4Imperial Prize and Japan Academy Prize to:
Kazushige Obara
Professor, Earthquake Research Institute,
The University of Tokyo
for “Discovery of Deep Low-Frequency Tremor at
Subducting Plate Interface and Development ofSlow Earthquake Seismology”

Outline of the work:
The research achievement of Dr. Kazushige Obara is summarized as the discovery of a new
type of earthquake phenomenon called a “slow earthquake” and the subsequent creation of a newresearch field known as the “science of slow earthquakes.” Similar to an ordinary earthquake, aslow earthquake is caused by fault slip; however, the fault slip speed in the latter is much slower
than that in the former.
Dr. Obara was responsible for constructing a data acquisition and processing system for thehigh-sensitivity seismograph network (Hi-net) deployed by the National Research Institute for
Earth Science and Disaster Prevention (NIED) after the 1995 Hanshin–Awaji Great Earthquake
Disaster. Based on the daily monitoring of continuous seismograph data obtained from 800
seismic stations to verify the status of the system, Dr. Obara found a strange seismic tremor
lasting for a long time with a very weak amplitude and a dominant frequency component lowerthan that of an ordinary earthquake. This tremor is now known as a “deep low-frequency tremor.”
Given that the usual detection and location method for ordinary earthquakes cannot be applied to the tremor analysis because of the lack of clear P or S wave, which are usually associatedwith ordinary earthquakes, Dr. Obara conceptualized a new method to detect and locate tremors.
This new method is based on the cross-correlation analysis of the envelope shapes of seismic
signals observed at many pairs of neighboring stations. Dr. Obara then clarified the source distribution of the tremor along a length of 600 km parallel to the strike of the subducting Philippine
Sea Plate in southwest Japan. The most impressive point is that the tremor is at the downdip
edge of the Nankai megathrust earthquake’s anticipated rupture area. Therefore, the tremor isexpected to be related to the occurrence of a major earthquake. The paper published in Sciencein 2002, which discussed the discovery of the tremor, has had a significant impact on the global
solid Earth science community. Following its publication, surveys of tremors were initiated inmany locations across the world.
After the discovery of the tremor, Dr. Obara worked on finding other related phenomena associated with the tremor by cooperating with young researchers. In the process, he successfully
discovered a short-term slow slip event, which had a very slow speed slip at the plate interface
with a duration of several days, along with deep very low-frequency earthquakes with a dominant period of several 10-s bursts. These slow earthquake phenomena occurred simultaneously5in space and time, and their focal mechanisms were estimated to be reverse faulting along theplate interface. Because these phenomena are all characterized as fault slips with slower speeds
compared with ordinary earthquakes, they are included in the “slow earthquake” family. In otherwords, Dr. Obara succeeded in identifying various members of slow earthquake phenomena andelucidating an entirely new perspective on slow earthquakes.
Moreover, Dr. Obara discovered a shallow, very low-frequency earthquake at the shallower
side of the megathrust seismogenic zone along the plate interface near the Nankai trough. Thisfinding indicated that the Nankai major earthquake seismogenic zone is sandwiched by shallow
and deep slow earthquakes along the dipping plate interface. When slow earthquakes occur, thestress at the slow earthquake source area is released but is then built up at the major earthquake
source area. Therefore, slow earthquake activity might be related to the occurrence of large
earthquakes. For 20 years since the publication of the first paper on the discovery of tremors in
2002, Dr. Obara and his co-authors have published seven significant papers in Science. Theseresearch results are applied to disaster mitigation policies formulated by the Japanese government
for the coming Nankai trough major earthquake.
Theoretically speaking, the discovery of several types of slow earthquakes by Dr. Obara
drastically changed our perceptions of earthquakes. Based on the new image of earthquakes
derived from Dr. Obara’s work, the newly created research field named “science of slow earthquakes” is expected to deepen our understanding of the earthquake phenomenon through multidisciplinary and interdisciplinary approaches.
