Modeling focusing effects in Iceland: an alternative method of estimating plume parameters.

Richard M. Allen (1), Guust Nolet (1), W. Jason Morgan (1), Bergur H. Bergsson (2), Palmi Erlendsson (2), Gillian R. Foulger (3), Steinunn Jakobsdottir (2), Bruce R. Julian (4), Matt Pritchard (3), Sturla Ragnarsson (2), Ragnar Stefansson (2), Kristin Vogfjord (1).

(1) Dept. Geosciences, Princeton University, USA. (2) Vedurstofa Islands, Reykjavik, Iceland. (3) Dept. Geological Sciences, University of Durham, UK. (4) U.S. Geological Survey, Menlo Park, CA, USA.

Abstract submitted to IRIS Workshop, June 1997.

___________________________________________________________________________

Seismic tomography has been used to image the low velocity region beneath Iceland (Wolfe et al 1997). Such techniques however require both damping and smoothing of the model, this results in lower amplitude velocity anomalies and broadening of the structure. In order to estimate the plume radius and the maximum velocity perturbation we use 2-D finite difference modeling to study the focusing effects around a cylindrical plume, and compare the results with observations from data recorded on Iceland.

High quality teleseismic arrivals from five events recorded on the HOTSPOT network (a PASSCAL array currently deployed in Iceland) were used in the study. The spectral ratios of S arrivals were used to asses these focusing effects. Attenuation, expected to be particularly strong due to the high temperature plume, would result in a relative decrease in high frequencies for rays sampling the plume. The focusing effect however causes increased amplitude high frequencies. This effect is observed in the teleseismic events which show a gradational increase in the amplitudes of higher frequencies with distance beyond the plume.

In our finite-difference calculations we use a cylindrical plume with a Gaussian velocity deviation, dv, from some background; dv = dvmax.exp{-x/a}. A plume with a = 175 km and dvmax = -4.2%, (a good fit to the Wolfe et al (1997) velocity model) fails to produce the focusing observed in the data. Instead a narrower plume with a higher velocity perturbation is required to satisfy the data. A plume with a = 100 km and dvmax = -12% produces the necessary high frequency amplitudes, and a focal point approximately 150 km beyond the center of the plume. The amplitudes are a good fit to the data from two events in the Mediterranean region with arrivals perpendicular to the ridge in Iceland. The fit for the three events with arrivals oblique to the ridge are not as good.

Our results show that geometry effects on amplitude variations can be very strong. Modeling of these variations can be used to constrain the real amplitude of the velocity anomalies. The data from Iceland is compatible with a narrow plume with a = 100 km and maximum S-velocity perturbation of -12%.

____________________________________________

© Richard M Allen