Engineering geological investigation of a recent mud volcano eruption, Waimata Valley

Abstract

Mud volcanoes typically form in sedimentary basins both onshore and offshore, by localized expulsion of mud that has been remobilized from the subsurface. The extrusion of mud forms topographic features that are representative of the release of fluids and mud and overpressure. The research that is presented in this thesis focuses on the recent formation of a mud volcano within the Waimata Valley near Gisborne from a single eruptive event on 15 December 2018, extruding approximately 11,200m3 of mud at the surface. The eruption occurred from a completely new location though it is inferred that this feature is part of a mud volcano system that has previously formed other nearby mud volcanoes. The eruption of a mud volcano from the day of its occurrence is only something that has been documented a handful of times in New Zealand’s written history. Prior to eruption, this new eruptive centre had gained the attention of local scientists following a significant MW7.1 earthquake that occurred offshore of Te Araroa in northern Tairawhiti region on 2 September 2016. It was also noted that following the earthquake many other mud volcanoes within the region demonstrated increased activity whilst others remained quiescent, highlighting an obvious relationship between mud volcanism and seismic activity. Through utilising an engineering geological investigation of the site, an insight into the processes and mechanisms that trigger mud volcanism, as well establishing a database of the geological and geotechnical properties of the associated materials has been achieved. The materials were identified to contain a significant clay fraction, contributing to their low permeabilities and strength. These properties are inferred to have an important influence on how the that materials withstand significant overpressures remaining unconsolidated at depth leading to their migration toward the surface. By developing this greater understanding of the mud volcano, multiple potential geohazards were identified and can be applied to future events that will inevitably continue to occur throughout the region. The most significant hazard identified resulting from this eruption was the formation a several highly mobile mud flows that led to the formation of a small lake.