Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS V)

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Barker, D.H.N.; Wallace, L.M.; Woods, K.; Savage, M.; TAN1809 Science Party 2019 Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS V). Lower Hutt, N.Z.: GNS Science. GNS Science report 2019/16. 29 p. + 4 appendices; doi: 10.21420/Z4FK-SK46. 

 

Abstract:
HOBITSS V was a 12-day (6–18 October 2018) Wellington to Wellington R/V Tangaroa voyage (TAN1809). Objectives of TAN1809 were to undertake seafloor geodetic and seismic instrument deployments, recoveries, and surveys offshore the Gisborne, Mahia, and Wairarapa coasts. The seafloor geodetic aims involve using Bottom Pressure Recorders (BPRs) to determine the cm-level vertical movement (upward or downward) of the seafloor during slow slip events and to deploy two new precision transponder arrays to monitor cmlevel horizontal tectonic movements of the seafloor (using GPS-Acoustic techniques) near the Hikurangi Trench. We also surveyed an existing seafloor transponder array offshore Gisborne using GPS-Acoustic methods. Ocean bottom seismometers (OBSs) were deployed and will be used to compare the generation of “fast-slip” earthquakes with slow-slip earthquakes and will complement data coming from the BPRs and monitoring instruments installed in International Ocean Discovery Program boreholes offshore Gisborne. Next generation seafloor instruments (developed by Lamont Doherty Earth Observatory) that combine pressure and seismic sensors (POBS) were also deployed to test their effectiveness. The offshore Gisborne to Hawkes Bay region was targeted due to the large and frequent slow slip events that occur there. Offshore Wairarapa and Hawkes Bay were targeted for GPS-A array deployments to determine the degree of locking near the Hikurangi Trench, which is currently not constrained. Five Bottom Pressure Recorders (BPRs) belonging to Kyoto University and four BPRs belonging to University of Texas and GNS Science were recovered during the voyage (nine recoveries). These BPRs were deployed in June 2017 (on TAN1705), and recorded data for one year. Five BPRs belonging to Kyoto University, five BPRs belonging to Lamont Doherty Earth Observatory and five BPRs belonging to the University of Texas and GNS Science were deployed. Five OBS belonging to the Earthquake Research Institute, University of Tokyo (ERI) were deployed on the voyage; these will be retrieved on a voyage planned for 2019. Two new GPS-A transponder arrays (consisting of three precision transponders per array) were deployed by Scripps Institution of Oceanography and GNS Science. They will be surveyed with a Waveglider in January of 2019. In all, 15 BPRs, five OBS, three POBS and two GPS-A arrays (consisting of a total of six transponders) were deployed (29 instrument deployments). A GPS-A survey of a transponder array offshore Gisborne was undertaken for approximately 22 hours, which will result in a position for the array accurate to within a few cm. All voyage objectives underpin research funded by an MBIE Endeavour Fund programme: “Diagnosing peril posed by the Hikurangi subduction zone: New Zealand’s largest plate boundary fault.” (auth)