Volcano Seismology

3D velocity modeling and earthquake relocations along the Alaska Volcanic Arc

Volcano seismic networks typically have few stations and marginal coverage, providing challenges for earthquake location in a complex, three-dimensional setting.  Routine catalog locations are performed using analyst phase picks and an approximate, 1D velocity model.  To improve earthquake location precision we compute a three-dimensional P-wave velocity model using double-difference tomography combined with waveform cross-correlation techniques. Waveforms recorded at volcanoes are often noisy and/or emergent. We use waveform cross-correlation techniques to improve the pick accuracy of catalog data.  Differential travel times for well-constrained events are used to simultaneously invert for hypocenter location and P-wave velocity structure. The double-difference tomography method provides significantly improved absolute and relative earthquake locations. Synthetic models are used to assess the accuracy of the resulting 3D models.  In association with the Alaska Volcano Observatory and Univ. Wisconsin Madison, we are simultaneously computing relative earthquake locations and 3D velocity models for some of the better monitored volcanoes along the Alaska Volcanic Arc, including Spurr (J. Brown), Redoubt and Augustine (H. DeShon), Shishaldin (N. Meyer), and Pavlof and Great Sitkin (J. Pesicek).  This work takes advantage of advances in earthquake location and inversion techniques (tomoDD) as well as waveform cross-correlation (BCSEIS).  Principle Investigators: Cliff Thurber, John Powers, and Stephanie Prejean. Arenal Volcano, Costa Rica

From May 1995 through February 1999, UCSC and OVSICORI-UNA collected time-continuous seismic and geodetic data at Arenal Volcano, Costa Rica.  Arenal, a young active stratovolcano located in northern Costa Rica, began erupting in 1968 after a 450 year period of quiescence and is currently in a Strombolian phase, producing small summit explosions, intermittent lava flows, and occasional pyroclastic flows.Seismic signals recorded at Arenal include long period (1-3 Hz) transients related to summit explosions and almost continuous, and frequently harmonic, volcanic tremor.  Daily statistics calculated for explosions and tremor provide a near continuous record of seismic activity at Arenal and show a decrease in summit activity from 1995-99, possibly reflecting a change within the volcano’s magmatic system.  My current work with the Arenal dataset includes exploring statistical relationships within the explosion and tremor datasets to better understand interactions between the physical mechanism(s) producing these seismic signals within the volcanic system. Frequency-amplitude analysis for summit explosions indicate explosion occurrence may follow power-law scaling, analogous to the Gutenberg-Richter relation for earthquake occurrence, suggesting a scale-invariant process produces summit explosions.  Previous work on Arenal’s volcanic tremor also suggests a scale-invariant, possibly chaotic, process produces tremor as well [Julian, 2000].  We are exploring temporal relationships between volcanic tremor amplitude and frequencies and summit explosion occurrence to lend further insight into the degassing processes active within the volcano during periods of high and low summit activity.