Updates to the Regional Seismic Travel Time (RSTT) Model: 1. Tomography

A function of global monitoring of nuclear explosions is the development of Earth models for predicting seismic travel times for more accurate calculation of event locations. Most monitoring agencies rely on fast, distance-dependent one-dimensional (1D) Earth models to calculate seismic event locations quickly and in near real-time. RSTT (Regional Seismic Travel Time) is a seismic velocity model and computer software package that captures the major effects of three-dimensional crust and upper mantle structure on regional seismic travel times, while still allowing for fast prediction speed (milliseconds). We describe updates to the RSTT model using a refined data set of regional phases (i.e., Pn, Pg, Sn, Lg) using the Bayesloc relative relocation algorithm. The tomographic inversion shown here acts to refine the previous RSTT public model (rstt201404um) and displays significant features related to areas of global tectonic complexity as well as further reduction in arrival residual values. Validation of the updated RSTT model demonstrates significant reduction in median epicenter mislocation (15.3 km) using all regional phases compared to the iasp91 1D model (22.1 km) as well as to the current station correction approach used at the Comprehensive Nuclear-Test-Ban Treaty Organization International Data Centre (18.9 km).

A New Bayesian Chronology for Postclassic and Colonial Occupation at Xaltocan, Mexico

This article proposes a new four-phase chronology for Postclassic and colonial occupation at Xaltocan, Mexico, using Bayesian statistical modeling of a suite of 54 radiometric dates. Of these, 46 samples come from recent extensive excavations of sealed, stratified household deposits, facilitating improved understanding of sample context and resulting in a more accurate chronology. The timing of the adoption of major ceramic wares at the site and intrasite level is outlined and contextualized within broad, regional phases. These results are particularly valuable given recent research that repudiates a uniform chronological framework for the Basin of Mexico in the Postclassic period, and indicates instead a significant amount of chronological overlap and regional variation in the use of distinct ceramic types.