Impacts of Slow Slip Events on Volcanic Areas in the Far-field: An Example from the Taupo Volcanic Zone, New Zealand

Abstract:

Volcanic and geothermal areas have been shown to be more susceptible to dynamic triggering from large earthquakes than normal tectonic regions. In this study we examine whether the Taupo Volcanic Zone (TVZ) in New Zealand, is susceptible to strains induced by slow slip events (SSEs) on the Hikurangi subduction zone. The Hikurangi margin experiences diverse SSE behaviour. We categorize the SSEs into three types: 1) Long duration SSEs (years) downdip of the geodetically locked portion of the subduction zone and southwest of the TVZ; 2) More frequent, short duration SSEs (weeks) in the partially locked and unlocked portions of the subduction zone close to the trench offshore of the TVZ; 3) Deep short duration SSEs (months) also trenchward of the TVZ, but downdip from type-2 SSEs. The type-3 SSEs are much closer to the TVZ (<100 km) than the more frequent type-2 SSEs (~200km). We use continuous GPS from the New Zealand GeoNet network to analyse changes in strain and strain-rate around Taupo volcano for the two type-3 SSEs closest to Taupo to date, in 2006 and 2008, and a recent SSE of type-1 and 2 (Manawatu 2010, and Southern Hawkes Bay 2013). The observed strains are compared to finite element models of strains predicted from the slip distribution of the individual SSEs. The strain and strain rate changes are also compared to long-term seismicity rates in the TVZ to determine their influence on the timing of volcanic unrest. Seismicity and gas changes at Taupo volcano accompanied a deep (>20 km) SSE in 2008. Despite a correlation in the timing of the 2008 deep SSE and unrest at Taupo volcano, there is no equivalent unrest observed during the 2006 deep SSE. It may be that the state of the volcano is important to whether changes in strain trigger unrest, therefore we also analyse seismic velocity variations using ambient noise cross correlations. The seismic velocity variations allow us to look for the influence of SSEs at times when other forms of unrest are not observed at the volcano.