APRIL 2013
AIRCURRENTS: REFORMULATING A
SEISMICITY MODEL FOR JAPAN
BY DR. MEHRDAD AND ROBERT ZALISK
EDITED BY NAN MA
EDITOR’S NOTE:
A previous AIR Currents article provided an overview of the update to the AIR Earthquake Model for Japan that
will be released in the summer of 2013, which is the first industry model to include tsunami, liquefaction, and fire following. In
this article, AIR Director of Earthquake Hazard Research Dr. Mehrdad Mahdyiar and Senior Science Writer Robert Zalisk outline
the changes in Japan’s seismicity and how the update accounts for them.
Japan is especially susceptible to earthquakes. But the 2011 Tohoku
earthquake that ruptured off the northeast coast of Honshu, the
most powerful earthquake in Japan ever recorded, was different. Its
magnitude, its location, and the formidable tsunami it generated
stunned scientists and caught public safety officials by surprise (read
the AIR Currents article, Rethinking the Unthinkable: Modeling
Unprecedented Ruptures Like the Great Tohoku Earthquake, for an
explanation).
Since then, seismologists have had a double-edged question
to unravel: First, what can the Tohoku earthquake reveal about
seismicity in Japan that was not known or clearly understood prior
to the March 11, 2011, catastrophe? And second, how did the very
THE ARTICLE: Gives
an overview of how
the 2011 Tohoku earthquake changed the state
of seismicity in Japan, especially in northern Honshu, and
describes how those changes have been incorporated into the
updated AIR model.
HIGHLIGHTS: Going
far beyond simply revising the rupture
occurrence probabilities of the previous model, AIR’s updated
model reflects a new understanding of many aspects of seismicity
in Japan. It represents the industry’s most comprehensive solution
to modeling earthquake risk, post-Tohoku.
occurrence of that event affect the seismic dynamics of the region:
where has stress increased, where has it lessened or remained the
concerning seismicity in post-Tohoku Japan and how AIR’s 2013
same, and which faults may now be more likely to rupture—and to
model is the first in the industry to include the new conditions the
rupture on what timescale? This article discusses the latest research
Tohoku catastrophe brought about.
JAPAN’S TECTONIC GEOMETRY
Japan is dominated by subduction activity, the movement of one plate
under another. Figure 1 presents a simplified outline of where the four
major tectonic plates—the Pacific Plate and the Philippine Plate, and
the Amurian Plate and the Okhotsk Plate—come together at Japan.
The small triangles along the Pacific and Philippine Plate boundaries
on the map indicate the direction the plates are moving.
All four plates come together at one place, the Sagami Trough.
This is where the most complex interactions among the plates are
happening, where some plates are themselves being subducted even
while subducting others. The effects of these actions extend under
mainland Honshu (Japan’s largest island) at the Kanto Plain. The Kanto
Plain is the most densely populated and heavily industrialized region of
Japan. Tokyo is located here; thus, understanding the intricate tectonic
dynamics of this area is central to being able to estimate earthquake
risk for the greater Tokyo area.
Figure 1. Four tectonic plates meet at Japan. The Pacific Plate is subducting both the
Okhotsk Plate and the Philippine Plate along the length of the Japan Trench at about
eight centimeters per year. At the same time, the Philippine Plate is subducting the
Okhotsk Plate along the Sagami Trough, while also subducting the Amurian Plate at the
Nankai Trough. (Source: AIR)
APRIL 2013 | REFORMULATING A SEISMICITY MODEL FOR JAPAN
BY DR. MEHRDAD MAHDYIAR AND ROBERT ZALISK
EDITED BY NAN MA
RECONFIGURING SEISMIC SOURCE ZONES
ALONG THE JAPAN TRENCH
The Japanese agency responsible for periodically publishing
estimates of seismicity for Japan is the Headquarters for Earthquake
Research Promotion (HERP). Based on the National Seismic Hazard
Maps HERP issued in 2007, the area along the Japan Trench (which
parallels northern Honshu) was configured into eight seismic source
zones (Figure 2a) more or less following the pattern of historical
earthquake ruptures as they were understood at the time. Six of the
eight zones experienced full or partial ruptures during the Tohoku
quake. The amount of seismic energy released varied considerably
from zone to zone, and these differences have helped to determine
not only new states of seismicity at the different locations, but also
help clarify the conditions that actually existed prior to Tohoku.
THE NANKAI TROUGH AND
TSUNAMIGENESIS
The greatest changes in seismicity caused by the Tohoku earthquake
were in the northern part of Honshu and between Honshu off the
Tohoku region and the Japan Trench. The prevailing understanding
is that because Tohoku released so much accumulated energy,
another “megathrust” earthquake is not likely to happen along
that part of the Japan Trench any time soon. Given the current state
of stress in the Japan region, such an event, which also would have
the potential for causing a very destructive tsunami, is more likely to
happen along the Nankai Trough to the south.
