Sediments May Control Location, Magnitude of Megaquakes
Boulder, Colo., USA: The world’s most powerful earthquakes strike at
subduction zones, areas where enormous amounts of stress build up as one
tectonic plate dives beneath another. When suddenly released, this stress
can cause devastating “megaquakes” like the 2011 Mw 9.0 Tohoku event, which
killed nearly 16,000 people and crippled Japan’s Fukushima Dai-ichi Nuclear
Power Plant. Now a study published in Geology suggests that
sediments atop the downgoing slab can play a key role in determining the
magnitude and location of these catastrophic events.
In this newly published study, a team led by Gou Fujie, a senior scientist
at the Japan Agency for Marine-Earth Science and Technology, used a trio of
geophysical methods to image the subducting sediments in the northeastern
Japan arc, where the Tohoku event occurred. The findings suggest that
variations caused by volcanic rocks intruded into these sediments can
substantially influence the nature of subduction zone earthquakes.
“Our imaging shows that the enormous amount of slip that occurred during
the 2011 Tohoku earthquake stopped in an area of thin sediments that are
just starting to subduct,” says Fujie. “These results indicate that by
disturbing local sediment layers, volcanic activity that occurred prior to
subduction can affect the size and the distribution of interplate
earthquakes after the layers have been subducted.”
Researchers first began to suspect that variations in subducting sediments
could influence megaquakes after the 2011 Tohoku event, when international
drilling in the northeastern Japan arc showed that giant amounts of slip
during the earthquake occurred in a slippery, clay-rich layer located
within the subducting sediments. To better understand the nature of the
downgoing slab in this region, Fujie’s team combined several imaging
techniques to paint a clearer picture of the subseafloor structure.
The researchers discovered there are what Fujie calls “remarkable regional
variations” in the sediments atop the downgoing plate, even where the
seafloor topography seems to be flat. There are places, he says, where the
sediment layer appears to be extremely thin due to the presence of an
ancient lava flow or other volcanic rocks. These volcanic intrusions have
heavily disturbed, and in places thermally metamorphosed, the clay layer in
which much of the seismic slip occurred.
Because the type of volcanism that caused sediment thinning in the
northeastern Japan arc has also been found in many areas, says Fujie, the
research suggests such thinning is ubiquitous—and that this type of
volcanic activity has also affected other seismic events. “Regional
variations in sediments atop descending oceanic plates appear to strongly
influence devastating subduction zone earthquakes,” he concludes.
FEATURED ARTICLE
Spatial variations of incoming sediments at the northeastern Japan arc
and their implications for megathrust earthquakes
Gou Fujie et al., fujie@jamstec.go.jp. URL:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G46757.1/583388/Spatial-variations-of-incoming-sediments-at-the
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