Deformation of an island arc: Rates of moment release and crustal shortening in intraplate Japan determined from seismicity and Quaternary fault data

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<jats:p>The historical record of large (M > 6.9) earthquakes and geologically determined rates of slip on Quaternary faults in intraplate Japan (Honshu and Shikoku) are used to estimate the average rate of seismic moment release ( <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrb4118-math-0001.gif" xlink:title="urn:x-wiley:01480227:media:jgrb4118:jgrb4118-math-0001" />) for the last 400 years and during the late Quaternary, respectively. Values of <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrb4118-math-0002.gif" xlink:title="urn:x-wiley:01480227:media:jgrb4118:jgrb4118-math-0002" /> estimated from the two data sets are similar in regions where seismic activity is concentrated on land. We interpret this observation to suggest that <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrb4118-math-0003.gif" xlink:title="urn:x-wiley:01480227:media:jgrb4118:jgrb4118-math-0003" /> in intraplate Japan has been constant during the late Quaternary and is relatively free from secular variation when averaged over periods of several hundreds of years. <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrb4118-math-0004.gif" xlink:title="urn:x-wiley:01480227:media:jgrb4118:jgrb4118-math-0004" /> in Shikoku may be attributed almost solely to right‐lateral slip of the median tectonic line (MTL). The easterly strike of the MTL is consistent with a compressive stress field that trends northwest. Crustal shortening of the Izu Peninsula taken up on a set of strike slip faults that show north by northwest compression is ≈1 mm/yr. Northeast Honshu is characterized by a set of reverse‐type faults that trend northerly. Conversion of <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/jgrb4118-math-0005.gif" xlink:title="urn:x-wiley:01480227:media:jgrb4118:jgrb4118-math-0005" /> in northeast Japan to strain rates suggests that about 5% of the relative plate motion between Japan and the Pacific plate (≈9.7 cm/yr) is accommodated as a permanent east‐west shortening (≈5 mm/yr) of northeast Honshu. The predominant deformation in central and western Honshu takes place as slip on a conjugate system of strike slip faults that strike northeast and southeast and show right‐lateral and left‐lateral motion, respectively. Crustal shortening, resulting from slip on faults, in central and western Honshu is 5 and 0.5 mm/yr, respectively, in an easterly direction. Central and western Honshu are in closest proximity to the Nankai trough, and hence, the stress field in these regions cannot simply be attributed to the accommodation of the relative (northwesterly) convergence of the Philippine Sea plate. The northward impingement of the Izu Peninsula into Honshu may influence stresses in central and western Japan, but a conclusive explanation of the stress field in central and western Honshu remains an enigma.</jats:p>

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