  A high-resolution tomography study for the mantle beneath the New Madrid
Seismic Zone (NMSZ), a major intraplate earthquake zone in the Central and
Eastern US, reveals 3 - 5 % low Vp and Vs anomalies in the upper mantle in the
depth range 100 to 250 km. When attributed only to temperature variations, such
low velocities lead to temperatures higher than the olivine solidus, for which
consistent observational support is lacking. Similar magnitudes of the Vp and
Vs anomalies also suggest that temperature anomalies are unlikely to be the
sole factor because Vs is more sensitive to temperature than Vp. In this study,
we attribute the velocity anomalies to elevated water and orthopyroxene (Opx)
contents as well as temperature variations. We then compute differential
stresses using three-dimensional numerical models subjected to a loading
similar to the regional stresses. The models assume a Maxwell viscoelastic
crust and mantle with viscosities based on the temperature, water and Opx
content converted from the tomography. We find that the presence of water
allows for sub-solidus variations in temperature. However, any combination of
water content and temperature anomalies fails to yield Vp and Vs anomalies of a
similar magnitude. Thus, we consider Opx enrichment in place of water
content.Our calculations show that reasonable Opx content and sub-solidus
temperatures can explain both Vp and Vs anomalies. We speculate that the Opx
enrichment could have been produced by fluids released from a stagnant piece of
the Farallon slab imaged at around 670 km beneath the NMSZ
