Temporal Changes of Surface Wave Velocity Associated with Major Sumatra Earthquakes from Ambient Noise Correlation

Temporal Changes of Surface Wave Velocity Associated with Major Sumatra Earthquakes from Ambient Noise Correlation Time shifts of various station pairs, relative to reference stack of all time periods. (A-C) Rayleigh wave for CHTO-PSI from vertical (Z) component correlation (A), Love wave for CHTO-PSI from tangential (T) component correlation (B), and a stable reference pair HIA-MDJ outside Sumatra region in northeast China (C). (D) Enlarged view of time shifts of CHTO-PSI Rayleigh wave before Dec. 2004 event.
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Detecting temporal changes of medium associated with major earthquakes has major implications for understanding earthquake genesis. The recent development of passive imaging through cross correlation of diffusive wavefield provides great opportunity in monitoring such changes because of the complete repeatability of the data processing procedures. We acquired 54 months of long period continuous data from Jan. 2004 to Jun. 2008 for stations in the Southeast Asia region from IRIS DMC. The stations are operated by GSN, Malaysia, Japan, and China. We retrieved empirical Green’s functions (EGFs) of surface wave between stations through cross correlation of continuous ambient noise data [Bensen et al., 2007]. We compared EGFs at different time periods before and after earthquake with a reference EGF. We observed clear temporal changes up to 1.44 s in Rayleigh wave after three major earthquakes in Sumatra region in Dec. 2004, Mar. 2005, and Sep. 2007 and a plausible precursor signal before Dec. 2004 event. However such changes were absent in Love waves. The temporal changes in Rayleigh wave appeared over a broad area near the source for a few months after the earthquakes and are frequency dependent. The observations are interpreted as stress changes and subsequent relaxation in upper-mid crust in the immediate vicinity of the rupture and the broad area near the fault zone.
</p><p>References
</p><p>Bensen, G. D., et al. (2007) Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements. Geophys. J. Int. 169: 1239-1260
</p><p>Acknowledgements: All data were downloaded from IRIS DMC. This work was supported by NSF (EAR8038188) and AFRL (FA8718-07-C-0006)</p>

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