Localized fluid discharge by tensile cracking during the post-seismic period in subduction zones

Makoto Otsubo,Ayumu Miyakawa,Asuka Yamaguchi,Gaku Kimura,Jeanne L Hardebeck

doi:10.1038/s41598-020-68418-z

Abstract

It is thought that extensional structures (extensional cracks and normal faults) generated during the post-seismic period create fluid pathways that enhance the drainage of the subducting plate interface, thus reducing the pore pressure and increasing fault strength. However, it remains to be elucidated how much pore fluid pressure decreases by the extension crack formation. Here we examined (i) the pore fluid pressure decrease, and (ii) the degree fault strength recovery by the extension crack formation during the post-seismic period by analyzing extension quartz veins exposed around the Nobeoka Thrust, southwestern Japan. The Nobeoka Trust is an on-land analog of the modern splay fault at shallow depths (~ 8 km) in the Nankai Trough. The poro-elastic model of extensional quartz vein formation indicates that the formation of extensional cracks only releases up to ~ 7–8% of the total pore fluid pressure at ~ 8 km depth. The pore pressure around the Nobeoka Thrust was close to lithostatic pressure during the entire seismic cycle. The estimated effective frictional coefficient along the Nobeoka Thrust after this small fluid-loss by the extensional crack formation does not exceed 0.15. Hence, the pore fluid pressure reduction due to the post-seismic extensional cracks contributes little to increase the fault strength of the megasplay fault.

Highlights

It is thought that extensional structures generated during the post-seismic period create fluid pathways that enhance the drainage of the subducting plate interface, reducing the pore pressure and increasing fault strength
The key question arises: To what degree is pore fluid pressure reduced by the extension crack formation? The temporal change in pore fluid pressure given by Skempton’s relationship[7,8] is of the same order as the stress drop related to trench type earthquakes[6]
We examine temporal changes in pore fluid pressure during seismic cycles by analyzing extension mineral veins that are exposed along an ancient megasplay fault

Summary

Model for estimating pore fluid overpressure

The character of the tectonic stress regime (i.e., the principal compressive stresses, σ1 > σ2 > σ3) plays a critical role in the containment of pore fluid overpressure, with overpressures being much more sustained in compressional stress fields[20]. Extensional veins are formed when pore fluid pressure exceeds the sum of the minimum principal stress (σ3) and Ts26. The driving pore fluid pressure ratio (P*) is the ration between the pore fluid overpressure ΔPo and Δσ as follows: P∗ = Po/ σ = (Pf − σ3 − Ts)/(σ1 − σ3). We can calculate Δσ from the driving pore fluid pressure ratio P* and the pore fluid overpressure ΔPo rearranging Eq (4):. If the extensional veins are formed under a normal faulting type stress regime, σ1 = σv calculated as ρgz where ρ is the rock density, g is the gravitational acceleration and z is the depth.

Geological setting and extensional quartz veins around Nobeoka Thrust

Pore fluid overpressure in postfailure period around Nobeoka Thrust

Temporal change of pore fluid pressure around Megasplay fault

Limited increase of fault strength after formation of mode I cracks

Additional information