Upper mantle structure beneath central Eurasia using a source array of nuclear explosions and waveforms at regional distances

Abstract

We have observed very consistent broadband, regional, three‐component P waveforms from a set of 11 explosions that occurred at the former Soviet test site in Kazakhstan during 1988 and 1989 and have modeled most of the prominent features in these waveforms to determine upper mantle structure beneath central Eurasia. Using a subset of events as a source array, we have identified four consistent arrivals during the first 15 s of P waves recorded at the Soviet/Incorporated Research Institutions for Seismology (IRIS) stations Arti (ARU) (d ≈1500 km) and Garm (GAR) (d ≈1380 km). These arrivals show consistent variations in frequency content, relative timing, and amplitudes from event to event despite a range in source magnitude (mb) between 4.9 and 6.1. We have also identified consistent features in P waveforms of these events recorded at Obninsk (OBN) (d ≈2880 km). We argue that most of the prominent features in these waveforms can be explained by reflections at or refractions near discontinuities or large velocity gradients in the upper mantle. We show that a model with discontinuities of approximately 3.0% and 6.5% near 200 km and 400 km, respectively, produces a better fit to the broadband data at ARU and GAR than previous models for this region. This model also produces a good match to waveforms recorded at OBN and NORESS, however, this model is not unique. For example, we show that it is also possible to produce similar waveforms by replacing the 200 km discontinuity with a large, notch shaped, low velocity zone between approximately 100 and 140 km. Based on reflectivity synthetics, we suggest that additional data, especially from distances less than 1000 km, should indicate which of these models is more accurate. Although our models are not unique, our observations do constrain the following features. (1) There must be reflectors between about 140 and 200 km and near 400 km depth in the upper mantle to explain the large secondary arrivals at both ARU and GAR. (2) Based on the relative size of some of the secondary arrivals at ARU and GAR, the discontinuity near 400 km must be large. However, it cannot be much greater than about 6.5% because there is no evidence, in our data, of a reflection from 400 km at OBN. (3) The relatively small amplitude of the first arrivals at both ARU and GAR suggests that either the gradient in the uppermost mantle is very small or that a shallow low velocity zone is present. Comparison of our results with other upper mantle studies shows that the central Eurasian upper mantle is similar to the upper mantle of the central and eastern United States.