Article
Volume 11, Number 12
24 December 2010
Q12010, doi:10.1029/2010GC003305
ISSN: 1525‐2027
Rapid, continuous streaking of tremor in Cascadia
Abhijit Ghosh, John E. Vidale, Justin R. Sweet, Kenneth C. Creager, Aaron G. Wech,
and Heidi Houston
Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA
(aghosh.earth@gmail.com)
Emily E. Brodsky
Department of Earth and Planetary Sciences, University of California, Santa Cruz,
California 95064, USA
[1] Nonvolcanic tremor is a recently discovered weak seismic signal associated with slow slip on a fault
plane and has potential to answer many questions about how faults move. Its spatiotemporal distribution,
however, is complex and varies over different time scales, and the causal physical mechanisms remain
unclear. Here we use a beam backprojection method to show rapid, continuous, slip‐parallel streaking
of tremor over time scales of several minutes to an hour during the May 2008 episodic tremor and slip
event in the Cascadia subduction zone. The streaks propagate across distances up to 65 km, primarily par-
allel to the slip direction of the subduction zone, both updip and downdip at velocities ranging from 30 to
200 km/h. We explore mainly two models that may explain such continuous tremor streaking. The first
involves interaction of slowly migrating creep front with slip‐parallel linear structures on the fault. The
second is pressure‐driven fluid flow through structurally controlled conduits on the fault. Both can be con-
sistent with the observed propagation velocities and geometries, although the second one requires unlikely
condition. In addition, we put this new observation in the context of the overall variability of tremor behav-
ior observed over different time scales.
Components: 5400 words, 9 figures, 1 animation.
Keywords: nonvolcanic tremor; tremor streak; spatiotemporal distribution; migration; fluid flow; Cascadia subduction zone.
Index Terms: 7230 Seismology: Seismicity and tectonics (1207, 1217, 1240, 1242); 8104 Tectonophysics: Continental
margins: convergent; 8045 Structural Geology: Role of fluids.
Received 23 July 2010; Revised 12 October 2010; Accepted 18 October 2010; Published 24 December 2010.
Ghosh, A., J. E. Vidale, J. R. Sweet, K. C. Creager, A. G. Wech, H. Houston, and E. E. Brodsky (2010), Rapid, continuous
streaking of tremor in Cascadia, Geochem. Geophys. Geosyst., 11, Q12010, doi:10.1029/2010GC003305.
1. Introduction
[2] Episodic tremor and slip (ETS) in the Cascadia
subduction zone (CSZ) provides an excellent oppor-
tunity to study the transition zone seismicity that takes
the form of nonvolcanic tremor (NVT). Slow slip
events may contribute to seismic hazard analyses, as
they occur downdip of the locked part of the sub-
duction fault, which produces large and destructive
earthquakes. Geodetic observations during slow slip
episodes delineate slipping patches on the plate
boundary smoothed over several days in time, and
several tens of kilometers in space. But recent high‐
resolution observations from a dense seismic array
indicate a more complex evolution of NVT (and
therefore ETS) spanning over a broad range of time
and length scales [Ghosh et al., 2009a; Ghosh et al.,
2010]. Hence, understanding styles of NVT propa-
Copyright 2010 by the American Geophysical Union 1 of 10

gation across a range of length and time scales may
give new insight into the tremormechanism, and help
clarify the physics of slow slip events.
[3] The broad region of tremor activity is well
mapped by envelope cross correlation (ECC) meth-
ods [e.g., Nadeau and Dolenc, 2005; Obara, 2002;
Wech and Creager, 2008]. But the spatial resolution
of ECC methods does not resolve fine‐scale spatio-
temporal details of NVT evolution. Different patterns
of tremor migration possibly indicate complex inter-
actions of various processes that govern the subduc-
tion boundary system. Interestingly, it appears that
tremor behavior markedly varies over the time scales
of observation. Over time scales of several days,
tremor activity releases moment from several distinct
patches [Ghosh et al., 2009a]. Over time scales of
several hours, slip‐parallel bands of tremor activity
migrate along strike with a velocity of ∼10 km/d
[Ghosh et al., 2010]. Over the time scale of several
minutes, low‐frequency earthquake (LFE) activity
suggests sporadic faster tremor migration in the
western Shikoku, Japan subduction zone [Shelly
et al., 2007a], and beneath the San Andreas Fault,
near Parkfield [Shelly, 2009]. How these different
NVT behaviors are linked to produce the overall
large‐scale ETS activity is poorly understood.
[4] Here we show rapid, continuous, slip‐parallel
faster tremor migration that produces streaks of
tremor within an ETS event in Cascadia, and explore
the physics of two simple models for generating
such tremor migration over the time scale of several
minutes to an hour. In addition, we combine dif-
ferent elements of spatiotemporal tremor distribu-
tion (i.e., tremor streaks, bands, and moment
patches), illustrate their relationship with each other,
and the slow slip event, and provide amore complete
picture of tremor distribution in space and time.
2. Data and Method
[5] We use seismic recordings of theMay 2008 ETS
event in northern CSZ by an 84‐element, small‐
aperture, vertical component seismic array, hence-
forth, the Big Skidder array. It was installed on the
Olympic Peninsula, Washington, USA, above the
migration path of this ETS event [Ghosh et al.,
2009a] (Figure 1). We use a beam backprojection
(BBP) method [Ghosh et al., 2009a] to detect and
locate tremor. The BBP method applied here stacks
and beamforms 3–8 Hz seismic energy using 5 min
sliding time windows with 50% overlap. Tremor is
detected and located from the beamformer output
assuming that NVT is occurring at the plate inter-
face. There is a growing consensus that tremor
occurs at the plate interface as a result of shear slip
on the fault plane [Brown et al., 2009; Ghosh et al.,
Figure 1. Location map and tremor streaks: colored circles in the maps represent tremor locations using the beam
backprojection method [Ghosh et al., 2009a]. Time is color coded to show tremor migration. Black solid square marks
the Big Skidder array. Arrows indicate overall slip direction of CSZ. Dashed contour lines shows plate interface depth
in km. Gray patches in Figures 1b and 1c show tremor moment patches [Ghosh et al., 2009a]; the darker the patch, the
higher the moment release. (a) Location map of the study area. Lines AB and CD are oriented parallel and per-
pendicular to the slip direction, respectively, and are used to generate Figure 3. Inset shows the station distribution of
the Big Skidder array. (b) Slip‐parallel tremor streak showing rapid downdip short‐term migration of tremor with a
horizontal velocity of 60 km/h. (c) Slip‐parallel tremor streak rapidly propagating updip with a horizontal velocity of
35 km/h.
Geochemistry
Geophysics
Geosystems G3 GHOSH ET AL.: TREMOR STREAKS IN CASCADIA 10.1029/2010GC003305
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