There were three blasts, a large ``quarry blast'', with 1500 pounds of explosives in a number of different shot holes (this blast is used by the quarry to dislodge rock) and two smaller single charges of 300 and 100 pounds that were set off for experimental purposes. USGS seismometers in the vicinity of our array clearly see the quarry blast (indeed, the quarry blast is visible for hundreds of kilometers) but not the smaller blasts.
Figure shows portions of the seismograms recorded by one of our 169 groups for the three blast records. The quarry blast appears on the middle trace at around 10 seconds, but is not very obvious on a single-trace display. The smaller blasts arrive around 10.5 seconds on the other two traces, and are not at all apparent. The signal-to-noise ratio in our data, at least for daytime recording, is obviously quite small.
We were lucky to record these blasts at all. Our recording equipment consisted of 169 seismic group recorders (SGRs), each recording data on its own cassette tape and powered by a battery. This equipment was donated to Stanford by Amoco. The rechargeable batteries have a lifetime of about a day of normal operation. They worked fine for our nighttime recording, but had to sit using power until the middle of the next day for the blast recordings, and battery failure began to take its toll. About half the SGRs were still working for the first blast at 11 AM, and only 38 of the 169 were still working for the third blast at 12 Noon.
While the blasts are not readily apparent on individual traces, beam steering has been a very useful tool for detecting and locating them. Figure gives a schematic view of the disc-shaped beam steering plots which will be used frequently throughout this paper. Stacking semblance is displayed as function of arrival direction (azimuth) and apparent slowness in polar coordinates. An apparent slowness of zero, corresponding to vertically incident events, is plotted at the center. The highest slowness value, at the edge, corresponds on these plots to an apparent velocity of 2 km/sec. These plots could also be described in terms of ray parameters px and py as shown in the figure, illustrating the equivalence of beam steering and slant stacking.
The orientation of the square in Figure is the same as the orientation of the square indicating our array on the map in Figure . Thus it is straightforward to relate azimuth directions on these beam steering plots to real-world directions.
Figure shows the result of beam steering the data from the three blasts and summing over 100 msec windows centered around the first arrivals from the blasts. It can be seen that the three blasts arrive from the same direction, all with an apparent velocity of around 4 km/sec. In Figure , I gained the three panels independently so that the three blasts would have the same relative strength. In fact the blast from the middle panel, the quarry blast, is much stronger than the other two. If we were to perform the same summation over time as shown in Figure , but using the entire 32 second records instead of a small window, the quarry blast would still dominate the middle plot, while the two smaller blasts would be lost in the background noise. Restricting the summation to a small window located at the right time has enabled us to see all three blasts.
While the blasts arrive in a consistent direction, that direction surprisingly differs considerably from the direction of the quarry, as can be seen by comparing these plots and the quarry direction indicated on the map in Figure . The difference is on the order of 45 degrees. One possibility is that these first arrivals have traveled along a path that does not follow a straight line from the quarry to the array. The local geology gives some support for this theory. A direct path from the quarry to our survey would pass through large amounts of sandstone and conglomerate, while a path that initially turned further to the west and then came back to our array would remain for the most part in a faster, more well-consolidated basalt formation that parallels the San Andreas fault, which passes about 5 km to the southwest of our survey area. Another possibility is that the energy follows a more direct path and is then rerouted by the near surface, for instance by a tilted weathered layer beneath the survey.
Another interesting fact to note about the blasts is that there is strong evidence of energy from the blast following different paths to reach our array. Figure shows many 100 msec frames from the large quarry blast following the first arrival shown in Figure . All these frames have been gained identically, so the later arrivals are truly quite strong. I haven't shown frames from before the blast arrived, but they are relatively very quiet. Thus we can assume that all the strong events shown in Figure are due to the blast. Note that this late-arriving energy comes in a variety of directions, some of it arriving in a direction much closer to the direction of the quarry. One possibility is that the earliest arrivals traveled through the faster basalts to the west, and then later energy traveling in the straight-line direction arrived.
Figure shows the same sequence of plots following one of the two smaller blasts, record 44. Although these plots have been gained so that the first arrivals of the two blasts are similar in appearance, the smaller blast does not seem to have the same large amount of energy from the blast arriving at later times.