PROF. DR. GREGORY MOORE | EXCELLENCE-AWARD OF THE PETERSEN FOUNDATION, MAY 2016
TOWARD UNDERSTANDING SUBDUCTION ZONES
THROUGH MARINE GEOPHYSICAL SURVEYS,
LAND FIELD MAPPING AND OCEAN DRILLING
Prof. Dr. Gregory Moore
Professor of Marine Geophysics,
Department of Geology and Geophysics,
University of Hawaii, U.S.A.
The most powerful and destructive earthquakes on Earth occur along convergent margins where one litho­
spheric plate slides beneath another in a process called subduction. As subduction proceeds, sediments are
stripped off the descending oceanic plate and are accreted to the landward plate to form subduction complexes.
If large amounts of sediment are accreted over many millions of years, the subduction complex is pushed above
sea level. Given that the enormous earthquakes generated during subduction often spawn devastating tsuna­
mis, such as those that killed thousands of people around the Indian Ocean in 2004 and along the east coast of
Japan in 2011, it is critical for scientists to gain a better understanding of the conditions that cause these natural
hazards. To do so scientists turn to study ancient and recent subduction terranes that span the boundary be­
tween oceans and their adjacent land masses.
Subduction Zones, Earthquakes and Tsunamis
Professor Moore’s career had been dedicated to studying convergent margins around the world. His research has focused on the development of subduction complexes and their evolution using data acquired through both land and
marine field expeditions. While conducting geological fieldwork on an uplifted terrane on an island west of Sumatra,
it was apparent the islands represented only the “tip of the iceberg” while most of the terrane was actually under the
ocean. He thus began a study of the deeper portion of the region using seismic reflection techniques to image the
internal structure of the ridge. Unfortunately, the “experts” thought that the Sumatra region was slowly slipping with
no likelihood of generating great earthquakes, so Prof. Moore joined colleagues to investigate active subduction
zones off Cascadia (west coast of US and Canada), Middle America, Barbados and Japan. All of these research efforts
have involved a combination of seismic reflection imaging and “ground-truth” verification by deep sea drilling.
Figure 1: The D/V Chikyu is the world’s first riser drilling-equipped science vessel,
capable of drilling deeper at sea than any other science drilling vessel to date. Since
2007, Chikyu has participated in the Integrated Ocean Drilling Program, and since
2013, the International Ocean Discovery Program. Photo: JAMSTEC
These initial studies led subduction
zone “experts” to believe that the
­Nankai Trough was the best place for
intensive study because it has a 2000
year recorded history of great earthquakes and resulting tsunamis, and the
last big quakes occurred in 1944 and
1946. Thus, both Japanese and American groups combined to study N
­ ankai.
Initial seismic surveys revealed the
complexity of the accretionary zone
and pointed to the need for three-dimensional (3D) seismic surveys. An
initial 3D survey, collected by a US
academic vessel, R/V Maurice Ewing,
provided insight into the lateral variation of this complex region and imaged
targets for drilling on Ocean Drilling
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PROF. DR. GREGORY MOORE | EXCELLENCE-AWARD OF THE PETERSEN FOUNDATION, MAY 2016
Kii Peninsula
Earthquake (1944)
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KU ASI
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Research area
JAPAN
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Earthquake (1946)
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Figure 2: Location map showing the regional setting of the Nankai Trough and the research area in the Kumano basin
Program Legs 190 and 196. Prof. Moore joined Leg 190 as Co-Chief Scientist, drilling the frontal regional of the accretionary prism and learning much about the deformation along the along the plate boundary fault (décollement)
and frontal thrust.
When the Japanese government built an industry-standard “riser” drilling ship, D/V Chikyu, it was natural that the
Nankai Trough was chosen as the first target for ultra-deep drilling into an active subduction zone. Plans for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) included drilling through the plate boundary fault at depths
of 5-7 km to return samples for laboratory
studies and to instrument the borehole to
document in-situ temperature, pressure
and seismicity variations.
Before the next phase of operations in
Nankai took place, the 2004 Sumatra
earthquake jolted the subduction community – we were wrong that the Sumatra region could not generate such a large
event. The study at Nankai thus intensified because we were now sure that NanFigure 3: 3D seismic data volume of the research area depicting the location of
kai would be the next location of a great
the megasplay fault (black lines) and its relationship to older insequence thrusts
subduction earthquake. Preparation for
of the frontal accretionary prism (blue lines). Source: Gregory Moore
deep riser drilling included acquisition
of the first commercial 3D seismic survey. The resulting data volume changed the interpretation of the regional
geology and illuminated targets for drilling by the Integrated Ocean Drilling Program. Prof. Moore participated in
the initial Nankai drilling (Exp. 314) in 2007. But, before our Nankai drilling could be completed, the “experts” were
surprised again as the Japan Trench ruptured with a great earthquake and tsunami in 2011.
Much was learned from the 2011 quake and the drilling effort at Nankai was resumed. As of 2014, 10 shallow non-riser holes and two deep riser holes have been drilled. Prof. Moore was Co-Chief Scientist for Exp. 338 in 2012/3 that
continued drilling the “ultimate” deep riser hole that has now reached a depth of 3500m, with plans to deepen the
hole to at least 5000 m in 2017/8. So far, NanTroSEIZE drilling has shown that slip during earthquakes that occurred
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PROF. DR. GREGORY MOORE | EXCELLENCE-AWARD OF THE PETERSEN FOUNDATION, MAY 2016
along the basal décollement propagated all the way to the trench, as well as up to the surface along a branching
(splay) fault. This slip thus pushed an enormous undersea land mass seaward several meters, thus generating large
tsunamis.
Recent seismic and drilling investigations of the Japan Trench showed that this mechanism was the cause of the
huge tsunami that devastated eastern Japan in 2011. The Japanese government continues to support additional
research efforts to understand to what extent such a mechanism might soon affect the Nankai Trough.
Subduction Zones and Mud Volcanoes
One of the lesser understood phenomena in subduction zones is that of upward
(diapiric) rise of mud from deep within
the accretionary complexes due to strong
geo-pressures. When the pressurized mud
reaches the surface, it erupts as mud volcanoes. Prof. Moore began a study of mud
volcanoes along the west coast of Myanmar in 2013, now having conducted three
field seasons of collecting mud samples
and analyzing the regional geologic setting
of the mud volcanoes. Prof. Dr. Achim Kopf
of MARUM analyzed the isotopic chemistry
of the muds, which indicates that the mud
Figure 1: Mud vulcano at the west coast of Myanmar. Photo: Gregory Moore
volcanoes originate at depths greater than
1-2 km. They are closely associated with strongly sheared flysch sediments (mélanges), but additional field work and
analyses will be necessary to determine whether the mud diapirism has caused the mélanges or whether the mud is
passively rising along the shear zones.
Further along this line of research, Prof. Moore joined a cruise with Japanese colleagues last year to study a mud
volcano on the seafloor in the Nankai region using an autonomous underwater vehicle (AUV). The AUV was able to
map the topography of the seafloor and image its surface features with side-scan sonar. These maps will be used
later this year by an international team led by Prof. Kopf to sample this active feature along with other mud volcanoes
farther landward during an expedition of the new R/V Sonne. Prof. Moore will join the cruise to help decipher the
relationship of the mud volcanoes and deeper structures imaged on seismic profiles.
We are hopeful that our continuing studies of subduction zones will lead to a better understanding of the conditions
that caused the 2004 and 2011 great earthquakes and resulting megatsunamis with the aim of quantifying potential
future hazards, not only in Nankai, but in all of Earth’s active subduction zones.
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