Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We report measurements of long-period P wave polarization (P pol) in Australia and Tahiti made by combining modeling of the polarization deviation and harmonic analysis. The analysis of the deviation of the horizontal polarization of the P wave as a function of event back azimuth may be used to obtain information about (1) sensor misorientation, (2) dipping discontinuities, (3) seismic anisotropy, and (4) velocity heterogeneities beneath a seismic station. The results from harmonic analysis and a grid search using Snell's law suggest the presence of a dipping seismic discontinuity beneath stations CTAO and CAN in Australia. These results are consistent with published receiver function studies for these stations. The P pol fast axis orientation is close to the N–S absolute plate motion direction at station TAU (Tasmania), which may be due to plate-motion-driven alignment of olivine crystals in the asthenosphere. Interestingly, measurements of SKS splitting at Tahiti (French Polynesia) show an apparent isotropy, whereas an inversion of P pol observations at PPTL seismic station located in Tahiti suggests the presence of two anisotropic layers. The fast axis azimuth is oriented E–W in the upper layer, and it is close to the NW–SE orientation in the lower layer. Since P pol orientations are used for real-time earthquake locations, especially in poorly instrumented areas such as the South Pacific, we show that the bias from anisotropy and sensor misorientation determined here can be corrected to improve the location accuracy, which yields fundamental data for rapid location necessary for effective tsunami warning.
ISSN: 2169-9313

hal-01389113
http://hal.univ-reunion.fr/hal-01389113
http://hal.univ-reunion.fr/hal-01389113/document
http://hal.univ-reunion.fr/hal-01389113/file/Fontaine_Ppol_JGR2009.pdf
DOI : 10.1029/2008JB005709

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Analysis of continuous seismic data recorded by a dense passive seismological network (Hi-CLIMB) installed across the Himalayas reveals strong spatial and temporal variations in the ambient seismic energy produced at high frequencies (>1 Hz). From June to September 2003, the high-frequency seismic noise is observed to increase up to 20 dB (relative to (m/s)(2)/Hz) for all the stations located along a steep 30-km-long narrow and deeply incised channel of the Trisuli River, a major trans-Himalayan river. The early summer increase in high-frequency energy is modulated by a 24-h periodicity where the minimum of seismic noise level is reached around noon and the maximum is reached late in the evening. A detailed study of seismic noise amplitude reveals a clear correlation with both regional meteorological and hydrological data along the Trisuli River. Seasonal increase in ambient noise coincides with the strong monsoon rainfall and a period of rapid melting of snow and ice in the high elevations. The observed 24-h cyclicity is consistent with the daily fluctuation of the precipitation and river discharge in the region. River-induced seismic noise is partly generated by stream turbulence, but this mechanism fails to explain the observed clockwise hysteresis of seismic noise amplitude versus water level. This pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bed load transport. This points out the potential of using background seismic noise to quantify in continuous river bed load and monitor its spatial variations, which remain difficult with classical approaches.
ISSN: 2169-9313

hal-00412170
https://hal.archives-ouvertes.fr/hal-00412170
DOI : 10.1029/2007JB005034

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Upper mantle structures and flow related to the South Pacific Superswell and to the associated short-lived hotspots are investigated using seismic stations deployed in French Polynesia. Beside island stations, broadband ocean bottom seismometers ( BBOBS) have been deployed between the various Polynesian archipelagos to investigate the oceanic upper mantle "unperturbed'' by thickened crust or hotspot activity. We investigate seismic anisotropy measured at BBOBS stations and found it rather similar to that from island stations. This confirms the ability of OBS to be used for anisotropy purposes and suggests some homogeneity in the upper mantle pervasive deformation. The two OBS installed on the head of the Society hotspot provide different anisotropy signatures than most of the other French Polynesian stations: one is "isotropic'' to SKS waves and the other provides a fast split direction normal to the average value observed in this area, suggesting a more complex mantle flow induced by the plume upwelling.
ISSN: 0094-8276

hal-00420907
https://hal.archives-ouvertes.fr/hal-00420907
https://hal.archives-ouvertes.fr/hal-00420907/document
https://hal.archives-ouvertes.fr/hal-00420907/file/Barruol_SKS_PLUME_BBOBS_GRL2009.pdf
DOI : 10.1029/2009GL038139