NEAR-VERTICAL EVENTS

One of the most interesting features of the dataset, discussed in SEP-60 (Nichols et al, 1989) was the abundance of near-vertically incident events in the nighttime records. These events are weak and not noticeable on individual seismograms. However, Lin Zhang showed that when the data were stacked along different trajectories (a special case of beam steering) these events showed up quite well.

Our first thought was that since the events are almost vertically incident, they must be due to some electrical interference. More careful beam steering now suggests that this is not so; these events seem to have a consistent direction of propagation that is close to vertical but not quite vertical. If that is so, they cannot be due to electrical noise.

Figure  shows the result of beam steering 12 different nighttime records. The entire 32 second records were used to produce each of these plots. The dominant feature of most of them, near the center, is the near-vertically incident events. While these events are close to the center, they are consistently just off-center. Their azimuth and apparent slowness indicate a fairly consistent arrival direction, from the east, with an apparent velocity of around 12 km/sec.

To illustrate why I think the events are not due to electrical interference, I took the same data and randomly re-ordered the positions of the 169 traces in our array. If the events are real, and not perfectly flat, then re-ordering the traces in this way should effectively eliminate them. Figure  shows the result of beam steering the re-ordered traces. There are some strong semblance values near the center, but they are not as strong as the values in Figures . More importantly, they are no longer in the same place, but are mostly clustered around the center. This is precisely what we would expect to happen; the semblances are not going to drop to zero because there was strong, near-vertically incident energy present. After re-ordering it will still stack in to some extent. But the directional consistency should be, and is, lost. Thus I think it's clear that these events are not due to electrical interference.

Figure  shows the result of beam steering one of the 12 records, with and without trace re-ordering, at a larger scale. As is the case in Figures  and , the two frames have been gained identically to preserve differences in semblance values. I think it's clear that re-ordering has reduced the semblance values considerably, and removed their directional dependence.

What could cause such events? If they were due to a surface source, the source would have to be very distant in order for the energy to arrive so steeply. Yet energy from any surface source that distant should be attenuated long before it reaches our array. Another possibility is that they are weak events coming from somewhere within the earth that typically go undetected but have been noticed by us because we have so many geophones in a compact area.