Real-time earthquake monitoring during the second phase of the deep fault drilling project, Alpine fault, New Zealand

The Deep Fault Drilling Project (DFDP) is a multinational scientific drilling effort to study the evolution, structure, and seismogenesis of the Alpine fault, New Zealand, via in situ measurements of fault rock properties. The second phase of drilling (DFDP-2), undertaken in the Whataroa Valley in late 2014, was intended to intersect the Alpine fault at a depth of around 1 km. In conjunction with the drilling and on-site science activities, a real-time seismic monitoring scheme and traffic-light response protocol were established to detect, locate, and if necessary respond to seismicity within 30 km of the drill site. This network was operated around the clock between late August 2014 and early January 2015, and we detected and located 493 earthquakes of ML 0.6-4.2. None of these earthquakes occurred within 3 km of the drill site, and nor did any of the seismicity detected require changes to drilling operations. The monitoring was undertaken using open-source software operated by an international team of 16 seismologists (including eight postgraduate students) working in 7 institutions and 3 countries to provide rapid on- and off-site manual checking and relocating of events. The team's standard response time between detection and final location was less than 30 min under normal background seismicity conditions and up to 1 hr during swarm activity and for low-priority, distant (≥30 km epicentrally from the drill site) earthquakes. This article documents the methodology, infrastructure, protocols, outcomes, and key lessons of this monitoring.