of 2
Bulletin
of
the
Seismological
Society
of
America,
Vol.
75,
No.3,
pp.
891-892,
June
1985
REPLY
TO
H.
ACHARYA'S
"COMMENTS
ON
'SEISMIC
POTENTIAL
ASSOCIATED
WITH
SUBDUCTION
IN
THE
NORTHWESTERN
UNITED
STATES"'
BY
THOMAS
H.
HEATON
AND
HIROO
KANAMORI
Subduction
in
the
northwestern
United
States
presents
us
with
a dilemma.
Although
there
is good
evidence
of
3 to
4 cm/yr
of
convergence
between
the
Juan
de
Fuca
and
North
American
plates,
the
occurrence
of
either
historic
or instrumen-
tally
recorded
shallow
thrust
earthquakes
is remarkably
low.
Why
aren't
there
more
earthquakes?
Aseismic
slip
along
the
entire
plate
boundary
provides
a convenient
explanation
for
this
dilemma.
Aseismic
slip
appears
to
be
the
predominant
mode
for
plate
interaction
for
many
subduction
zones.
However,
the
Juan
de
Fuca
subduction
zone
is
clearly
different
from
the
most
common
class
of
aseismic
subduction
zone
that
is characterized
by
the
subduction
of
very
old
oceanic
lithos-
phere
(Heaton
and
Kanamori,
1984).
We
noted
that,
in
general,
the
subduction
of
young
lithosphere
is characterized
by
strong
interplate
seismic
coupling.
The
Juan
de
Fuca
subduction
zone
can
be
considered
as
an
end
member
in
that
it involves
some
of
the
youngest
subducted
lithosphere
observed
anywhere.
This,
in
itself,
suggests
that
the
Juan
de
Fuca
subduction
zone
belongs
in
a class
(perhaps
aseismic)
by
itself.
However,
there
are
several
other
localities
where
comparably
young
crust
appears
to
be
subducting.
These
are
southern
Chile
between
42°
and
45°
south
latitude,
Colombia
near
2o
north
latitude,
and
the
Rivera
plate
off
western
Mexico
.
.All
of
these
regions
are
seismically
active
and,
in
the
case
of
Colombia
and
southern
Chile,
have
involved
earthquakes
with
energy
magnitudes
of
8.8
(1906)
and
9.5
(southern
half of
the
1960
rupture
zone),
respectively.
Furthermore,
these
regions
of
Colombia
and
southern
Chile
do
not
have
bathymetric
trenches,
and
there
is no
significant
seismic
activity
deeper
than
100
km
observed
on
their
Benioff-
Wadati
zones.
There
is also
evidence
that
these
regions
have
experienced
significant
periods
of
seismic
quiescence.
The
NOAA
catalog
shows
a remarkable
absence
of
shallow
activity
between
41
o and
45°
south
latitude
along
the Chile
trench
for
at
least
30
yr
prior
to
the
1960
Mw
9.5
Chilean
earthquake.
Unfortunately,
this
catalog
is
not
sufficiently
complete
to
allow
a
comparison
of
seismicity
at
small
magnitude
earthquakes.
However,
at
a magnitude
cutoff
of
6,
the
rate
of
seismicity
in
the
50
yr
preceding
the
1960
earthquakes
in
the
region
between
41
o
and
45o
south
latitude
seems
comparable
to
that
reported
for
the
Juan
de
Fuca
subduction
zone's
150-yr
history
(the
Juan
de
Fuca
convergence
rate
is
about
one-third
that
of
southern
Chile).
Acharya
(1981)
has
estimated
that
1.8
cm/yr
of
aseismic
slip
on
the
Juan
de
Fuca
subduction
zone
is
indicated
by
the
rate
of
historic
volcanism
observed
in
the
Cascade
ranges.
The
formula
he
uses
is
AS
=
5.55N
+
0.83,
where
AS
is
the
aseismic
slip
rate
(cm/yr),
and
N
is
the
number
of
eruptions
per
year
per
1000
km
of
trench.
Although
this
relationship
may
be
approximately
valid
for
many
subduction
zones,
it
leads
to
the
conclusion
that
the calculated
aseismic
slip
must
always
be
greater
than
0.83
cm/yr,
regardless
of
the
plate
convergence
rate
or
the
volcanism
rate.
For
the
Juan
de
Fuca
subduction
zone,
Acharya's
estimated
aseismic
slip
value,
1.8
cm/yr,
is
not
much
larger
than
this
minimum
891
892
LETTERS
TO
THE
EDITOR
value.
Furthermore,
a model
in
which
the
Juan
de
Fuca
subduction
zone
is
strongly
coupled
would
require
an
absence
of
Cascade-type
volcanism.
However,
Cascade-
type
volcanism
is common
in
regions
of
great
subduction
earthquakes.
For
example,
the
volcanism
rates
in
Colombia
and
southern
Chile
are
similar
to
that
observed
in
the
northwestern
United
States
when
scaled
for
convergence
rate.
Thus,
it is difficult
to
interpret
the
presence
of
such
volcanoes
as
evidence
that
large
earthquakes
will
not
occur.
We
feel
that
the
basic
problem
remains.
How
can
there be
convergence
at
3 to
4
em/year
and
such
a remarkable historic
paucity
of
shallow
thrust
earthquakes?
Lacking
direct
evidence
of
great
prehistoric
earthquakes
or
of
aseismic
slip,
we
have
no
clear
answer
to
this
question,
and
we
reiterate
our
previous
conclusion:
"Although
we
cannot
completely
rule
out
the
possibility
that
the
plate
motion
is
being
accommodated
by
aseismic
creep,
we
find
that
the
Juan
de
Fuca
subduction
zone
shares
many
features
with
other
subduction
zones
that
have
experienced
great
earthquakes
...
and
there
is
sufficient
evidence
to
warrant
further
study
of
the
possibility
of
great
subduction
zone
earthquakes
in
the
Pacific
Northwest."
REFERENCES
Acharya,
H.
(1981).
Volcanism
and
aseismic
slip
in
subduction
zones,
.f.
Geophys.
Res.
86,
335-344.
Heaton,
T. H.
and
H.
Kanamori
(1984).
Seismic
potential
associated
with
subduction
in
the
Northwestern
United
States,
Bull.
Seism.
Soc.
Am.
74,
933-941.
U.S.
GEOLOGICAL
SURVEY
EARTHQUAKES,
VOLCANOES
AND
ENGINEERING
525
SOUTH
WILSON
AVENUE
PASADENA,
CALIFORNIA
91106
(T.H.)
Manuscript
received
29
January
1985
SEISMOLOGICAL
LABORATORY
CALIFORNIA
INSTITUTE
OF
TECHNOLOGY
PASADENA,
CALIFORNIA
91125
(H.K.)