HR: 14:55h
AN: S32C-06    [PDF]
TI: Vertical Distribution of Seismic Anisotropy Beneath the Western Flank of the Gulf of Suez.
AU: Margheriti, L
EM: margheriti@ingv,it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143 Italy
AU: * Levin, V
EM: vadim@ldeo.columbia.edu
AF: Department of Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, 08854 Italy
AU: Pondrelli, S
EM: pondrelli@bo.ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via D. Creti 12, Bologna, 40128 Italy
AB: We investigate the vertical distribution of seismic anisotropy beneath the site on the western flank of the Gulf of Suez. The area was affected by rifting during early stages of the Arabian-Nubian Shield breakup, but became quiescent at $\sim$15 Ma when the locus of rifting shifted to the Gulf of Aqaba. We use two complimentary techniques sensitive to seismic anisotropy: birefringence in teleseismic SKS phases, and receiver function analysis focused on directional and epicentral systematics of SH-polarized converted waves. While the first method provides an integral measure of anisotropic properties, the second is sensitive to abrupt changes in them. Data used in the analysis were collected by the station KEG (Kottamya, Egypt) operated as a part of the MEDNET project from 1990 through 1999. Measurements of shear wave splitting yield fast directions in the range of 20$\deg$-40$\deg$NE for the majority of observed backazimuths. A significant minority of observations, clustered around backazimuths of 85$\deg$-90$\deg$ and 260$\deg$-270$\deg$, displays more northerly fast directions. Such systematic variations in the value of fast direction are expected if the anisotropic fabric varies with depth. Indeed, numerous phases indicative of anisotropy are evident in the receiver functions. Most conspicuously, the crust-mantle transition displays characteristic traits of anisotropy, with transversely polarized pulses of opposite polarity observed from the northeast and from the west. The likely orientation of the symmetry axis for these phases is within 15$\deg$ of south. Additionally, a shallow converted phase (2 s delay) exhibits polarity changes near backazimuths of 60$\deg$ NE and 310$\deg$ NW, while a deeper feature ($\sim$12 s delay) seen from the west change polarity around backazimuth of 300$\deg$. We construct a vertical profile of seismic properties (velocity and anisotropy) beneath station KEG by jointly modeling the receiver functions and the pattern of fast shear wave directions. Inferred depths, types and orientations of anisotropy-inducing fabrics are interpreted in the context of the tectonic history of the region.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2003 Fall Meeting