ÔØ ÅÒÙ×Ö ÔØ Volcano collapse and tsunami generation in the Bismarck Volcanic Arc, Papua New Guinea Eli Silver, Simon Day, Steve Ward, Gary Hoffmann, Pilar Llanes, Neal Driscoll, Bruce Appelgate, Steve Saunders PII: S0377-0273(09)00260-1 DOI: doi: 10.1016/j.jvolgeores.2009.06.013 Reference: VOLGEO 4348 To appear in: Journal of Volcanology and Geothermal Research Received date: 12 November 2008 Revised date: 5 June 2009 Accepted date: 30 June 2009 Please cite this article as: Silver, Eli, Day, Simon, Ward, Steve, Hoffmann, Gary, Llanes, Pilar, Driscoll, Neal, Appelgate, Bruce, Saunders, Steve, Volcano collapse and tsunami generation in the Bismarck Volcanic Arc, Papua New Guinea, Journal of Volcanology and Geothermal Research (2009), doi: 10.1016/j.jvolgeores.2009.06.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. 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ACCEPTED MANUSCRIPT Volcano collapse and tsunami generation in the Bismarck Volcanic Arc, Papua New Guinea Eli Silver1*, Simon Day1,2, Steve Ward1, Gary Hoffmann1, Pilar Llanes3, Neal Driscoll4, Bruce Appelgate4, Steve Saunders5 1Earth and Planetary Sciences Dept., University of California, Santa Cruz, CA 95064 *Corresponding Author: [email protected] 2Dept of Earth Sciences, University College London, London, UK 3Universidad Complutense de Madrid, Facultad de Geologia, Ciudad Universitaria, Madrid, Spain 4Scripps Institution of Oceanography, La Jolla, CA 92093 5Rabaul Volcanic Observatory, Papua New Guinea Abstract During a cruise on the R/V Kilo Moana in 2004, we mapped 12 debris avalanches from volcanoes in the Bismarck volcanic arc, estimated their sizes and computed the size of potential tsunami run-up in major local population centers from these features. We used the towed side-scan instrument Hawaii-MR1, the hull-mounted EM120 system for swath bathymetry and backscatter intensity, a shallow penetration chirp system, several bottom camera tows and selected cores. We calibrate our computations with the known tsunami run-up of the Ritter collapse. Even the small collapses may have had significant run-up on near-by coastlines. Had any of the collapses we have identified occurred in modern times each would affect a presently populated region of the coastline to a moderate or significant degree. 1. Introduction ACCEPTED MANUSCRIPT Sector collapses are common occurrences for many volcanoes, such as the Aleutian volcanoes (Coombs et al., 2007), the Lesser Antilles (Boudon et al., 2007), Japan (Satake and Kato, 2001), Tonga-Kermadec (Wright et al., 2006), and the Cascades (Crandell et al., 1984; Glicken, 1996). Enormous debris avalanche deposits have been mapped around all of the Hawaiian Islands (Moore et al., 1989) and the Cape Verde - Canary Islands (Watts and Masson, 1995; Masson et al., 2002; Masson et al., 2008), which represent major oceanic island events. Collapse events into the ocean often result in massive tsunami generation (Ward and Day, 2003). The 1888 AD collapse of nearly the entire western flank of Ritter Island in the Bismarck volcanic arc was described by 1 7/8/2009 ACCEPTED MANUSCRIPT Johnson (1987), Ward and Day (2003), and Silver et al. (2005). Here we report on the results of a major multibeam and side-scan sonar study of a large part of the offshore volcanic islands in the Bismarck volcanic arc, and we present observations for debris avalanches from 11 volcanoes in the Bismarck chain: Garove, Tolokiwa, Sakar, Ritter, Dakataua, Crown, Karkar, Manam, Kadovar, Bam, and a small seamount near Lolobau Island off East New Britain. We explore implications for the possible tsunami effects they may have had on the sites of present population centers. 2. Regional Setting The Bismarck volcanic arc overlies the northward subducting Solomon Sea, a small ocean basin that is rapidly disappearing (Davies et al., 1987; Silver et al., 1991). The rate of subduction is highest in the eastern part of the arc and slowest in the west (Taylor, 1979; Tregoning et al., 2000; Wallace et al., 2004). In addition, the Solomon sea oceanic crust disappears from view at the surface to the west of New Britain island, as it is overrun by the collision of the South Bismarck Sea plate with the Australian plate (Abers and McCaffrey, 1988; Silver et al., 1991; Abbott et al., 1994). Nonetheless, subduction of the Solomon Sea continues westward, as seen by both seismicity data (Cooper and Taylor, 1987; Abers and Roecker, 1991) and the presence of 10Be in the lavas from volcanoes both east and west of West New Britain (Gill et al., 1993). Both the western and eastern ends of the Bismarck volcanic arc pass close to or are cut by transform faults of the Bismarck Sea Seismic Lineation (BSSL). Our data do not include the westernmost ACCEPTEDend, but they