Digital algorithms for robust detection of phase arrivals in the presence of stationary and nonstationary noise have a long history in seismology and have been exploited primarily to reduce the amount of data recorded by data logging systems to manageable levels. In the present era of inexpensive digital storage, however, such algorithms are increasingly being used to flag signal segments in continuously recorded digital data streams for subsequent processing by automatic and/or expert interpretation systems. In the course of our development of an automated, near-real-time, waveform correlation event-detection and location system (WCEDS), we have surveyed the abilities of such algorithms to enhance seismic phase arrivals in teleseismic data streams. Specifically, we have considered envelopes generated by energy transient (STA/LTA), Z-statistic, frequency transient, and polarization algorithms. The WCEDS system requires a set of input data streams that have a smooth, low-amplitude response to background noise and seismic coda and that contain peaks at times corresponding to phase arrivals. The algorithm used to generate these input streams from raw seismograms must perform well under a wide range of source, path, receiver, and noise scenarios. Present computational capabilities allow the application of considerably more robust algorithms than have been historically used in real time. However, highly complex calculations can still be computationally prohibitive for current workstations when the number of data streams become large. While no algorithm was clearly optimal under all source, receiver, path, and noise conditions tested, an STA/LTA algorithm incorporating adaptive window lengths controlled by nonstationary seismogram spectral characteristics was found to provide an output that best met the requirements of a global correlation-based event-detection and location system.

[1] Detailed P wave velocity (V p ) and S wave velocity (V s ) models and V p /V s ratios were determined for a major portion of the New Madrid seismic zone using arrival times recorded by the New Madrid seismic network and Portable Array for Numerical Data Acquisition (PANDA) stations. We performed a simultaneous inversion for P and S wave velocities and hypocentral locations, yielding the most detailed tomographic image of the upper portion of the crust to date. Low V p and high V s anomalies resulted in low V p /V s ratios that correspond to the major arms of seismicity north of the intersection of the Cottonwood Grove-Blytheville Arch (CG-BA) fault with the Reelfoot fault. The unusual low V p /V s values can be attributed to the presence of quartz-rich rocks. Two regions contain anomalous V p and V s values and V p /V s ratios that cannot be attributed to variations in rock composition and are probably produced by overpressured fluids. One region is located on the hanging wall of the northern portion of the Reelfoot fault and is aseismic. The other region corresponds to a portion of the southern Reelfoot fault that experiences swarm activity. A distinct velocity contrast exists across the CG-BA fault at depths exceeding 4.65 km; basement rocks southeast of the fault have V p values that are 4%-6% slower than values for rocks located to the northwest. The most logical explanation is that the fault follows a preexisting structural feature or lithologic change in basement rocks.

Two strong-motion arrays were deployed for the October 2002 Embayment Seismic Excitation Experiment to study the spatial variation of strong ground motions in the deep, unconsolidated sediments of the Mississippi embayment because there are no comparable strong-motion data from natural earthquakes in the area. Each linear array consisted of eight three-component K2 accelerographs spaced 15 m apart situated 1.2 and 2.5 km from 2268-kg and 1134-kg borehole explosion sources, respectively. The array data show distinct body-wave and surface-wave arrivals that propagate within the thick, unconsolidated sedimentary column, the high-velocity basement rocks, and small-scale structure near the surface. Time-domain coherence of body-wave and surface-wave arrivals is computed for acceleration, velocity, and displacement time windows. Coherence is high for relatively low-frequency verticalcomponent Rayleigh waves and high-frequency P waves propagating across the array. Prominent high-frequency PS conversions seen on radial components, a proxy for the direct S wave from earthquake sources, lose coherence quickly over the 105-m length of the array. Transverse component signals are least coherent for any ground motion and appear to be highly scattered. Horizontal phase velocity is computed by using the ratio of particle velocity to estimates of the strain based on a plane-wave-propagation model. The resulting time-dependent phase-velocity map is a useful way to infer the propagation mechanisms of individual seismic phases and time windows of three-component waveforms. Displacement gradient analysis is a complementary technique for processing general spatial-array data to obtain horizontal slowness information.

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