Comparison of Repeating Earthquake Activity during the 2014 and 2019 Gisborne Slow Slip Events and Implications for Seamount Subduction and Fluid Migration at the Northern Hikurangi Subduction Margin, New Zealand

Abstract:

Fault slip is a spectrum, ranging from continuous creep to rapid seismic ruptures. Within this spectrum are transient slow slip events (SSEs) that occur in conditionally stable frictional regimes. Pore fluid pressure and subducting topography are two of many factors thought to influence the transitional fault zone properties required for slow slip in subduction zones. At the northern Hikurangi margin, New Zealand over-pressurized sediments spatially correlated with quasi-periodic shallow SSEs are imaged down-dip of subducted seamounts. Immediately following a 2014 SSE in this region, Shaddox & Schwartz (2019) detected burst-type repeating earthquakes coincident with tremor using template matching and ocean-bottom seismic data from the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) array. These repeating earthquakes located within an upper-plate fracture network above the down-dip edge of a subducted seamount offshore of Gisborne. They proposed that during the large plate-boundary SSE, fluids migrated from downdip over-pressurized sediments into the fracture network, diverting slow slip to multiple faults in the upper plate. Five ocean-bottom seismometers (OBS) were subsequently deployed in Oct. 2018–Oct. 2019 near the subducted seamount. An offshore SSE occurred in March/April 2019 during the OBS deployment, presenting an excellent opportunity to compare the relationship between burst-type repeating earthquakes and the 2019 SSE with the results of the 2014 study. Using the OBS and template matching, we find shallow burst-type repeating earthquakes with similar characteristics to those identified in 2014, but with higher intensity activity than in 2014. Most of the repeating events occur toward the end and after the 2019 SSE and concentrate at the down-dip edge of the subducted seamount. We conclude that similar to the 2014 event, the 2019 SSE also triggered fluid migration into the upper plate fracture network above the subducted seamount, causing slow slip on multiple upper plate faults and driving repeating earthquake activity. This is additional evidence that fluid migration and seamount subduction are major influences on the mechanics of shallow slow slip and microseismicity at the northern Hikurangi margin.