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Stress drop variations among small earthquakes before the 2011 Tohoku-oki, Japan, earthquake and implications for the main shock

TitleStress drop variations among small earthquakes before the 2011 Tohoku-oki, Japan, earthquake and implications for the main shock
Publication TypeJournal Article
Year of Publication2014
AuthorsUchide T, Shearer PM, Imanishi K.
JournalJournal of Geophysical Research-Solid Earth
Date Published2014/09
Type of ArticleArticle
ISBN Number2169-9313
Accession NumberWOS:000343874600024
Keywordsbeneath; fault; High-frequency Radiation; megathrust; microearthquakes; northeastern japan; pacific coast; rupture; slip; source spectra; stress drops; structure; the 2011 Tohoku-oki earthquake; Tohoku-oki; zone

It is important to assess the likely rupture characteristics of future megathrust earthquakes. One approach is to study the spatiotemporal variation of geophysical properties in active subduction zones. We explore this idea by examining stress drops of 1563 small earthquakes (M-w 3.0-4.5) shallower than 80km in the Tohoku-oki region before the 2011 Tohoku-oki earthquake. Although individual stress drop estimates exhibit considerable scatter, we find a strong increase in stress drop with depth between 30km and 60km, whereas stress drops for shallower and deeper events, respectively, are nearly constant. We also identify lateral variations in stress drop along strike. Higher-than-average stress drops are found in East Aomori-oki and Miyagi-oki, whereas Sanriku-oki is a moderate stress drop area. The high stress drop zone in Miyagi-oki is located just south of the large slip area of the 2011 Tohoku-oki earthquake, and possibly acted as a barrier to further rupture propagation during the event. In addition, the frequency dependence of the seismic radiation observed during the main shock, with proportionally higher frequencies coming from the deeper parts of the fault, mimics the depth dependence we see in small earthquakes in the same region. These results imply that smaller pre-main shock earthquakes can provide insight into the fault properties and consequent rupture processes of future megathrust earthquakes.

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