Lyman and a Note on the Revised Moments for Pahute Mesa Tectonic Release

In two earlier papers (Wallace et al., 1983, 1985), we discussed the evidence for tectonic release from underground nuclear explosions on Pahute Mesa at the Nevada Test Site (NTS) as observed in long-period body waves. It has been shown for some time that the nonisotropic component of the surface waves from most of these events could be explained by an equivalent double-couple source; namely strike-slip motion on north-striking faults. In an attempt to get better resolution on the mechanism of the tectonic release, we modeled the SH wave forms using the constraints imposed by reversals in the SV polarity and distortions in the longperiod P waves. We found that the orientation inferred from the body waves closely matched those determined from the surface waves. Further, we were able to determine the seismic moments for the tectonic release by comparison with several shallow, strike-slip earthquakes. Although the SH waveforms provide the most direct measure of the tectonic release, the tectonic release mechanism adequately explains a wide range of long-period body wave observations. In particular, we identified a distortion in the upper mantle long-period P waves which we attribute to the phase SP from the tectonic release. The amplitude of this phase correlates very well with the moment of the tectonic release; it can be modeled by adding a strike-slip synthetic to the waveform of a low tectonic release event (see Figures 6 and 8 in the 1983 paper); and finally it shows a change in polarity (see Figure 9 in the 1983 paper; Figure 5 in the 1985 paper) as expected by the strike-slip radiation pattern. The identification of the sP phase provided the highest frequency signature of tectonic release ( -0.33 Hz) yet known and suggested that short-period P waves could also be affected by tectonic release. With this in mind, Lay et al. (1984) studied the first cycle (ab) amplitude of the teleseismic short-period P waves from the Pahute Mesa events and found a systematic azimuthal variation which is consistent with a distortion which could be caused by the tectonic release. Douglas et al. (1985) suggest that we have misinterpreted the complexity in the P waves, and actually the distortion is caused by slap down, and that there is no evidence to support that tectonic release is influencing the first P wave arrivals from explosions at NTS. The principal data Douglas cites are the broadband recordings at EKA of the large Pahute Mesa events (Figure 1 in Douglas et al., 1985). If our interpretation of tectonic release is correct, Douglas contends that the deflection he has marked Ao should have a negative polarity, which it does not. It is important to note here that these backswings in the P waves are interference patterns between the explosion P, pP, and any contribution of slapdown with that of the sP phase is from the tectonic release. Such interference patterns are very sensitive to the relative timing of the different episodes. A small shift in the location of the tectonic release relative to the explosion can produce a "misleading" polarity. (It is also difficult to interpret Figure 1 in Douglas et al. since explosions of much different size are all plotted with a normalized amplitude scale.) A direct comparison between Figure 1 in Douglas et al. and Figure 5 in Wallace et al. shows that, in fact, the backswing in question is greatly reduced relative to direct P for the high tectonic release events such as MUENSTER and BENHAM at EKA which is consistent