Ileana M. Tibuleac

Unexplained Sets of Seismographic Station Reports and A Set Consistent with a Quark Nugget Passage

In 1984 Edward Witten proposed that an extremely dense form of matter composed of up, down, and strange quarks may be stable at zero pressure (Witten, 1984). Massive nuggets of such dense matter, if they exist, may pass through the Earth and be detectable by the seismic signals they generate (de Rujula and Glashow, 1984). With this motivation we investigated over 1 million seismic data reports to the U.S. Geological Survey for the years 1990-1993 not associated with epicentral sources. We report two results: (1) with an average of about 0.16 unassociated reports per minute after data cuts, we found a significant excess over statistical expectation for sets with 10 or more reports in 10 min; and (2) in spite of a very small a priori probability from random reports, we found one set of reports with arrival times and other features appropriate to signals from an epilinear source. This event has the properties predicted for the passage of a nugget of strange quark matter through the Earth, although there is no direct confirmation from other phenomenologies. Manuscript received 20 June 2002.

Evaluation of Short-Period, Near-Regional Ms Scales for the Nevada Test Site

Surface wave magnitude (M_s) estimation for small events recorded at near-regional distances will often require a magnitude scale designed for Rayleigh waves with periods less than 10 sec. We have examined the performance of applying two previously published M_s scales on 7-sec Rayleigh waves recorded at distances less than 500 km. First, we modified the Marshall and Basham (1972) M_s scale, originally defined for periods greater than 10 sec, to estimate surface wave magnitudes for short-period Rayleigh waves from earthquakes and explosions on or near the Nevada Test Site. We refer to this modification as ^(M+B) M_s(7), and we have used short-period, high-quality dispersion curves to determine empirical path corrections for the 7-sec Rayleigh waves. We have also examined the performance of the Rezapour and Pearce (1998) formula, developed using theoretical distance corrections and surface wave observations with periods greater than 10 sec, for 7-sec Rayleigh waves ^(R+P) (M_S(7)) as recorded from the same dataset. The results demonstrate that both formulas can be used to estimate M_s for nuclear explosions and earthquakes over a wider magnitude distribution than is possible using conventional techniques developed for 20-sec Rayleigh waves. These M_s(7) values scale consistently with other Ms studies at regional and teleseismic distances with the variance described by a constant offset; however, the offset for the ^(M+B) M_s(7) estimates is over one magnitude unit nearer the teleseismic values than the ^(R+P) M_s(7) estimates. Using our technique, it is possible to employ a near-regional single-station or sparse network to estimate surface wave magnitudes, thus allowing quantification of the size of both small earthquakes and explosions. Finally, we used a jackknife technique to determine the false-alarm rates for the ^(M+B) M_s(7)-m_b discriminant for this region and found that the probability of misclassifying an earthquake as an explosion is 10%, while the probability of classifying an explosion as an earthquake was determined to be 1.2%. The misclassification probabilities are slightly higher for the ^(R+P) M_s(7) estimates. Our future research will be aimed at examining the transportability of these methods.

A Semiautomatic Calibration Method Applied to a Small-Aperture Alaskan Seismic Array

Small-to-medium aperture (25 km or less) seismic arrays are of great importance for event location and characterization. Effective use of these arrays requires calibration, preferably with large numbers of events, to account for local and distant structural and propagation effects. We implement a cross-correlation method as a semiautomatic procedure applicable to any small array, able to process thousands of events with several days of computer time on a Sun Blade 1000 workstation. We analyze a database of 1228 P and PcP arrivals recorded between 1997 and 1998 at a 19-element Alaskan seismic array. The arrivals are picked by the Prototype International Data Centre for events well located by the United States Geological Survey. Backazimuth and horizontal-velocity residuals are calculated for all events. Complicated geology beneath the elements and elevation differences among the array stations make static corrections necessary. We use 328 core phases (including PcP , PKiKP , PKP , PKKP ) to determine the static corrections. We present first-order structural interpretations of our calibration results, including a Moho discontinuity dipping to the north with a 10.5° dip angle and a strike of 109°. Our method allows for resolution of phase arrivals within 2–3 sec and has potential to be used as an automatic detector of primary and secondary seismic phases.

An Automated Short-Period Surface-Wave Detection Algorithm

We describe an automated short-period Rayleigh wave ( Rg ) detector designed to work on local (<2.5° epicentral distance) events recorded at three- component stations. The detector was modeled after an automatic 17- to 22-sec Rayleigh-wave detection method; however, we have modified the algorithms for local distance and short-period applications. We have tested the detector on a well-located cluster of mining events from central India and on a set of ground-truth events in the area. The Rg detector was also integrated into a semiautomatic event detection and location algorithm and applied on continuous data. Fourier and wavelet-based methods are evaluated for prefiltering. We observe that sample standard deviations of backazimuth estimates using the Rg detector, after wavelet prefiltering, are comparable to fk 3C P backazimuth estimates from event clusters. Our results indicate that using the Rg -phase backazimuths for event location is a promising alternative to using small signal-to-noise ratio first-arrival backazimuths. We recommend wavelet prefiltering versus Fourier prefiltering because it is more consistent for the detection of low signal-to-noise ratio events at local distances.