Day Three

It was a nice break from the rigours of the earthquake story to start the final day with the air balloon test. Ellen and I have been hearing rumours of billowing bin bags that never really take off, so when we join mission control for the lift off we’re not sure what to expect. In the end it was great fun. Just as we got there with Mike, we could see Jonathan and Kathy desperately trying to keep the balloon in check. In the end it took off before we or most of the camera crew could get there – frustration for Kathy and Jonathan who now needed to have a repeat performance of their only success of the last couple of days. There was no need to worry – right on cue the balloon took off carrying its camera payload. The relief for all of us was clear as we danced around like idiots below. God knows what the pilot of a low-flying F-16 thought as he buzzed us.

Still, it was just putting off the inevitable for Ellen and I. Kate was pressing for an answer for our challenge. Some decisions had to be made. In the end, we plumped for the assumption that since the biggest ground movements were in the vicinity of Lone Pine then that was a reasonable guess for where the centre of the 1872 quake was. The magnitude was more tricky. Did we assume just one great quake and use the values of 3 m vertical and 20 m horizontal, or did we go for a more geological assumption of three large quakes with 1 m vertical and 7 m horizontal, the most recent of which was our 1872 event. It’s at times like that that you see the faces of your academic peers bearing down on you, and that it is the most likely reason for me opting for the more cautious choice. So, a slip value of 7.5 m was added to a fault length of 100 km and inserted into a standard geological calculation that also assumed that the fault extended down into the Earth for 15 km and that the rocks had a particular coefficient of friction – basically a measure of how easy they are to break. The result is what seismologists refer to as moment magnitude, which isn’t the same as what Kate asked for – Richter magnitude. Since Richter magnitude is based on the record that the seismic waves have on a seismograph, it is impossible for us to determine that. But thankfully, at moderate and large quakes the moment magnitudes are roughly comparable to Richter magnitudes, so we could just assume a straight conversion between the two.

So, our answer to Kate was a magnitude of 7.6 and a location close to Lone Pine. To be honest it was a best guess with what we had, so I wasn’t that confident as Kate opened the envelope. Then, hearing that the answer was supplied by legendary earthquake geologist Burt Slemmons, I knew that I wasn’t going to be in a position of contesting the result. In the end Burt placed the centre of the quake a bit further north, roughly mid-way along the fault (maybe that would just have been a simpler way to do it) but gave a lovely magnitude range of 7.5-8.0. Given that the magnitude scale is logarithmic, so a magnitude 8 event is ten times larger than a magnitude 7, that is a fairly generous range, but it all because we’re talking about a quake that happened 130 years ago before modern seismic measurements. Given the time we had, I don’t think Ellen and I did that bad.

And neither did Mike. We hadn’t witnessed the torture that Mike had gone through trying to get his carbon dioxide filter to work with the dodgy limestone, but it was clear that he really wasn’t sure if this was going to work. In fact, in previous series the Mike’s chemistry was famed for its glorious failures. This series, however, he was on a roll, and so it was brilliant to see that this was no exception. A glorious success. And it was nice to see chemistry taking centre stage at the finale of a Rough Science programme.