Double Difference Earthquake Relocation and Tomography at Mount Spurr Volcano, Alaska, 1991 to 2004

Mount Spurr, one of the northernmost active volcanoes monitored by the Alaska Volcano Observatory, is located approximately 120 km west of Anchorage in the Cook Inlet region. Mount Spurr erupted three times in 1992. In the subsequent decade the volcano was relatively quiet. However, seismicity rates began increasing again in early 2004, indicating that the volcano is entering a new stage of unrest. In order to gain a better understanding of the temporal and spatial distribution of seismicity at Mount Spurr, we relocated earthquakes from 1991 through 2004 using the double difference relocation code hypoDD (Waldhauser and Ellsworth, 2000). We also solved for the P-wave velocity structure of Spurr with the same dataset using the adaptive grid double difference tomography algorithm, tomoADD (Zhang and Thurber, 2004), in order to image the internal structure of the volcano and evaluate the applicability of this technique in a volcanic setting. Even though the seismic array at Mount Spurr is relatively sparse (10 short period stations) earthquake relocations result in significantly more precise hypocenters. The hypocenters from the 1992 eruptive sequence define a conduit-like structure extending from the Crater Peak vent (~2 km above sea level) to over 20 km depth, dipping steeply to the southeast. Earthquakes under the summit of Spurr (~3 km above sea level), whose vent has been quiet historically, are more scattered epicentrally and generally restricted to depths of < 5 km below sea level. Using tomoADD we imaged a low velocity body beneath the Crater Peak vent from depths of roughly 1 km above sea level to up to 10 km below sea level. This low velocity body is co-located with the steeply dipping 1992 seismicity lineation defined by relocations and likely reflects the magmatic conduit and/or the presence of a hydrothermal system southeast of Crater Peak. The spatial distribution of low velocity bodies determined using tomoADD is consistent with that determined previously by more traditional finite difference tomography techniques (Power et al., 1994). Both hypoDD and tomoADD appear to be useful methods even in relatively sparsely instrumented volcanic settings.