Nat. Hazards Earth Syst. Sci., 6, 671–685, 2006 www.nat-hazards-earth-syst-sci.net/6/671/2006/ Natural Hazards © Author(s) 2006. This work is licensed and Earth under a Creative Commons License. System Sciences Numerical simulation of tsunami generation by cold volcanic mass flows at Augustine Volcano, Alaska C. F. Waythomas1, P. Watts2, and J. S. Walder3 1U.S. Geological Survey, Alaska Volcano Observatory, Anchorage, AK, USA 2Applied Fluids Engineering Inc., Long Beach, CA, USA 3U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, WA, USA Received: 18 April 2006 – Revised: 22 June 2006 – Accepted: 22 June 2006 – Published: 26 July 2006 Abstract. Many of the world’s active volcanoes are situated 1 Introduction on or near coastlines. During eruptions, diverse geophysical mass flows, including pyroclastic flows, debris avalanches, Many of the world’s active volcanoes are located within a and lahars, can deliver large volumes of unconsolidated de- few tens of kilometers of the sea or other large bodies of wa- bris to the ocean in a short period of time and thereby gen- ter. During eruptions, large volumes of volcaniclastic debris erate tsunamis. Deposits of both hot and cold volcanic mass may enter nearby water bodies, and under certain conditions, flows produced by eruptions of Aleutian arc volcanoes are this process may initiate tsunamis (Tinti et al., 1999; Tinti exposed at many locations along the coastlines of the Bering et al., 2003). Worldwide, tsunamis caused by volcanic erup- Sea, North Pacific Ocean, and Cook Inlet, indicating that tions are somewhat infrequent (Latter, 1981); however, doc- the flows entered the sea and in some cases may have ini- umented historical cases illustrate that loss of life and prop- tiated tsunamis. We evaluate the process of tsunami gener- erty has been significant, sometimes involving thousands of ation by cold granular subaerial volcanic mass flows using fatalities (Blong, 1984). Thus, it is generally recognized that examples from Augustine Volcano in southern Cook Inlet. tsunami generation by volcanic processes is an important and Augustine Volcano is the most historically active volcano in credible hazard. the Cook Inlet region, and future eruptions, should they lead Volcanic mass flows such as debris avalanches, lahars to debris-avalanche formation and tsunami generation, could (volcanic mudflows), and pyroclastic flows and surges com- be hazardous to some coastal areas. Geological investiga- monly develop during moderate to large eruptions (>VEI 2; tions at Augustine Volcano suggest that as many as 12–14 Newhall and Self, 1982) and they may reach the sea tens of debris avalanches have reached the sea in the last 2000 years, kilometers from their source. Other volcanic processes, such and a debris avalanche emplaced during an A.D. 1883 erup- as flank collapse, lateral blast, and pyroclastic fall may in- tion may have initiated a tsunami that was observed about troduce material directly into the water which can also trig- 80 km east of the volcano at the village of English Bay (Nan- ger tsunamis (Tinti et al., 1999; Ward and Day, 2001; Watts walek) on the coast of the southern Kenai Peninsula. Nu- and Waythomas, 2004). In Alaska, many of the volcanoes merical simulation of mass-flow motion, tsunami generation, of the Aleutian volcanic arc (Fig. 1) are partly or entirely propagation, and inundation for Augustine Volcano indicate surrounded by water, and volcanic mass flows of various only modest wave generation by volcanic mass flows and lo- types have entered the sea many times in the past 5–10 ka calized wave effects. However, for east-directed mass flows (Waythomas and Watts, 2003). Although only a few histori- entering Cook Inlet, tsunamis are capable of reaching the cal accounts of water waves generated by subaerial volcanic more populated coastlines of the southwestern Kenai Penin- mass flows and other volcanic processes have been reported sula, where maximum water amplitudes of several meters are in Alaska (Lander, 1996), the combination of an active is- possible. land arc setting surrounded by deep water suggests that vol- canogenic waves may be more significant than presently re- alized. Correspondence to: C. F. Waythomas ([email protected]) Published by Copernicus GmbH on behalf of the European Geosciences Union. 