Upper mantle structure of western North America from apparent velocities ofPwaves

Abstract

In a study of variation of the compressional velocity with depth in the C region of the upper mantle in western North America, we determined apparent velocities of first arrivals across the Tonto Forest array in Arizona for about 70 shallow‐focus earthquakes. The epicenters ranged from 10° to 30° in distance and were mostly south of the array. The method gives the slope of the travel‐time curve directly, the parameter required for a velocity‐depth determination. For this distance range we found that although the slope of the P wave travel‐time curve decreased substantially with distance, i.e., increasing apparent velocity, the variation was not uniform. Two relatively abrupt changes were observed at distances of about 17° and 24°. These are most readily interpreted as two second‐order discontinuities in the region of the mantle above 1000 km. Assuming an average crustal structure as derived from seismic refraction measurements and taking the uppermost 200 km of the mantle given by Gutenberg or Jeffreys, we investigated various possible forms of variation of the P wave velocity with depth. The discontinuities in the rate of change of velocity with depth which cause the observed breaks in the apparent velocity curve are found to be at depths of about 320 and 640 km. Although no absolute times are required in the method, the travel times for the various derived models are computed and compared with standard tables.

Low-velocity zone under long valley as determined from teleseismic events

“…The recording of distant earthquakes is a commonly used method of extracting velocity information about the crust and upper mantle. For example, such studies have been made using the large aperture seismic array (Lasa) in Montana [Iyer and Healy, 1972; Chinnery and Toksoz, 1967], the Tonto Forest Seismic Observatory in Arizona [Niazi and Anderson, 1965], and the U.S. Geological Survey (USGS) seismic network in Yellowstone [lyer et al, 1974; lyer, 1975]. The investigation at Yellowstone is the first such study in a known geothermal area, and indeed the Yellowstone network was installed for the purpose of investigating passive seismic techniques in such areas.…”

Model P Wave Multipathing at Regional Distances in Southeast Asia

“…The example seismograms presented in Niazi & Anderson (1965) show clear later arrivals, but the authors were unable to include these observations in the modeling because of uncertainties in the depths of the sources. Presumably, the authors were worried about misinterpreting depth phases as triplicated arrivals.…”

“…Presumably, the authors were worried about misinterpreting depth phases as triplicated arrivals. However, the potential value of these arrivals was recognized, with the final sentence of Niazi & Anderson (1965) stating, "Clearly, later arrivals must be used to determine details of transition zones".…”

“…The derivative information (dt/d∆ as a function of ∆) was necessary to obtain a radial velocity model via the Herglotz-Weichert method. One of the earliest such studies was that of Niazi & Anderson (1965). These authors used data recorded at the Tonto Forest array in Arizona to measure dt/d∆ for 70 shallow events occurring at distances of 10…”

Electrical conductivity under Western North America in relation to heat flow, seismology, and structure.

“…The use of an array of seismometers for inversion of the apparent phase velocity to a velocity-depth profile was initiated by NIAzI and ANDERSON (1965) and developed by JOHNSON (1967). The array used was in Arizona and the velocity profiles refer to the western superprovince.…”