2:15 PM - 2:30 PM
[SCG58-08] Temporal Variation of P- and S-wave attenuation in Fluid Pathway
Keywords:Slow Slip event, fluid, attenuation
We investigate the long-term temporal variation of Qp-1, Qs-1, and Qp-1/Qs-1 beneath the southwestern part of Ibaraki Prefecture, one of the most seismically active area in the Kanto district, Japan, from October 2009 to August 2021, and discuss the spatiotemporal correlation with the surrounding seismic activity and the dominant attenuation mechanism.
A megathrust cluster develops beneath the southwestern part of Ibaraki Prefecture in the NW-SE direction along the upper boundary of the subducting Philippine Sea plate at a depth of 40-60 km. The Megathrust cluster contains repeating earthquakes, and its activity suggests the short-term slow slip events with a period of about one year. Within the overlying plate, a seismic cluster (supra-slab cluster) has been observed at depth of 25-35km directly above the Megathrust cluster, and the number of supra-slab earthquakes increases periodically. Nakajima and Uchida (2018) found that the SSE on the megathrust, the Qp-1 and supra-slab cluster within the overlying plate are spatio-temporally correlated, and proposed drainage from the plate boundary to the overlying plate by the SSE.
Comparison of Q-1 for P and S waves can provide constraints on the dominant attenuation mechanism due to differences in the propagation characteristics of the two body waves. For example, in the interior of the mantle (the athenosphere), which comprises most of the Earth, rocks exhibit ductile behavior, so mechanisms due to grain boundary interactions and intragranular relaxation (simplified in the Burgers model ) dominate the internal attenuation and enhance the attenuation of S-waves rather than P-waves. On the other hand, higher attenuation of P waves than S waves is often observed in the lithosphere, such as the crust and the uppermost mantle, for which other dominant mechanisms should be considered. The dissipation of wave energy due to thermoelastic relaxation and fluid flow is a typical example. In this study, we investigate the time variation of Qp-1 and Qs-1 within the overlying plate in the southwestern part of Ibaraki Prefecture, where the existence of liquid fluid has been suggested by previous studies, and consider the dominant mechanism of attenuation.
Using 2156 seismic waveforms recorded from October 2009 to August 2021 at eight stations of the MeSO-net (Metropolitan Seismic Observation network), we adopted the method of Nakajima and Uchida (2018) and investigate the temporal variation of Qp-1 and Qs-1 at 20-45 Hz. For seismic activity analysis, we used supra-slab earthquakes from the hypocenter catalog of the Japan Meteorological Agency (JMA) and the catalog of repeating earthquakes determined by Igarashi (2020). The analysis showed that both Qp-1 and Qs-1 show a temporal variation of about one year cycle, and a spatio-temporal correlation between SSE, Q-1, and the number of supra-slab earthquakes was observed. Furthermore, Qp-1>Qs-1 was observed throughout the analysis period, and Qp-1/Qs-1 increased accompanied with the occurrence of SSEs. Given the presence of liquid fluid pre-existing fractures in the study region , these results may suggest that compressional waves generate fluid flow (wave-induced fluid flow, WIFF) on a mesoscopic scale between cracks and background pores filled with liquid fluid. Generally, it is believed that the high confining pressure in our target area renders WIFF unlikely to occur due to crack closure, but the hypotheses may be reinforced by examining the time variation of S-wave anisotropy, etc. in future studies.
A megathrust cluster develops beneath the southwestern part of Ibaraki Prefecture in the NW-SE direction along the upper boundary of the subducting Philippine Sea plate at a depth of 40-60 km. The Megathrust cluster contains repeating earthquakes, and its activity suggests the short-term slow slip events with a period of about one year. Within the overlying plate, a seismic cluster (supra-slab cluster) has been observed at depth of 25-35km directly above the Megathrust cluster, and the number of supra-slab earthquakes increases periodically. Nakajima and Uchida (2018) found that the SSE on the megathrust, the Qp-1 and supra-slab cluster within the overlying plate are spatio-temporally correlated, and proposed drainage from the plate boundary to the overlying plate by the SSE.
Comparison of Q-1 for P and S waves can provide constraints on the dominant attenuation mechanism due to differences in the propagation characteristics of the two body waves. For example, in the interior of the mantle (the athenosphere), which comprises most of the Earth, rocks exhibit ductile behavior, so mechanisms due to grain boundary interactions and intragranular relaxation (simplified in the Burgers model ) dominate the internal attenuation and enhance the attenuation of S-waves rather than P-waves. On the other hand, higher attenuation of P waves than S waves is often observed in the lithosphere, such as the crust and the uppermost mantle, for which other dominant mechanisms should be considered. The dissipation of wave energy due to thermoelastic relaxation and fluid flow is a typical example. In this study, we investigate the time variation of Qp-1 and Qs-1 within the overlying plate in the southwestern part of Ibaraki Prefecture, where the existence of liquid fluid has been suggested by previous studies, and consider the dominant mechanism of attenuation.
Using 2156 seismic waveforms recorded from October 2009 to August 2021 at eight stations of the MeSO-net (Metropolitan Seismic Observation network), we adopted the method of Nakajima and Uchida (2018) and investigate the temporal variation of Qp-1 and Qs-1 at 20-45 Hz. For seismic activity analysis, we used supra-slab earthquakes from the hypocenter catalog of the Japan Meteorological Agency (JMA) and the catalog of repeating earthquakes determined by Igarashi (2020). The analysis showed that both Qp-1 and Qs-1 show a temporal variation of about one year cycle, and a spatio-temporal correlation between SSE, Q-1, and the number of supra-slab earthquakes was observed. Furthermore, Qp-1>Qs-1 was observed throughout the analysis period, and Qp-1/Qs-1 increased accompanied with the occurrence of SSEs. Given the presence of liquid fluid pre-existing fractures in the study region , these results may suggest that compressional waves generate fluid flow (wave-induced fluid flow, WIFF) on a mesoscopic scale between cracks and background pores filled with liquid fluid. Generally, it is believed that the high confining pressure in our target area renders WIFF unlikely to occur due to crack closure, but the hypotheses may be reinforced by examining the time variation of S-wave anisotropy, etc. in future studies.