HR: 0800h
AN: U41A-0712    [Abstracts]
TI: Characterizing Deep ($>$ 500 km) Earthquake Regions to Investigate the Fate of Subducting Slabs
AU: * Reif, C
EM: creif@ucsd.edu
AF: UCSD, 9500 Gilman Drive MS-0225, La Jolla, CA 92093-0225 United States
AU: Dziewonski, A
EM: dziewons@seismology.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138 United States
AU: Ireland, T
EM: tireland@geol.umd.edu
AF: University of Maryland, Geology Department, College Park, MD 20742 United States
AU: Hammond, J
EM: j.hammond@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Lekic, V
EM: lekic@seismo.berkeley.edu
AF: UC Berkeley, 215 McCone Hall, Berkeley, CA 94720-4760 United States
AB: Subducting oceanic lithosphere is the main driving force of plate tectonics and the main source of upper mantle chemical heterogeneity. Here we present a comprehensive characterization of all regions of deep seismicity ($>$ 500 km): Japan, Izu-Bonin, Marianas, Philippines, Java, Solomon, New Hebrides, Tonga, and South America. Regional tomographic studies in subduction zones find fast velocity anomalies associated with the Wadati-Benioff zone of earthquakes, indicating continuity of some slabs to the bottom of the transition zone. Global tomographic studies find evidence of fast velocity anomalies in the mid-mantle, but often these anomalies are not obviously connected to descending slabs. The data we have assembled include the regional patterns of seismicity, CMT focal mechanisms, and discontinuity topography. Tomographic and dynamic models are also available for some regions. Most dynamic models show that, in the absence of complicated plate geometries and reorganizations, slabs should remain continuous into the lower mantle retaining a seismically detectable shear velocity anomaly of at least 1%. However, this expected anomaly is not observed below 700 km in many of these regions of deep seismicity. Rather, in most of the Western Pacific we find large-scale fast anomalies above 700 km along with focal mechanisms that indicate slab ponding rather than slab penetration. Most high velocity anomalies that do exist below the transition zone are no longer planar features, also indicating past slab ponding. In South America and New Hebrides there is deep seismicity seemingly isolated from the shallower slab seismicity. The New Hebrides seismicity is difficult to explain as it occurs outside of the current slab geometry with no consistent pattern in the focal mechanisms. Whereas, the South American seismicity is likely happening within the slab and exhibits down-dip compression. In Java and the Marianas, the steep angle of the slabs and the nearly vertical compression axes of the focal mechanisms indicate current slab penetration. Further dynamic modeling is needed to understand the stability of slabs with regard to their thermal and chemical structure as they pass through the transition zone. The unique structure of seismicity and mantle heterogeneity in each region suggests that the subduction process is highly time dependent and difficult to fully interpret over short time scales.
DE: 7230 Seismicity and seismotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
SC: Union [U]
MN: 2004 AGU Fall Meeting