A. Douglas

Seismic Discrimination in Southern Xinjiang: The 13 March 2003 Lop Nor Earthquake

Earthquakes that occur near known nuclear test sites are invaluable for evaluating the capabilities of the International Monitoring System (ims), currently being established to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (ctbt). On 13 March 2003 a seismic disturbance with magnitude m b (National Earthquake Information Center [neic]) 4.8 occurred near the Chinese nuclear test site at Lop Nor, southern Xinjiang. Here we attempt to identify this disturbance as an earthquake by using three types of seismic data: (1) teleseismic P waves recorded at ims stations; (2) long-period surface waves recorded at a network of stations in Eurasia that simulates the proposed ims seismic network in this region, and (3) long-period full waveform ( P, S , and surface wave) modeling at station wmq, which is approximately 250 km from the reported epicenter of the 13 March 2003 disturbance. We find that all impulsive teleseismic P -wave onsets show compressive first motion and that discrimination using the m  b : M s criterion requires assumptions that cannot be justified on theoretical grounds. However, by combining the three data types, we conclude that the observations are consistent with a double-couple source with strike φ = 125 ± 10°, dip δ = 40 ± 10°, rake λ = 90 ± 10°, and moment M 5.5 ± 1 × 1015 N m. That said, consistency of the seismic wave field with a double couple does not immediately rule out an explosion source, because tectonic release accompanying an explosion can make it impossible to resolve the isotropic moment using long-period data. The strongest argument for an earthquake is that both the regional surface waves, and waveform modeling at wmq, give an estimated focal depth of 6 ± 1 km. This result reinforces the importance of focal depth determination to ctbt monitoring, in particular, for shallow earthquakes with mechanisms that are close to perfect 45° reverse-dip-slip, which are difficult to discriminate by using other methods. Fortunately, depth determination using surface waves is especially favorable for this type of earthquake, if a sufficient number of stations can be used. We therefore recommend that long-period data recorded at auxiliary seismic stations of the ims be utilized by the International Data Centre to monitor compliance with the ctbt.

An Analysis of P Travel Times for Nevada Test Site Explosions Recorded at Regional Distances

The uncertainty in hypocenters and origin times depends on measurement error (the wrong onset is picked) and error in the travel-time tables (and thus earth model) used. The errors in the tables comprise a baseline shift (the average difference over all stations is not zero) and the residuals about the baseline. The residuals are usually referred to as model error. Baseline error only affects origin time and so is usually ignored. It is model error that can result in epicenter error. A priori variances of the model and measurement error are usually used to estimate the uncertainty on epicenters. Few estimates of these variances have been published. Here the two variances are estimated, relative to International Association for Seismology and the Physics of the Earth9s Interior (iaspei) 91, for the P times from explosions at the Nevada Test Site at stations at regional distances. The analysis shows that at a large signal-to-noise ratio (snr) the variance of the measurement error is 0.01 sec 2 . The measurement error increases as snr decreases. Further, the travel times appear to increase as snr decreases. Model error has a formal variance of up to 1.9 sec 2 , but this variance is irrelevant to assessing the uncertainty in epicenter estimates. It is systematic bias (if any) caused by model error that contributes to epicenter uncertainty. Without knowing the bias it is only possible to estimate the precision of an epicenter and this depends on the measurement error. The analysis gives estimates of model error for each source-to-station path, and these path effects can be used to correct the travel times to give a revised model. With correction for path effects the estimated uncertainty in the epicenters becomes a measure of the accuracy of a location. The results presented here show that when estimating these effects, care must be taken to ensure that variations in snr do not bias the estimates.

Comment on “Unexplained Sets of Seismographic Station Reports and a Set Consistent with a Quark Nugget Passage” by David P. Anderson, Eugene T. Herrin, Vigdor L. Teplitz, and Ileana M. Tibuleac

Anderson et al. (2003) interpret a set of unassociated seismic arrivals observed on 24 November 1993 as a line source, which they claim is consistent with the passage of a “strange quark nugget” through the Earth. In fact, these arrivals can be convincingly interpreted as an earthquake source on the Pacific-Antarctic ridge. We attempt to form an event using the arrivals listed in table 6 of Anderson et al. (2003) and find, as the authors do, that this is not possible without a large residual at station stk. We then use the bulletin of the ISC (International Seismological Centre, 2001) to investigate residuals at stk during November 1993. The results are shown in Figure 1. We find that the residuals show a clear trend with values around −9 sec on 24 November. Consequently we believe …