Characterizing the 410 km discontinuity low‐velocity layer beneath the LA RISTRA array in the North American Southwest

Abstract

[1] Receiver functions recorded by the 54-station 920 km long Program for Array Seismic Studies of the Continental Lithosphere-Incorporated Research Institutions for Seismology Colorado Plateau/Rio Grande Rift Seismic Transect Experiment (LA RISTRA) line array display a pervasive negative polarity P to S conversion (P d s) arrival preceding the positive polarity 410 km discontinuity arrival. These arrivals are modeled as a low-velocity layer atop the 410 km discontinuity (410-LVL) and are inverted for a velocity profile via a grid search using a five-parameter linear gradient velocity model. Model parameter likelihood and correlations are assessed via calculation of one-and two-dimensional marginal posterior probability distributions. The maximum likelihood model parameter values found are top velocity gradient thickness of 0.0 km with a 4.6% (−0.22 km/s) shear velocity reduction, a 19.8 km constant velocity layer, and bottom gradient thickness of 25.0 km with a 3.5% (+0.17 km/s) shear velocity increase. The estimated mean thickness of the 410-LVL is 32.3 km. The top gradient of the 410-LVL is sharp within vertical resolution limits of P to S conversion (<10 km), and the diffuse 410 km velocity gradient is consistent with hydration of the olivine-wadsleyite phase transformation. The 410-LVL is interpreted as a melt layer created by the Transition Zone Water Filter model. Two secondary observations are found: (1) the 410-LVL is absent from the SE end of the array and (2) an intermittent negative polarity P 525 s arrival is observed. We speculate that upper mantle shear velocity anomalies above the 410 km discontinuity may manifest Rayleigh-Taylor instabilities nucleated from the 410-LVL melt layer that are being shed upward on time scales of tens of millions of years.Components: 9241 words, 15 figures, 2 tables.

Deep Earth Structure - Transition Zone and Mantle Discontinuities

“…Schmandt et al (2011) found in P receiver functions a low-velocity zone at 25-60 km above the 410 in several regions in the western United States. Jasbinsek et al (2010) found a low-velocity zone above the 410 at the Colorado Plateau and Rio Grande Rift. Chu et al (2012) found also a low-velocity zone atop the 410 in the United States using wideangle earthquake records.…”

Multiple transition zone seismic discontinuities and low velocity layers below western United States

“…We note however that this is less than reported in the studies of Jasbinsek and Dueker [] and Jasbinsek et al . [].…”

A sporadic low‐velocity layer atop the 410 km discontinuity beneath the Pacific Ocean

“…The LVL‐410 was initially discovered beneath northeast Asia by Revenaugh and Sipkin [] using seismic waves reflected at the core‐mantle boundary (multiple‐ ScS reverberation). Later seismic studies using a similar technique [ Courtier and Revenaugh , ; Bagley et al , ], receiver functions [ Vinnik and Farra , ; Vinnik et al , ; Fee and Dueker , ; Jasbinsek and Dueker , ; Vinnik and Farra , ; Wittlinger and Farra , ; Leahy , ; Jasbinsek et al , ; Schaeffer and Bostock , ; Tauzin et al , ; Vinnik et al , ; Schmandt et al , ; Huckfeldt et al , ; Bonatto et al , ; Morais et al , ; Thompson et al , ; Liu et al , ], and body wave triplications [ Song et al , ; Gao et al , ; Obayashi et al , ] suggest the widespread existence of this LVL (Figure and supporting information Table S1). In addition, an electromagnetic study in the southwestern U.S. also suggests a layer with high conductivity near the 410 km discontinuity [ Toffelmier and Tyburczy , ].…”

“…Previous observations of an LVL‐410 under continents and continental margins [ Revenaugh and Sipkin , ; Vinnik and Farra , ; Vinnik et al , ; Fee and Dueker , ; Song et al , ; Gao et al , ; Obayashi et al , ; Courtier and Revenaugh , ; Jasbinsek and Dueker , ; Toffelmier and Tyburczy , ; Vinnik and Farra , ; Wittlinger and Farra , ; Bagley et al , ; Leahy , ; Jasbinsek et al , ; Schaeffer and Bostock , ; Tauzin et al , ; Vinnik et al , ; Schmandt et al , ; Huckfeldt et al , ; Tauzin et al , ; Bonatto et al , ; Morais et al , ; Thompson et al , ; Liu et al , ]. Digits indicate the reference numbers listed in supporting information Table S1.…”