Is where great trench earthquakes occur controlled by subduction
parameters or largely a sampling bias?
Seth Stein, Emile Okal, and Laura Swafford
Northwestern University
Hiroo Kanamori told the first author, then a first year graduate
student, that "any time you can explain 50% of the data, you should
write a paper." Kanamori himself did much better. Among his many
accomplishments, he led the development of methods for studying great
earthquakes and provided many of the most important results about
them. In addition, he pioneered most current ideas about the role of
such earthquakes in the subduction process, including the relation
between plate convergence rates and the seismic slip history and the
concept of "comparative subductology" - comparing different trenches'
behavior to explore physical processes.
An
important
question
about
subduction is whether the largest
(Mw > 8.5) trench earthquakes can
occur only at certain trenches as a
result
of
their
mechanical
properties, which may be describable
in terms of easily observed factors
such as convergence rate, plate age,
and trench sediment thickness. This
issue
is
important
both
for
understanding the mechanics of the
subduction process and for assessing
tsunami hazards. The challenge in
the latter application is that
although most trenches seem capable
of generating Mw > 8 earthquakes,
the danger of an oceanwide
or farfield (as opposed to local) tsunami,
is low for earthquakes with Mw <
8.5, significant for larger moment
magnitude, and extreme for Mw > 9.
Okal, 2005; Geller, 1976
Prompted by the great December 2004 Sumatra earthquake, we have been
revisiting these issues. Although Ruff and Kanamori (1980) proposed
that Mw 9 earthquakes occur only when young lithosphere subducts
rapidly, much of this correlation vanishes using new plate motion
rates and other data not then available. Among the problems are that
we do not know if the largest trench events were interplate thrusts,
and the possibility that due to short historic record some large
events were missed.
R=0.8
R=0.4
Another possibility is that thick trench sediments lubricate the
interface & allow rupture to propagate long distances, giving Mw >
8.5. Although the correlation still looks good, there are
counterexamples. For example, the Makran zone has 6000m sediment, but
the maximum Mw is 8. Hence thick sediment may be a necessary but not
sufficient condition. A further complication is that sediment
thickness varies along the long rupture zones of such earthquakes.
An
alternative
that
seems likely is that
instead of some trench
segments but not others
being prone to Mw > 8.5
events, these apparent
differences may simply
reflect
the
short
earthquake
history
sampled.
This
possibility
is
suggested
by
the
variability in rupture
mode
along
trench
segments. For example,
the
Nankai
Trough
history
shows
that
sometimes the entire
region slipped, whereas
in other intervals slip
was
divided
into
several events over a
few years (Ando, 1975).
Mw
~ 8.6
~ 8.4
~ 8.4
~ 8.2
~ 8.4
Not yet!
Similarly, because the seismic slip rate inferred assuming the Mw 9.6
1960 Chilean earthquake is a characteristic Chilean subduction
earthquake exceeds the convergence rate, then either earlier
earthquakes were smaller than the 1960 event, the average recurrence
interval is greater than observed in the last 400 years, or both.
PLATE MOTION
Stein et al., 1986
Hence if Mw > 8 events are three times more common than Mw 8.5,
following a Gutenberg-Richter prediction, then Mw > 8.5 will be rarer
and thus absent from the short record available for some trenches.
This effect will be enhanced if the larger earthquakes are rarer than
this prediction, which is the case globally. We explore this
possibility using numerical simulations of trench seismicity in which
the spatial variation along strike results simply from magnitude 8
earthquakes being rarer than magnitude 9’s.
Thus much remains to be learned about the relation of great thrust
fault earthquakes to subduction processes, and Hiroo Kanamori’s
insights and methods will remain at the core of such studies for
years to come.