As they did for the Japan Trench, AIR researchers determined new
configurations for the sources of seismic activity along the Nankai
Trough, which, as seen in Figure 3, runs roughly parallel to the
coast of southern Honshu. New data indicate that the angle of dip
(the angle between the fault plane and the horizontal) at Nankai is
more acute, or shallower, than previously thought. The significance
of this possibility is that a rupture along the Nankai subduction
zone—thought to be at a depth of about 35 km—would occur
much closer to land, if not actually beneath southern Honshu
in some locations. In addition, AIR’s newly configured seismic
source zones allow for the possibility that Nankai is capable of
producing earthquakes that could rupture as many as four fault
segments—one more than previously considered. It is a possibility
also considered by new scenarios published by the government of
Japan.
To locate likely sources of tsunamigenesis (tsunami-formation),
Figure 2. a) HERP 2007 seismic source zones for northern Honshu; b) AIR 2013
seismic source zones for the same area. (Source: AIR)
AIR researchers identified highly coupled areas within the Nankai
subduction zone. (“Coupling” refers to the degree of fault-locking
between adjacent rock surfaces along a fault; the greater the
AIR researchers used HERP’s latest findings, other published
coupling, the greater the resistance and locking—and thus the
research, and their own studies to reconfigure the seismic source
associated buildup of stress.) To do this, the researchers constructed
zones to reflect an emerging new understanding of seismicity along
a kinematic block model—a physically-based model that treats a
the Japan Trench (as also for along the Nankai Trough, as outlined
seismically active region as if it were a system of rigid blocks—for
below). Figure 2b illustrates AIR’s new configuration of seismic
the length of the Nankai Trough. Researchers used this block model
sources for Japan that have been incorporated into the model
to invert the GPS-derived ground movement data into a coupling
update.1
coefficient footprint over the subduction zone (Figure 3). Areas with
high coupling coefficients are the most likely to have the largest
The changes are immediately apparent, not simply with respect
displacement in future large earthquakes.
to the boundaries, dimensions, and the number of the zones, but
also for the greater complexity they display, representing zones
where large interplate earthquakes, tsunamigenic earthquakes, and
intraplate normal faulting are characteristic.
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APRIL 2013 | REFORMULATING A SEISMICITY MODEL FOR JAPAN
BY DR. MEHRDAD MAHDYIAR AND ROBERT ZALISK
EDITED BY NAN MA
THE SUBDUCTING PLATES
The chief source of seismicity on the Plain has been the movement
of the underlying subducting Philippine and Pacific Plates. The
rupture of large-magnitude earthquakes like Tohoku alters the
stress borne by other faults nearby, out to a distance of at least
several hundred kilometers (read The Tohoku Earthquake and Stress
Transfer for more information). Indicating such changes in stress,
the rate of earthquake occurrence increased significantly after the
Tohoku earthquake struck—mostly close to the epicenter, but also
in the Kanto Plain region (see area circled in green in Figure 5b).
Figure 3. Regions of high coupling (in red) along the Nankai Trough indicate where
tsunamigenic earthquakes could occur. (Source: AIR)
SEISMOGENIC INFLUENCES ON THE KANTO
PLAIN
The Kanto Plain region, shown in Figure 4, stretches across the
middle of Honshu, midway between the northern half of the
island (where seismicity is dominated by the Japan Trench) and the
southern half (where seismicity is dominated by the Nankai Trough).
As noted earlier, this is where Tokyo is situated, and it holds the
major concentration of exposures for all of Japan. Significantly, the
plain is exposed to earthquake hazard from diverse sources, most
traceable to the relentless subducting action of the Pacific and
Figure 5. a) Seismic activity pre-Tohoku; b) seismic activity post-Tohoku
Philippine Plates, the complex interaction of the plates with each
This change was used by AIR researchers to provide values for some
other, and the still uncertainly understood interaction of the plates
of the parameters of a “rate-state” model, a physical model that
with the region off the Boso Peninsula referred to as the Boso
simulates earthquake genesis. The researchers could thus generate
Segment.
scenarios that show the likelihood that different faults within the
Kanto Plain might rupture. They employed several procedures in
this effort. For example, a “Coulomb failure stress change” analysis
of the Plain was also conducted, using values obtained for the
Tohoku region. (Coulomb failure stress change is a physics-based
mathematical operation that produces estimates of changes in the
state of stress in a seismic system).
The results of the studies indicate that the Tohoku earthquake
increased the state of seismic stress on both the Pacific-Philippine
plate interface and Pacific intraslab faults in the region. Indeed,
they suggest that the 30-year probability of an M ≥ 6.7 earthquake
happening has potentially increased from 72% to approximately
81-93%, depending on the assumptions.
