3D modeling of the Buhi debris avalanche deposit of Iriga Volcano, Philippines by integrating shallow-seismic reflection and geological data Likha G. Minimo a, ⁎, Alfredo Mahar Francisco A. Lagmay a,b a Volcano-Tectonics Laboratory, National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines b Department of Science and Technology-Project NOAH (Nationwide Operational Assessment of Hazards), National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines abstract article info Article history: Received 19 August 2015 Received in revised form 25 February 2016 Accepted 6 March 2016 Available online 11 March 2016 Numerical models for simulating volcanic debris avalanches commonly lack a critical initiation parameter, the source volume, which is difficult to estimate without data on the deposit thickness. This, in turn, limits how rheology can be characterized for simulating flow. Leapfrog Geo, a 3D geological modeling software, was used to integrate shallow-seismic reflection profiles with field and borehole data to determine the volume of the Buhi debris avalanche and the pre-collapse structure of Iriga Volcano. Volumes of the deposit calculated in this way are 34–71% larger than previous estimates. This technique may improve models of debris avalanches elsewhere in the world, and more precisely depict landslide runout and lateral extent, thus improving disaster prevention and mitigation for the many cities located near volcanoes. © 2016 Elsevier B.V. All rights reserved. Keywords: Debris avalanche deposits Buhi DAD Iriga Volcano Seismic reflection Volume 3D modeling 1. Introduction Debris avalanches, “rapidly moving masses of unsorted rock, debris, and soil mobilized by gravity (Schuster and Crandell, 1984)”, are major volcanic hazards. Volcanic debris avalanches are generally larger in volume than non-volcanic examples. Their sources are weak, heteroge- neous mixture of massive lava flow units, unconsolidated ash, clay, pumice, scoria, and water (e.g. Siebert, 2002; Dufresne and Davies, 2009; Carrasco-Núñez et al., 2011). Non-volcanic debris avalanches, although characteristically less mobile are also hazardous. Aside from being prone to hydrothermal alteration, volcanoes may be underlain by weak substrata and are often cut by faults that further destabilize their edifices (e.g., López and Williams, 1993; Frank, 1995; Lagmay et al., 2000; Cecchi et al., 2004). Once triggered by a phreatic eruption, magmatic intrusion, earthquake or heavy rainfall (e.g., Voight et al., 1983; McGuire, 1996; Ui and Yoshimoto, 2000; Siebert, 2002) volcanic debris avalanches can be generated by flank collapses. Volcanic debris avalanches may transform into debris flows or lahars when incorporated with snow or water (e.g., Carrasco-Núñez et al., 1993; Bernard et al., 2009; Cortés et al., 2010). They have also been associated with pyroclastic flows, lateral blasts, and Plinian erup- tions (e.g., Siebert, 2002; Carrasco-Núñez et al., 2011). A large debris avalanche can even produce a tsunami when it enters the ocean (e.g., Katayama, 1974; Ward and Day, 2001; Tappin et al., 2008; Whelan and Kelletat, 2003). Of the 49 active and potentially active volcanoes in the Philippines listed by the Philippine Institute of Volcanology and Seismology (PHIVOLCS), five have experienced debris avalanches (Fig. 1). The active ones are Banahaw in southern Luzon (Geronimo-Catane, 1994), Kanlaon in Negros Oriental (PHIVOLCS, 2008d), Biliran in Eastern Visayas (PHIVOLCS, 2008c) and Iriga (Aguila et al., 1986; Geronimo-Catane, 1994; Lagmay et al., 2000; PHIVOLCS, 2008a; Paguican et al., 2012) in Bicol Region. Isarog Volcano (Fontijn and Newhall, 2013) is classified as potentially active. Among these debris avalanche deposits (DADs), two at Iriga have been most studied (e.g., Aguila et al., 1986; Geronimo-Catane, 1994; Belousov et al., 2011; Paguican et al., 2012). Iriga City was built on one in the southwest sector (Fig. 2). The other, in the southeast sector, underlies the town and the lake of Buhi. Based on their type localities, Paguican et al. (2012) renamed the older one ‘Iriga DAD1’, and the younger and more prominent one ‘Buhi DAD2’ (Fig. 2). For clarity, this report calls them Iriga DAD and Buhi DAD, respectively. Sosio et al. (2012) have numerically modeled large landslides including Buhi DAD to test the applicability of their numerical code to the volcanic varieties. Yoshida (2013) compared his calculated volume with that estimated by Aguila et al. (1986). Woelfl et al. (2014) checked their simulations with the actual extent of the Buhi DAD mapped by Paguican (2012). De Vries et al. (2001) have pointed out that volumes Journal of Volcanology and Geothermal Research 319 (2016) 106–123 ⁎ Corresponding author. E-mail address: lgminimo@nigs.upd.edu.ph (L.G. Minimo). http://dx.doi.org/10.1016/j.jvolgeores.2016.03.002 0377-0273/© 2016 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores