Temporal Variations of the S-Wave Attenuation Field in the Area of the Lop Nor Nuclear Test Site

Resumo

The characteristics of the shear wave attenuation field in the area of the Lop Nor nuclear test site, China, are considered. Methods are used that analyze the relative level of S_n and P_n waves (parameter S_n/P_n) and the slope of P-coda envelopes. An S_n wave is formed by shear waves reflected from multiple boundaries in the upper mantle, while P-coda is formed by S–P type conversion in the lithosphere of the source region. Twenty-six records of underground nuclear explosions (UNEs) and earthquakes obtained by station BRVK at epicentral distances of ∼1800–2000 km have been processed. It is found that from the late 1960s until the mid-1990s, the mean S_n/P_n values in the records of UNEs have substantially decreased and the slope of the P-coda envelope has increased in the area of the test site. These effects indicate an increase in S-wave attenuation in the lithosphere of the source region. A decrease in S_n/P_n with time in earthquake records for the test site region has also been observed after the termination of UNE series. Similar results were obtained earlier for the areas of the Semipalatinsk and Nevada nuclear test sites. The revealed effects are supposedly caused by deep fluid migration as a result of intensive long-term anthropogenic impact.