Attenuation of high‐frequency shear waves in the crust: Measurements from New York State, South Africa, and southern California

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 .