16:15 〜 16:30
[SCG45-09] ヒクランギ沈み込み帯北部におけるテクトニック微動のエネルギーの時空間分布
キーワード：ヒクランギ沈み込み帯、微動、海域地震観測
In the Hikurangi subduction zone off the eastern coast of the North Island of New Zealand, the Pacific Plate is subducting beneath the Australian Plate at a rate of ~45 mm per year. The Hikurangi Plateau with the oceanic crust of a thickness of 10-15 km on the Pacific Plate is subducting. The relatively shallow subduction of the Pacific Plate has yielded higher resolution about the structure along the plate interface, and subducted seamounts are clearly imaged. The Hikurangi subduction zone can be divided into northern, central, and southern regions based on the characteristics of the depths, recurrence intervals and durations of Slow Slip Events (SSEs). SSEs in the northern region have the shortest recurrence intervals of about 1.8 years and durations of a couple of weeks, whereas SSEs in the southern region occurs along the deeper plate interface with longer durations.
At the northern part of this margin, tectonic tremor activities accompanying SSEs have been observed around one of the subducted seamounts. It is important to know the spatio-temporal distribution of the tectonic tremors to reveal the relationship among the subducted seamount, SSEs and tectonic tremors. Because those slow fault slip events occur offshore, it is difficult to observe tectonic tremors by the onshore seismic network.
In this study, we detected offshore tectonic tremors by applying the envelope correlation method and estimate the energy of tremors using ocean bottom seismometer (OBS) data acquired by the international collaborative offshore seismic and geodetic observation, HOBITSS(Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip) experiment, from May 2014 to June 2015 at the northern part of the Hikurangi subduction margin. As a result, we detected tectonic tremors 20 times as many as in a previous study. We eestimated that the seismic energy rate and seismic moment rate of those tremors are approximately 102-104 [ J ] and ~103 [ J/s ], respectively.
Because we used a smaller offshore observation network and set a lower threshold for detection, we could detect more events than the previous study. The seismic energy rates of the detected tremors are equivalent to those of deep tectonic tremors in other subduction zones such as the Nankai subduction zone, and this value is 1 to 2 order of magnitude smaller than those of shallow tectonic tremors.
The expected seismic moment rates of very low frequency earthquakes (VLFE) are expected to be approximately 1010~1012 [Nm/s] when the scaled energy between the seismic energy rate of tremor and the seismic moment rate of a VLFE is constant, ~10-9. The value is much smaller than those in other subduction zones, and this may be a reason why VLFEs have not been identified in the Hikurangi subduction zone.
There is a spatial seismic gap of the tectonic tremor activity accompanying the large SSEs at the top of the subducted seamount. We found that the average of the seismic energies of the events in the downdip of the seamount across the gap is twice as large as those in its updip. And when compared with the P-wave velocity structure, the tectonic tremors are located in the area where P-wave velocity around the plate interface is relatively higher.
At the northern part of this margin, tectonic tremor activities accompanying SSEs have been observed around one of the subducted seamounts. It is important to know the spatio-temporal distribution of the tectonic tremors to reveal the relationship among the subducted seamount, SSEs and tectonic tremors. Because those slow fault slip events occur offshore, it is difficult to observe tectonic tremors by the onshore seismic network.
In this study, we detected offshore tectonic tremors by applying the envelope correlation method and estimate the energy of tremors using ocean bottom seismometer (OBS) data acquired by the international collaborative offshore seismic and geodetic observation, HOBITSS(Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip) experiment, from May 2014 to June 2015 at the northern part of the Hikurangi subduction margin. As a result, we detected tectonic tremors 20 times as many as in a previous study. We eestimated that the seismic energy rate and seismic moment rate of those tremors are approximately 102-104 [ J ] and ~103 [ J/s ], respectively.
Because we used a smaller offshore observation network and set a lower threshold for detection, we could detect more events than the previous study. The seismic energy rates of the detected tremors are equivalent to those of deep tectonic tremors in other subduction zones such as the Nankai subduction zone, and this value is 1 to 2 order of magnitude smaller than those of shallow tectonic tremors.
The expected seismic moment rates of very low frequency earthquakes (VLFE) are expected to be approximately 1010~1012 [Nm/s] when the scaled energy between the seismic energy rate of tremor and the seismic moment rate of a VLFE is constant, ~10-9. The value is much smaller than those in other subduction zones, and this may be a reason why VLFEs have not been identified in the Hikurangi subduction zone.
There is a spatial seismic gap of the tectonic tremor activity accompanying the large SSEs at the top of the subducted seamount. We found that the average of the seismic energies of the events in the downdip of the seamount across the gap is twice as large as those in its updip. And when compared with the P-wave velocity structure, the tectonic tremors are located in the area where P-wave velocity around the plate interface is relatively higher.