George E. Randall

Moment tensor estimation using regional seismograms from a Tibetan Plateau portable network deployment

The authors present the results of moment tensor inversions using regional seismograms recorded during the 1991-1992 PASSCAL passive seismic experiment on the Tibetan Plateau. Using data recorded over a one-year period, they estimate the source mechanisms for 38 earthquakes within and near the Tibetan Plateau with moment-magnitudes (M{sub w}) ranging from 3.8 to 5.0. With only one exception, all the events within the Plateau, but away from the Plateau boundary, indicate either strike-slip or extensional faulting, a result consistent with previous studies. Three-fourths of all the events occurred at depths less than 10 km, and all but two at depths less than 15 km. Several events, located near the southeastern section of the Plateau, exhibit an unusual m{sub b}-M{sub w} difference of approximately one magnitude unit. 19 refs., 3 figs.

1 Hz Lg Q tomography in central Asia

We have applied tomographic techniques to a data set of over 1700, regional distance, Lg amplitudes from 12 stations for paths inside a 30° by 40° region of Central Asia. Our purpose is to create high resolution path correction maps for use in regional distance source discrimination and magnitude determination, as well as to study Lg propagation characteristics in one of the most heterogeneous portions of the earth. For 1.0 Hz data, tomography reduced variance 33%, relative to a best fit, uniform Q model. LgQ varied with geologic region, with low values in Tibet, the Tien Shan and the Pamir range (Q=200 to 400), intermediate values in basins such as the Tarim (Q=500) and high values for platforms and older crust (Q>800). The relatively efficient propagation across the Tarim differs from the poor propagation that is generally observed across sedimentary basins.

Path correction using interpolated amplitude residuals: An example from central China

To isolate effects of the laterally varying lithosphere on regional seismograms, we geographically smoothed source- and distance-corrected amplitudes from 853 events recorded at station LZH in central China. This procedure was applied to P n , P g , S n and L g phases and various coda windows for eight bands between 0.5 and 8 Hz. We assume smoothing reduces effects such as source radiation, leaving an estimate of the path effect that can be used to correct data. A cross-validation procedure showed that path corrections performed best with low-frequency L g data, reducing variance up to 39% for 1 Hz L g , and up to 57% for 0.75 Hz P/L g ratios, compared to correction for distance alone. These corrections reduce scatter in magnitude estimates and discriminant ratios without needing to know detailed geological structure and will be important for effective test-ban monitoring.

Application of regional phase amplitude tomography to seismic verification

Abstract — We have applied tomographic techniques to amplitude data to quantify regional phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to apply path corrections into regions devoid of calibration events, as well as raising levels of confidence in the corrections because of their more physical basis. Our tomography technique solves for resolvable combinations of attenuation, source-generation, site and spreading terms. First difference regularization is used to remove singularities and reduce noise effects.¶In initial tests the technique was applied to a data set of 1488, 1.0 Hz, Pg/Lg amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power-law distance correction traditionally applied to amplitude ratios. Relative Pg/Lg attenuation varied with geologic region, with low values in Tibet, intermediate values in basins and high values for platforms and older crust. Spatial patterns were consistent with previous path effect studies in Asia, especially local earthquake coda-Q. Relative spreading was consistent with expected values for Pg and Lg. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident.¶Tomography residuals followed systematic trends with distance, which may result from the evolution from direct to coda phases, focusing, model tradeoff or data windowing effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects. The residual patterns often follow geological boundaries, which could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These results will guide efforts to reparameterize tomography models to more effectively represent regional wave attenuation and blockage. The interpolated residuals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.

Preliminary Study of Crust-Upper Mantle Structure of the Tibetan Plateau by Using Broadband Teleseismic Body Waveforms

