Leiph Preston

Observation and modeling of source effects in coda wave interferometry at Pavlof volcano

Sorting out source and path effects for seismic waves at volcanoes is critical for the proper interpretation of underlying volcanic processes. Source or path effects imply that seismic waves interact strongly with the volcanic subsurface, either through partial resonance in a conduit (Garces et al., 2000; Sturton and Neuberg, 2006) or by random scattering in the heterogeneous volcanic edifice (Wegler and Luhr, 2001). As a result, both source and path effects can cause seismic waves to repeatedly sample parts of the volcano, leading to enhanced sensitivity to small changes in material properties at those locations. The challenge for volcano seismologists is to detect and reliably interpret these subtle changes for the purpose of monitoring eruptions.

Seismic Spatial-Temporal Character and Effects of a Deep (25–30 km) Magma Intrusion below North Lake Tahoe, California–Nevada

In 2003, a magma intrusion event occurred at 25-30 km in the lower crust under the northwestern corner of Lake Tahoe, as evidenced by both an earth- quake swarm and a geodetic displacement. This study examines the seismicity as- sociated with that event and subsequent seismicity in the upper crust. HYPODD relocations showed that the deep swarm of approximately 1600 microearthquakes at the intrusion site was concentrated on a planar area with a strike of N42°W, dipping at 39° to the northeast. The largest microearthquake in this swarm was M 2.2, and an anomalously high b-value of 2.0 is seen in the recurrence-versus-magnitude plot. The swarm progressed over this plane in a somewhat irregular pattern for a period of roughly 5 months. Focal mechanisms of the deep-swarm events are highly variable and do not reflect the known regional stress field. Two months after the deep-swarm activity started, a shallow swarm of approximately 1100 microearthquakes began at 10-12-km depths in the shallow crust almost immediately above the deep swarm and continued through 2005. This swarm had a maximum M of 2.4 and a relatively high b-value of 1.5. Based on HYPODD relocations, hypocenters in this swarm are con- centrated in a narrow pipelike volume, and event depths progressed steadily upward over the more than 2 yr of observation. Focal mechanisms in this shallow swarm are more consistent with the regional stress field than those of the deep swarm. Within one focal depth horizontally of the deep swarm, postintrusion seismic activity increased significantly compared to prior years. Stress triggering from the deep magma intru- sion, although based on sub-bar stress changes in the shallow crust, is a feasible ex- planation of the observed increase.

Seismic-acoustic finite-difference wave propagation algorithm

An efficient numerical algorithm for treating earth models composed of fluid and solid portions is obtained via straightforward modifications to a 3D time-domain finite-difference algorithm for simulating isotropic elastic wave propagation.