P wave amplitudes in 3D Earth Guust Nolet, Caryl Michaelson, Ileana Tibuleac, Ivan Koulakov, Princeton University

The objective: Assess the possibility of incorporation of focusing/defocusing effects by lateral heterogeneity in the tomographic inversions

Method Compare a set of amplitude variations from deep earthquakes to predicted amplitude variations by 3D global models DATA n 29 events recorded by IRIS GSN n January May 2000 n 5.6 < mb < 7.6 n Depth 88 km km n deg epicentral distance Hand picked P arrivals on vertical component of broadband instruments

Method

We measured: -maximum zero-to-peak amplitude -maximum peak-to-peak amplitude -integrated amplitude over pulse duration -integrated squared amplitude over the pulse duration on raw and 0.2 Hz low pass filtered data We excluded stations/events with: -interference of P and another phase -low SNR -short pulse duration ( < 2 sec) -bad fit of polarity predicted by CMT solution

Method

Volcano Islands Region event, 03/28/2000, mb=7.6, km deep Stack of P wave pulse

Method Theoretical expression for the P wave energy: E = S 2 R 2 G- 2 a s a r S - source term from Harward CMT solution R - free surface effect at the receiver G- 2 - geometrical spreading coefficient calculated for a particular model a s a r - source and receiver corrections respectively

Method Can we use frequency independent correction factors?

Ray tracing results event 1999/07/21, 13:46, mb =5.6, 175 km depth

Ray tracing results event 2000/03/28, 11:00, mb =7.6, km depth

Results

Results raw data

Results low pass filter 0.2 Hz

Results Raw data Low pass 0.2 Hz

Conclusions Causes for the misfit observed - predicted energy:  3D attenuation in the upper mantle  Attenuation in the lower mantle  Variation of instrument gain  Frequency dependence of correction factors  Complex radiation patterns  Strong focusing and defocusing by 3D structure

Conclusions What we think about them:  Absorbed in the correction factors  Not strong enough for observed variations of up to 50%  less than 5%  Small for frequencies < 0.5 Hz  Wavelength of P waves was comparable to the fault size for most of the events  that must be it but …

Conclusions What we think about them:  Absorbed in the correction factors  Not strong enough for observed variations of up to 50%  less than 5%  Small for frequencies < 0.5 Hz  Wavelength of P waves was comparable to the fault size for most of the events  that must be it but …

Conclusions The actual tomographic models fail to explain the misfit in observed and calculated energy because the predicted focusing is too small