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Plate motions are governed by equilibrium between basal and edge forces. Great earthquakes may induce differential static stress changes across tectonic plates, enabling a new equilibrium state. Here we consider the torque balance for idealized circular plates and find a simple scalar relationship for changes in relative plate speed as a function of its size, upper mantle viscosity, and coseismic stress changes. Applied to Japan, the 2011 $mathrm{M}_{mathrm{W}}=9.0$ Tohoku earthquake generated coseismic stresses of $10^2-10^5$~Pa that could have induced changes in motion of small (radius $sim100$~km) crustal blocks within Honshu. Analysis of time-dependent GPS velocities, with corrections for earthquake cycle effects, reveals that plate speeds may have changed by up to $sim3$ mm/yr between $sim3.75$-year epochs bracketing this earthquake, consistent with an upper mantle viscosity of $sim 5times10^{18}$Pa$cdot$s, suggesting that great earthquakes may modulate motions of proximal crustal blocks at frequencies as high as $10^-8$~Hz.
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The giant Sumatra-Andaman earthquake of December 26 2004 caused permanent deformations effects in a region of previously never observed extension. The GPS data from the world wide network of permanent IGS sites show significant coseismic displacements in an area exceeding 107 km^2. The effects of the permanent residual deformation field could be detected as far as Australia, the Phillipines and Japanese archipelagos, and, on the West, as far as the indian continent. The synthetic simulations of the coseismic displacement field obtained by means of a spherical model support the hypothesis that a considerable amount of energy has been released aseismically.
Sedimentary records of tsunamis are a precious tool to assess the occurrence of past events, as attested by an abundant literature, which has seen a particular boom in the aftermath of the 2004 Indian Ocean tsunami and the 2011 Tohoku tsunami. Despite an extensive literature, there is very little to no understanding of the role that the changing coastal environment is playing on the record of a tsunami, and for a given location, it is still unclear whether the largest tsunamis leave the largest amount of deposits. To research this question, the present study took place in Japan, in the Tohoku Region at Agawa-pond, because the pond act as a sediment trap. Using a sediment-slicer, a 1 m thick deposit was retrieved, from which 4 tsunami sequences were identified, including the latest 2011 tsunami. Using a series of sedimentary proxies: the AMS (Anisotropy of Magnetic Susceptibility), grain size analysis, quartz morphoscopy (morphology and surface characteristics) and the analysis of microfossils, disparities between the tsunami deposits were identified and most importantly a clear thinning of the tsunami deposit towards the top. Provided the present evidences, the authors discuss that the upward fining is due to at least two components that are seldom assessed in tsunami research (1) a modification of the depositional environment, with the progressive anthropization of the coast, providing less sediments to remobilize; and (2) a progressive filling of the Agawa pond, which progressively loses its ability to trap tsunami materials.
The Multifractal Stress-Activated (MSA) model is a statistical model of triggered seismicity based on mechanical and thermodynamic principles. It predicts that, above a triggering magnitude cut-off $M_0$, the exponent $p$ of the Omori law for the seismic decay of aftershocks is a linear increasing function $p(M) =a M+b$ of the main shock magnitude $M$. We previously reported empirical support for this prediction, using the Southern California SCEC catalog. Here, we confirm this law using an updated, longer version of the same catalog, as well as new methods to estimate $p$. One of this methods is the newly defined Scaling Function Analysis, adapted from the wavelet transform. This method is able to measure a singularity ($p$-value), erasing the possible regular part of a time series. The Scaling Function Analysis also proves particularly efficient to reveal the coexistence of several types of relaxation laws (typical Omori sequences and short-lived swarms sequences) which can be mixed within the same catalog. The same methods are used on data from the worlwide Harvard CMT and show results compatible with those of Southern California. For the Japanese JMA catalog, we still observe a linear dependence of $p$ on $M$, yet with a smaller slope. The scaling function analysis shows however that results for this catalog may be biased by numerous swarm sequences, despite our efforts to remove them before the analysis.
We report the observation of the ground rotation induced by the Mw=9.0, 11th of March 2011, Japan earthquake. The rotation measurements have been conducted with a ring laser gyroscope operating in a vertical plane, thus detecting rotations around the horizontal axis. Comparison of ground rotations with vertical accelerations from a co-located force-balance accelerometer shows excellent ring laser coupling at periods longer than 100s. Under the plane wave assumption, we derive a theoretical relationship between horizontal rotation and vertical acceleration for Rayleigh waves. Due to the oblique mounting of the gyroscope with respect to the wave direction-of-arrival, apparent velocities derived from the acceleration / rotation rate ratio are expected to be always larger than, or equal to the true wave propagation velocity. This hypothesis is confirmed through comparison with fundamental-mode, Rayleigh wave phase velocities predicted for a standard Earth model.
An article for the Springer Encyclopedia of Complexity and System Science
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