I. [Heidi Houston] Introduction
A new, multi-scale framework for understanding the coupled phenomena of slow slip and low-frequency seismic radiation in Cascadia. As a consequence of the major investments in instrumentation and focused research in Cascadia, particularly in the last 5 years, we can now view these coupled phenomena over an unprecedented range of temporal and spatial scales. Emerging findings about slow slip and low-frequency seismic radiation in Cascadia are revealing:

    A. similarities and differences with regions elsewhere in the world.
    B. spatial coherence implying underlying organization over distances of kilometers to the entire span of the subduction zone.
    C. significant temporal variations in propagation velocities, durations of energy release, etc. range from minutes to weeks.
    D. the true degree of repeatability and periodicity.
    E. tantalizing correlations with when and where earthquakes occur.

II. Background

    A. [Heidi Houston] Cartoon illustrating the ‘basics’
    B. [Heidi Houston] Implications for basic science. What hypotheses about underlying physical processes can we now test?

        i. Frictional model predictions.
        ii. [Simon Peacock] Postulated roles of fluids.


    C. [David Schmidt] Implications for earthquake hazards.

III. The spatial and temporal evolution of slow slip and tremor from 2007 and beyond.

    A. [Ken Creager, Evelyn Roeloffs, Tim Melbourne] Multi-year, Cascadia-wide evolution of slow slip and tremor. Comparison to elsewhere (e.g. Japan), implies processes coherent over 100s of km, analogous to how earthquakes start/stop?

        i. 2007-2008 Time-history of tremor and slow slip from Mendocino to northern Vancouver Island
        ii. 2007-2008 Map with slip and time/color-coded tremor?


    B. [Ken Creager, Evelyn Roeloffs, Tim Melbourne] A high-resolution look at the 2008 ETS event. Kilometer-scale heterogeneity, variability in propagation velocities, degree to which tremor tracks slip or visa versa, parallels with earthquake ‘patchiness’.

        i. Map showing migration of tremor constrained by Big-Skitter array, time-series of slip from strain meter data, etc.

IV. What we now know.

    A. Patterns are recurrent, but with variability that is becoming quantifiable.
    B. [Simon Peacock, Michael Bostock] High pore-pressures would explain triggered and tidally modulated tremor and elevated Vp/Vs ratios. The former also could imply critically stressed conditions.
    C. Correlation exists between moment release in slow slip and radiated seismically
    D. Tremor is a likely robust proxy for slow slip; detectable slow slip always accompanied by tremor
    E. Earthquakes and tremor seem to be anti-correlated spatially.

V. What we now know that we wouldn’t ever have known without the investments of the last 5 years.

    A. [John Vidale, Simon Peacock] Slip and tremor are ‘patchy’ on scales of tens of km, activity migrates both smoothly and in jumps
    B. [Heidi Houston] The same menagerie of low-frequency radiating seismic events (LFEs, VLFs, etc.) occur in Cascadia as in Japan
    C. Slip and tremor occur beneath Oregon.
    D. [John Vidale] Tremor likely occurs all the time at some level.
    E. Radiating low frequency seismic events likely have limited maximum sizes but no lower limit.
    F. [Paul Bedrosian] Map of instrumentation (likely not to include in a paper, but for our own use).

VI. What we now know we don’t know (or know well).

    A. [Michael Bostock] What and where is the plate interface? How localized is slip on this interface?
    B. [Simon Peacock] How are the tremor sources distributed in depth?
    C. How much energy is released seismically and what fraction of the accumulating moment budget does it represent? How to define ‘size’ for seismically manifest ‘events’?

VII. Future directions/questions

    A. What controls scale; limits the size and heterogeneity of slow slip and low frequency seismic events?
    B. What do these phenomena tell us about the seismogenic potential of the Cascadia interface (i.e. about plate coupling)
    C. Are there ‘communications’ with crustal faulting and seismicity?
    D. Do very long-duration transient occur?