The clustered distribution of shallow slow earthquakes in the Nankai Trough has been attributed to different factors such as seamount subduction, pore fluid pressure, fluid migration, and sediment input. However, there is still a lack of comprehensive understanding of how these factors interact to generate slow earthquakes. We examined the seismic reflection profiles crossing four subducted seamounts off Muroto to understand how it deforms the accretionary wedge. Along-trough reflection profiles within the accretionary wedge were also used to infer the lithology of the underthrusted sediments. The seamounts are at different stages of subduction and their associated underplated sediments were identified. Comparison with sandbox models indicate that the underplated sediments are likely comprised of fluid-rich trench fill sediments. A negative polarity decollement and transparent underthrust is observed off Muroto. The transparent underthrust is interpreted as mudstone, while stratified underthrust sediments in other regions is interpreted as turbidites. By comparing with numerical simulations, we propose the following deformational history off Muroto: (1) subduction of the first seamount resulted in underplating of a large volume of fluid-rich trench fill sediments, (2) the underplated sediments are undergoing horizontal compression from subsequent subduction of the three seamounts resulting in high pore pressure consistent with previously reported low velocity zones, and (3) the horizontal compression may also result in fluid expulsion and these fluids migrate updip and get trapped because the mudstones serve as an impermeable cap. This mechanism accounts the aforementioned factors associated with slow earthquakes and likely controls the clustered distribution off Muroto.

A document by Paul Caesar M Flores. Click on the document to view its contents.

The giant 2011 Tohoku earthquake (M9.0) could be expected to induce an 8-class outer-rise earthquake at the Japan Trench. In order to assess the risk of tsunamis from outer-rise events, we carried out tsunami simulations using 33 simple rectangular fault models with 60 degrees dips based on geophysical studies of the Japan Trench. The largest tsunami resulting from these models, a fault 332 km long producing a 8.66 normal-faulting event, had a maximum height of 27.0 m. We tested variations of the predictions due to the uncertainties in the assumed parameters. Because seismic observations and surveys show that the dip angles of outer-rise faults range from 45 to 75 degrees, we calculated tsunamis from events on fault models with 45-75 degree dips. We tested a compound fault model with 75 degrees dip in the upper half and 45 degrees dip in the lower half. Rake angles were varied by plus-minus 15 degrees. We also tested models consisting of small subfaults with dimensions of about 60 km, models using other earthquake scaling laws, and models including dispersive tsunami effects. Predicted tsunami heights changed by 5-10% for dip angle changes, about 5-10% from considering tsunami dispersion, about 2% from rake angle changes, and about 1% from using the model with subfaults. The use of different earthquake scaling laws changed predicted tsunami heights by about 50% on average for the 33 fault models. We emphasize that the earthquake scaling law used in tsunami predictions for outer-rise earthquakes should be chosen with great care.

Seismic velocity structures have been estimated using ambient noise records observed by distributed acoustic sensing (DAS) technology. Previous studies have obtained S-wave velocity (Vs) structures along submarine cables using surface waves; however P-wave velocity (Vp) structure have seldom been constructed because ambient noise primarily contains surface waves. Here, we show P and Rayleigh wave extractions from ambient seafloor noise observed by DAS, and also estimate the Vp and Vs structures along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. To extract the P waves, we calculated the cross-correlation functions (CCFs) of the ambient noise and applied a frequency-wavenumber filter to the CCFs to remove the effects of slower surface waves. The results indicate that similar features can be obtained in the Vp and Vs profiles, and that the P wave extractions can be performed on days with poor weather conditions.

Seismic wave extractions have been performed using ambient noise records observed by distributed acoustic sensing (DAS) technology. Extractions of microseisms can be investigated at a local scale using such DAS records observed in the ocean. Here, we show P and Scholte wave extractions from ambient seafloor noise observed by DAS along a submarine cable deployed off Cape Muroto in the Nankai subduction zone, Japan. The P waves can be observed at a frequency band of 0.1–0.3 Hz and up to a distance of 6–7 km. The distance in which the P waves can be observed is controlled by the P incident angle and the DAS sensitivity to the observable apparent velocity. The effective extractions of P and Scholte waves are performed in correspondence to large significant wave heights of the sea surface, which indicate that these waves are originated from fluid distturbances at the sea surface.