Deep and Clustered Microseismicity at the Edge of Southern New Zealand's Transpressive Plate Boundary

Deep ($>\sim$20 km) crustal seismicity is typically associated with cold Precambrian lithosphere. However, previous studies have reported that the depth above which 90% of seismicity occurs (D90) in the Southland region of New Zealand is 30–35 km, even though it is situated in relatively young Mesozoic crust at the eastern edge of the transpressive Pacific-Australian plate boundary. To examine this unusual lower crustal seismicity further, we deployed 19 short period seismometers between October 2022 and 2023 through the Southland Otago Seismic Array (SOSA). Supplemented by the permanent GeoNet network and an ongoing array to the west in Fiordland, SOSA provided a station spacing of $\sim$30 km across this 150 km wide region. In total, SOSA recorded 85 events in and around Southland. These events are not uniformly distributed, with an abrupt decrease in seismicity east of Fiordland. We also identified six spatial-temporal clusters of elevated seismicity across eastern Fiordland and Southland. The distribution of SOSA event depths confirms that seismicity extends into Southland's mid-lower crust and possibly into the upper mantle (D90 = 32 km). This D90 estimate is significantly deeper than suggested by event depths to the north in Otago (D90 = 14 km). Using 1D lithospheric strength profiles, we suggest that the embrittlement of Southland's mid-lower crust reflects the emplacement of mafic terranes in this region during Mesozoic subduction at the Gondwana margin. Our study therefore highlights that a mafic lower crust can lead to deep seismicity within a region undergoing transpression.