17:15 〜 19:15
[SCG52-P06] Monitoring the electrical features of the overlying plate in the Northern Hikurangi subduction zone, New Zealand, using ocean bottom electromagnetometers
キーワード：ヒクランギ沈み込み帯、海底電磁気探査、電気伝導度、モニタリング、スロースリップイベント
The Hikurangi subduction zone off the North Island of New Zealand offers significant opportunities for observing various types of fault slips along the plate interface. There is a prominent fault segment boundary in the central part of the Hikurangi subduction zone, which shows a sharp contrast in interplate locking strength, coinciding with the direction of plate convergence. In the north of the segment boundary, slow slip events occur with a relatively constant interval of approximately two years. The seismicity associated with these events has been determined at a depth of approximately 10 km below the seafloor of the overlying plate, which is relatively shallow compared to other subduction systems in which the occurrence of slow slip events has been recognized. These features benefit the monitoring of the events and studying their mechanism by seafloor geophysical observations because the closeness to the event source would provide more accurate observations and the repeatable nature would facilitate hypothesis testing. Therefore, for more than a decade, an international collaborative project between marine geophysical groups in Japan and New Zealand has been conducting continuous observations using ocean bottom seismometers and ocean bottom pressure gauges in this area. We initiated electromagnetic (EM) observations using ocean bottom electromagnetometers (OBEMs), joining this project.
A leading hypothesis for the cause of slow slip events is the migration of fluids in the crust. EM exploration is an alternative method for imaging the distribution of fluid in the crust, independently of other geophysical surveys such as seismological observations. Because fluids are much more conductive than the crust forming rocks, the high conductivity zone imaged in the crust can be interpreted to the zone with high porosity filled with fluid. Chesley et al. (2021) demonstrated a two-dimensional electrical conductivity structure model across the northern Hikurangi Trough obtained by joint analysis of magnetotelluric (MT) and controlled-source EM (CSEM) data. They argued that a high conductivity anomaly over a bulged plate boundary, which is coincident with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event, can be interpreted as a fluid-rich damage zone formed by modulating the fluid overpressure associated with the subducting seamount. However, the electrical conductivity structure model is only a snapshot and therefore it cannot give information of temporal variation of fluid overpressure.
In this study, we aimed to determine the temporal variation of electrical conductivity by a long-term continuous EM observation. A sensitivity study by forward modeling showed that the MT responses significantly change in the periods between 10 and 1000 s if the conductivity value (~0.5 S/m) of the fluid rich zone imaged by Chesley et al. (2021) changes 1.8 times more conductive or 1.8 times more resistive, which corresponding to 40% and 20% in porosity, respectively. We focused on this conductive anomaly. Our strategy is to monitor the MT responses over the conductive anomaly for several years, which covers slow slip events in the future by iterating a one-year deployment of OBEMs. To achieve one-year observation, we improved the existing OBEMs so as to mount more batteries and to introduce a sampling mode in which the electric filed measurements can continue with high sampling rate (8Hz) but the magnetic field measurement, which consumes power, is conducted intermittently (every 10 minutes). In October 2023, we deployed three OBEMs along the survey line of Chesley et al. (2021) where one site was just over the conductivity anomaly and the other two sites located approximately 5 km northwest and 17 km southeast of the first site. These OBEMs were successfully recovered in October 2024 and deployed at the same sites again. In addition, we built a land magnetic station in Waitārere, the west coast of the Northern Island, in September 2024. The magnetometer is driven by a commercial power supply, and the data can be retrieved in real time via internet. The land magnetic data can be used to complement the low sampling rate of the magnetic field measurement by the OBEM.
We check the repeatability and temporal variation of the MT responses by comparing them with the responses reported by Chesley et al. (2021). A brief summary of the preliminary analysis of the OBEM data retrieved in 2024 will be demonstrated in the presentation.
In December 2024, a large slow slip event occurred in the study area. We anticipate that the OBEMs on the seafloor recorded signals associated with the event.
A leading hypothesis for the cause of slow slip events is the migration of fluids in the crust. EM exploration is an alternative method for imaging the distribution of fluid in the crust, independently of other geophysical surveys such as seismological observations. Because fluids are much more conductive than the crust forming rocks, the high conductivity zone imaged in the crust can be interpreted to the zone with high porosity filled with fluid. Chesley et al. (2021) demonstrated a two-dimensional electrical conductivity structure model across the northern Hikurangi Trough obtained by joint analysis of magnetotelluric (MT) and controlled-source EM (CSEM) data. They argued that a high conductivity anomaly over a bulged plate boundary, which is coincident with the location of burst-type repeating earthquakes and seismicity associated with a recent slow slip event, can be interpreted as a fluid-rich damage zone formed by modulating the fluid overpressure associated with the subducting seamount. However, the electrical conductivity structure model is only a snapshot and therefore it cannot give information of temporal variation of fluid overpressure.
In this study, we aimed to determine the temporal variation of electrical conductivity by a long-term continuous EM observation. A sensitivity study by forward modeling showed that the MT responses significantly change in the periods between 10 and 1000 s if the conductivity value (~0.5 S/m) of the fluid rich zone imaged by Chesley et al. (2021) changes 1.8 times more conductive or 1.8 times more resistive, which corresponding to 40% and 20% in porosity, respectively. We focused on this conductive anomaly. Our strategy is to monitor the MT responses over the conductive anomaly for several years, which covers slow slip events in the future by iterating a one-year deployment of OBEMs. To achieve one-year observation, we improved the existing OBEMs so as to mount more batteries and to introduce a sampling mode in which the electric filed measurements can continue with high sampling rate (8Hz) but the magnetic field measurement, which consumes power, is conducted intermittently (every 10 minutes). In October 2023, we deployed three OBEMs along the survey line of Chesley et al. (2021) where one site was just over the conductivity anomaly and the other two sites located approximately 5 km northwest and 17 km southeast of the first site. These OBEMs were successfully recovered in October 2024 and deployed at the same sites again. In addition, we built a land magnetic station in Waitārere, the west coast of the Northern Island, in September 2024. The magnetometer is driven by a commercial power supply, and the data can be retrieved in real time via internet. The land magnetic data can be used to complement the low sampling rate of the magnetic field measurement by the OBEM.
We check the repeatability and temporal variation of the MT responses by comparing them with the responses reported by Chesley et al. (2021). A brief summary of the preliminary analysis of the OBEM data retrieved in 2024 will be demonstrated in the presentation.
In December 2024, a large slow slip event occurred in the study area. We anticipate that the OBEMs on the seafloor recorded signals associated with the event.