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Superquakes, Supercycles and Global Earthquake Clustering
Chris Goldfinger, Yasutaka Ikeda and Robert S. Yeats
EARTH Vol. 58 (No. 1), p. 34
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recent research and recent quakes
reveal surprises in major fault systems
damage in honshu, Japan, from the 2011 tsunami.
Chris Goldfinger, Yasutaka ikeda and robert s. Yeats
A
number of recent big earthquakes around the world have humbled many earthquake researchers.
The March 2011 magnitude-9 superquake off Tohoku, Japan, and the December 2004 magnitude9-plus temblor off Sumatra were both far larger than what scientists expected those fault systems to produce. Based on these quakes, and on recent research that contradicts long-held paradigms,
it is becoming clear that the types and sizes of large earthquakes that a given fault system is capable of
producing remain poorly known for most major fault systems.
34

EARTH January 2013
www.earthmagazine.org
u.s. navy photo by naval Air Crewman 1st Class Jay okonek/released
superquakes,
supercycles,
and global
Earthquake
clustering
Clockwise from top left: Chris Goldfinger; Jay Patton; U.S. Navy photo by Photographer’s Mate 2nd Class Philip A. McDaniel
Oregon State University scientists and Indonesian colleagues recover
a piston core from the Sunda trench off northern Sumatra on board
the Scripps vessel R/V Roger Revelle. Right: The core shows a long history of earthquakes.
The problem is that we are relying on
short historical records, and even shorter
instrumental records. Basing estimations
of maximum earthquake size or models of earthquake recurrence on such
records or on the behavior of smaller
earthquakes does not encompass the
full range of fault behavior. We can no
longer hold onto the notion that magnitude-9 earthquakes might be likely to
occur in some areas, but not in others.
So much remains to be learned about
great earthquakes
There are a few places with long and
detailed enough records to examine
long-term behavior of major fault systems: Northeastern Japan and Cascadia,
off the coast of Oregon, Washington and
British Columbia, are two of the beststudied. And thanks to new research
techniques, those two areas are offering
new lessons and hints at earthquakes
to come.
theory that plate age and convergence
velocity determine maximum earthquake size. And we have tried to apply
recurrence rate models, based on the
relationships between time before or
after an earthquake, to inform us about
the next earthquake. But all of these
models are built on the short instrumental and sometimes historical records.
As we begin to gather more long
records from around the world, we are
beginning to see more complex behavior. Strain may accumulate at even very
slow rates, sometimes in settings previously considered unlikely, eventually to
be released in very large earthquakes.
Such long-term cycling is suggested for
Cascadia, Hokkaido, northeastern Japan,
Chile and Sumatra.
In the case of northeastern Japan,
earthquakes of the past few hundred
years had dimmed the memory of the
Jogan superquake of A.D. 869, which
produced a tsunami that killed about
1,000 people on the Sendai Plain and was
probably similar in size to the Tohoku
quake. Models were largely based on
more recent quakes.
In Cascadia, models showing a limited range of expected quake sizes have
A village near the coast of Sumatra after the tsunami that struck Southeast Asia
the day after Christmas 2004.
