High-precision earthquake location and three-dimensional P wave velocity determination at Redoubt Volcano, Alaska

Redoubt Volcano, Alaska poses significant volcanic hazard to the Cook Inlet region and overlying flight paths. During and following the most recent eruption in 1989 - 1990 the Alaska Volcano Observatory deployed up to 10 seismometers to improve real- time monitoring capabilities at Redoubt and continues to produce an annual earthquake catalog with associated arrival times for this volcano. We compute a three-dimensional P wave velocity model using double- difference tomography combined with waveform cross-correlation techniques to identify families of similar earthquakes and increase earthquake location precision at Redoubt. Absolute and differential times for well-constrained events are used to simultaneously invert for hypocenter location and P wave velocity structure. Shot and earthquake arrival times recorded by temporary stations deployed on Redoubt in July 1991 supplement the catalog picks. Double-difference tomography reduces uncertainties associated with absolute and relative earthquake locations and provides a clearer picture of three-dimensional velocity heterogeneity. All Redoubt data available through November 2005 are relocated through the three- dimensional model, and we investigate seismicity associated with the 1989 - 1990 eruption and compare cross-correlation coefficients to identify similar earthquakes within the catalog. Waveform similarity is high at Redoubt, especially during the eruptive phases, and event families include long-period shallow summit, hybrid, and both shallow summit and midcrust level volcano-tectonic earthquakes. We do not resolve a low-velocity feature in the three-dimensional P wave model that can be interpreted as a source magma chamber and prefer the idea of an interconnected body of dikes and sills as the source of magma erupted in 1989 - 1990.