Cascadia Transition Zone: Tremor as a Fault Strength Indicator

Cascadia Transition Zone: Tremor as a Fault Strength Indicator Displacement history profiles and transition zone model. (a) Cumulative tremor profiles in each strike-perpendicular bin showing a transition from small, frequent slip downdip to larger, less frequent slip updip. (b) Profiles of displacement timelines from the locked zone to stable sliding with results from a inserted in the transition zone. (c) Profile schematic showing how the different regions accommodate plate convergence. Our results may predict long-term slow slip updip (purple dashed line), which would shift the downdip limit of the megathrust (blue dashed line) updip. (d) Schematic profile of stress timelines illustrating our stress transfer model. Stable sliding loads the downdip tremor region, which is weakly coupled and slips easily. Each slip relieves stress locally and transfers stress updip to a stronger portion of the fault with a higher stress threshold. This is a fractal-like process where the local stress is the integrated effect of downdip slip.
As oceanic lithosphere descends beneath continents in subduction zones worldwide, the contact between the two plates under- goes a transformation in response to a variety of physical parameters that vary with increasing depth. The result of this transformation is a transition in fault coupling from fully locked on the shallow, updip side to stable sliding downdip where the oceanic plate descends into the mantle. But how this transition zone works is not entirely understood. Updip of tremor, the fault yields no displacement for hundreds of years, constantly accumulating stress before breaking in the form of a megathrust earthquake. On the downdip side, the plates are thought to stably slide past each other at a constant rate without increasing stress. By accumulating stress for months to years between discrete episodes of stable moment release, episodic tremor and slip (ETS) provides an intermediate mechanism for accommodating plate convergence between the locked and stable sliding end members in relatively young, warm subduction zones. We use automatically detected tectonic tremor as a slow slip indicator in northern Cascadia to observe updip migration and a depth-dependent transition in slip size and periodicity. Our observations fill in the transition zone spectrum with a continuum of slow slip behavior that reflects the fault strength. This behavior is explained by a fractal-like stress transfer model controlled by friction, which provides a new and intuitive understanding of subduction zone dynamics.
</p><p>Wech, A.G., and K.C. Creager (2010) Cascadia transition zone: tremor as a fault strength indicator, IRIS Meeting.</p>


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