Earthquakes
Reveal Model
Shortcomings
Scientists have developed detailed
conceptual models of quake behavior
and recurrence rates. We have the characteristic earthquake model, which suggests that faults have “typical” repeatable ruptures. We also have the seismic
gap theory, which suggests we should
look to holes in the instrumental record
for future events. We have relied on a
www.earthmagazine.org
EARTH January 2013 35

144°
146°
0
-5
Japan Sea
Hok
kai
do
44°
-10
1 1961 Mw 7.0
2
42°
2004
Mw 7.0
2.5
2003 Mw 8.3
1952 Mw8.1
0
-1
1901
Mw 7.5
1989
Mw 7.4
1896
Mw 8.2
1915
Mw 7.5
1978
Mw 7.6
6 1936
Mw 7.1
2011
Mw 9.0
1938
Mw 7.7
1933
Mw 8.4
1897
Mw 7.8
Trench
3
4
Long-term Cycling
in the Northeast
Japan Trench
82 mm/yr
Japan
7
hu
1938
Mw 7.8
Pacific Plate
1938
Mw 7.9
-2.5
Tokyo
5
Sendai
8
36°
lT
ri
u
K
1968 Mw 7.8
ch
ren
1968
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Ho
ns
NF
M
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ts
40°
1994
Mw 7.7
-5
uf
o ld
and th rust belt
1931
Mw 7.7
38°
1973 Mw 7.8
140°
142°
144°
146°
Map showing recent vertical crustal movements and source areas of large intraplate earthquakes. Blue line contours indicate rates of subsidence in millimeters
per year, as revealed by tide gauge observations from 1955 to 1981. Orange
lines indicate source areas of subduction earthquakes of magnitude 7 or greater
since 1896. The epicenter and source area of the 2011 Tohoku earthquake (magnitude 9) are indicated by a star and orange shading, respectively.
36 EARTH January 2013

been based on excellent but relatively
short paleoseismic records onshore.
However, recent research on undersea
deposits indicates that a wide range of
earthquake sizes has occurred there,
including quakes larger than previously
thought possible and smaller ones limited to southern Cascadia. In Sumatra,
models suggest that plate convergence
was so oblique that the plates were moving parallel to each other, and that a
subduction-type earthquake like the one
that occurred in 2004 was unlikely.
In general, relationships — such as
that between plate age and convergence
rate — were appealing but had never
really been tested. They were based on
dubious physical concepts and the short
instrumental record. These modeled
relationships were on shaky ground in
terms of both statistics and the unknown
nature of long-term energy storage and
release in subduction zones.
Scientists are now working to rewrite
these models based on new evidence.
There are a couple of ways to look
at the seismic hazard in northeastern
Japan, near where the Tohoku quake
struck. Using the historical record, especially of the most recent earthquakes
in the Sendai area — a magnitude 8.4
in 1933, a magnitude 7.1 in 1936, and a
magnitude 7.6 in 1978 — most scientists
considered the largest likely quake in
the region to be about 8.4. Those earlier
quakes did not leave widespread tsunami records. More numerous and perhaps better remembered are the smaller
events in recent decades and centuries
that make up the majority of the historical record; it was these smaller events
that drove the underestimation of the
seismic and tsunami hazard in northeastern Japan.
The historical record can be helpful,
however, if you look far enough back
in time; and if you include paleoseismic
and paleotsunami evidence, the picture
changes. The older historical record in
the Tohoku region, for example, reveals
a number of large earthquakes and
www.earthmagazine.org
Chris Goldfinger
142°
-2.5
140°
Clockwise from bottom left: Chris Goldfinger; Chris Goldfinger; Masato yamada; Chris Goldfinger
above: chris goldfinger digs a pit on the
sendai plain in 2011 after the tohoku
quake to sample tsunami deposits. left:
tsunami deposits from the a.d. 869
Jogan superquake can be seen in the
shallow subsurface on the sendai plain.
such deposits help create a more complete record of past quakes.
high tide in ishinomaki (shown in June
2011), a coastal town that was affected
by the march 2011 tsunami. permanent subsidence put most streets in this
neighborhood underwater at high tide.
www.earthmagazine.org
associated tsunamis, including large
events in A.D. 869, 1611 and 1896. The
largest of these events was the A.D. 869
Jogan tsunami, based on the presence
of tsunami deposits in a coastal lake
and across the Sendai and Ishinomaki
plains. The landward inundation of the
March 2011 tsunami penetrated 3.5 to
4 kilometers inland on the Sendai Plain,
similar to the Jogan tsunami (although at
the time of the Jogan event, the shoreline
was approximately 1 kilometer inland of
where it presently is). The paleotsunami
evidence also includes two predecessors
to the Jogan event, which also penetrated
approximately 4 kilometers inland. That
evidence supports the existence of “outsized” events — those about magnitude
9 or so — along the Tohoku coast with
recurrence times in the range of 800 to
1,200 years.
