Possible methods for monitoring Mount Ruapehu volcano: shear-wave splitting and ground deformations using GPS

Abstract

Stress and strain on Mt. Ruapehu is examined using shear-wave splitting, which detects the orientation of stress aligned cracks within the crust and the Global Positioning System, which can monitor ground deformations. The splitting data are from two deployments of seismometers, in 1994 and 1998. Shallow hypocentre events (<30km) show fast directions of NW-SE, with greater variation for the 1998 results when compared to those from 1994. Deep hypocentre events (>50km) from 1994 show a fast direction of NW-SE and from 1998, ENE-WSW. The fast direction of the shallow data is almost perpendicular to the maximum compressive stress axis of the Taupo Volcanic Zone (TVZ), (NNE-SSW). This is interpreted as resulting from a localised stress regime induced by the volcano. The strong alignment in 1994 is interpreted as an increase in pressure from magma accumulation and the variation in 1998 as a result of the stress release during the 1995/6 eruption. The deep 1998 results are parallel to the TVZ compressive axis. The deep 1994 results are consistent with the shallow results. The deep 1994 results may have been affected by the strong shallow alignment at this time. These changes in splitting may provide a means for monitoring stress changes induced by magma movements. Global Positioning System (GPS) data is used to determine if continuous real-time deformation monitoring could be used on Ruapehu. Several processing runs were carried out to study the effects of ionospheric and tropospheric modelling and the use of predicted orbits. This determined that a real-time (using predicted orbits) system that solved separately at each station for tropospheric effects and created a local ionosphere model using a dual frequency receiver could detect vertical deformations above 30mm. Modelling of expected deformations due to a 10 6Pa eruption indicated that vertical displacements of 68.2mm could be expected at 300-600m from the conduit centre. Thus we would expect to observe ground deformation on Mt. Ruapehu with a real-time GPS monitoring system.