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We analyzed temporal changes in site response associated with the strong ground motion of the 2004 Mw 6.6 Mid-Niigata earthquake sequence in Japan. The seismic data were recorded at a site with accelerometers at the surface and a 100-m-deep borehole. We computed the empirical surface-to-borehole spectral ratios and used them to track temporal changes in the top 100 m of the crust. We observed that the peak spectral ratio decreases by 40%-60% and the peak frequency drops by 30%-70% immediately after large earthquakes. The coseismic changes are followed by apparent recoveries, with the time scale ranging from several tens to more than 100 sec. The coseismic peak frequency drop, peak spectral ratio drop, and the postseismic recovery time roughly scale with the input ground motions when the peak ground velocity is larger than ~5 cm/sec (or the peak ground acceleration is larger than ~100 Gal). Our results suggest that at a given site the input ground motion plays an important role in controlling both the coseismic change and the postseismic recovery in site response. [ABSTRACT FROM AUTHOR]

The article discusses four reports within the issue regarding geophysics research, including one by Sato and colleagues, Simons and colleagues, Ide and colleagues, and Vigny and colleagues. The teams illustrate the use of a network of global positioning system (GPS) detectors to show how the Earth's surface deformed before and after the Tohoku-Oki magnitude 9 (M9) earthquake in Japan on March 11, 2011 and the M9 earthquake in central Chile (Maule) on February 27, 2010, in which thousands of lives were saved as a result of early warning systems for tsunamis.

The article discusses three separate online reports published in "Science" regarding seismology research analyzing the March 11, 2011 magnitude-9.0 earthquake in Sendai, Japan. Topics include results from the reports which gauged where and by how much the fault slipped, the pattern of the slip, and concern by seismologists about the southern portion of the fault, which broke in the Tohoku earthquake.