THE BOSO SEGMENT
The “Boso Segment” was an unnumbered zone or area on the
2007 HERP maps, located where the Philippine, Pacific, and
Figure 4. The Kanto Plain region.
Okhotsk Plates meet (see Figure 4).
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APRIL 2013 | REFORMULATING A SEISMICITY MODEL FOR JAPAN
BY DR. MEHRDAD MAHDYIAR AND ROBERT ZALISK
EDITED BY NAN MA
Historically, the Boso Segment has been quiescent. Indeed, the
100% coupled—the model can estimate (using kinematic inversion
only large historical event that might have occurred there is an
algorithms) what degree of coupling is most consistent with the
earthquake in 1677, about which little is known. Based on this
empirical GPS data.
history, HERP, in 2007, considered the segment to be weakly
coupled or not coupled at all and did not formally identify the
Based on such estimates and similar results from other efforts, it
segment as a source of seismic activity. However, seismologists
was concluded that the Boso Segment is, in fact, at least partially
can no longer automatically assume that the Boso Segment’s long
coupled and is capable of generating an earthquake as large as
history of quiescence is attributable only to seismic inactivity; on
M8.6. An earthquake of this magnitude would be the largest ever
the contrary, the segment may actually be strongly coupled and
to occur in this region and would cause significant shake damage
accumulating increasingly more energy—as was the case in Tohoku.
in the greater Tokyo area. Moreover, it also could generate large
tsunami waves around the Boso and Izu Peninsulas, which would
Figure 6 depicts ground movement for all of Japan as indicated by
GPS-derived data (that is, ground movement measured by satellite
result in additional damage.
with laser precision). However, this surface movement does not
CLOSING THOUGHTS
directly reflect the underlying deep seismicity of the subducting
The Tohoku earthquake in March of 2011 wrought extensive
plates—especially given the distorting influence of a 10 kilometer-
changes to the seismicity of Japan. The full extent of those changes,
thick section of plate of uncertain origin that recent studies indicate
how some of the changes have been identified, and what remains
is wedged between the Philippine and Pacific Plates, strongly
to be determined, have been only touched on here. In summary
coupled to one or perhaps both.
to the explanations offered here, Figure 7 provides a comparison
of seismic hazard in northern Honshu as generated by AIR’s 2008
Japan Earthquake Model and AIR’s 2013 model update.
Figure 7. AIR’s 2008 and 2013 seismic hazard maps for northern Honshu (showing
peak ground acceleration, or PGA, with a 10% probability of exceedance in 50 years).
(Source: AIR)
Figure 6. GPS measurements of surface movement in Japan.
To resolve this conundrum, AIR seismologists used the GPS results
shown in Figure 6 to determine the state of coupling in the region.
As with their analysis of the Nankai Trough, they first constructed
a kinematic block model (this time for the Kanto Plain, including
the Boso Segment). Again, a block model normally uses GPS data
to generate estimates of how coupled a seismic system is. By
Looking at the two maps, it is possible to discern the generally
diminished seismicity to the north, where the Tohoku region lies
around Sendai, and also some increased seismicity near Tokyo
and on the Boso Peninsula on the Kanto Plain. The changes are
more apparent in Figure 8, however, which shows only the relative
increases (in gradations of red) and decreases (in gradations of blue)
in seismic hazard between the two maps above.
inputting test coupling values—that the system was 0% or 50% or
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APRIL 2013 | REFORMULATING A SEISMICITY MODEL FOR JAPAN
BY DR. MEHRDAD MAHDYIAR AND ROBERT ZALISK
EDITED BY NAN MA
A central lesson Tohoku has taught the research community is
that it cannot rely on historical data alone to determine the risk
from large damaging earthquakes: a region believed to be weakly
coupled based solely on the historical record may actually be
strongly coupled. This lesson, along with other insights on seismicity
in Japan and the fact that so much destruction was caused by
earthquake risk factors that previously had not been modeled—a
massive tsunami, liquefaction, fire following—shaped the objectives
of AIR researchers and modelers in developing the update to the
AIR Earthquake Model for Japan.
Figure 8. Relative increase and decrease in PGA (with a 10% probability of
exceedance in 50 years) in northern Honshu as indicated by the AIR 2008 and 2013
seismic hazard maps. (Source: AIR)
MORE DETAILED DISCUSSION OF THESE FINDINGS (AND THE OTHER QUESTIONS CONSIDERED IN THIS AIR CURRENT)
IS GIVEN IN THE AIR WHITE PAPER, “UNDERSTANDING EARTHQUAKE RISK IN JAPAN FOLLOWING THE TOHOKU-OKI
EARTHQUAKE OF MARCH 11, 2011.”
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