Provocation and reaction: A study into the influence of slow slip earthquakes and volcanic activity in the southern Taupo Volcanic Zone, New Zealand

Abstract

Slow slip events (SSEs) occur regularly on the Hikurangi subduction zone along the East coast of the North Island. These SSEs sometimes trigger small local earthquakes, usually offshore. To study the general influence SSEs have on seismicity rates in New Zealand, we did a statistical analysis of earthquake rates comparing times when SSEs were occurring to all other times. We found that although seismicity rates remain relatively low, for most regions there is a significant increase in the number of earthquakes that occur during SSEs. This shows that while the changes associated with SSEs are too small and too slow to be felt, they are large enough to influence tectonic processes and earthquake rates. Future New Zealand hazard models that take into account time variations could incorporate this information. Given the connection to earthquake rates, we have investigated whether these SSEs affect the timing of volcanic activity at Taupo caldera beneath Lake Taupo. In 2008 a small SSE was accompanied by a large increase in seismicity and gas release at Taupo caldera. Despite other similar and larger SSEs in the area, no other accompanying volcanic unrest has been observed at Taupo in association with these events. By calculating strain from GPS instruments around Lake Taupo we have shown that the 2008 SSE was unique in that it produced dilational strain, which acted to stretch out the caldera. All other SSEs examined appear to have the opposite affect, pushing the caldera together. We also looked for changes in seismic velocity associated with these SSEs, but have not found any links. The seismic velocity does vary with time and some of these changes may be seasonal. We suggest that the 2008 SSE pulled apart the caldera enough to allow existing gases to rise to the surface and be released. In the future increases in extension at the caldera should trigger increased monitoring and scrutiny of other volcanic parameters.