We compare the attenuation of high‐frequency (3–30 Hz) shear waves for crustal paths in New York State, South Africa, and southern California over source‐receiver distances of about 10–400 km. The data consist of digital recordings of S waves (Δ = 5–100 km) and Lg waves (Δ = 100–400 km) produced by earthquakes. We use a coda normalization method to remove the effects of site amplification and source excitation from the amplitudes of the S and Lg waves. Over the entire distance range studied (10–400 km), the amplitude decay of 3‐Hz shear wave energy is considerably less for the tectonicaily stable areas of New York and South Africa than for the tectonicaily active region of southern California. High‐frequency (30 Hz) S wave attenuation is significantly less for New York and South Africa than for southern California, for distances between 15 and 90 km. We parameterize the decay with distance (R) of coda‐normalized shear wave amplitudes with a frequency‐independent Q and geometrical spreading exponent γ, where geometrical spreading is proportional to R−γ. For New York State the S wave amplitude decay (3–30 Hz) is well described by a frequency‐independent Q of 2100−330+490 and γ of 1.3±0.1. The decay of Lg wave amplitudes from 3 to 15 Hz in the New York State region is fit with a frequency‐independent Q of 1600−280+330 γ of 0.70±0.2. The S wave amplitudes (3–30Hz) in South Africa yield a Q of 1500−190+380 and γ of 1.3±0.1. Fixing the geometrical spreading at R−0.5 produces an Lg wave Q estimate at 3 Hz in South Africa of 360−50+80. This Lg wave Q is low considering that South Africa is a cratonic, tectonicaily stable area. The S wave amplitudes from southern California are described with a frequency‐independent Q of 800−150+240 and a large geometrical spreading exponent of γ of 1.9±0.2. We find an Lg wave Q at 3 Hz of 260±30 for southern California, after constraining the geometrical spreading at R−0.5.

No abstract

Seismic body waves, from tamped chemical explosions, two with yields of 50 and one of 150 kg, were compared with corresponding data from three mining‐induced tremors with a view to testing methods of discriminating between the two types of events. Detonated at depths of about 2 km, all three explosions generated P waves for which the low‐frequency spectral asymptotes agree well with corresponding results scaled down from nuclear shots at the Nevada Test Site. For the two smaller explosions, recorded underground at hypocentral distances ranging from 234 to 871 m, the P wave corner frequencies, defined by the intersections of the low‐ and high‐frequency asymptotes, are also in fine agreement with scaled‐down results from the Nevada Test Site; for the 150‐kg explosion, which was recorded only at the surface, it appears that 2.39 km of upward propagation caused the corner frequency to be at least a factor of 4 lower than anticipated due to attenuation. All three explosions generated S waves that appear to be a consequence of deviatoric stress release in the immediate environs of the explosions. The three tremors analyzed here generated P and S waves whose spectra and source parameters agree well with standard earthquake source models. With regard to discrimination based on P wave spectra, we conclude that for events of fixed low‐frequency spectral asymptotes, the explosions typically have higher corner frequencies than tremors or earthquakes, although counterexamples certainly exist. Interestingly, the 150‐kg explosion was identified as such on the basis of P and S wave polarities that are incompatible with the normally expected double‐couple source model; instead, these initial motions are consistent with an explosion in conjunction with normal faulting. The body wave spectra of this explosion and those of a nearby tremor, however, were indistinguishable.

No abstract

No abstract