Project Title: Interferometric Monitoring and Imaging of Recent Activity at Mount Spurr Volcano, Alask
Mendenhall Fellow: Matthew Haney, (907) 786-7460, firstname.lastname@example.org
Duty Station: Anchorage, AK
Start Date: March 19, 2007
Education: Ph.D. (Geophysics), Colorado School of Mines, Golden, CO, 2005
Research Advisors: Stephanie Prejean, (907) 786-7462, email@example.com; John Power, (907) 786-7426, firstname.lastname@example.org; Cliff Thurber, University of Wisconsin, Madison, (608) 262-6027, email@example.com
Project Description: Recent advances in the understanding of seismic wave propagation in noisy environments and complex geologic structures holds potential for the field of volcano seismology. These new techniques broadly fall under an emerging part of seismology known as interferometry, named so because of the common underlying process of cross-correlation. For instance, several studies have demonstrated that the cross-correlation of continuous seismic noise recordings at two stations yields portions of the impulse response, or Green’s function. The impulse response is the data which would have been recorded had a controlled seismic source been activated at one of the stations and the resulting seismic waves measured at the other station. The idea of cross-correlating seismic noise can be traced back to the spatial auto-correlation (SPAC) method first introduced by Keiiti Aki in 1957. In contrast to the SPAC method, contemporary studies have popularized the use of temporal cross-correlations between pairs of seismic stations.
We plan to conduct an initial study into the use of seismic noise for imaging at Mount Spurr volcano, located 100 km west of Anchorage, Alaska. In part because of its proximity to Anchorage, Mount Spurr is one of the most densely instrumented volcanoes in the network run by the Alaska Volcano Observatory, with three permanent broadband and thirteen short period stations. In addition, data from eight temporary broadband stations exist from a temporary deployment during three months in the summer of 2005. The complete data set, including permanent/temporary and broadband/short period stations, provides good station coverage and makes surface wave tomography using cross-correlated seismic noise recordings feasible at Mount Spurr. After using the Spurr network as a testing ground, we foresee applying the cross-correlation technique at other Alaska volcanoes to gain insight into how the method performs under varied noise conditions and in different geologic settings.
In addition, the research for this project includes an investigation of a related, but different, technique called Coda Wave Interferometry (CWI). The aim of CWI is to use late-arriving seismic waves, measured at different times, to detect small changes occurring during the intervening time period. These late-arriving seismic waves are referred to as coda waves by analogy to musical notation where the coda denotes the closing part of a musical piece. Figures 1 and 2 shows a comparison of the average volcanic explosion signals at Pavlof volcano (Alaska) during its eruption of late August and early September 2007. Both figures show the signals taken from one day (blue solid line) and four-and-a-half days later (red dashed line). At early times, as seen in figure 1, the signals are repeatable and overlap with each other. In contrast, figure 2 shows that at late times the signals do not overlap. Moreover, the later signal (red dashed line) is to a great degree simply a time-shifted version of the earlier signal. Once measured, this time-delay can be used to make inferences about small, time-lapse changes occurring within the interior of Pavlof. We plan to apply CWI to repeating volcanic earthquakes at Mount Spurr in the future.
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