HR: 0800h
AN: G21A-0102    [Abstracts]
TI: SPATIAL AND TEMPORAL VARIATIONS OF INTERSEISMIC STRAIN FROM GPS MONITORING (CGPS AND CAMPAIGN DATA) AND SEISMIC MONITORING IN THE NEPAL HIMALAYA
AU: * Bettinelli, P
EM: pierre.bettinelli@cea.fr
AF: CEA, LDG, BP12, Bruyeres-Le-Chatel, 91680 France
AU: Avouac, J
EM: avouac@gps.caltech.edu
AF: Caltech, GPS department, MC 100-23, Pasadena, CA 91125 United States
AU: Flouzat, M
EM: mireille.flouzat@cea.fr
AF: CEA, LDG, BP12, Bruyeres-Le-Chatel, 91680 France
AU: Bollinger, L
EM: laurent.bollinger@cea.fr
AF: CEA, LDG, BP12, Bruyeres-Le-Chatel, 91680 France
AU: NSCDMG, S
EM: nscdmg@mos.com.np
AF: NSCDMG, Lainchaur, Kathmandu, 00000 Nepal
AB: We investigate interseismic processes in the Himalaya from geodetic and seismic monitoring. Geological investigations have shown that the shortening rate across the Nepal Himalaya is taken up by a major thrust fault, the Main Himalayan Thrust fault, which emerges along the Himalayan piedmont. GPS campaign data are reasonably well adjusted from a model in which the fault is assumed to be fully locked over a width of the order of 100km with only minor lateral variations. However there is no necessity that interseismic strain be stationary, due to possible variation of the width the Locked Fault Zone (LFZ) or variations of ductile shear rates downdip of the LFZ in the postseismic period or throughout the interseismic period. In addiation we cannot exclude the possibility of slow events. Indeed, if the locked fault Zone were to slip only during seismic events, it would require a seismic release rate, on the long term average, well in excess of what is estimated from historical catalogues. This implies that the LFZ might slip during transient seismic events on some occasion. So geodetic strain rates might vary with time, and differ from that predicted form the simple kinematic model above. Since background seismicity in the Himalaya is driven by stress build up along the downdip end of the LFZ, seismicity rate might also show meaningful temporal variations. To investigate this problem, we have deployed a network of permanent GPS stations that complements the existing seismic network and GPS campaign data. Four stations (including one from IDYLHIM Team) have been in operation since 1997 and have been used to evaluate the best processing strategy. These stations are situated along a transect across the Himalaya of Central Nepal at the latitude of Kathmandu. All data were processed with Bernese in the International Terrestrial Reference Frame 2000 (ITRF2000). We have tested various processing strategy and found that strain within our network was best resolved if only one IGS station is jointly processed, probably because common mode in seasonal variations then cancel out. We used LHASA as the reference IGS station. The position of our stations in ITR2000 was next estimated from the position of LHASA relative to ITRF2000, with account for observed seasonal effects. We find that to the data are consistent with a present rate of shortening across the range of 17.5mm.yr$^{-1}$ slightly less than the Holocene slip rate of 21$\pm$1.5mm.yr$^{-1}$ along the MHT. In addition, significant temporal variations are observed at GUMBA, and DAMAN. These stations lies in the area that is most sensitive to strain accumulation near the downdip end of the locked fault zone, where interseismic strain induced a well clustered seismicity. This suggests that strain accumulation in the interseismic period might not be a stationary process.
DE: 9320 Asia
DE: 7230 Seismicity and seismotectonics
DE: 8102 Continental contractional orogenic belts
DE: 8123 Dynamics, seismotectonics
DE: 1206 Crustal movements--interplate (8155)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting