Effects of smectite to illite transformation on the frictional strength and sliding stability of intact marine mudstones

Geophysical Research Letters
By: , and 



At subduction zones, earthquake nucleation and coseismic slip occur only within a limited depth range, known as the “seismogenic zone”. One leading hypothesis for the upper aseismic-seismic transition is that transformation of smectite to illite at ∼100–150°C triggers a change from rate-strengthening frictional behavior that allows only stable sliding, to rate weakening behavior considered a prerequisite for unstable slip. Previous studies on powdered gouges have shown that changes in clay mineralogy alone are unlikely to control this transition, but associated fabric and cementation developed during diagenesis remain possible candidates. We conducted shearing experiments designed specifically to evaluate this hypothesis, by using intact wafers of mudstone from Ocean Drilling Program Site 1174, offshore SW Japan, which have undergone progressive smectite transformation in situ. We sheared specimens along a sawcut in a triaxial configuration, oriented parallel to bedding, at normal stresses of ∼20–150 MPa and a pore pressure of 1 MPa. During shearing, we conducted velocity-stepping tests to measure the friction rate parameter (a-b). Friction coefficient ranges from 0.28–0.40 and values of (a-b) are uniformly positive; both are independent of clay transformation progress. Our work represents the most direct and comprehensive test of the clay transformation hypothesis to date, and suggests that neither illitization, nor accompanying fabric development and cementation, trigger a transition to unstable frictional behavior. We suggest that strain localization, in combination with precipitation of calcite and quartz, is a viable alternative that is consistent with both field observations and recent conceptual models of a heterogeneous seismogenic zone.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Effects of smectite to illite transformation on the frictional strength and sliding stability of intact marine mudstones
Series title Geophysical Research Letters
DOI 10.1029/2012GL051761
Volume 39
Issue 11
Year Published 2012
Language English
Publisher Wiley
Contributing office(s) Earthquake Science Center
Description 6 p.
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Geophysical Research Letters
Country Japan
Other Geospatial Philippine Sea Plate;Eurasian Plate
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