Frequency dependent focusing and seismic attenuation in low velocity regions such as Iceland

Richard M. Allen (1), Guust Nolet (1), W. Jason Morgan (1), Kristin Vogfjord (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).

(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 AGU Spring Meeting, May 1997.

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Low velocity regions such as that beneath Iceland are generally associated with higher seismic attenuation due to higher temperatures and partial melt. The parameter t* is a measure of seismic attenuation, higher seismic attenuation results in increased t*.

We present the results of 2-D finite difference modeling which indicate the importance of frequency dependent focusing effects on t* measurements in low velocity regions. Focusing around such regions causes reduced t* measurements, the opposite effect to high seismic attenuation which would increase t*. The focusing effect is very sensitive to the relative position of seismographs and the low velocity region resulting in large t* variations.

Teleseismic arrivals recorded on the HOTSPOT array (a PASSCAL array currently deployed across Iceland) are used to obtain t* values for a number of events. The spatial variation in t* cannot be explained by seismic attenuation models based on plume velocity models such as Wolfe et al (1997). Our measurements show large variations more akin to the variations caused by focusing than attenuation.

Our results indicate there are a lot of difficulties in detecting high seismic attenuation associated with low velocity regions using t* techniques. This is due to the detrimental effect of frequency dependent focusing on the measured t*.

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© Richard M Allen