Monitoring spatio-temporal variation in the Earth's crust using seismic coda and ambient noise
Zusammenfassung der Projektergebnisse
Ambient seismic noise was used as a continuous seismic source to monitor changes in the Earth’s crust caused by several large crustal earthquakes in Japan. We observed a similar behavior for all analyzed earthquake source areas: At the time of the earthquake seismic wave velocity is reduced by about 0.5 to 1 percent. We can neither observe areas of increasing velocity nor any kind of radiation pattern indicating a systematic azimuthal dependence on the fault plane. All observations indicate a reduction of seismic velocity decreasing with distance to the earthquake source. The co-seismic velocity drop increases with increasing frequency. This is modeled under the assumption of a near surface layer of velocity reduction using different penetration depths of surface waves with different frequencies. A quantitative modeling showed that for some seismometer sites a layer of velocity reduction with a depth down to a few kilometers best explains the observed frequency dependence. Using borehole seismometers installed in a depth range between 1 and 3 km in Tokyo metropolitan area we revealed a velocity drop caused by the Tohoku earthquake. Since these deep sensors have little sensitivity to changes near the Earth’s surface, this is another indication for deep changes in the order of kilometers. All earthquakes show a post-seismic recovery (velocity increase) after the co-seismic velocity decrease. The recovery is fast shortly after the earthquake, but within the analyzed time period of between 3 and 8 years after the earthquake the recovery is incomplete. Some ’permanent’ velocity drop remains. To separate ’permanent’ and ’recovering’ velocity changes, we fitted a theoretical curve with exponential recovery function and with a permanent offset to the data. The result is that around 50% of the velocity change recovers and 50% is permanent using this exponential recovery model. (The second Chuetsu earthquake of 2007 is an exception: 90% of the velocity drop of this earthquake recovers and only 10% is permanent.) The recovery time is about 0.6 years independent of the size of the co-seismic velocity drop, of the earthquake region, as well as of the frequency band. It is well known from laboratory experiments that changes in static stress (such as caused by large crustal earthquakes) changes seismic velocity. However, we found that static strain changes do not correlate with co-seismic velocity changes, because of a missing ’radiation pattern’ in the velocity drop data. PGA and PGV values, as a measure for the strength of shaking caused by the earthquakes, better correlate with the co-seismic velocity drops. Such a model of nonlinear effects due to strong ground shaking is consistent with the fact that the co-seismic velocity change is always negative. PGV slightly better correlates with the velocity drops than PGA. This could be revealed by a comparison of Tohoku earthquake (M=9) data with the other earthquakes (6.5 < M < 7.0). Seasonal changes of seismic velocity were also observed, but the cause of these changes was not analyzed within this project.
Projektbezogene Publikationen (Auswahl)
- Monitoring co- and postseismic velocity changes caused by the 2008 Iwate-Miyagi Nairiku earthquake using Passive Image Interferometry. In Workshop on recent developments in seismic wave scattering and heterogeneities in the Earth, proceedings, Tohoku University, Sendai, Japan, 27.– 28.10. 2011
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
- Using ambient noise correlations to monitor coseismic and postseismic velocity changes caused by the 2008 Iwate-Miyagi Nairiku earthquake, Japan. In American Geophysical Union (AGU) Fall Meeting, San Francisco, USA, 05.–09.12. 2011
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
- Coseismic and postseismic elastic wave velocity variations caused by the 2008 Iwate-Miyagi Nairiku earthquake, Japan. J. Geophys. Res., 117:B09313, 2012
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
(Siehe online unter https://doi.org/10.1029/2012JB009402) - Estimation of the spatial distribution of coseismic velocity changes caused by large crustal earthquakes in japan. In C. Hadziioannou, E. Niederleitinger, A. Schmidt, C. Sens-Schönfelder, and U. Wegler, editors, Noise and Diffuse Wavefields - Neustadt Workshop, volume IV of DGG-Mitteilungen, Sonderband, pages 48–49. Deutsche Geophysikalische Gesellschaft, 2012. ISSN 0947- 1944
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
- Noise and Diffuse Wavefields, volume IV of DGG-Mitteilungen, Sonderband, 2012. Deutsche Geophysikalische Gesellschaft. ISSN 0947-1944
C. Hadziioannou, E. Niederleitinger, A. Schmidt, C. Sens-Schönfelder, and U. Wegler, editors
- Passive Image Interferometry and temporal velocity changes near five large crustal earthquakes in Japan. In C. Hadziioannou, E. Niederleitinger, A. Schmidt, C. Sens-Schönfelder, and U. Wegler, editors, Noise and Diffuse Wavefields - Neustadt Workshop, volume IV of DGG-Mitteilungen, Sonderband, pages 99–100. Deutsche Geophysikalische Gesellschaft, 2012. ISSN 0947- 1944
U. Wegler, M. Hobiger, K. Shiomi, and H. Nakahara
- Systematische Untersuchung von durch große Erdbeben (Mw > 6.5) erzeugte seismische Geschwindigkeitsänderungen in Japan. In 72th annual meeting of the German Geophysical Society (DGG), Hamburg, Germany, 05.–08.03. 2012
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
- Depth distribution of coseismic velocity changes caused by large earthquakes in Japan. In 73. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, Leipzig, Germany, 04.–07.03. 2013
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
- Depth distribution of coseismic velocity changes caused by large earthquakes in Japan. In Ambient Noise Imaging and Monitoring Workshop, Cargese, France, 22.–27.04. 2013
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
- Single-station cross-correlation analysis of ambient seismic noise: Application to stations in the surroundings of the 2008 Iwate-Miyagi Nairiku earthquake. Geophys. J. Int., 198(1):90–109, 2014
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
(Siehe online unter https://doi.org/10.1093/gji/ggu115) - Coseismic and post-seismic velocity changes detected by Passive Image Interferometry: Comparison of one great and five strong earthquakes in Japan. Geophys. J. Int. Geophysical Journal International, Volume 205, Issue 2, 1 May 2016, Pages 1053–1073
M. Hobiger, U. Wegler, K. Shiomi, and H. Nakahara
(Siehe online unter https://doi.org/10.1093/gji/ggw066)
