Rockfalls at Augustine Volcano, Alaska: The influence of eruption precursors and seasonal factors on occurrence patterns 1997–2009 Nicole DeRoin ⁎, Stephen R. McNutt Geophysical Institute, University of Alaska Fairbanks, PO Box 757320, 903 Koyukuk Drive, Fairbanks, AK 99775, United States abstract article info Article history: Received 10 May 2011 Accepted 7 November 2011 Available online 13 November 2011 Keywords: Eruption precursor Rockfall Rockfall trigger Steaming Freeze-thaw weathering Rockfalls have been recorded in seismic data at Augustine Volcano from 1997 to the present. Typical events last about 30 s and have frequencies >4 Hz on stations within 5 km of the summit. Many rockfalls are well recorded on summit seismic stations, suggesting that they originate from the steep summit dome. Typical background years such as 2003 or 2004 had several dozen events in the summer and fall (June to November) that were strong enough to trigger an automatic event detection system. For example, 17 rockfalls were recorded in 2003; mostly in late summer when air temperatures were warm and rainfall rates were highest, and 28 events were recorded in 2004, also in late summer. In 2005, about eight months before the onset of the eruption of Augustine in January 2006, there was a significant increase in the number of rockfalls detected. This increase of surface rockfall activity occurred at nearly the same time as precursory earthquake activity increased beneath Augustine. Overall there were more than 340 rockfalls in 2005, consisting of both short (less than 30 s) and long (greater than 30 s) duration events. The high rate of rockfalls in 2005 constitutes a new class of precursory signal that needs to be incorporated into long-term monitoring strategies at Augustine and elsewhere. During the eruption, numerous rockfalls contin- ued to occur, and block-and-ash flows dominated the seismic records when the volcano began a phase of dome growth and collapse. The high rates of rockfalls continued after the eruption ended, due the new unstable lava dome and adjacent tephra at the summit. As of 2009 the rockfall rates are still high, but are declining steadily. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Rockfalls and landslides are a common occurrence at many volcanoes around the world. The events may be small (single falling blocks), or very large avalanches. A scheme for classifying mass wasting events in general proposed by Sharpe (1938) includes slow-flowage and rapid flowage types, landslides and subsidence. Rockfalls, rockslides, debris falls and debris slides fall into the landslide category. Landslide movements are classified into five types by Varnes (1978): falls, topples, slides, spreads and flows. Within these types are further subdivisions, i.e., falling mo- tions can include free-falling, bouncing, and rolling motions, and sliding movements can be mean rotational or translational sliding (Cruden and Varnes, 1996). Causes of landslides can be geologic (weak, sensitive, or weathered slope materials), morphological (tectonic or volcanic uplift, glacial rebound, deposition loading), physical (rainfall, snow melt, draw- down of tides, volcanic eruption, earthquake, thawing, freeze-and-thaw weathering, shrink-and-swell weathering) or human (irrigation, defor- estation, mining, excavation of slope, etc.) (Cruden and Varnes, 1996). Several of these factors may occur together to cause a landslide, however, only one trigger will exist which causes a landslide as a near-immediate response (Wieczorek, 1996). Triggers may be intense rainfall, rapid snowmelt, earthquake shaking, volcanic eruption, or water-level change (Wieczorek, 1996). Seismic characterization of rockfalls is important for several reasons. Seismic monitoring of rockfalls can provide real-time hazard assessment at volcanoes. Seismic characterization of rockfalls can make it easier to distinguish them and remove them when monitoring other seismic ac- tivity (Surinach et al., 2005). Rockfalls can also be useful for monitoring volcanic activity, as sudden or gradual changes in their locations, sizes, or rates may be related to subsurface magma movements. Rockfalls occur frequently at Augustine Volcano and occurred in high numbers before, during and after its eruption in 2006. Many of the rockfalls generated seismic signals large enough to trigger the au- tomatic seismic event detection system (Johnson et al., 1995). Photo- graphs of several smaller rockfalls at Augustine Volcano exist that show motion in the fall/rolling/bouncing category, but most of the rockfalls in this study were identified by the seismic signals they pro- duced only, and not observed visually. For this reason, we refer to the events discussed here as rockfalls in general, except when it is known more specifically what type of event is occurring. Rockfalls and flow events from four periods are studied in this paper. The first period (the background period) includes events occurring at low rates (average 28 per year) during the years from 1997 to 2004. The second and third periods include rockfalls as well as block-and- ash flows that occurred at high rates during the 2005 precursory and 2006 eruption periods. Some eruptive events were also recorded on a Journal of Volcanology and Geothermal Research 211–212 (2012) 61–75 ⁎ Corresponding author. Tel.: + 1 907 474 5517; fax: +1 907 474 5618. E-mail addresses: nderoin@alaska.edu (N. DeRoin), steve@giseis.alaska.edu (S.R. McNutt). 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.11.003 Contents lists available at SciVerse ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores