High precision relocation of earthquakes at Iliamna Volcano, Alaska Patrick Statz-Boyer a , Clifford Thurber a, ⁎, Jeremy Pesicek a , Stephanie Prejean b a Department of Geology and Geophysics, University of Wisconsin–Madison, Madison, WI 53706, United States b USGS Alaska Science Center, Alaska Volcano Observatory, Anchorage, AK 99508, United States abstract article info Article history: Received 27 October 2008 Accepted 20 April 2009 Available online 7 May 2009 Keywords: volcano seismicity waveform correlation double-difference location In August 1996, a period of elevated seismicity commenced beneath Iliamna Volcano, Alaska. This activity lasted until early 1997, consisted of over 3000 earthquakes, and was accompanied by elevated emissions of volcanic gases. No eruption occurred and seismicity returned to background levels where it has remained since. We use waveform alignment with bispectrum-verified cross-correlation and double-difference methods to relocate over 2000 earthquakes from 1996 to 2005 with high precision (~100 m). The results of this analysis greatly clarify the distribution of seismic activity, revealing distinct features previously hidden by location scatter. A set of linear earthquake clusters diverges upward and southward from the main group of earthquakes. The events in these linear clusters show a clear southward migration with time. We suggest that these earthquakes represent either a response to degassing of the magma body, circulation of fluids due to exsolution from magma or heating of ground water, or possibly the intrusion of new dikes beneath Iliamna's southern flank. In addition, we speculate that the deeper, somewhat diffuse cluster of seismicity near and south of Iliamna's summit indicates the presence of an underlying magma body between about 2 and 4 km depth below sea level, based on similar features found previously at several other Alaskan volcanoes. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Iliamna Volcano is a glacially carved stratovolcano located adjacent to the Cook Inlet in south central Alaska, about 220 km from the city of Anchorage (Fig. 1). It has a cone-shaped edifice at the north end of a 5-km-long, north–south oriented ridge (Miller et al., 1998). No his- torical eruptions have been documented; however, the volcano has several fumaroles that regularly emit steam and gas (Waythomas and Miller, 1999). The volcano experiences background seismicity (typi- cally 1–3 events per day) consisting mainly of shallow (b 5 km) low- magnitude (b M L 2) earthquakes beneath the summit region (e.g., Dixon et al., 2005). A field study in 1999 reported evidence of at least two minor eruptions in the last 300 years (Waythomas et al., 2000). Iliamna is one of six volcanoes in the Cook Inlet region known to have had eruptions during the past 10,000 years. In 1996 and 1997 there were two main periods of elevated seismicity within a volume of crust beneath the edifice of Iliamna. Roman et al. (2004) hypothesized that this period of unrest, which did not lead to an eruption, was probably caused by an episode of magmatic intrusion. The 1996–1997 activity was recorded by a sparse network of six stations located within 15 km of the volcano's summit (Fig. 1), operated by the Alaska Volcano Observatory (AVO). Most of the seismicity occurred in two swarms. The first swarm occurred in the month of May 1996 and consisted of about 90 events. The second swarm, consisting of over 2800 events, began in early August 1996, peaked late that month, and tapered off in early 1997. The largest event in the swarm was M L 3.2, and the rate of detected earthquakes reached a peak of about 75 per day during the second swarm (Fig. 2). Most earthquakes were shallower than 6 km depth (below sea level, here and elsewhere), and virtually all of them were reported to be volcano- tectonic (VT) in nature with very few long-period (long period, LP) or hybrid (VT plus LP) earthquakes identified (Roman et al., 2004). Although increased emissions of CO 2 and SO 2 were measured during 1996, no other signs of unrest were observed (Roman et al., 2004). Roman et al. (2004) examined seismicity patterns and focal mechan- isms associated with the 1996 swarm and showed that the swarm coincided with a period of increased degassing of CO 2 and SO 2 . Based on these observations, they postulated that a new dike was emplaced beneath the southern flank of Iliamna at this time. However, the locations used in their study are quite scattered and do not provide a clear picture of the seismic activity beneath Iliamna. Here we use waveform alignment and hypocenter relocation using double-difference (DD) techniques to obtain much more precise locations. These methods have been success- fully applied to other Alaskan volcanoes, such as Mount Spurr, Mount Redoubt, and Great Sitkin (Brown et al., 2004; DeShon et al., 2007; Pesicek et al., 2008). At Iliamna, our precise relocations provide a clearer picture of the seismogenic processes beneath the volcano, and show a detailed image of the nature of the 1996–1997 earthquake swarms. 2. Methods In standard earthquake catalogs, seismic wave arrival-time picks have varying accuracy largely due to event-to-event variability in signal- Journal of Volcanology and Geothermal Research 184 (2009) 323–332 ⁎ Corresponding author. Tel.: +1 608 262 6027. E-mail address: thurber@geology.wisc.edu (C. Thurber). 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.04.016 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores