Fault Zone Guided Wave generation on the locked, late interseismic Alpine Fault, New Zealand

Fault Zone Guided Waves (FZGWs) have been observed for the first time within New Zealand's transpressional continental plate boundary, the Alpine Fault, which is late in its typical seismic cycle. Ongoing study of these phases provides the opportunity to monitor interseismic conditions in the fault zone. Distinctive dispersive seismic codas (~7–35 Hz) have been recorded on shallow borehole seismometers installed within 20 m of the principal slip zone. Near the central Alpine Fault, known for low background seismicity, FZGW‐generating microseismic events are located beyond the catchment‐scale partitioning of the fault indicating lateral connectivity of the low‐velocity zone immediately below the near‐surface segmentation. Initial modeling of the low‐velocity zone indicates a waveguide width of 60–200 m with a 10–40% reduction in S wave velocity, similar to that inferred for the fault core of other mature plate boundary faults such as the San Andreas and North Anatolian Faults. Key Points Fault Zone Guided Waves observed within the late interseismic Alpine Fault Transpressional Alpine Fault zone comparable at depth to San Andreas Fault