Looking at both long-term geology
and new instrument-based research
also provides a new view of the seismic
hazard in this region. For at least the
past 5 million years, the Japan arc has
been subjected to east-west compression due to convergence of the Pacific
Plate at a rate of 80 millimeters per year.
Investigating the structural geology of
northeastern Japan reveals that a foldand-thrust belt permanently relieves
an unsecured petroleum storage tank
swept across ishinomaki Bay by the
march 2011 tsunami.
some of this compression, but only
at a rate of about approximately 3 to
5 millimeters per year. Including other
active faults and folds, the total rate of
permanent horizontal shortening over
the Northeast Japan arc is about 5 to
7 millimeters per year. Permanent uplift
of northeastern Japan occurs at a rate
that is in agreement with the permanent shortening. This indicates that less
than 10 percent of plate convergence is
accommodated in the Japan arc as permanent deformation.
In the short term, however, triangulation, trilateration and GPS observations
show that the Japan arc has been shortening at a rate as high as several tens of
millimeters per year over the last century, much higher than geological rates.
At the same time, abnormally high rates
(up to 10 millimeters per year) of subsidence along the coast have been observed
by tide gauges during the last 80 years or
so. Despite the fact that marine terraces
along the Pacific coast of northeastern
Japan show permanent uplift, the best
explanation is that in the short term,
it is being dragged downward by the
subducting Pacific Plate.
The difference between short-term
and long-term deformation rates indicates that most of the plate convergence
is not deforming northeastern Japan.
Instead, that energy is elastic and is being
stored in the way that a spring stores
energy. Eventually, this energy must be
released by slip on the subducting plate
EARTH January 2013

37
the 2011 tsunami swept up this canyon to more than
60 meters above sea level. the canyon leads to one of
the hundreds of “tsunami stones” (right), some more than
600 years old, that are found along the coast of Japan. the
inscription on this stone, near the village of aneyoshi, warns
against building houses in areas lower than where the stone
is set and describes destruction caused by past tsunamis.
boundary as giant earthquakes. Yet in
recent times, earthquakes with magnitudes of only 7 to 8.4 have occurred in
the same region, and did not result in
the release of very much of the accumulated energy. To release the excess
energy and balance plate motion, much
larger earthquakes must occur. Thus,
the Tohoku magnitude-9 earthquake
should not have been unexpected with
longer-term information taken into consideration. Nonetheless, very few geologists accurately forecasted the size of the
Tohoku earthquake.
38

EARTH January 2013
In Hokkaido, north of Sendai, a similar
relationship has been observed between
the shorter historical and instrumental records and the paleoseismic record
along the Kuril Trench. Prehistoric
tsunamis that most likely were generated from long ruptures along the
Kuril Trench were significantly larger
than those existing in the historical
and instrumental records. Over the
past 2,000 to 7,000 years, such outsized
events occurred on average about every
500 years, with the most recent event
approximately 350 years ago.
The best-known large earthquake
along the Cascadia Subduction Zone
occurred in 1700. The magnitude of the
earthquake is estimated to be approximately 9, based on the arrival of an
“orphan tsunami” — meaning there
is a tsunami record without a corresponding local earthquake — along
the Japanese coast. The perception of
Cascadia quickly underwent a paradigm
shift, from a place thought to host no
earthquakes at all to a region capable
of great earthquakes, based largely on
this quake. More recently, the view of
Cascadia’s hazard has been changing
again due to new paleoseismic work.
Because there haven’t been any large
quakes along Cascadia in recent history,
the entire earthquake record is based on
several decades of paleoseismic work.