Based on his outstanding contributions to science and research, Dr. Obara has received
several awards from Japan, including the Seismological Society of Japan Award in 2018 andthe Medal with Purple Ribbon in 2022. He also received distinctions from the US scientificcommunity, including being recognized as an American Geophysics Union Fellow in 2013 and aGutenberg Lecturer by the American Geophysics Union in 2015.

List of Main Publications
1. K. Obara and H. Sato, Existence of an S wave reflector near the upper plane of the double
seismic zone beneath the southern Kanto District, Japan. J. Geophys. Res., 93, 15037–15045, 1988.
2. K. Obara, Regional extent of the S wave reflector beneath the Kanto district, Japan. Geophys.
Res. Lett., 16, 839–842, 1989.
3. K. Obara and H. Sato, Regional diﬀerences of random inhomogeneities around the volcanic
front in the Kanto-Tokai area, Japan, revealed from the broadening of S wave seismogramenvelopes. J. Geophys. Res., 100, 2103–2121, 1995.
4. K. Obara, Simulations of anomalous seismogram envelopes at coda portions. Phys. EarthPlanet. Interior, 104, 109–125, 1997.
5. K. Obara, Hi-net: High sensitivity seismograph network, Japan. Lect. Notes Earth Sci., 98,79–87, 2002.
6. K. Obara, Nonvolcanic deep tremor associated with subduction in Southwest Japan. Science,296, 1679–1681, 2002.

6

7. K. Obara, H. Hirose, F. Yamamizu and K. Kasahara, Episodic slow slip events accompanied
by non-volcanic tremors in southwest Japan subduction zone. Geophys. Res. Lett., 31,L23602, 2004.
8. K. Obara, Y. Haryu, Y. Ito and K. Shiomi, Low frequency events occurred during the sequence of aftershock activity of the 2003 Tokachi-Oki earthquake; a dynamic process of thetectonic erosion by subducted seamount. Earth Planets Space, 56, 347–351, 2004.
9. K. Obara, K. Kasahara, S. Hori and Y. Okada, A densely distributed high-sensitivity seismograph network in Japan: Hi-net by National Research Institute for Earth Science and Disaster
Prevention. Rev. Sci. Instrum., 76, 021301, 2005.
10. K. Obara and Y. Ito, Very low frequency earthquakes excited by the 2004 oﬀ the Kii peninsula earthquakes: A dynamic deformation process in the large accretionary prism. EarthPlanets Space, 57, 321–326, 2005.
11. H. Hirose and K. Obara, Repeating short- and long-term slow slip events with deep tremor
activity around Bungo channel region, southwest Japan. Earth Planets Space, 57, 961–972,2005.
12. Y. Ito and K. Obara, Dynamic deformation of the accretionary prism excites very-lowfrequency earthquakes. Geophys. Res. Lett., 33, L02311, 2006.
13. K. Obara and H. Hirose, Non-volcanic deep low-frequency tremors accompanying slow slips
in the southwest Japan subduction zone. Tectonophysics, 417, 33–51, 2006.
14. H. Hirose and K. Obara, Short-term slow slip and correlated tremor episodes in the Tokairegion, central Japan. Geophys. Res. Lett., 33, L17311, 2006.
15. Y. Ito, K. Obara, K. Shiomi, S. Sekine and H. Hirose, Slow earthquakes coincident withepisodic tremors and slow slip events. Science, 315, 503–506, 2007.
16. Y. Asano, K. Obara and Y. Ito, Spatiotemporal distribution of very-low frequency earthquakes in Tokachi-oki near the junction of the Kuril and Japan trenches revealed by usingarray signal processing. Earth Planets Space, 60, 871–875, 2008.
17. K. Obara and T. Maeda, Reverse propagation of T waves from the Emperor seamount chain.
Geophys. Res. Lett., 36, L08304, 2009.
