Geos596F Subduction Zones: An Integrated View
Sept. 8, 2004
Topics discussed in Session 2
(Notes by Jerome Guynn, edited by Zandt)
Discussion Papers:
Miller, M. M., et al., Periodic Slow Earthquakes from the Cascadia Subduction Zone, Science,
295, 2002.
Dragert, H., et al., A Silent Slip Event on the Deeper Cascadia Subduction Interface, Science,
292, 2001.
Obara, K., Nonvolcanic Deep Tremor Associated with Subduction in Southwest Japan, Science,
296, 2002.
Julian, B., Seismological Detection of Slab Metamorhpism, Science, 296, 2002. (Perspective on
Obara, 2002)
Rogers, G. and Dragert, H., Episodic Tremor and Slip on the Cascadia Subduction Zone: The
Chatter of Silent Slip, Science, 300, 2003.
Melbourne, T.I. and Webb, Frank H., Slow But Not Quite Silent, Science, 300, 2003.
(Perspective on Roger & Dragert, 2003)
Calvert, A. J., Seismic reflection imaging of two megathrust shear zones in the northern Cascadia
subduction zone, Nature, 428, 2004.
One of the major points that came out of the meeting is that while silent slip and nonvolcanic tremors have been found to be correlated (Rogers & Dragert, 2003; see short
description and Figure on page 3), the current resolution of the data does not enable us to resolve
the exact relationship. Do the tremors start just before, at, or just after the start of the slip event?
Another unresolved question is what is the exact cause of the tremor? Is it due to a mechanical
effect along the fault (thus, “silent slip” would not be so silent) or is it an acoustical effect due to
fluids? If the latter, is it a result of liberated fluid as a result of dehydration reactions, which
might induce the slip event (Obara, 2002), or could the readjustment of stress open Mode I
cracks into which existing fluids can flow, as suggested by Mihai?
So far, only the Cascades and some areas in Japan have been studied for tremors and silent
slip events. Unfortunately, this is likely to remain the case for a while, since the resolution for
this work requires dense spacing of GPS and seismic stations, as well as very sensitive
seismometers. Thus, historical data may be of little use. The Earthscope and Plate Boundary
Observatory programs may help, as they will provide extra instrumentation along the western
North American plate boundaries, the Cascadia subduction zone and San Andreas transform fault
system. There are aseismic events along the San Andreas Fault, but it does not appear that
anyone has looked to see if tremors occur. If there were no tremors along transform faults during
silent slip events, this might add evidence for metamorphic reactions associated with tremors in
subduction zones.
Questions were brought up concerning the accuracy of the location of the earthquakes and
whether they could be confidently assigned to the fault (Obara, 2002). Megan mentioned that
while cross-correlation techniques are very good at determining the accuracy of the earthquakes
relative to each other, they were not as accurate for determining absolute depth. Thus, the whole
group could be moved up or down, taking them off the fault plane. However, Sue and George
pointed out that due to the dense network in Japan, there were always stations located more or
less above the earthquakes, which allows for accurate depth determination, unlike most globally
determined earthquake locations.
There were some questions regarding the Obara (2002) paper. The particular study area was
chosen partly because there is no volcanic activity above the slab, so that there was no confusion
between volcanic tremors and those related to slab events. However, it was pointed out that
since the volcanoes are thought to be a result of melting due to dehydration of the slab, if the
tremors are due to metamorphic dehydration reactions, why are there no volcanoes? A
counterpoint was that maybe it’s too shallow; the tremors occurred around 30 km depth.
However, why this would not lead to melting was not resolved. (Maybe not hot enough at 30
km?). Lara pointed out that the gap in tremors around the Kil Channel was mentioned, but no
attempt was made to explain it. A possible explanation is that the slab appears to only exist to
about 40 km at this location. The shallow edge of the Philippine Sea plate is a result of very
young subduction. Finally, the paper made a casual inference between the tremors and actual
earthquakes. However, the data was not particularly convincing, since some earthquakes
appeared to occur just before tremor events, some after and some during.
The question of triggering is important, though, because silent slip along the lower portion of
the “great thrust” will transfer stress to the upper, locked part of the fault (Dragert et al., 2001).
Therefore, silent slip events could precede an actual earthquake by increasing the stress along the
seismic portion of the fault. For example, this is thought to have occurred for the 1960 Chile
earthquake. Of course, at this point, data is too limited to say much on this issue. One of the
other major points is that there is simply not enough data, of enough quality, to currently address
some of the issues. In addition to further GPS and seismic data, some modeling or
experimentation could help resolve the issue of what causes the tremors, though George pointed
out that experiments involving fluids and rocks are quite difficult. Scale is also a major iss when
dealing with fault slip.
The Calvert (2004) paper
was not really discussed. It
interprets seismic reflection data
that appears to show the plate
interface at 40 km depth splits into
two splays that bounds a zone of
imbricated crust. Slow slip and the
associated tremor may be
occurring at both the roof and floor
thrusts (Fig. 4, right).