Science of Slow Earthquakes

Science of Slow Earthquakes - Toward a unified understanding of all earthquake events from low-speed deformation to high-speed slip

Project OverviewGroup C02: Physical Modeling (Area number: JP16H06478)
Unified Understanding of Slow and Regular Earthquakes from Nonequilibrium Physics Point of ViewLeader: Takahiro HATANO, School of Science, Osaka University

  1. Admin, Int
  2. A01
  3. A02
  4. B01
  5. B02
  6. C01
  7. C02

Purpose of the Research Project

"Slow earthquakes," which are regarded as non-abrupt slips of plate interfaces, were first discovered in Japan by taking advantage of the high precision seismograph network and the geodetic observation network developed after the Great Hanshin Earthquake. Soon after the discovery in the Nankai Trough, similar phenomena have been detected along plate subducting zones all over the world. Like ordinary earthquakes, slow earthquakes occur regularly to reduce elastic strains accumulated by plate motion. In the Nankai Trough, a great variety of slow earthquakes have occurred along the rim of mega earthquake occurrence areas. It calls a strong attention to the relation between dynamics of strain-reduction by slow earthquakes and mega earthquake occurrence.
The sizes of various slow earthquakes and their slip dynamics have been gradually revealed by accurate observation studies. Both slow and ordinary earthquakes are slips along plate interfaces. Also, magnitudes (M) and durations of slow earthquakes vary widely similarly to those of ordinary earthquakes. "Slow Slip Events (SSEs)" characterized by relatively long duration and the absence of seismic waves can be as large as M7. A "Tremor", which generates feeble seismic waves, may consist of smaller events called "low-frequency earthquakes" of M1 or less. SSEs and tremors often occur in the same site and the time. Also known is "Very-low frequency earthquake," which is of intermediate size between them.
What causes such a wide variety of slow earthquakes? The mechanisms are still unknown. Unified understanding of the diversity of slow earthquakes from statistical-physics point of view is one of the major tasks of our project.
Diversity of slow earthquakes could be more interesting when we take ordinary (or "fast") earthquakes (slip fast and generate seismic waves) into account. Although fault rupture dynamics in fast earthquakes is quite diversified, we could find a simple scaling law if we focus on the average behavior. Scaling of earthquakes involves duration (t) and seismic moment (Mo) (proportional to the product of the ruptured area and the average displacement). In fast earthquakes, Mo is proportional to t3 on average. At the same time, for slow earthquakes Mo appears to be proportional to t1 (Ide et al. 2007). These scaling relations imply that a slow earthquake is not a single unstable event but consists of many marginally-stable events that interact with each other. However, unified understanding based on their physical entities has not been gained yet. One of the major tasks in this research field is to seek such universal scaling laws and/or statistical rules for various slow/fast earthquakes.
Neighboring of slow and fast earthquakes in the Nankai Trough suggests that both types of earthquakes are closely related. Although slow earthquakes are often observed along other subducting zones prior to mega earthquakes, we haven’t come to any conclusion regarding the relationship between slow and fast earthquakes. In order to explain mutual interaction between slow and fast earthquakes, we must combine observation data, experiments, and physical theories in a proper way. This is another important task of this research field. We aim at providing perspectives to slow and fast earthquake from the viewpoint of statistical physics.

Details of the Research

Some research topics are outlined with emphases on the relations with other research groups in this project.

  1. We will draw up general terms on mechanisms of cooperation, competition and switching of slow phenomena and fast events by laboratory analog experiments and mathematical model analyses. We do not intend to model actual plate interface sites in detail. We rather wish to trim the details, reveal the nature of phenomena, and provide brief understanding to the mechanism of slow earthquakes. This may provide Group C01 with a simpler understanding of their model studies.
  2. Law of friction is often used irresponsibly to extrapolate plate interfaces. However, with a large gap of 3-digits or more, it is unreasonable to apply the results of laboratory experiments straightaway to plate interfaces. We will intermediate between laboratory experiments by Group B02 and geophysical models by Group C01 by identifying physical processes on which laboratory law of friction relies and theoretically resolving its scale conversion.
  3. We intend to clarify the role of fluid migration dynamics which is considered significant for slow earthquakes. This should be done by describing slow and ordinary earthquakes collectively using nonlinear mutual interacting models of slip, void and fluid. Besides, we will study time evolution of fault structure to clarify hydraulic/geological formation process of subducting zones. We eventually aim to provide theoretical description on those what to be unraveled through observations such as electromagnetic structures by Group B01 and fault structures and fluid movement by Group B02.

Members

Project Leader
  • HATANO TakahiroSchool of Science, Osaka University
Co-investigators
  • YAMAGUCHI TetsuoGraduate School of Engineering, Kyushu University
  • SUMINO YutakaFaculty of Science Division 1, Tokyo University of Science
  • SUZUKI TakehitoCollege of Science and Engineering, Aoyama Gakuin University
Research Collaborators
  • OTSUKI MichioGraduate School of Engineering Science, Osaka University
  • YOSHINO HajimeCybermedia Center, Osaka University
  • KATSURAGI HiroakiGraduate School of Environmental Studies, Nagoya University
  • TAKADA SatoshiInstitute of Engineering, Tokyo University of Agriculture and Technology
  • SETO RyoheiMathematical Soft Matter Unit, Okinawa Institute of Science and Technology Graduate University
  • HORI TakaneResearch and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology
  • NIIYAMA TomoakiGraduate School of Natural Science and Technology, Kanazawa University
  • ISHII AkioGraduate School of Engineering Science, Osaka University
  • SUMITA IkuroGraduate School of Natural Science and Technology, Kanazawa University
  • MATSUKAWA HiroshiCollege of Science and Engineering, Aoyama Gakuin University

Group Research Subthemes

Co-investigater's Subjects
  • HATANO Takahiro Nonlinear dynamics
  • YAMAGUCHI Tetsuo Studies on control of frictional constitutive laws and earthquakes cycles using polymer gels
  • SUMINO Yutaka Fluid system with drastic rheological change mimicking slow earthquake
  • SUZUKI Takehito Analytical treatment of nonlinearity generated by the interaction among heat, fluid pressure and pore creation associated with earthquake source process