Mantle transition zone structure beneath the western Gulf of Mexico and central southern US from triplicated P waveforms

Abstract:

Triplicated P-waveforms from the discontinuities at 410 and 660 km depths are used to understand thermal processes, chemical composition, and dynamics of the upper mantle beneath the western Gulf of Mexico and central southern US. We choose an intermediate-depth Mw 5.6 southern Mexican earthquake, which is recorded by 190 USArray Transportable Array stations, to probe lateral variations in the thickness and velocity of the mantle transition zone. We find that the triplicated P phases by both 410 and 660 discontinuities (410 and 660) arrive 2 s faster in 342°-348° azimuths and 4 s faster in 351°-360° azimuths than the expected arrival times based on the global 1-D average velocity model. Such faster arrival times can be explained if the topography of 410 and 660 is elevated by <20 km and 20-50 km with respect to the global average depth, respectively. Furthermore, the P phase waveforms at the epicentral distance 18°-21° appear to be more complex, and it cannot be fully explained by the global 1-D model. We plan to carry out forward modeling by computing both 1-D frequency-wavenumber (FK) synthetics and 2-D finite-difference synthetics to fit the arrival times and amplitudes of the complex waveforms. We suspect that the cause of the observed waveform complexity may arise from (1) the change in the discontinuity depth, (2) the presence of low velocity zone above 410 and 660, and (3) variable impedance contrast across the 520 km discontinuity.