EMPIRICAL OBSERVATIONS OF EARTHQUAKE-EXPLOSION DISCRIMINATION USING P/S RATIOS AND IMPLICATIONS FOR THE SOURCES OF EXPLOSION S-WAVES
We continue exploring methodologies to improve earthquake-explosion discrimination using regional amplitude ratios such as P/S. The earliest simple source models predicted P/S wave amplitudes for explosions should be much larger than for earthquakes across the body wave spectrum. However empirical observations show the separation of explosions from earthquakes using regional P/S amplitudes is strongly frequency dependent, with relatively poor separation at low frequencies ({approx} 1 Hz) and relatively good separation at high frequencies (> {approx}3 Hz). We demonstrate this using closely located pairs of earthquakes and explosions recorded on common, publicly available stations at test sites around the world e.g. Nevada, Lop Nor, Novaya Zemlya, Semipalatinsk, India, Pakistan, and North Korea. We show this pattern appears to have little dependence on the point source variability revealed by longer period surface wave modeling. For example regional waveform modeling shows strong tectonic release from the May 1998 India test in contrast with very little tectonic release in the recent North Korea test, but the P/S discrimination behavior is similar in both events, using the limited regional data available. While accepted explosion P-wave models have been available for many years, the frequency behavior of the P/S discriminant has inspired a variety of competing models to explain how explosions generate S-waves. We briefly review some of these models in the context of the P/S discriminant observations. One hypothesis is that S-waves are generated mainly from conversion of P-waves and surface waves, so S-waves from explosions can be predicted from the P-wave models via a frequency dependent transfer function. A different hypothesis is that significant generation of S-waves comes from the CLVD (compensated linear vector dipole) component created by spall above the explosion. A recent model by Fisk (2006) shows the explosion S-wave spectra can be modeled using the P-wave spectra with the corner frequency reduced by the ratio of the wave velocities, which seems to imply a direct generation of S-waves in the source region. We examine a number of nuclear tests from around the world in light of these models. Given the importance of depth on some of the model predictions we reexamine some of the overburied explosions in Nevada. We also look at chemical explosions, including dedicated single shots at different depths and mining shots at adjacent open pit and underground mines to look at depth effects. Finally we examine a subset of Nevada data with signal above noise up to 16 Hz to determine if discrimination performance saturates at frequencies around 6 Hz as some models predict, or continues to improve at higher frequencies.
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 957613
- Report Number(s):
- UCRL-CONF-232450; TRN: US201016%%57
- Resource Relation:
- Conference: Presented at: MRR2007 - 29th Research Review on Nuclear Explosion Monitoring Technologies, Denver, CO, United States, Sep 25 - Sep 27, 2007
- Country of Publication:
- United States
- Language:
- English
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