Collaborative Research: Geodetic Constraints on Moment Deficit and Physics-based Earthquake Cycle Models in the Source Region of the M 9 Tohoku, Japan Earthquake

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The Mw 9 March 11, 2011 Tohoku earthquake, resulting in 25,000 fatalities, was the largestearthquake to strike Japan in modern times. The failure of long-term forecasts to anticipate anevent of this magnitude is leading to a reevaluation of our scienti c understanding of subductionzone fault dynamics. At the same time, the enormous Japanese investment in seismic andgeodetic monitoring provides an unprecedented opportunity to study processes leading to giantearthquakes. The investigators will focus here on two aspects of the quake that raise central issues for our abilityto forecast future hazards globally: 1) the constraints that geodetic strain accumulation placeon the seismic moment budget, and 2) how the distribution of mechanical properties on theplate interface control seismic versus aseismic slip.Forecasts of seismic hazard rely primarily on the historical record, paleoseismology, andgeodetic strain accumulation. While a strict time-predictable model has proven inconsistentwith geodetic data in some areas, the rate of strain accumulation must strongly inuence the recurrence rate and/or size of large earthquakes. Previous analyses of the extensive JapaneseGPS data set regularize the underdetermined inverse problem; the inferred plate coupling isthus conditional on the choice of regularizing functional. In addition, most assumed no coupling at the trench, an area that effectively lies in the model null-space. The researchers propose rather to estimate rigorous bounds on the moment deficit rate, independent of any regularizing functional. They also propose to systematically explore the effects of time-dependent viscoelastic effects on estimates of slip deficit.The Tohoku earthquake also raises profound questions about the frictional properties ofthe subducting plate interface. Mw 7 events along the Japan-Kurile trench have repeatedin nearly the same locations. Substantial afterslip following these events suggested that thefault surrounding these seismic `asperities' have different frictional properties, e.g., velocityweakening `asperities' surrounded by velocity strengthening regions. The fact that the March11 earthquake seems to have ruptured through both areas raises important questions. Is theasperity paradigm fundamentally flawed? Or did the M 9 activate other weakening mechanisms(thermal pressurization?) that allowed it to rupture through velocity strengthening regions?To distinguish between competing hypotheses the researchers will use a combination of GPS data basedhypothesis testing and physics based fault modeling. They will test whether or not post-seismicGPS data can be fit with afterslip only in regions that have not experienced seismic slip inthe last several decades. They will test alternative physical models that could accountfor the observed behavior, by extending our numerical codes that couple rate-state friction,thermal pressurization, and dilatant strengthening to three dimensions. This will allow them totest for example whether dilatancy could stabilize slip in otherwise velocity weakening regions,or thermal pressurization could allow ruptures to extend into velocity strengthening areas.