Geological Survey of Japan, AIST
Earthquakes occur under the conditions of fault, such as applied stress, fault strength, pore pressure, etc. Conversely characters of earthquakes will be clues to know such fault properties contributing earthquake occurrence. The fault properties will help us understand ongoing tectonic process and expand our capability to assess the rupture process of future large earthquakes. From this point of view, we study characteristics of number of small earthquakes to take the following advantages: (1) More opportunities to study thanks to more frequent occurrence of small earthquakes than large ones; (2) Spatial resolution is usually better than the distribution of source properties within one large earthquake. Since we study so many earthquakes, we simply extract the characteristics of earthquake source process as parameters of simple source model. One of them is stress drop of a circular crack model inferred from source spectra, which is the effect of earthquake source process reflected in observed seismograms in addition to the propagation path effect and the site effect.
One of interest in earthquake science is that the properties of faults on which large earthquakes. We addressed this problem from the view point of stress drops of small earthquakes that occurred before a large earthquake. The Tohoku-oki region, northeastern Japan was struck by the 2011 Tohoku-oki earthquake (Mw 9.0). More than 1500 small earthquakes in this region before the mainshock show the depth dependency as well as the horizontal variation [Uchide et al., JGR, 2014]. A comparison with the slip distribution of the mainshock [Uchide, GRL, 2013] found that a high stress-drop area at the edge of the large slip area of the mainshock acted as a barrier due to the high strength of the fault.
Recently more careful analysis investigates the omega-square model that is a standard model of source spectra and often used for fitting spectra from observation. The result of multiple spectral ratio method indicates that the source spectra may have two corner frequencies for some of small earthquakes, while single corner-frequency model works for some other earthquakes [Uchide and Imanishi, BSSA, 2016]. This result implies the intrinsic character we cannot overlook by a simplification. The multiple spectral ratio method also indicates that the magnitude scale of Japan Meteorological Agency (a kind of local magnitude scale) underestimates the size of microearthquakes [Uchide and Imanishi, under revision]. This result affects the quantitative evaluation of seismicity by, for example, b-values of the Gutenberg-Richter law, etc.
The source spectral analyses now explorer the frontier of earthquake seismology in many aspects.
Uchide, T. (2013), High-speed rupture in the first 20 s of the 2011 Tohoku earthquake, Japan, Geophys. Res. Lett., 40, 2993-2997, doi:10.1002/grl.50634.
Uchide, T., P. M. Shearer, and K. Imanishi (2014), Stress drop variations among small earthquakes before the 2011 Tohoku-oki, Japan, earthquake and implications for the main shock, J. Geophys. Res. Solid Earth, 119, 7164-7174, doi:10.1002/2014JB010943.
Uchide, T., and K. Imanishi (2016a), Small earthquakes deviate from the omega-square model as revealed by multiple spectral ratio analysis, Bull. Seismol. Soc. Am., 106, 1357-1363, doi:10.1785/0120150322.
Uchide, T., and K. Imanishi (2016b), Underestimation of microearthquake size by magnitude scale of Japan Meteorological Agency: Influence on earthquake statistics, under revision.