Monday, October 22, 2018

Slow Slip and Fluid Drainage could be a Reason for Megathrust Earthquakes

Megathrust Earthquakes
The most powerful earthquakes are the Megathrust Earthquakes that occur at the subduction zones. Here we see one tectonic plate is pushed below the other. On the other hand, there is the slow slip events (SSEs) which release seismic stress at a much lower rate as compared with the large earthquakes. These occur in a cyclical manner across months to years. These SSEs can occur along the megathrust or any other areas which experience weakness due to loading. As a result, there can be release of low frequency seismic waves.

How the fluid which drainsfrom the SSEs can influenceseismic activity has been a topic of research for scientists fromthe Tokyo Institute of Technology and Tohoku University.

Impact of Fluid Drainage on Megathrust earthquakes

It is known that when megathrust earthquakes occur, the megathrusts open up. The fluid that drains out from the megathrusts is due to deformation. Not much is known whether the fluid movements that occur is due to the SSEs or not.

According to professor Junichi Nakajima at Tokyo Institute of Technology and associate professor Naoki Uchida at Tohoku University, the megathrust seismic activity can also be due to the fluid draining from the slow slip.

In order to find whether there is a relationship between SSEs and seismic activities, they have conducted various studies with regard to the seismic activities that occur around the Philippine Sea Plate. They found that the boundary of the plate is where the earthquakes occurred time and again. They also found a relationship between seismic activity and estimated slip-rates. From this they concluded that the seismic activity that was seen above the megathrust of the Philippine Sea Plate kept changing according to the episodic cycles of the SSEs. There was a great amount of draining during the SSEs which occurred every year together with fluid draining into the overlying plate.

The researchers have studied waveform data from beneath Kanto, Japan from 2004 to 2015. Their analysis has been published in Nature Geoscience.

They have explained the role of pore-fluid pressure. The areas of slow slip have high pore-fluid pressure and hence are able to release the fluid into the other areas of the rock. The SSEs could cause the fluid to flow into the overlying rock areas if there was a fracture or a space in the rock bodies. This in turn will cause weakness in those areas resulting in earthquakes.

On the other hand, if there were no fractures or open spaces on the overlying plate, then it would not be possible for the fluid to move into it. The fluid would then be forced on to the megathrust area. This could eventually trigger megathrust earthquakes. From this they inferred that the slow slip could be a catalyst in triggering megathrust earthquakes.

Though stress modulation is a major contributory factor for the megathrust earthquakes, the fluid that flows due to the episodic SSE may also play an important role than what was believed earlier.

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