New evidence from onshore subsidence
and tsunami deposits and offshore turbidites (submarine slope failure records)
has yielded an unprecedented record of
great earthquakes along the Cascadia
Subduction Zone. The offshore records
www.earthmagazine.org
Bottom: Chris Goldfinger; top: kyodo via Ap Images
palEosEisMic
rEcord in cascadia
suBduction ZonE
scientists can use turbidites (submarine slope failure deposits) collected
from the ocean floor to understand
past earthquakes. turbidites provide
clues to the timing and magnitude
of past events. the authors have collected turbidites from 6,000 meters
below the sea surface off the coast
of sumatra and 3,000 meters deep in
the cascadia subduction Zone in the
pacific northwest.
Large earthquakes trigger
underwater landslides.
Density currents flow into
deep water and settle to
form turbidite deposits.
Repeated layers of turbidites
under the seafloor record
past earthquake events.
are in good agreement with onshore
paleoseismology evidence, and both
offer consistent information about the
relative size of paleoearthquakes, lending confidence that both are recording the same phenomena. The longest
records available, those from deep-sea
turbidites, reveal information about much
longer-term behavior of this subduction
zone going back 10,000 years.
In 1999, we started looking at turbidite
deposits along the Cascadia margin to
see what they could tell us about ancient
earthquakes. Canadian geologist John
Adams had proposed in 1990 that turbidite deposits first discovered along the
Ca
e
ver
I
sla
nd
ne
Zo
WASHINGTON
Juan
eF
u
aR
id g
CANADA
Van
cou
n
tio
uc
bd
Su
ia
ad
sc
Fra S
ctu ov
re anc
Zo o
ne
Cascadia margin in the 1960s reliably
recorded 13 past great earthquakes. We
thought otherwise, and collected numerous cores from Canada to Northern
California to test the idea. It turned out
that we were wrong, and a decade of
work and three coring cruises have led
to a detailed earthquake record along
the 1,000-kilometer length of Cascadia.
We now have a 10,000-year integrated
onshore-offshore earthquake record in
Cascadia, including 41 probable earthquakes. This long record allows us to
c
d
OREGON
Both: kathleen Cantner, AGI
Pacific Plate
North American
Plate
Juan de Fuca Plate
mantle
upwelling
subducting
plate
hydrous melting
zone
theThe
historic
seismic
record from
the from
cascadia
is no longer
limited
to instrumental
earthquake
data (blue
historic
seismic
record
thesubduction
CascadiaZone
subduction
zone
is no
longer limited
to instrudots). it has been extended by the association of mega earthquakes with turbidite deposits identified in offshore sediment
mental
earthquake
data (blue dots). It has been extended by the association of mega earthcores
(orange
dots).
quakes with turbidite deposits identified in off-shore sediment cores (orange dots).
www.earthmagazine.org
EARTH January 2013

39
Dates
T12
Dates
H
6440
turbidite
slanted
(6280-6570)
T13
T14
T13
H
5810
(5680-5960)
T11
H
6440
H (6280-6570)
T12
T14
H
H
7120
(6990-7250)
H
T15
7610
(7500-7710)
8050
(7900-8200)
T15
7610 T13
(7500-7710)
H
6.0
8050 T14
coring
(7900-8200)
artifact
T15
coring
artifact
T12?
H
6.0
T13
7150
(7030-7270)
T15
Density
Magnetics
5.0
H
T17
9330
(9230-9410)
H 9530
(9340-9770)
H
T17a
T18
H
7150
(7030-7270)
H
H
6.0
T15
7.0
T16
H
T17
H
10040
(9960-10180)
H
9340
(9260-9410)
T18
9650
(9460-9860)
ge BasinforWest
the age.
