RESULTS FROM PRIOR NSF SUPPORT	WILLIAM H. MENKE 	Professor

Social Security No.:  054-50-4916

Seismic Studies in Iceland (NSF, 7/88, 1 yr, $55,000; 6/94, 1 yr, $6,000; 3/96, 1 yr, $50,000)
	Purpose:  Understand the various tectonic and volcanological processes that create the crust of earth at the Iceland hot spot, through active and passive seismic experiments.

Results:  My collegues and I have demonstrated that the old, 1970's view of the crust of Iceland as a thin layer of solid rock overlying a huge pool of partial melt is incorrect. Our new model of Iceland has the following elements:

• The top-most 20-30 km of the mantle is sub-solidus. (Other investigators have identified melt at deeper depth).

	• The crust of Iceland is very thick, with the older regions being thicker  (30-35 km) than the present-day volcanic zones (19-25 km).

• No partial melt occurs in the lower crust, which is found to have low attenuation (Qs>500) and "normal" Vp/Vs ratios (1.76-1.78).

• Several of the central volcanos (e.g. Katla, Krafla) possess shallow (2-4 km) magma chambers containing 5-50 cubic kilometers of melt.

• The magma chambers are underlain by high-velocity material, interpreted as cumulates, formed at the bottom of the magma chamber and advected to deeper depths.

• The shallow crust is pervasively cracked, which accounts for its low seismic velociy (Vp 2-3 km/s), high attenuation (Qs<50) and shear wave splitting (15% velocity anisotropy).

Inversion of Polarization Anomalies for the Laterally Heterogenous Structure of the Lithosphere (NSF, EAR-9104199, 8/91, 1 yr, $55,000).

	Purpose: Develop inversion method for determining the elastic velocity structure
	of the lithosphere from  measurements of the azimuth and angle of incidence of
	seismic waves.  Apply the method to data from the SCARLET array.

	Results:  We have applied the method to southern California.  While our results are similar to Humphrey's travel time inversion, they show shallower slow regions beneath the Salton Trough.   The code that we developed is capable of imaging a 3-D structure using traveltime and/or polarization data.