HR: 16:30h
AN: T52F-03    [PDF]
TI: Breaking into the Plate: Seismic and Hydroacoustic Analysis of a 7.6 Mw Oceanic Fracture Zone Earthquake Adjacent to the Central Indian Ridge Plate Boundary
AU: * Bohnenstiehl, D R
EM: del@ldeo.columbia.edu
AF: LDEO Columbia Univ., Box 1000, Palisades, NY 10964
AU: Tolstoy, M
AF: LDEO Columbia Univ., Box 1000, Palisades, NY 10964
AU: Chapp, E
AF: LDEO Columbia Univ., Box 1000, Palisades, NY 10964
AB: Where oceanic spreading segments are offset laterally from one another, the differential motion of the plates is accommodated by strike-slip motion along ridge-perpendicular transform faults. Off-axis from the ridge-transform intersection, no differential motion is require, and the fracture zone trace is thought to be inactive except where reactivated by intra-plate stresses. On 15 July 2003, an earthquake with a magnitude of 7.6 Mw occurred near the northern Central Indian Ridge (CIR), the divergent boundary separating the Somalian plate from the Indian and Australian plates. The size of this event places it within the 99th quantile of magnitude for shallow ($<$ 40 km depth) strike-slip events (null axis plunge $>$45 deg) within the global Harvard CMT catalog. The earthquake's epicenter is near 2.5 deg S, 68.33 deg E, where the CIR is marked by a series of short ($<$100 km long) right-stepping transforms that offset the northwest trending spreading segments (20 mm/yr). Seismic signals associated with the mainshock and its largest aftershocks were recorded well by land-based seismic networks. Regional seismic phases (Pn, Sn), as well oceanic T-waves, where also recorded at an IMS hydroacoustic station to the north of the Diego Garcia atoll. T-wave signals recorded at Diego Garcia were cross correlated to determine accurate travel time differences. These traveltime differences were used in a plane wave fitting inversion to determine the horizontal slowness components and estimate the back azimuth to the epicenter. Aftershock locations are derived using the azimuthal information and Pn-T traveltime differences. Together, the seismically- and hydroacoustically-derived epicenters show a linear band of aftershocks extending more than 200 km along the off-axis trace of a right stepping transform. We interpret these aftershock events as delineating the length of the mainshock rupture. As the well-constrain hypocenter of the mainshock lies near the western edge of this aftershock zone and the centroid location lies within its eastern portion, the rupture appears to have propagated from the plate boundary, breaking into the Indian plate along the fracture zone. The moment tensor solution for the mainshock event indicates dominantly right-lateral slip with a near vertical nodal plane striking parallel to the trend of the aftershocks. The right-lateral sense of slip opposes the left-lateral slip along the adjacent active transform and is consistent with reactivation due to north-south extension in association with a diffuse plate boundary that separates the Indian and Australian Plates. Coulomb failure modeling indicates that the right-lateral mainshock should promote left-lateral slip on the active portions of neighboring transforms. Consistent with this prediction, the largest aftershock, a 5.6 Mw left-lateral earthquake, is located on the active portion of a transform approximately 150 km northwest of the mainshock rupture. These models also show that left-lateral slip on the adjacent active transform, extending southwest from mainshock rupture, should be inhibited. No aftershock activity is observed along this active transform. Static stress changes will tend to reduce ridge-normal compressive stresses along the portion of the CIR axis to the north of the mainshock, with an increase in ridge-normal compression to the south. This will promote and impede axis-parallel diking events within these respective regions.
DE: 3035 Midocean ridge processes
DE: 7230 Seismicity and seismotectonics
DE: 8020 Mechanics
DE: 8150 Plate boundary--general (3040)
DE: 8164 Stresses--crust and lithosphere
SC: Tectonophysics [T]
MN: 2003 Fall Meeting