Silt
Hemipelagic clay
Very fine sand
Turbidite silty mud
range Channel
Rogue
Oldest Mazama ash bearing turbidite
-128
(5870-6120)
Radiocarbon
Gamma
Density (g/cm
T11 3)sample location
5920
(5790-6050)
Core break
T12
Shell
Wood fragment
H
4.0
6900
High-resolution
point mag. susc (SI)
T12a (6780-7030)
H
6.0
H
7940
H
H

T18
46
6590
JUAN DE FUCA
PLATE
(8
(8150-8350)
42
5.0
T16
8850
(8660-8990)
T16a
9070
(8910-9230)
T17
8400
(8260-8550)
H
H
10040
(9960-10180)
9150
(8990-9260)
H
H
7.0
9340
(9260-9410)
9190
(9070-9300)
H
T16a
9650
(9460-9860)
- 122
H
- 124
T17
T18
S
Astoria
Canyon
Hydrate Ridge
Basin West core site
Burrows
Wood
fragment
7100
(7090-7260)
22/23 PC
44
7670
(7530-7820)
9830
(9640-9990)
Cascadia
Channel
core site
56 PC
-
Rogue Channel
core site
30/31 PC
42
GORDA
PLATE
Hydrate Ridge
Basin West
Eel
Canyon
40
OREGON
Rogue
Canyon
Smith
Canyon
Klamath
Canyon
Trinidad
Canyon
Mendocino
Channel
Noyo
www.earthmagazine.org
40
T17a
Vancouver
Island
OCEAN
Quinault
Canyon
Grays
Canyon
WASHINGTON
Willapa
Canyon
(8
(8
9830
(9640-9990)
T18
A
RNI
9650
11/12 PC
T15
T15a
(6440-6750)
Mottled
clay
Above: Correlation
of four Cascadia turbidite cores spanT14a (7800-8080)
ning 550 kilometers of the Cascadia margin. Every spike
with8150
a T and T15
a number shows an
earthquake.
H individual 8250
(7940-8360)
Earthquakes
T11-T18 (dates shown above,(8150-8350)
with possible
8650
T15a
8460
(8470-8820)
ranges
based on (8270-8600)
different dating 5.0techniques in parentheses) are shown to illustrate the two extreme earthquakes
in this record, T11 and T16, which happened roughly
T16 years ago, respectively, and are
5,900 and 8,900
both esti8400
8850
H
(8260-8550)
mated to have been
about magnitude
9.1. Dark
blue areas
(8660-8990)
10040
T16a
9070 light blues show the density of
show
magnetic
properties;
(9960-10180)
(8910-9230)
the sediment. Magnetic and density spikes indicate more
9150
T17
dense
sandy material
in the turbidites,
which
correlates to
9340
(8990-9260)
H
(9260-9410)
earthquakes.
Right: The location map shows the four cores
9190 shown
in large yellow-bordered
symbols, with other cores
T17a
(9070-9300)
H
as gray and white dots.
40 EARTH January 2013
(9460-9860)
H
8460
(8270-8600)
SAF
H
T14
Juan de Fuca Channel
core site
Shell
3
7670
8150
(7530-7820)
(7940-8360)
8650
(8470-8820)
8250
H
T14
T14a44(7
IFO
H
7.0
T15
T15a
H
T13
CAL
H
7220Gamma Density (g/cm )
(7140-7310)
T13
7650
(7480-7820)
H
Barclay
Canyon
Juan de Fuca
Canyon
Very fine sand
Sand
7100
(7090-7260)
PACIFIC
Mottled clay
High-resolution
point mag. ash
susc (SI)
Burrows
Oldest Mazama
bearing turbidite
H
H
-126
48
Density
MagneticsPurple ages indicate erosion correction
applied. Magnetics
Sand
Radiocarbon sample location
Event
Dates
Dates
Blue
ages indicate
age from
benthic species.
Silt Dates
Coreages
breakindicates reversed age.
Gray
6000
7650
(7480-7820)
6.0
T13
T18
-130
T12
6590
7220
(6440-6750)
(7140-7310)
4.0
6900
T12a (6780-7030)
T17a
H
48
46
H
T17a
T18
T11
T12a (6
H
T17
7.0
Explanation
Turbidite silty mud
Purple
ages indicate
erosion correction applied.
