SEISMIC IMAGE OF THE MOUNT-SPURR MAGMATIC SYSTEM

The three-dimensional P-wave velocity structure of Mount Spurr is dete rmined to depths of 10 km by tomographic inversion of 3,754 first-arri ving P-wave times from local earthquakes recorded by a permanent netwo rk of 11 seismographs. Results show a prominent low-velocity zone exte nding from the surface to 3-4 km below sea level beneath the southeast ern flank of Crater Peak, spatially coincident with a geothermal syste m. P-wave velocities in this low-velocity zone are approximately 20% s lower than those in the shallow crystalline basement rocks. Beneath Cr ater Peak an approximately 3-km-wide zone of relative low velocities c orrelates with a near-vertical band of seismicity, suggestive of a mag matic conduit. No large low-velocity zone indicative of a magma chambe r occurs within the upper 10 km of the crust. These observations are c onsistent with petrologic and geochemical studies suggesting that Crat er Peak magmas originate in the lower crust or upper mantle and have a short residence time in the shallow crust. Earthquakes relocated usin g the three-dimensional velocity structure correlate well with surface geology and other geophysical observations; thus, they provide additi onal constraints on the kinematics of the Mount Spurr magmatic system.