Earthquakes, Mud Volcano Eruptions, and Fracture Formation Hazards in the South Caspian Basin: Statistical Inferences from the Historical Record

Abstract

Based on data since 1832 from 533 earthquakes and 220 mud volcanoes in the Azerbaijan region, an analysis is given of: (a) the occurrence likelihood of weak, medium and strong earthquakes, the latter capable of causing significant damage; (b) the likely directions from which damaging earthquake waves can arrive; (c) the likelihood of a mud volcano hazard (ejected breccia and/or mud flows and/or flame ignition) in temporal association with an earthquake; and (d) the likelihood of fracture formation associated with mud volcanic eruptions. The Chirag region of the South Caspian Basin is used to illustrate application of the methods because of the potential significance of the region for hydrocarbon exploration involving rigs susceptible to the above hazards. The statistical information would indicate that the occurrence likelihood for a 7-balls or higher damaging earthquake in the Chirag region corresponds to an average waiting time of around 5,000 yrs; a medium strength (6-balls or higher) earthquake should occur, on average, in the region every 1,200 yrs, while a weak earthquake (5-balls or higher) is likely to occur, on average, every 110 years. The most likely direction of longitudinal seismic waves from earthquakes of sufficient strength to cause significant damage in the Chirag region, be the earthquake epicenter in the region or at a remote focus, is roughly east and west, with a slight prevalence for a westward origin. There is some correlation between earthquake actively and mud volcano activity, suggesting that mud volcanoes occur between zero to five years prior to earthquakes. But the correlation is not sharply delineated due to the paucity and quality of currently available data. For surface fractures, associated with mud volcano eruptions, which can be meters wide and can stretch for a kilometer or more, only volcanoes with five or more eruptions were used to estimate the likelihood of fracture occurrence, yielding an average of about 30% chance of occurrence. The low number and low quality of data did not permit any more detailed investigation of fracture parameters – such as average width, length or offset. In view of the potential hazards for oil rigs in the offshore South Caspian Basin, it would seem that organized, high quality, data collection both offshore and onshore should be rapidly undertaken as a vital adjunct to drilling operations in order to sharpen the assessments of risk factors presented here.