18. K. Obara and K. Shiomi, underground structural anomalies and slow earthquake activities
around seismogenic megathrust earthquake zone as revealed by inland seismic observations.
J. Disaster Res., 4, 83–93, 2009.
19. K. Obara, Inhomogeneous distribution of deep slow earthquake activity along the strike ofthe subducting Philippine Sea Plate. Gondwana Res., 16, 512–526, 2009.
20. K. Obara and S. Sekine, Characteristic activity and migration of episodic tremor and slowslip events in central Japan. Earth Planets Space, 61, 853–862, 2009.
21. T. Maeda and K. Obara, Spatiotemporal distribution of seismic energy radiation from lowfrequency tremor in western Shikoku, Japan. J. Geophys. Res., 114, B00A09, 2009.
22. Y. Ito, K. Obara, T. Matsuzawa and T. Maeda, Very low frequency earthquakes related tosmall asperities on the plate boundary interface at the locked to aseismic transition. J. Geophys. Res., 114, B00A13, 2009.
23. T. Matsuzawa, K. Obara and T. Maeda, Source duration of deep very low frequency earthquakes in western Shikoku, Japan. J. Geophys. Res., 114, B00A11, 2009.7

24. K. Obara, Phenomenology of deep slow earthquake family in southwest Japan: Spatiotemporal characteristics and segmentation. J. Geophys. Res., 115, B00A25, 2010.
25. K. Obara and M. Matsumura, Reverse propagation of surface waves reflected from a sea
mountain range in the northwestern pacific. Bull. Seism. Soc. Am., 100, 1342–1349, 2010.
26. K. Obara, T. Maeda, S. Tanaka and T. Matsuzawa, Depth-dependent activity of non-volcanic
tremor in southwest Japan. Geophys. Res. Lett., 37, L13306, 2010.
27. S. Aoi, B. Enescu, W. Suzuki, Y. Asano, K. Obara, T. Kunugi and K. Shiomi, Stress transfer in the Tokai subduction zone from the 2009 Suruga Bay earthquake in Japan. NatureGeoSci., 3, 496–500, 2010.
28. H. Hirose and K. Obara, Recurrence behavior of short-term slow slip and correlated nonvolcanic tremor episodes in western Shikoku, southwest Japan. J. Geophys. Res., 115,
B00A21, 2010.
29. H. Kimura, T. Takeda, K. Obara and K. Kasahara, Seismic evidence for active underplating
below the megathrust earthquake zone in Japan. Science, 329, 210–212, 2010.
30. H., Hirose, Y. Asano, K. Obara, T. Kimura, T. Matsuzawa, S. Tanaka and T. Maeda, Slowearthquakes linked along dip in the Nankai subduction zone. Science, 330, 1502, 2010.
31. K. Obara, T. Matsuzawa, S. Tanaka, T. Kimura and T. Maeda, Migration properties of nonvolcanic tremor in Shikoku, southwest Japan. Geophys. Res. Lett., 38, L09311, 2011.
32. K. Obara, Characteristics and interactions between non-volcanic tremor and related slow
earthquakes in the Nankai subduction zone, southwest Japan. J. Geodynamics, 52, 229–248,
2011.
33. T. Maeda, K. Obara, T. Furumura, and T. Saito, Interference of long-period seismic wavefieldobserved by dense Hi-net array in Japan. J. Geophys. Res., 116, B10303, 2011.
34. A. Kato, K. Obara, T. Igarashi, H. Tsuruoka, S. Nakagawa and N. Hirata, Propagation ofslow slip leading up to the 2011 Mw 9.0 Tohoku-Oki Earthquake. Science, 335, 705–708,
2012.
35. K. Obara, T. Matsuzawa, S. Tanaka and T. Maeda, Depth-dependent mode of tremor migration beneath Kii Peninsula, Nankai subduction zone. Geophys. Res. Lett., 39, L10308,2012.
