Lianli Cong

Lg coda Q variation across Eurasia and its relation to crustal evolution

An extensive set of broadband and short-period seismograms from regional seismic events has been used to derive a tomographic image of broad-scale variations of Lg coda Q for most of Eurasia. The image indicates that Lg coda Q, at a frequency of 1 Hz, varies between about 200 and 1000. Lowest Q values ( 800) are found in four regions, the East European shield, the Siberian shield, the Indian shield, and the southwestern (and oldest) portion of the Altaid belt. The pattern of Lg coda Q variation across Eurasia suggests that crustal Q in any region is directly proportional to the length of time that has elapsed since the most recent episode of large-scale compressional tectonic activity or upper mantle heating that occurred there. We interpret the low Q as resulting largely from hydrothermal fluids, generated by tectonic activity or heating, that now reside in cracks and permeable portions of the Eurasian crust. Crustal Q increases with time, after that activity, as fluids are lost to the surface or absorbed by retrograde metamorphism.

Spectral characteristics of the excitation and propagation of Lg from underground nuclear explosions in central Asia

The nonlinear inverse method of Xie [1993] is applied to analyze Lg spectra from many recent underground nuclear explosions in central Asia. The analysis simultaneously determines Lg seismic moments (M0) and corner frequencies (fc) of events, as well as path-variable Lg Q values and their frequency dependences at 1 Hz. Lg Q at 1 Hz estimated for many paths from the Lop Nor and Balapan test sites to various broadband Incorporated Institution for Seismology, Chinese Digital Seismic Network, and Kyrghizstan Network stations agree well with path-averaged Lg coda Q values from previously obtained tomographic maps for Eurasia. This suggests that the Lg Q values obtained in this inversion have not been significantly affected by focusing/defocusing or other effects of three-dimensional structural complexities. The logarithms of Lg M0 values obtained in this study correlate linearly, with a slope of about 1.2, with International Seismological Centre (ISC) body wave magnitudes (Mb). The linear relationship is consistent with a previous empirical relationship found between the logarithm of P wave seismic moments and ISC Mb values. The slope of the linear relationship also suggests that the logarithm of Lg M0 is a more direct measure of event size than is Mb, which, for larger events, is affected by high-frequency amplitude falloff. Another important finding of this study is that the M0 values for the explosions scale with fc−4, rather than with fc−3, as would be expected if stress drops were constant. This is consistent with previous scaling relations derived using Lg from earthquake sources and suggests that the difference between Lg and local S excitation by the same source types (explosion or earthquake) may overwhelm that between the excitation of Lg by different source types. A comparison between Lg M0 and fc values obtained using an explosion source model with those obtained using an earthquake source model suggests that the M0 values estimated using the explosion source model are systematically lower (by a factor of 0.27) than those estimated using the earthquake source model. This occurs even though the data used are the same in the inversions with the two types of source model. The factor of 0.27 is therefore due to the source model dependence of the M0, fc estimates. The overshoot and lower spectral levels at lower frequencies in the explosion source model tend to reduce both the estimated M0 and fc values. This suggests that in any nuclear monitoring program, the same type of source model (either an earthquake or explosion) should always be used to discriminate explosions using Lg. The source model dependence also suggests that more effort should be made to estimate the amount of overshoot in the Lg excitation caused by various source types.

Excitation and propagation of Lg from earthquakes in central Asia with implications for explosion/earthquake discrimination

Broadband Lg records from 52 shallow earthquakes that occurred near the Balapan and Lop Nor nuclear test sites have been used to study spectral characteristics of the excitation and propagation of the Lg phase in central Asia. Using a source model that falls off as ω2 at high frequencies and a nonlinear inversion method, we simultaneously inverted for Lg seismic moments (M0), corner frequencies (fc), and path-variable values of Q0 and η (QLg at 1 Hz and its power law frequency dependence). The logarithms of M0 obtained using Lg correlate linearly with logarithms of fc, the slope being about −3.56 when all data are used and −4.04 when data were sufficient that full inversions could be performed. These slopes differ only slightly from the slope of −3.83 obtained in our earlier studies using underground nuclear explosions. The fc values in this study are systematically lower than those found for explosions at the same M0 level. A comparison of corner frequencies and Lg moment values for unknown events with the plots for known events of this study therefore provides means to discriminate between earthquakes and explosions. The mb values are systematically lower for earthquakes of this study than for previously studied explosions at the same seismic moment; moreover, the logarithms of Lg moments obtained in this study correlate linearly, with a slope of 1.04, with mb values when all data are used and 1.21 when only well-recorded events are used. These slopes are close to the slope of 1.19 obtained for explosions. The average QLg values along the paths used in this study agree well with those obtained using explosion data in central Asia and with reported Lg coda Q values.

Complexities in high‐frequency seismic waveforms due to three‐dimensional structure in the New Madrid Seismic Zone

Effects of three-dimensional structure on 2 to 10 Hz seismic signals in the New Madrid Seismic Zone (NMSZ) are investigated using waveforms from similar microearthquakes that occurred near Ridgely, Tennessee. Slowness power spectral (SPS) analysis is used to study the plane wave composition of P, S, and coda waves. The observed SPS at four stations suggest that the early S coda is composed mainly of wavelets leaving the source with slownesses of the direct S wave. At greater lapse times, coda is increasingly affected by random scattering. SPS observed on the vertical components and/or near the S wave nodes are more comlplex due to practical limitations of the algorithm. P coda and early S coda at a fifth station, located southwest of the junction, show the most complex SPS. Analysis of that complexity, together with waveform synthetics and travel time analysis, strongly suggests that scattering/multipathing due to source zone velocity heterogeneities occurs near the path, probably due to dilatancy occurring southwest of the Ridgely junction, or due to internal fault zone complexities. Coda Q, when averaged over three components, show no station variations. We infer that (1) coda at 1 Hz is mainly caused by scattering in the sedimentary layer where Q is frequency independent, (2) at higher frequencies the coda contains an increasing amount of scattered body waves, and (3) the source radiation pattern and path variable scattering/multipathing effects in early coda do not affect coda Q.