7290
(7220-7380)
Blue ages indicate age from benthic species.
Radiocarbon
ageage.
and the error
Gray ages indicates reversed
Dates
T16
T17a
T17a
Hemipelagic clay
Radiocarbon age and the error range
for the age.
H
T17
9900
(9680-9980)
H
Explanation
T16
7.0
T18
7290 (7220-7380)
H
T14?
8150
(7940-8360)
8650
(8470-8820)
H
5920
Magnetics
(5790-6050)
Density
7650
T15 T14
7940
(7480-7820)
T14a (7800-8080)
T14?
H
Dates
T11
5920
(5790-6050)
T13
T12
7220
(7140-7310)
H
9330
T17
(9230-9410)
9530
T17a
(9340-9770)
9900
(9680-9980)
H
T18
8990
T16
(8860-9140)
8990
(8860-9140)
Event
-13
6000
(5870-6120)
T16
T17a
5.0
DatesT12? Event
6530
(6400-6700)
H
6530
(6400-6700)
T14
Dates
Rogue
H Channel
T11
H
T16
H
T17
T12
Event
T11
7120
(6990-7250)
Magnetics
5.0
Hydrate Ridge Basin West
Dates
T13
H
T16
Magnetics
H
Density
Event
(5
in core
T11
T12
T11
5.0
Dates
5.0
Cascadia Channel
5810
Density
Event
(5680-5960)
Magnetics
Chris Goldfinger
H
Magnetics
Density
Density
turbidite
slanted
in core
Juan
T11de Fuca Channel
Event
Event
ICAN PLATE
Magnetics
NORTH AMER
Density
Astoria
Asto
Ast
As
A
sto
sst
to
to
orriiaa C
Channel
haan
nne
nn
n
neeell
n
Event
research on the cascadia subduction Zone has caused scientists to raise the earthquake hazard assessment for cities along
the zone, like seattle, wash., (above) and portland, ore. (right).
Cascadia Supercycles
Potential
Energy Gain
(Plate Motion)
10500 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500 1000
Time (years before present)
500
0
Kinetic
energy loss
(Earthquakes)
From bottom: Chris Goldfinger; ©istockphoto.com/David Birkbeck; ©istockphoto.com/Mark Hatfield
energy cycles along the cascadia margin, based on 10,000 years of earthquakes: the sawtooth pattern represents the storage of energy from plate motion (upticks) and energy loss from earthquakes (downticks). the complexity of this pattern
suggests that simple models based on the most recent events underestimate earthquake potential, as occurred in tohoku.
look more deeply into how this great
fault behaves over long periods of time.
One thing that is obvious is that the turbidites show an apparent clustering of
great earthquakes into groups of four to
five events, with 700- to 1,000-year gaps
between the clusters. The turbidites also
provide clues to the magnitudes of past
events, and to the buildup and release
of energy over time.
In the turbidite record, the benchmark 1700 earthquake is roughly average relative to 19 similar ruptures, in
terms of turbidite thickness and mass
over the past 10,000 years. About a half
dozen turbidites are larger than the
1700 event and a half dozen smaller; the
rest are similar. Two obviously larger
events are the 11th and 16th events back
in time, at about 5,960 and 8,810 years
ago. These turbidites are consistently
two to five times larger than “average”
turbidites at all core sites along the
length of Cascadia, and may have been
triggered by a magnitude-9.1 quake,
which is 44 percent larger than a magnitude-9 quake.
Our research shows that, despite the
obvious simplifications involved, a
www.earthmagazine.org
connection between turbidite mass and
energy release of the source earthquakes
can be made because of consistency in
the turbidite records along the length
of the margin at multiple sites and multiple depositional environments. We also
found a good correlation between turbidite size and tsunami size at Bradley
Lake along the southern Oregon coast,
where an excellent 4,600-year tsunami
record is found. This suggests that when
paleoseismic records are long enough
and of sufficient quality, we may be able
to tease out more information about past
earthquakes than just their timing.
