The 1912 eruption of Novarupta was the largest of the 1900s (Fierstein and Hildreth 2001, Hildreth et al. 2003). A century later, fundamental questions remain regarding the source of the magma for that eruption. A previous seismic study of the Katmai area (Jolly et al. 2007) identified a single large area of anomalous structure in the subsurface centered beneath Katmai Pass (Figure 2), but the magma source for the 1912 eruption is thought to have been beneath Mt. Katmai (Hildreth et al. 2003). This mystery was a prime motivation for the research project described here.
In summer 2008, scientists and staff from the Alaska Volcano Observatory (AVO) and the University of Wisconsin- Madison installed 11 temporary seismic recording instruments around the Katmai Pass area, complementing the existing AVO seismic network stations (Figure 3). The primary goal of the deployment was to record data from local earthquakes in order to yield an improved model of the three-dimensional structure of the upper crust beneath and surrounding Katmai Pass, using an analysis method known as double-difference seismic tomography (Zhang and Thurber 2003). The method yields a threedimensional image of the velocity of seismic waves in the subsurface, and also produces improved estimates of the locations of the earthquakes beneath the seismic stations.
Our main finding is that there is not a single large anomalous zone centered beneath Katmai Pass; rather there are several separate anomalous zones, one each beneath Katmai, Trident-Novarupta, and Martin-Mageik. Furthermore, the earthquakes are tightly clustered beneath the various volcanic centers, and are found to be systematically deeper than previously thought. Linear trends of earthquakes are also revealed, similar to features observed at other volcanoes, possibly outlining previously unidentified fault structures or indicating the path of migrating magma or magmatic fluids and gases.
