VOLCANIC HAZARDS ASSESSMENT OF SAVAI’I, SAMOA Paul Taylor’ and Lameko Talia August 1999 SOPAC Technical Report 295 ’Australian Volcanological Investigations Apia Observatory, Samoa Funding for this Project was provided by the United Nations Development Programme through the South Pacific Disaster Reduction Programme [3] CONTENTS SUMMARY.................................................................................................................................... 5 ACKNOWLEDGEMENTS............................................................................................................. 6 INTRODUCTION.......................................................................................................................... 7 THE GEOLOGY OF SAVAI’I......................................................................................................... 7 AGE RELATIONSHIPS OF PREHISTORIC VOLCANIC ACTIVITY.............................................8 RECENT VOLCANIC ACTIVITY......................................_...........................................................10 FEATURES OF VOLCANIC ACTIVITY ON SAVAI’I................................................................... 10 STRUCTURAL INFLUENCES ON RECENT VOLCANIC ACTIVITY......................................... 14 FREQUENCY AND DISTRIBUTION OF VOLCANISM THROUGH GEOLOGICAL TIME......... 14 VOLCANIC HAZARDS ON SAVAI’I........................................................................................................ 16 VULNERABLE ELEMENTS ON SAVAI’I..................................................................................... 19 CONCLUSIONS AND THE RISK OF FUTURE VOLCANIC ACTIVITY ON SAVAI’I.................. 22 RECOMMENDATIONS.................................................................................................................. 23 REFERENCES......................................................................................................................... 24 APPENDIX Volcanic Hazards Map of Savai’i, Samoa Introduction...........................................................................................................................25 ExplanatoryNotes.................................................................................................................. 25 Precautionary Notes...............................................................................................................26 [SOPAC Technical Report 295 -Taylor &Talia] [4] LIST OF FIGURES Figure 1 Locality Maps.......................................................................................................................8 2 Simplified Geological Map...................................................................................................8 3 Savai'i.1. Topography.............................................................................................................. 9 4 1760 Lava Flows................................................................................................................ 11 5 1902 Flows and Vents....................................................................................................... 11 6 1905 -11 Flows........................................................ 11 7 Features of the 1905-11 Eruption................................................................................ 12-13 8 Features of Savai'i Volcanism............................................................................................ 13 9 The Ologogo Fault System................................................................................................ 14 10 Temporal Distribution of Volcanic Cones and Lavas on Savai'......................................... 15 11 Vulnerable Elements..........................................................................................................20 LIST OF TABLES Table .,. 1 Volcanic Stratigraphy of Savai'i........................................................................... 9 2 Historic Eruptions onSavai'i...............................................................................................,. 12 3 Frequency and Distribution of Volcanism on Savai'i.......................................................... 16 4 Summary of Volcanic Hazards on Savai'i.......................................................................... 17 [SOPAC Technical Report 295-Taylor &Talia] [5] SUMMARY The island of Savai’i is the largest and westernmost of the Samoan archipelago and is the home for 50,000 Samoans living in villages located around the coast. Savai’i is a mature volcanic environment, but exhibits the features of recent “monogenetic” volcanism. Geological studies conducted on Savai’i suggest that volcanism has been occurring, more or less continuously, since at least Pliocene times. It was suggested that two volcanic episodes have occurred during the development of Savai’i. The initial episode, the ”shield-building phase”, produced the basal lava shield during Pliocene to early-Pleistocene times. Following a period of relative quiescence during the early to mid-Pleistocene the “post-erosional phase” has produced a sequence of lavas and pyroclastics that overly the marked erosional boundary. This phase of volcanism has continued during recent times. The most recent eruption occurred during 1905-11 and resulted in the destruction of large areas of arable land and a number of villages located along the northeast coast. A volcanic hazard assessment conducted by the authors has concluded that activity is likely to continue. Future eruptions will be effusive (lava-forming), explosive (tephra-forming) or a combi- nation of both. Due to the lack of comprehensive chronological data for eruptions prior to the mid-1700's it is difficult to forecast when future eruptions will occur. Furthermore, because of the “monogenetic” nature of the volcanism it is also difficult to forecast where future vents will form. A preliminary volcanic hazards map has been constructed using the spatial density of prehis- toric and recent vents. The hazard zones shown thus define the relative probability of a vent erupting in that zone. However, due to the possibility that a vent may erupt anywhere on Savai’i, the entire island must therefore be considered vulnerable to the hazards associated with future activity. The likely hazards include: lava flows, pyroclastic flows/surges, lahars, tephra falls, volcanic gases, ballistic ejecta, lightning and volcanic edifice collapse. The recommendations put forward in the report provide a firm basis for planning for the future. The priority activities include the development of a comprehensive chronological database of prehistoric activity, the development of mitigation programs aimed at reducing the impact of future activity and public awareness programs to increase the general level of knowledge of all levels within the community. [SOPAC Technical Report 295 -Taylor & Talia] [6] ACKNOWLEDGEMENTS This report would not have been possible without the assistance of the Assistant Director of the Apia Oservatory, Ausetalia Titimaea, and his staff who provided support during the project, and allowed the junior author of this report to take part in fieldwork on Savai’i. Warren Jopling, of Safua Tours in Savai’i provided much information during our many discus- sions about the geology of Savai’i. Many of the localities could not have been visited without his advice. David Kear of Ohope, New Zealand is thanked for his assistance on the available 14C dates for the Puapua Volcanics. Ian all of the Australian National University in Canberra is also thanked for his valuable insights on K/Ar dating of the older rocks of Savai’i. Thoughtful reviews by Shane Cronin, of Massey University and Russell Blong of the Natural Hazards Research Centre at Macquarie University helped to improve this report. Russell Howorth, Program Manager with SOPAC is also thanked for co-ordinating the project and making many arrangements with the Government of Samoa. Funding for this project was provided by the United Nations Development Program through the South Pacific Disaster Reduction Program. [SOPAC Technical Report 295 -Taylor &Talia] [7] INTRODUCTION This report on the volcanic hazard assessment of Savai’i was initiated as a recommendation put forward at the ”Workshop on Volcanic Hazards and Emergency Management in the South Pacific”, held in Port Vila, Vanuatu from 24-28 February 1997 (Howorth and Elaise, 1997). The workshop identified eight volcanic areas within the South Pacific region that require immediate attention. One of these was Savai’i, Samoa, specifically the recent vent of Matavanu' located on the northeast slopes of Savai’i. Matavanu erupted almost continuously from August 1905 to November 1911 destroying several villages, large areas of arable land as well as extensive areas of the lagoon and fringing/barrier reef along the northeast coast. The activity at Matavanu was not the only activity to occur on Savai’i in recent times, Mauga Afi(1760 est.) and Mauga Mu (October - November 1902) have also erupted. Savai’i is the westernmost and largest island of Samoa. It supports a population of approxi- mately 50,000, living mainly in villages along the coastal regions. The economy of Savai’i, as part of Samoa, is based on subsistence agriculture, fishing and tourism. Hence, future destruc- tion of arable land or reef systems by volcanic activity, could have a serious effect on the economy of Savai’i and Samoa. This report will refer to both published information on the geology of Savai’i and observations