structural data are useful both for understanding Cascade Range volcanism and for geothermal development in progress on the volcano.