As part of a joint Sino-U.S. research project to study the deep structure of the Tibetan Plateau, 11 broadband digital seismic recorders were deployed on the Plateau for one year of passive seismic recording. In this report we use teleseimic P waveforms to study the seismic velocity structure of crust and upper mantle under three stations by receiver function inversion. The receiver function is obtained by first rotating two horizontal components of seismic records into radial and tangential components and then deconvolving the vertical component from them. The receiver function depends only on the structure near the station because the source and path effects have been removed by the deconvolution. To suppress noise, receiver functions calculated from events clustered in a small range of back-azimuths and epicentral distances are stacked. Using a matrix formalism describing the propagation of elastic waves in laterally homogeneous stratified medium, a synthetic receiver function and differential receiver functions for the parameters in each layer can be calculated to establish a linearized inversion for one-dimensional velocity structure.Preliminary results of three stations, Wen-quan, Golmud and Xigatze (Coded as WNDO, TUNL and XIGA), located in central, northern and southern Plateau are given in this paper. The receiver functions of all three stations show clear P-S converted phases. The time delays of these converted phases relative to direct P arrivals are: WNDO 7.9s (for NE direction) and 8.3s (for SE direction), TUNL 8.2s, XIGA 9.0s. Such long time delays indicate the great thickness of crust under the Plateau. The differences between receiver function of these three station shows the tectonic difference between southern and north-central Plateau. The waveforms of the receiver functions for WNDO and TUNL are very simple, while the receiver function of XIGA has an additional midcrustal converted phase. The S wave velocity structures at these three stations are estimated from inversions of the receiver function. The crustal shear wave velocities at WNDO and TUNL are vertically homogeneous, with value between 3.5–3.6 km/s down to Moho. This value in the lower crust is lower than the normal value for the lower crust of continents, which is consistent with the observed strong Sn attenuation in this region. The velocity structure at XIGA shows a velocity discontinuity at depth of 20 km and high velocity value of 4.0 km/s in the midcrust between 20–30 km depth. Similar results are obtained from a DSS profile in southern Tibet. The velocity under XIGA decreases below a depth of 30 km, reaching the lowest value of 3.2 km/s between 50–55 km. depth. This may imply that the Indian crust underthrusts the low part of Tibetan crust in the southern Plateau, forming a “double crust”. The crustal thickness at each of these sites is: WNDO, 68 km; TUNL, 70 km; XI-GA, 80 km.

Chapter 12 Source Effects From Broad Area Network Calibration of Regional Distance Coda Waves

Abstract We have applied regional coda techniques to a network of 64 stations across central and east Asia to isolate source effects for bands from 0.03 to 8 Hz. The heterogeneity of the study region required us to determine two‐dimensional (2‐D) path and transfer function corrections. The importance of the 2‐D path corrections increased with frequency and distance, and for continental paths, became critical beyond 500 km for 1 Hz data. We propose a new spreading model for coda amplitudes, termed the extended Street‐Herrmann (ESH) model, to which attenuation can be added, facilitating the use of tomographic techniques for path correction. The 2‐D transfer function varied between continents and oceans, as well as within continents in areas of poor Lg propagation, reflecting differing excitation of Lg and Sn coda. We also demonstrate the use of empirically determined coda shapes, or type curves , to measure coda amplitudes, adding precision and flexibility for source regions of special interest. We applied these techniques to 112,000 records from 35,000 events, magnitudes 2–7, depths 0–50 km, between latitudes 0° and 60° and longitudes 60° and 150°. The resulting coda source spectra were used to derive moments and, for the better recorded events, corner frequencies, allowing computation of apparent stress for just under 6700 earthquakes. Preliminary apparent stress results ranged from 10 −2 to 1 MPa and showed some regional variation. For example, stress increased from south to north across the Tian Shan, perhaps reflecting deformation in varying crustal rheology or effects of prior slip history. Low stress observed in Tibet could be an artifact of under correction for high attenuation; however, the correlation also could be physical. Low stress observed in oceanic regions is inconsistent with local studies and indicates that upgrades to the coda methodology to more explicitly account for mixed Lg and Sn coda will be needed. The regional network coda results should be further tested by comparing to ground‐truth spectra obtained by applying coda techniques to data from local scale networks within the study region.

Propagation or Source? Analysis of a Moderate-Magnitude Seismic Event in the Qinghai Province, China

We investigate a moderate-size ( m b 4.5) seismic event that occurred in the Qinghai Province, China on 6 December 1997. Observations of large Pn and little Sn energy at high frequency (>3 Hz) show explosion-like characteristics at the Chinese Digital Seismic Network station WMQ. We perform high- and low-frequency analyses using both body and surface waves. The longer-period surface wave analysis shows that the event was indeed an earthquake. Several factors appear to be contributing to the large Pn amplitude and small Sn amplitude. A zone of high Sn attenuation identified in the Tibetan plateau appears to be contributing to the low-amplitude Sn observations if the zone lies further north than previously identified. The focal mechanism and rupture directivity may be contributing to the large Pn observation. Furthermore, focusing of the Pn wave is possibly given propagation from a source region with a thick crust to thinner crust at station WMQ.

Processing international monitoring system infrasound data to detect and locate global events using probabilistic algorithms

Automating the detection and location of events using the International Monitoring (IMS) System infrasound network is a significant challenge. Any algorithm must reliably detect nuclear tests in the atmosphere with a minimum number of false alarms. Here, we report on the application of probabilistic techniques for detection, association, and location of infrasound events to data from the IMS network. We compare our results with the SEL3 automatic event detections obtained by the CTBTO.