HR: 16:45h
AN: S52K-04    [PDF]
TI: Rupture Characteristics and Aftershocks of the July 15, 2003 Carlsberg `H' (Indian Ocean) $M_w$ 7.6 Earthquake
AU: * Antolik, M
EM: antolik@seismology.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
AU: Abercrombie, R E
EM: rea@bu.edu
AF: Department of Earth Sciences, Boston University, Boston, MA 02215
AU: Pan, J
EM: pan@seismology.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
AU: Ekstrom, G
EM: ekstrom@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138
AB: The occurrence of a $M_w$ 7.6 earthquake near the Carlsberg ridge (15 July, 2003) provides valuable information about earthquake rupture processes in oceanic lithosphere, which are not well understood, and the distributed deformation of the India-Australia plate. The earthquake had a strike-slip mechanism opposite to that of the transform faults on the ridge, and appears to have ruptured a fracture zone (designated as `H' by {\it Royer et al.}, 1997) within the India-Australia composite plate. We examine the rupture characteristics of this earthquake using the full spectrum of seismic radiation. Inversion of the body waves indicates rapid rupture propagation toward the NE, away from the Carlsberg Ridge, for a distance of $\sim$200 km. The average rupture velocity is well constrained and is $\sim$3.6 km s$^{-1}$. The total source duration is $\sim$60 s; however, nearly all of the moment release occurs in the last 30 s. The age of the lithosphere in the area of largest moment is release is 10-15 Ma. The body waves can be well fit with a simple rupture model and no jump of fracture zones is required, as has been suggested for some oceanic earthquakes (e.g., {\it McGuire et al.}, 1996). The source process is very similar to the well-studied 1994 $M_w$ 7.0 earthquake along the Romanche transform in the equatorial Atlantic ({\it Abercrombie and Ekstr\"{o}m}, 2001). We also analyze the aftershock distribution using multiple-hypocenter relocation techniques and moment-tensor analysis using intermediate-period surface waves. Only 15 aftershocks ($M >$ 4.5) are listed in the USGS catalog, which is typical of large oceanic earthquakes (e.g., {\it Boettcher and Jordan}, 2001). Moment tensors obtained from five of the aftershocks show a diversity of focal mechanisms. We interpret a cluster of aftershocks located at $\sim$1$^{\circ}$ S as representing extension which results from the stress field of the mainshock at the end of the rupture. This interpretation is consistent with the 200-km rupture length inferred from body waves. The focal mechanism of the July 15 earthquake indicates N-S extension within the oceanic lithosphere east of the Carlsberg Ridge. {\it Royer and Gordon} (1997) interpreted this extension as part of the deformation associated with a diffuse plate boundary between the Indian plate and a hypothesized Capricorn Plate. They placed the northern boundary of the diffuse region of deformation near 7$^{\circ}$ S. The occurrence of the July 15 earthquake indicates this diffuse boundary is wider than previously believed.
DE: 7215 Earthquake parameters
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting