The Seismic Structure of the Mantle Wedge under Cascade Volcanoes (original) (raw)

2011

Abstract

ABSTRACT Under a number of Cascade volcanoes we have identified a characteristic seismic signature in individual station Ps receiver functions and in Ps CCP image volumes made from USArray Transportable Array and Flexible Array stations. In the mantle wedge, the CCP images and the RFs show a strong negative event just below the Moho, paired with a weak to moderate positive event between 50-70 km, and a strong slab event. At most of these volcanoes, a strong negative signal also appears between 15 and 25 km depth in the crust. The signature is particularly clear under Mt. Lassen and Mt. Shasta in data from FAME (Flexible Array Mendocino Experiment), where instruments were close to the volcanic centers. Comparing the average Cascadia volcano signature to those of stations throughout the western U.S. and specifically those of the Cascadia backarc region, shows that this signal is unique to the Cascadia volcanoes. Joint inversion of the Ps receiver functions and ambient noise Rayleigh wave phase velocities (Porritt et al., 2011; Liu et al., submitted) for those volcanoes with the paired events provides 1D shear velocity profiles having common characteristics. A strong sub-Moho low velocity zone from 5 to 15 km thick gives rise to the paired negative-positive signals in the receiver functions. The sub-Moho low velocity zones, with velocities of 3.7 < Vs < 4.0 km/s, are evident in 15 of the 22 stations we examined. Stations not exhibiting this pattern also show a characteristic seismic structure: There is no abrupt velocity increase at Moho depths, instead Vs increases gradually from the lower crust to as deep as ~70 km, forming a thick, relatively high velocity layer (4.0 < Vs <4.5 km/s). This project was initiated as part of the CIDER 2011 summer program.

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