show the relati MANUSCRIPTonships between this fault system and the Schouten Islands (Llanes et al., 2009). Our easternmost data includes the region offshore of Ulawun and Lolobau volcanoes. Ulawun is quite active and has been a target of study for the United Nations (1987) International Decade of Natural Hazard Reduction program volcanoes (see also http://www.sveurop.org/gb/articles/articles/decade.htm). The volcanic line runs largely along the north coastal region of New Britain, with several remarkable exceptions (Fig. 1). One is the prominent N-trending Wilaumez Peninsula that extends about 50 km north of the average line of the island’s north coast. The second exception is the Witu group of islands that lies about 100 km off the north coast of New Britain. Most of the islands in 2 7/8/2009 ACCEPTED MANUSCRIPT this group (Garove, Mundua, and Narage) lie on a roughly east-west oriented platform, and the island of Unea is about 10 km to the south. West of New Britain, the islands of Umboi, Sakar, Tolokiwa, Ritter, Long, Crown, Bagabag, Karkar and Manam all form a northwest trend with minor scatter about the trend. Formed as part of the process of opening the Manus basin (Taylor, 1979), the Bismarck Sea Seismic Lineation (BSSL) extends westward across the Bismarck Sea to its intersection with the coast. West of this intersection, subduction is southerly directed beneath the margin, whereas to the east subduction is still northerly directed, toward the Bismarck Sea. The pole of rotation that describes the collision of the Finisterre-Adelbert Ranges with the rest of New Guinea lies near this intersection (Wallace et al., 2004). This fault system passes through the Schouten Islands and may have had an influence in their development. 3. Methods During November-December, 2004, we carried out a geophysical study of the Bismarck volcanic arc aboard the R/V Kilo Moana. We collected swath bathymetry and backscatter data with the Simrad EM-120 hull-mounted sonar and the Hawaii MR1 towed side-scan system. The minimum water depth allowable for the Hawaii MR1 towed side- scan instrument was 500 m, which limited our work in shallow water. We obtained shallow subsurface data with a Knudsen hull-mounted 4 kHz sub-bottom profiler (University of Hawaii) and an X-Star towed 1-5.5 kHz chirp sub-bottom profiler (Scripps Institution of Oceanography).ACCEPTED Several camera MANUSCRIPT tows were also carried out using a digital deep-tow camera (Woods Hole Oceanographic Institution). In addition to the marine geophysical study, onshore fieldwork was carried out by S. Day in September-November 2004 and July-August 2005. This work was principally directed at collection of data on tsunami deposits and oral traditions of tsunamis (to be reported elsewhere), but a number of volcanic islands and coastal volcanoes were visited: Talasea and other volcanoes of the Willaumez peninsula in 2004, and all of the Witu Islands; Arop (Long); Crown; Bagabag; Karkar; Manam; Boisa; Bam; Blupblup; Wei; Koil and Vokeo in 2005. Finally, a brief onshore visit was made to Ritter Island in December 2006 during the course of near-shore survey work. 3 7/8/2009 ACCEPTED MANUSCRIPT 4. Results The swath bathymetry and side-scan results from the 2004 cruise reveal a number of debris avalanche deposits that may have generated significant tsunamis. We describe each of these and discuss their volcanic and tectonic settings. Our interpretation of the potential size of tsunamis that could be generated from the collapse events is based on the size of the debris avalanche, as seen mainly on the side-scan images, although swath bathymetry, with lower spatial resolution, is used as well. On many of the figures in this paper that show debris avalanche deposits, we have outlined an area that is somewhat wider than the large blocks clearly visible on the image to indicate the extent of the debris field (as we interpret it). For very young collapse features the edges of the debris fields are very clear in the side-scan imagery (Ritter, for example), but older collapse deposits suffer partial or complete burial and a lack of clear definition of the edges of the original deposit. Most of the debris fields that we map are those of the debris avalanche stage of the collapse landslide and not of any debris flows that may extend farther from the source. Debris flows are quite evident off Ritter (discussed below) but are not evident in our data on the other, older collapse features. Debris avalanche deposits tend to show scattered blocks of various sizes, generally extended down-slope from the volcano. Conical structures sometime present are considered volcanic cones and not part of the debris avalanche, though the latter deposits may occur in the same region as the cones.