15:30 〜 15:45
[SCG44-07] Offshore tremor activities in relation to a slow slip event around the plate coupling transition zone in the central part of the Hikurangi subduction margin, New Zealand
キーワード：テクトニック微動、スロースリップイベント、プレート間固着強度
Onshore GNSS observation in the North Island of New Zealand has revealed episodic slow slip events (SSE) along the Hikurangi subduction margin. The recurrence interval, depth distribution and duration of the SSEs vary along the margin. The most distinctive and abrupt change in such SSE characteristics occurs at a marked change in plate coupling in the central part of the margin. To the north, SSEs occur at ~1-2 year intervals along the shallow part of the plate interface (< 12 km depth). To the south, SSE slip distribution occurs at much deeper depths (> 25 km), and the slip duration is longer. The plate interface geometry does not show noticeable variations across this transition zone, and, therefore, the along-strike temperature and lithostatic pressure conditions are considered uniform. While both changes in incoming plate roughness and upper-plate lithology have been suggested to explain the transition, the controlling factors of the plate coupling and slip behavior of the plate interface remain enigmatic.
The last SSE around the transition zone before our recent observation and research project, PULSE (Physically Understanding Localised Slip Episodicity), occurred in 2016. We deployed 5 short-period and 5 broadband ocean bottom seismometers (OBS) for a one year offshore observation in October 2020, aiming to capture the next occurrence of slow slip and accompanying seismicity in this transition region. During the deployment, in May 2021, an SSE occurred to the north of the plate coupling transition zone directly beneath the OBS network. We successfully recovered all OBSs in October 2021. Good quality data were obtained at all OBS stations. The OBS data revealed two sequences of tremor activities in relation to the SSE. The first sequence appears to have started in February 2021, about three months before the SSE, and the activity became most intense at the end of March, stopping in mid-April. The second sequence appears to be coincident with the SSE, but the intensity appears much weaker than the first sequence. There may have been migrations of tremor locations in both along-strike and along-dip directions, and the locking transition boundary appears to have acted as a physical barrier to migrating tremor activity.
The last SSE around the transition zone before our recent observation and research project, PULSE (Physically Understanding Localised Slip Episodicity), occurred in 2016. We deployed 5 short-period and 5 broadband ocean bottom seismometers (OBS) for a one year offshore observation in October 2020, aiming to capture the next occurrence of slow slip and accompanying seismicity in this transition region. During the deployment, in May 2021, an SSE occurred to the north of the plate coupling transition zone directly beneath the OBS network. We successfully recovered all OBSs in October 2021. Good quality data were obtained at all OBS stations. The OBS data revealed two sequences of tremor activities in relation to the SSE. The first sequence appears to have started in February 2021, about three months before the SSE, and the activity became most intense at the end of March, stopping in mid-April. The second sequence appears to be coincident with the SSE, but the intensity appears much weaker than the first sequence. There may have been migrations of tremor locations in both along-strike and along-dip directions, and the locking transition boundary appears to have acted as a physical barrier to migrating tremor activity.