VOLUME BALANCING SEISMIC TOMOGRAPHY: QUANTITATIVE TESTS OF THE ORIGIN OF THE ISABELLA ANOMALY AND THE SIERRA NEVADA

Numerous colorful and provocative seismic tomography images have emerged in the past 10 years in large part because of the explosion of data associated with Earthscope's Transportable Array and Flex Array. Quantitative hypothesis testing with this data has been limited, and frequently analysis relies on the comparison or interpretation of shapes as defined by contours of velocity perturbation. Such interpretations stand on shaky ground as contours and shapes are often strongly dependent on starting models, inversion technique, damping parameters and subsets of data used. Differences between published images highlight the peril in relying too heavily on shape alone. Much as unbalanced geologic sections have fallen out of favor, aphysical interpretations of tomograms should be avoided. A robust measure in teleseismic tomography is the vertical travel time anomaly, which is generally close to the value being fit. A direct product from a tomographic inversion is the vertically integrated wavespeed anomaly; use of this value will be fairly robust in the face of other variations in inversion. I demonstrate how this can be applied to the high-wavespeed upper mantle Isabella anomaly adjoining the southwestern Sierra Nevada. Integrating over the anomaly from 95 to 245 km depth yields a volume of 7 ± 1 x10⁶ km³ at a mean anomaly of +1%. We expect from geological considerations that a volume on average 5% fast of 0.9-1.6 x 10⁶ km³ was removed from under the southern Sierra, equivalent to a volume of 4.4-8.4 x 10⁶ km³ at 1% fast, in close agreement with the equivalent volume of the Isabella anomaly. This lends support to the hypothesis that this body originated under the eastern Sierra Nevada. Caveats in this instance are that lateral wavespeed variations are maintained as material moves downwards and we have overlooked thermal erosion of the descending body.