36. K. Obara, New detection of tremor triggered in Hokkaido, northern Japan by the 2004
Sumatra-Andaman earthquake. Geophys. Res. Lett., 39, L20305, 2012.
37. T. Matsuzawa, K. Obara, T. Maeda, Y. Asano, and T. Saito, Love and Rayleigh wave microseisms excited by migrating ocean swells in the North Atlantic detected in Japan. Bull.
Seism. Soc. Am., 102, 4, 1864–1871, 2012.
38. A. Kato, and K. Obara, Step-like migration of early aftershocks following the 2007 Mw 6.7
Noto-Hanto earthquake. Japan, Geophys. Res. Lett., 41, 3864–3869, 2014.
39. K. Obara, Contribution of slow earthquake study for assessing the occurrence potential ofmegathrust earthquakes. J. Disaster Res., 9, 317–329, 2014.
40. Y. Asano, K. Obara, T. Matsuzawa, H. Hirose and Y. Ito, Possible shallow slow slip events inHyuga-nada, Nankai subduction zone, inferred from migration of very-low-frequency earthquakes. Geophys. Res. Lett., 42, 331–338, 2015.
41. Y. Yamashita, H. Yakiwara, Y. Asano, H. Shimizu, K. Uchida, S. Hirano, K. Umakoshi, H.

8Miyamachi, M. Nakamoto, M. Fukui, M. Kamizono, H. Kanehara, T. Yamada, M. Shinohara, K. Obara, Migrating tremor oﬀ southern Kyushu as evidence for slow slip of a shallow
subduction interface. Science, 348, 676–679, 2015.
42. T. Matsuzawa, Y. Asano and K. Obara, Very low frequency earthquakes oﬀ the Pacific coastof Tohoku. Japan, Geophys. Res. Lett., 42, 4318–4325, 2015.
43. A. A. Delorey, K. Chao, K. Obara, P. A. Johnson, Cascading elastic perturbation in Japandue to the 2012 Mw 8.6 Indian Ocean earthquake. Sci. Adv., 1, e1500468, 2015.
44. K. Obara and A. Kato, Connecting slow earthquakes to huge earthquakes. Science, 353,253–257, 2016.
45. K. Chao and K. Obara, Triggered tectonic tremor in various types of fault systems of japan
following the 2012 Mw 8.6 Sumatra Earthquake. J. Geophys. Res., 121, 170–187, 2016.
46. R. Takagi, K. Obara and T. Maeda, Slow slip event within a gap between tremor and lockedzones in the Nankai subduction zone. Geophys. Res. Lett., 43, 1066–1074, 2016.
47. S. Annoura, K. Obara and T. Maeda, Total energy of deep low-frequency tremor in theNankai subduction zone, southwest Japan. Geophys. Res. Lett., 43, 2562–2567, 2016.
48. K. Obara, Characteristic activities of slow earthquakes in Japan. Proc. Jpn. Acad. Ser. B,96, 297–315, 2020.
49. R. Kurihara and K. Obara, Spatiotemporal characteristics of relocated deep low-frequency
earthquakes beneath 52 volcanic regions in Japan over an analysis period of 14 years and 9months. J. Geophys. Res. Solid Earth, 126, e2021JB022173, 2021.
50. S. Baba, K. Obara, S. Takemura, A. Takeo snd G. A. Abers, Shallow slow earthquake
episodes near the trench axis oﬀ Costa Rica. J. Geophys. Res. Solid Earth, 126,e2021JB021706, 2021.
51. C. Aiken and K. Obara, Data-driven clustering reveals more than 900 small magnitude slow
earthquakes and their characteristics. Geophys. Res. Lett., 48, e2020GL091764, 2021.
小原一成「どんな地震だったのか
東北地方太平洋沖地震の地球科学的背景、概要と影、響」、佐竹健治・堀 宗朗（編著）『東日本大震災の科学』（2012 年、東京大学出版会）
pp. 1–40.