We have also done further research
on long-term patterns, looking at the
energy balance between the Juan de Fuca
and North American plates in Cascadia.
Because the turbidite size is so consistent among sites, the size of the turbidites can be used as a crude measure of
energy release and balanced against the
energy gain from plate motion. Plotting
the energy gain against energy loss, it
is possible to generate a 10,000-year
energy time series for Cascadia, showing how energy is stored and released
by the fault over time.
What this plot and a similar one created for the Sumatran subduction zone
show is that not only are earthquakes
clustered in time, but there appears to be
a long-term cycle of energy storage and
release. The resulting sawtooth pattern
reveals what we interpret as long-term
energy cycling on the subduction zone,
with some events releasing less energy
and others releasing more energy than
is generated from plate convergence
alone. Earthquakes that are larger than
the energy available from plate convergence may have “borrowed” stored
energy from previous cycles; at other
times, energy is stored when less energy
is released than is available.
The plots also show that when a fault
is in a high- or low-energy state, its
behavior can vary. For example, when
Cascadia (and perhaps other faults) is
at a high-energy state, it appears equally
likely to rupture in single very large
(maybe magnitude-9.1) earthquake or
in a series of somewhat smaller (but still
giant) magnitude-8.8 to -9 earthquakes
to relieve stress. When at a very lowenergy state, Cascadia may go for long
periods without any quakes, or it may
EARTH January 2013
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41
othEr proBlEMs
With short
oBsErvation tiMEs
the university of washington’s r/v
thomas g. thompson leaving the dock
in seattle, as researchers head out to
study the cascadia subduction Zone.
shake in a series of earthquakes that only
release a little of the accumulated energy
over time — something that also clearly
occurred for the 1,000 years prior to the
2011 Tohoku earthquake.
The plots also suggest that the size of
an earthquake is not that closely related
to the time since the last earthquake.
Think of a battery as representing the
energy storage of a plate boundary: You
can draw energy from it in any increment available at any time, limited only
by the total energy available.
What this means for Cascadia and
other subduction zones is that we cannot
say very much about the next earthquake
based on the few most recent ones. Long
paleoseismic records help get around
this problem. Turbidites from deep water
in subduction zones and from lakes offer
ways to build these long records. They
have the advantage of being able to link
the records between sites with physical
stratigraphy and extend them further
back in time.
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EARTH January 2013
Where long paleoseismic records
aren’t available, or in addition to
paleoseismic evidence, scientists are
frequently turning to geodetic data —
data from GPS, strain gauges and tide
gauges — to inform assessments of seismic hazards based on how the land is
deforming in real time. We now have
a wealth of geodetic data that directly
provides strain accumulation and slip
rates on faults. These geodetic rates are
commonly thought to be the best available data.
In some cases, geodetic rates over
the last few years agree with long-term
rates determined from geology, such
as slip rates on faults and uplift rates
of mountains; at other times, geodetic
rates undershoot or overshoot these
geologic rates. The reasons for the differences are not always clear, but given
the examples of very long-term cycling
of earthquake energy release, it’s likely
that one cause for the discrepancy in
rates is linked to the short time frame
of the observations.
Not only are the geodetic observations
made at random times in the seismic
cycle of nearby faults, they may well also
be at random times in a long-term multievent “supercycle,” making the data
less directly linked to average plate and
fault block motions, and more difficult
to interpret in terms of size and rates of
future earthquakes.
With short records, almost every significant earthquake is “new” to geologists. The magnitude-7.7 quake last
October in the Queen Charlotte Islands
off British Columbia was a thrust earthquake, a type not seen previously on
that strike-slip fault, and the magnitude-8.6 strike-slip quake off the coast
of Sumatra last April was the largest of
its type ever recorded.
