Seismic tomography of Shatsky Rise by adaptive importance sampling

Abstract

We present results from a wide‐angle seismic refraction survey over Shatsky Rise, a large oceanic plateau in the northwestern Pacific. A new Monte Carlo sampling scheme is developed to explore comprehensively the model space of crustal velocity structure by joint refraction and reflection seismic tomography. The new scheme, which is founded on the notion of adaptive importance sampling, is made possible by the combination of several independent developments, including the introduction of effective model parameters, the implementation of automated model regularization, and a polynomial expansion of the probability density function. From 21 ocean‐bottom instruments deployed along a 420‐km‐long refraction transect, we collect 16446 Pg and 4735 PmP travel times, which are inverted to construct a two‐dimensional compressional velocity model across the major axis of Shatsky Rise. The convergence of Monte Carlo sampling is tested by running parallel sampling chains, and the effective dimensions of the model space are estimated to be ∼10 by principal component analysis. The thickness of the rise crust varies from ∼9 km to ∼30 km, and the average velocity of the lower crust ranges from ∼7.0 km s−1 to ∼7.2 km s−1. One standard deviation uncertainty for whole‐crustal thickness and average lower‐crustal velocity is <1 km and ∼0.05 km s−1, respectively, for most of the model domain. Shatsky Rise crustal structure exhibits a negative correlation between crustal thickness and velocity, which implies that a chemically anomalous mantle may have been responsible for the formation of the rise.

Formation and evolution of Shatsky Rise oceanic plateau: Insights from IODP Expedition 324 and recent geophysical cruises

“…7; Korenaga and Sager, 2012). The observed thickness is consistent with complete Airy isostasy (Zhang et al, 2016).…”

“…Ocean Drilling Program (IODP) Expedition 324 2011a;2011b), seismic imaging by the R/V Marcus G. Langseth (Korenaga and Sager, 2012;Sager et al, 2013;Zhang et al, 2015), and geophysical data collection by R/V Yokosuka (Nakanishi et al, 2015). In this review, we encapsulate results of these recent research projects and explore their ramifications regarding the formation and evolution of Shatsky Rise and by inference, other oceanic LIPs.…”

“…rather than the ~30 km from refraction measurements (Korenaga and Sager, 2012)) can also fit the observed plateau elevation.…”

“…Data were recorded by 21 ocean bottom seismometers and these data were used to construct a tomographic velocity section (Korenaga and Sager, 2012). A total of ~3350 km of 2D multichannel seismic (MCS) reflection profiles were collected over southern Shatsky…”

Reflection/Refraction Seismology

“…Each subsequent iteration of the model is developed using both a finite-difference forward calculation of travel times and ray-paths and a regularized inversion incorporating a combination of smallest, flattest, and smoothest perturbation constraints, the weights of each being allowed to vary with depth. An advanced tomographic inversion based on a Monte Carlo scheme with an implementation of automated model regularization and a polynomial expansion of the probability function has been developed by Korenaga and and Sager (2012).…”