672 C. F. Waythomas et al.: Numerical simulation volcanic mass flows, Augustine Volcano Hayes Volcano 2000). We evaluate conditions required for tsunami genera- 154° 152° 150° Mount Spurr Volcano tion by debris avalanche, evaluate potential far-field effects, Crater Peak Anchorage and present a variety of maps depicting the extent of haz- Tyonek 61° ard associated with tsunamis reaching coastlines in southern Nikiski Cook Inlet. The analytical approach we present is flexible and may be easily applied to other volcanoes in the Aleutian Redoubt Volcano Kenai arc as well as to other coastal volcanic centers where debris Soldotna avalanche is a potential tsunami source. ninsula Iliamna Volcano Pe 60° Cook InletNinilchik enai K Anchor 2 Debris avalanche on Augustine Volcano: potential Point Homer tsunami source Seldovia Nanwalek Debris-avalanche deposits are ubiquitous in all quadrants on Augustine Volcano the flanks of Augustine Volcano (Beg´ et´ and Kienle, 1992; 0 50 100 KILOMETERS 59° Waitt et al., 1996). Most of these deposits, which record 0 30 60 MILES the collapse of summit lava domes and flows, consist of unsorted mixtures of bouldery rock debris, gravel, sand and silt. Debris-avalanche deposits are exposed in coastal bluffs around Augustine Island and are also inferred in areas ALASKA just off shore, particularly on the east side of the volcano, FAIRBANKS where zones of hummocky, irregular bathymetry are present (Fig. 2). Geologic studies of the deposits on Augustine Is- ANCHORAGE land have documented at least 12 debris-avalanche deposits, JUNEAU Active volcanoes of all younger than about 3500 cal yr. B.P. (Beg´ et´ and Kienle, the Aleutian arc Area shown 1992; Waitt et al., 1996). These deposits are exposed along Bering Sea by figure the modern shoreline of Augustine Island indicating that they Pacific Ocean traveled at least as far as the near shore zone around the is- land. Fig. 1. Location of Augustine Volcano in south-central Alaska and Augustine Volcano is the most historically active volcano towns and villages along the Cook Inlet coastline. Black triangles in the Cook Inlet region (Miller et al., 1998) and has had locate other volcanoes in the Cook Inlet region. six major eruptions since 1883, including an ongoing erup- tion that began in December 2005. However, only the 1883 eruption had a large associated debris avalanche, deposits Here, we focus attention on the process of tsunami gener- of which are found at Burr Point on the north coast of Au- ation by volcanic debris avalanches which are gravity driven gustine Island (Fig. 2). A slightly older eruption at 300– mass flows of cold, dry, volcaniclastic debris (Ui, 1983; 500 cal yr. B.P. also produced a debris avalanche that formed Siebert, 1984). We first define potential tsunami sources what is now West Island (Fig. 2; Siebert et al., 1995). The by making reference to the morphological characteristics of young West Island and Burr Point debris-avalanche deposits debris-avalanche deposits on Augustine Volcano, an island clearly record large (>0.5 km3) volcanic mass flows that en- volcano in southern Cook Inlet, Alaska (Fig. 1). Then we tered the water of southern Cook Inlet under oceanographic describe the process of tsunami generation and an analyti- conditions essentially the same as today. These flows also cal procedure for estimating initial tsunami amplitude and are representative of the size and extent of other debris- wavelength. We chose Augustine Volcano to illustrate our avalanche deposits on Augustine Volcano (Beg´ et´ and Kienle, method because the volcano is situated at the entrance to 1992) and are thus reasonable analogs for future events. A Cook Inlet, a major transportation and economic thorough- brief descriptionFigure 1 of the Burr Point and West Island debris- fare for Alaska, and is located within a few hundred kilome- avalanche deposits is given next to provide some context for ters of several towns and villages along the coastline (Fig. 1). our analysis of tsunami generation by debris avalanche. Augustine is also known to have produced numerous debris avalanches that reached the sea in the past several millen- 2.1 Burr Point debris-avalanche deposit nia (Beg´ et´ and Kienle, 1992; Waitt et al., 1996). It seems reasonable that some of these debris avalanches may have The Burr Point debris-avalanche deposit is located on the produced tsunamis (Kienle et al., 1987), although geologic north coast of Augustine Island (Fig. 2). Most of the de- evidence for tsunamis that may have reached the Cook In- posit is buried by younger volcaniclastic debris, mainly let coastline remains somewhat controversial (Waythomas, pyroclastic-flow deposits erupted in 1976, 1986, and 2006. Nat. Hazards Earth Syst. Sci., 6, 671–685, 2006 www.nat-hazards-earth-syst-sci.net/6/671/2006/ C. F. Waythomas et al.: Numerical simulation volcanic mass flows, Augustine Volcano 673 154° 00' 153° 30' Approximate debris avalanche flow path Iniskin Bay 59° 45' Approximate offshore extent of debris-avalanche deposits Debris-avalanche deposits 1 Burr Point 2 Northeast Point 3 East Point Oil Bay 4 Yellow Cliffs Iliamna Bay 5 Southeast Point 6 Southeast Beach 7 South Point 8 Long Beach 9 Lagoon 10 West Island 11 Grouse Point 25 12 North Bench 15 13 Rocky Point Ursus Cove 30 5 20 10 59° 30' 35 Augustine Island 40 25 20 45 Bruin Bay 15 131 12 1011 9 2 3 6 4 50 8 5 5 7 Kamishak 10 Bay 55 20 5 25 10 15 59° 15' 30 35 40 45 Base from USGS Iliamna Quadrangle, 1:250,000 scale 0 20 km BATHYMETRIC CONTOUR INTERVAL 5 METERS Fig.