Another issue with short observation
periods is the question of global clustering. Many people have noted that we
have had a lot of really large earthquakes
lately. Tohoku in 2011, Chile in 2010,
Sumatra in 2004. Before that, there is a gap
of 50 years following the 1964 Alaskan
earthquake, with several other giants
clustered between 1957 and 1964. These
great earthquakes are clearly clustered in
time, but is this just a random phenomenon, or does it mean something about
global plate dynamics?
A couple of recent studies suggest
global clustering of magnitude-9 earthquakes doesn’t exist, and can easily be
explained as part of a random process.
The problem, though, is that because
the recurrence times of magnitude-9
earthquakes can be hundreds of years,
and for the largest events can be 5,000 to
6,000 years, we mostly don’t have enough
data to actually test this idea either way.
Absent enough magnitude-9 events,
www.earthmagazine.org
Both: Chris Aikenhead
the oregon state university
coring team examines the
upper section of a turbidite
core recovered off the sumatran margin.
recent surprising earthquakes in Japan, sumatra and the Queen charlotte islands (circles), as well as new research along
the cascadia subduction Zone (triangle), indicate that most any fault zone could rupture significantly. locations that have
previously experienced “superquakes” are shown in orange. locations that researchers are starting to think could potentially experience magnitude-9 quakes are shown in red.
smaller earthquakes with higher frequencies from other fault systems are
used as proxies, but the comparison is
dubious at best.
The observation that magnitude-9
events worldwide have clustered twice
in the last 100 years is also questionable
because no mechanism to produce such
clusters is known (of course, observations
virtually always exist before explanations are found). It’s likely that this question will remain open for the foreseeable
future, until we have long records from
enough subduction zones to examine
whether this phenomenon really exists,
and if so, what it might mean and what
mechanism could explain it.
kathleen Cantner, AGI
thE nEXt Big onE
So, where will the next big one be?
That’s impossible to say. But the recent
surprises from Tohoku and Sumatra, coupled with new evidence of long-term seismic supercycles on faults like Cascadia
and Sumatra, imply that other subduction
zones and other major fault systems may
be capable of massive quakes.
www.earthmagazine.org
Based on older models, relatively old
subduction plate systems have been
ignored as potential producers of magnitude-9 earthquakes, but recent quakes
show these zones are indeed at risk. Such
areas include much of the west coast of
South America, the remainder of the
Japan Trench, the Himalayan Front, the
Kuril Islands and the western Aleutians
in the North Pacific, Java, Indonesia, in
the South Pacific, the Antilles Islands in
the Caribbean, the Makran coastal area
in Iran and Pakistan, and the Manila,
Sulu, Philippine and Hikurangi trenches,
among many others.
Finally, the underlying cause of
supercycles is unclear. They could be
generated by intrinsic properties of
the plate boundaries, cycling of energy
among segments of an individual fault,
transfer of energy between adjacent
faults, or even energy transfer acting
over larger distances. In addition, there
is clear evidence of global clustering of
magnitude-9 earthquakes, but we don’t
have long enough records from around
the world to say for sure whether this
phenomenon is real or random, or
what it means.
The earthquake community has had
to take a step back and abandon older
models of great earthquake occurrence.
Today it’s not clear whether new global
models will emerge, or whether local
geology has the final say and that there
are no universal governing relationships
to guide model-based hazard assessments. In either case, hazards can be
addressed directly by gathering long
earthquake records that don’t depend
on models. Such records can answer the
societal hazard questions in a straightforward way long before there is consensus on global plate dynamics.
Goldfinger and Yeats are geologists in
the College of earth, Ocean and atmospheric sciences at Oregon state University. ikeda is a geologist in the department of earth and planetary science at
the University of tokyo in Japan. their
work on long-term earthquake cycles will
appear in an upcoming issue of seismological research Letters.
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