Jeffrey J. McGuire, John A. Collins, Pierre Gouédard, Emily Roland, Dan Lizarralde,
Margaret S. Boettcher, Mark D. Behn and Robert D. van der Hilst, Variations in earthquake rupture properties along the Gofar transform fault, East Pacific Rise, Nature Geoscience, 2012
On a global scale, seismicity on oceanic transform faults
that link mid-ocean ridge segments is thermally controlled1,2.
However, temperature cannot be the only control because the
largest earthquakes on oceanic transform faults rupture only
a small fraction of the area that thermal models predict to
be capable of rupture3–5. Instead, most slip occurs without
producing large earthquakes3,4,6. When large earthquakes
do occur, they often repeat quasiperiodically7,8. Moreover,
oceanic transform faults produce an order of magnitude
more foreshocks than continental strike-slip faults7,9. Here we
analyse a swarm of about 20,000 foreshocks, recorded on an
array of ocean-bottom seismometers, which occurred before a
magnitude 6.0 earthquake on the Gofar transform fault, East
Pacific Rise. We find that the week-long foreshock sequence
was confined to a 10-km-long region that subsequently acted as
a barrier to rupture during the mainshock. The foreshock zone
is associated with a high porosity and undergoes a3%decrease
in average shear-wave speed during the week preceding the
mainshock. We conclude that the material properties of fault
segments capable of rupturing in large earthquakes differ from
those of barrier regions, possibly as a result of enhanced
fluid circulation within the latter.We suggest that along-strike
variations in fault zone material properties can help explain
the abundance of foreshocks and the relative lack of large
earthquakes that occur on mid-ocean ridge transform faults. |
