Back Azimuth Research Articles

Seismic evidence for continuity of the deep slab beneath central and eastern Peru

Seismograms of Peru‐Brazil deep focus earthquakes recorded on the nearby Carnegie broadband station at Cuzco, Peru (CUS) are characterized by an anomalous large‐amplitude P wave arrival that follows about 1.5 s after direct P. The anomalous arrival is interpreted to be an underside wide‐angle reflection from the upper surface of the descending Nazca plate somewhere in the depth range 150–400 km and in a region where the slab is wholly aseismic. Particle motion analysis of the CUS seismograms shows that the direct P wave has arrival angles as predicted from simple ray tracing. The large, impulsive anomalous phase (termed Pr) also arrives as a P wave but is of opposite polarity and about 1.5 times the amplitude of the direct P arrival. The back azimuth of Pr is about 20° clockwise from the path of direct P, and its angle of emergence is significantly larger. Forward modeling calculations indicate that the seismic observations are consistent with a model in which Pr originates as a P wave that travels up (inside) the slab from the source and is then reflected at wide angle from the steeply dipping upper plate boundary. Results from modeling lead to the following conclusions: (1) the descending Nazca plate exists in and is probably continuous through the aseismic region between 150 and 500 km depth; (2) the plate is very steeply dipping (∼70°) below depths of about 150 or 200 km; and (3) the velocity discontinuity at the reflection point must be relatively sharp, probably less than about 10 km in thickness.

Crustal structure of the Adirondacks determined from broadband teleseismic waveform modeling

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of the Department of Energy's Regional Seismic Test Network station RSNY are analyzed to examine the crustal structure beneath the Adirondack Highlands of upstate New York. Radial receiver functions are inverted in the time domain to determine the vertical shear velocity structure at four distinct back azimuths. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. Southeast of RSNY, our model consists of a thick crust, including a broad crust‐mantle transition. The most prominent structure is a high‐velocity zone (shear velocity >4.0 km/s) between 18 and 26 km depth which overlies a lower crust of low average shear velocity (< 3.7 km/s). The extent of the high‐velocity zone correlates in depth with a highly reflective zone in Consortium for Continental Reflection Profiling profiles SE of RSNY, while the deep low‐velocity zone may be correlated with a broad electrical conductivity anomaly in the Adirondacks. This general structure is also seen SW of RSNY, but is more difficult to document at NW and NE azimuths.

Detection of interfering Rayleigh waves at LASA

abstract The problem of detecting one Rayleigh wave in the presence of the coda of another larger Rayleigh wave is considered. A detection method is proposed in which a high-resolution, wave number analysis technique is applied to prefiltered data from the Large Aperture Seismic Array (LASA) to determine the direction of arrival of the 40-sec-period Rayleigh-wave group at the appropriate arrival time. The performance of this detection method is considered in great detail. A necessary concomitant of the study of the detection method is the determination of the phases which comprise the coda. It is shown that one component of the coda consists of fundamental-mode Rayleigh waves which propagate along multiple paths and may be caused by either reflections at continental margins or diffraction effects. As has been demonstrated many times, the coda is shown to consist also of fundamental-mode Rayleigh waves which have propagated around the Earth in the direction of the back azimuth. Multiply reflected P and S waves, which propagate primarily in the upper mantle, are detected easily.

THE FURTHER EXTENSION OF THE GAUSS INVERSE PROBLEM

AbstractIt would be reasonable to suppose that if the formulae given by Rainsford in [1] were extended still further, there would be terms of greater than fifth order, elliptic and otherwise; and that of these, the elliptic would be of less importance. Now it is easy to see that all such terms that are not elliptic, obtained by putting the eccentricity = 0, could be provided by “closed” spherical formulae. Rainsford's extension was tested at 1000 miles in [2], and showed differences of up to 16·7 metres in distance and 0·70″ in azimuth. The “Spherical” Gauss system came in to 0·0 metres in length and 0·00″ in forward and back azimuths for all of the nine test lines. The numerical results are given later in detail.