Akira Hasegawa Examines Earthquake Mechanisms in the NE Japan Arc
Special Topic of Earthquakes Interview, January 2011
According to our
Special Topics analysis on earthquakes research over the past
decade, the work of Professor Akira Hasegawa ranks at #3 by total
number of papers and #7 by total cites, based on 74 papers cited a
total of 1,087 times.
In Essential Science IndicatorsSM from Thomson Reuters, his work is among the top 1% of researchers in Geosciences, with 86 papers cited 1,305 times between January 1, 2000 and August 31, 2010.
Hasegawa is the former director of and currently Professor Emeritus at theResearch Center for Prediction of Earthquakes and Volcanic Eruptions in the Graduate School of Science of Tohoku University in Japan.
Please tell us about your educational
background and early research experiences.
After graduating from the Department of Geophysics, Faculty of Science, Tohoku University in March, 1967, I started research on seismology in the master's course in the Department of Geophysics, Graduate School of Science, Tohoku University. Upon finishing the master's course in March, 1969, I went on to the doctoral course. In April, 1971, I got a job as a research associate at Aobayama Seismological Observatory, Faculty of Science, Tohoku University (the predecessor of the present-day Research Center for Prediction of Earthquakes and Volcanic Eruptions).
Soon after starting the job, I was tasked with the data analysis of the high sensitivity seismograph network which Aobayama Seismological Observatory had deployed in northeastern Japan at that time. Through this data analysis, I discovered "the double-planed deep seismic zone," which was my first achievement.
What first drew your interest to earthquakes?
Photo taken by a collaborative researcher, Dr. Stephen
Kirby from USGS.
View larger image in the tab below.
Since Japan is located in subduction zones, we have repeatedly suffered from large earthquakes for many years. Born and raised in Japan, I wished to reduce the earthquake damages. For this, I have aimed to understand the mechanism of earthquakes and to apply that knowledge toward earthquakes prediction.
Even in the few years before my entrance to the graduate school, there had been several large earthquakes, such as the 1964 M6.9 Oga peninsula earthquake and the 1964 M7.5 Niigata earthquake, which caused considerable damage. These events strongly motivated me to advance my research on seismology at the graduate school.
One of your papers in our analysis is the 2000 Tectonophysics paper, "Seismic activity and deformation process of the overriding plate in the northeastern Japan subduction zone," (Hasegawa A, et al., 319[4]: 225-39, 20 April 2000). Please tell us about your methods and findings in this work.
Precise hypocenter relocations and seismic tomography studies in the central part of Tohoku of the NE Japan subduction zone brought us to the finding that the thickness of the seismogenic layer in the arc crust has considerable lateral variations, becoming locally thin in areas where seismic velocities are low and so temperatures are thought to be high. We also found that shallow seismic activity and relatively large contraction deformation estimated from triangulation measurements are concentrated in such areas.
Based on these observations, we proposed a simple model for earthquake generation in this volcanic arc: earthquake occurrence and deformation processes of the arc crust are governed by the thermal regime of the arc, which is characterized by a horizontally heterogeneous distribution of the temperature.
Another of your papers, the 2005 Tectonophysics paper "Deep structure of the northeastern Japan arc and its implications for crustal deformation and shallow seismic activity" (Hasegawa A, et al., 403[1-4]: 59-75, 5 July 2005), also looks at the northeastern Japan arc. What new data did this paper uncover compared with the 2000 paper?
After the 2000 Tectonophysics paper, we have continuously investigated the detailed structure of the northeastern Japan arc in order to deepen our understanding of the earthquake-generation mechanism. Dense seismic and GPS networks deployed later enabled us to confirm the model proposed in the 2000 paper.
Seismic tomography studies using data from this dense seismic network revealed the existence of an inclined sheet-like low-velocity zone in the mantle wedge oriented sub-parallel to the subducting plate, which perhaps corresponds to the upwelling mantle flow. This upwelling flow reaches the bottom of the arc crust right beneath the volcanic front, suggesting that the volcanic front is formed by this hot upwelling mantle flow.
"Since Japan is located in subduction zones, we have repeatedly suffered from large earthquakes for many years."
GPS data clearly showed a notable concentration of contraction deformation of the arc crust along the volcanic front. Shallow earthquakes are also concentrated in the upper crust along this contraction deformation zone.
All these observations support the model for earthquake generation; i.e., magmas or aqueous fluids expelled from solidified magmas weaken the lower crust there, resulting in local contractive deformation and generation of earthquakes in the upper crust right above it.
How much have we learned about earthquakes in the past decade? What advances would you like to see in the future of earthquake research?
During the last decade, significant progress has been made in seismology. That is, there has been excellent progress in the understanding of earthquake-generation mechanisms.
Regarding interplate earthquakes in subduction zones, it has become evident that the asperity model is applicable, which has provided a theoretical underpinning for long-term earthquake forecast.
As for intermediate-depth earthquakes among intraslab earthquakes, many observational results which support the dehydration embrittlement hypothesis—which suggests that the aqueous fluids provided by dehydration from the slab decrease the strength and cause earthquakes—have been reported and become widely accepted as their generation mechanism.
In the case of shallow inland earthquakes in subduction zones, researchers have presented the idea that aqueous fluids provided by the dehydration of the subducted plate reach the arc crust through the mantle wedge and decrease the strength of the lower crust, which causes the concentration of stress into the upper crust just above it and lead to earthquake occurrence, and the observational results which support this idea have been increasing.
Moreover, the aqueous fluid is estimated to play an important role also in
the cause of the asperity; I think that understanding the role of the
aqueous fluids in earthquake generation will lead to a further
understanding of the earthquake-generation mechanism. I hope that we will
deepen the understanding of it much more in the future and that the
accuracy of earthquake forecast will improve.
Professor Akira Hasegawa
Research Center for Prediction of Earthquakes and Volcanic Eruptions
Graduate School of Science
Tohoku University
Sendai, Japan
KEYWORDS: EARTHQUAKES, SEISMOGRAPH NETWORK, DOUBLE-PLANED DEEP SEISMIC ZONE, SUBDUCTION ZONES, JAPAN, EARTHQUAKE PREDICTION, 1964 OGA PENINSULA, 1964 NIIGATA, HYPOCENTER RELOCATIONS, SEISMIC TOMOGRAPHY, ARC CRUST, LATERAL VARIATIONS, SEISMIC VELOCITY, TEMPERATURE, CONTRACTION DEFORMATION, TRIANGULATION, NORTHEASTERN JAPAN ARC, MANTLE WEDGE, VOLCANIC FRONT, INTERPLATE, ASPERITY MODEL, INTRASLAB, DEHYDRATION EMBRITTLEMENT HYPOTHESIS, AQUEOUS FLUID.
