tephra, lava flow and nuée ardente hazard zoning of taftan volcano, se iran

1. introduction there are about 1500 active volcanoes across the world, among which only about 400 cases have erupted during last century, only one third of which have had recorded historical eruptions. also, over 10% of world population lives around active and semi-active volcanoes (tilling, 2005). the majority of hazardous volcanoes are of stratovolcano type. plio-quaternary stratovolcanoes of iran are considered as potential volcanic threats in iran. hence, it is helpful to evaluate the probability of their reactivation, their kind of threat as well as to determine locations at threat. 2. study area in this regard, we decided to evaluate the hazard aspects of taftan volcano. taftan volcano which is 4000 m in height lies at 61°,8ʹ eastern longitude and 28°,36ʹ northern latitude at 45 km north of khash town in sistan and baluchistan province of iran (bahrami, yamani & alavipanah, 2008). this stratovolcano belongs to the volcanic arc of makran subduction zone in which the oceanic lithosphere of oman sea is subducted northward beneath continental lithosphere of eastern iran and pakistan (farhoudi & karig, 1977). currently, this volcano exhibits post volcanic activity via sulfataric and fumarolic activities near its summit (moeinvaziri & aminsobhani, 1978). although previously several geological studies have been undertaken on taftan, but their purpose have been to understand it, so they have not dealt with its hazards and hazard zoning.since the potassium-argon and rubidium-strontium dating investigations undertaken by biabangard (2006) revealed that its latest flows have erupted about 6.95±0.02 thousand years ago, the conclusion is that we deal with a potentially active volcano. taftan has erupted with strombolian to plinian types of activity in the past (biabangard & moradian, 2002) and it may erupt with 3-5 intensity on vei scale in the future. moreover, it is probable that the eruptions have pyroclastic flows (nuée ardente) or ash (tephra). thus, hazard zoning must be conducted for this kind of threat. 3. material and methods after gathering the needed data, landsat images and digital elevation model (dem) of the study area were employed to create a 3d model and hill-shaded topographic model to enhance visibility. they also were used to prepare the land use layers of the area. in this study, we employed the voris software which is based on advection-diffusion model. in this model several parameters such as height of eruption column (h), the volume of erupting materials (m), eruption duration (d), particle size, velocity and wind direction at different elevations of the atmosphere, air temperature and some other are used. relevant parameters are needed and were determined for the studied area was introduced to the software and it was run. the output was the ash thickness map (isopach map) (mastin, 2009). the contributing factors on tephra movement the air include horizontal velocity of the wind, turbulence and the rate of vertical discendance which vary according the reynolds number of particles. the atmosphere is divided into several horizontal layers each of them having specific velocity and directions. ash particles follow the conditions of each atmospheric layer and when entering a new layer are affected by the conditions of this new one. this process continues until the ash particle reaches the ground surface (mortazavi, sparks, & amigo, 2009). the necessary wind data include its direction and velocity in five atmospheric levels (4000, 9000, 16000, 20000 & 35000 m) which in this study were taken from ncep/ncar national centers. in order to determine the route of lava flows from a point (the summit of volcano), the simulation model of lava flow was employed. this model is based on this presumption that topography plays the main role in the route of flows. this model uses two logics, the first one supposes that lava will flow from one cell (pixel) to the nearby pixel whenever their elevation differences are positive, and secondly, the volume of lava entering from one pixel to the other one depends on the value of positive differences between them. in preparing the zoning map with this model, the maximum distance for moving lava was considered to be 5 kms. the model used for simulating the maximum potential extent affected by pyroclastic density currents (pdc) is a very simple model proposed by malin and sheridan (1982). the principle is that the height of the starting point of the flow (hc) ratios to the length of the run out (l) as a type of friction parameter is termed the heim coefficient. the inclination of the energy cone is an angle (αc) defined by arctan (hc/l). the intersection of the energy cone, originating at the eruptive source, with the ground surface defines the distal limits of the flow (felpeto, 2009). in this study, the collapse equivalent height of nuée ardente was considered to be 200 m and the collapse equivalent angel which is the angel between collapse height and slope angel was presumed to be 6°. 4. conclusion according to this study, taftan is a potentially semi-active volcano which may reactive in the future. it has erupted lava flows, tephras and nuée ardentes in the past. according to the presented hazard zonation map, the flows and nuée ardentes threats some nearby villages. the ashes will disperse and extend eastward and will rise and accumulate in some villages. it is suggested that a seismological station be established near this volcano for forecasting its probable eruptions. the kind of threats identified and the hazard zoning maps may be used for emergency evacuation and hazard reduction programs.