Julian Lozos, Bernalwood’s Senior Seismologist, has spent much of this week at the American Geophysical Union’s AGU 2010 conference, which is taking place downtown at the Moscone Convention Center. Julian filed this report to update us on the latest scientific thinking about earthquakes, faults, probabilities, and what all that means for helpless mortals living in Bernal Heights:
I attended sessions during which several major debates in earthquake science were discussed. One was about determining probable endpoints of earthquake rupture, and what that means for overall event size. A lot of this is determined by the geometry of the fault trace — which ties in directly to my research! — but regardless of whether or not you believe geometry has a big effect, the whole panel agreed that there are segments of fault in California, even aside from the San Andreas, that are long enough to produce earthquakes up to magnitude 8.
The second debate had to do with whether or not faults have “characteristic earthquakes” — that is, whether or not a fault creates essentially the same event (same epicenter, same length of rupture, same magnitude) over and over. By a characteristic model, a fault would have this large specific event at some recurrence interval, with very few smaller events in the interim.
The opposite end of the characteristic model suggests that faults have a wider range of earthquake sizes, and that the largest events might happen on different parts of the fault from earthquake cycle to earthquake cycle.
Earthquakes can’t be predicted by any means, but faults that don’t behave characteristically are even harder to sort out. In terms of what that means for San Francisco: If the San Andreas is characteristic, we can expect repeats of the 1906 rupture (and maybe some of the smaller Santa Cruz-ish segment ruptures, like Loma Prieta and 1836). A repeat of 1906 wouldn’t be as bad now as it was then, since we have better building codes and disaster response, but it would still be a large-scale disaster. Who knows, maybe more people would move to Bernal Heights after that one as well, after catching on to the benefits of our bedrock? (But would we WANT the potential hipster exodus out of the liquefacted Mission? Ahhhh.)
Anyhow, if the San Andreas is not characteristic, its next big quake in the Bay Area could still be the same general segment as 1906, but with a different epicenter that could lead to rupture directivity that sends even more seismic energy into San Francisco proper than happened in 1906. (In terms of directivity, actually, 1906 was kind of the BEST case. Yikes.) Or, the next big one might be some smaller subset of the 1906 rupture — which was pinned to the south by a part of the fault that doesn’t slip in earthquakes, and to the north by the end of the fault. Yet that’s still big enough to cause a lot of problems.
Of course, the Hayward, the Calaveras, and all the faults to the north of us have similar issues. If the 1868 Hayward event was the characteristic earthquake, well, we’re still in a lot of trouble. But if it was a smaller event within a bigger cycle, the eventual main event could be larger than the ~M6.8 of the 1868 event, and then we’ll REALLY be in trouble — though not as much trouble as the East Bay.
So there you go. Have a nice day!
Photo: The 1906 earthquake as seen from the bedrock safety of Bernal Hill, via the Bernal History Project