SUMMARY
The depth of reflector layers in the Earth's crust is usually estimated using controlled sources or earthquake signals. Ambient seismic noise, however, can also be used for this purpose. We develop and apply a new method, based on continuous waveform analysis, to estimate the two‐way P‐wave reflection component of the Green's function beneath each station. The Green's functions are retrieved from continuous record autocorrelation stacks at broad‐band sensor locations within the USArray EarthScope Transportable Array in the western Great Basin and the Sierra Nevada, in a region with complex crustal and upper‐mantle structure. In this paper, we show evidence of a reflector at the crust–mantle boundary (Moho discontinuity) derived for the first time from ambient noise autocorrelations using short‐period (∼1 s) data. Our results compare well with earthquake and controlled source investigations, and with tomography findings in the region. Moho depth is difficult to resolve seismically because of the lack of favourable spatial distribution of source and receiver geometries. In contrast, our method can be applied at any desired sensor spacing to estimate Earth reflector depth beneath surface‐located sensors, providing unprecedented resolution.
[1] We report on measurements of characteristic extensional wavelength represented by ribbons in Venusian tessera terrain. Fourier power spectra for ribbons in 35 areas from 9 geographic regions show dominant wavelengths of 2 to 6 km. We used these values to estimate mechanical layer thickness during ribbon formation at 0.6 to 2.9 km. Because ribbons accommodate extension of a single strong layer overlying a ductile substrate, we conclude that the base of this mechanical layer corresponded to the local brittle-ductile transition (BDT) during ribbon formation. Maintaining a BDT at <3 km depth for a significant length of time requires a locally hot environment, as over a plume impinging on thin lithosphere. These results indicate that locally hot conditions prevailed at widely distinct locations in the past.
SUMMARY
Evidence is presented here that P wave amplitudes contain additional information on the Earth's heterogeneity and must be considered in future tomographic interpretations. We analyse the reliability and the variance of teleseismic P wave amplitudes recorded at well calibrated broadband Global Seismic Network (GSN) stations from intermediate to deep earthquakes (depth >46 km). The dataset contains 217 earthquakes with mb between 5.6 and 7.6, from 1993 January to 2000 May. Using pairs of closely located events with similar focal mechanisms as well as data recorded at the closely spaced MOMA seismic array we demonstrate the consistency of observations. We deduce that the magnification of the GSN instruments generally drifts by at most 2 per cent per year, and likely much less. P wave amplitudes have variations due to focusing/defocusing with a standard deviation of at least 38 per cent. This reduces to 19 per cent if only periods in excess of 10 s are considered. Tomographic P wave models with mantle velocity anomalies of the order of 1 per cent are unable to reproduce such large variations.
Calibration studies at TXAR (Lajitas, Texas) used a modified version of the correlation method described by Cansi et al. (1993) in order to estimate azimuth and horizontal phase velocity of 144 events for which USGS m b values were available. Modifications to the correlation method include the Fourier interpolation of the data by a factor of 8 to obtain a virtual sample rate of 320/sec, use of an L1 norm (least absolute deviation) to obtain estimates of the azimuth and phase velocity, and a moving window display to indicate those portions of the waveform that show strongest correlation across the arra)~ Corrected phase velocities normally associated with Pn (less than 8.6 km/s) are generally seen for events at epicentral distance as far as 2,000 km. Upper mantle refracted first arrivals (P) with corrected phase velocities greater than 8.6 km/s are generally observed for epicentral distances beyond about 1,600 km. Phase identification is essential in order to select a suitable magnitude scale.Based on the 144 well-located events by USGS and using the Denny et al. (1987) formula, the most reliable magnitude estimates are as follows:1. For horizontal phase velocity less than 8.6 km/sec: mb(D) = logA + 2.4 (log D) -3.95 + C, with C= +0.3 2. For horizontal phase velocity greater than 8.6 km/sec: mb(D ) = logA + 2.4 (log D) -3.95 + C, with C= -0.50The M-discontinuity beneath TXAR was determined to the first order to strike along an azimuth of 109 degrees (NW-SE) and dip 11 degrees to the northeast. This result is consistent with the tectonic setting for the area.
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