The Great San Francisco Earthquake(s)
On October 8th, 1865, the “Great San Francisco Earthquake” hit south of the city of San Francisco, magnitude 6.3.
On October 21st, 1868, the ‘Great San Francisco Earthquake” hit near Haywards, east of the city, across the bay, magnitude 6.8.
On April 18th, 1906, the “Great San Francisco Earthquake” hit the Bay Area, magnitude 7.6.
The death tolls were unknown (but small), 30, and about 3,000, respectively.
Eighteen significant earthquakes happened after that (and five or so had happened between the first “great quakes”) before February 9th, 1971, when the Sylmar earthquake (magnitude 6.7, death toll 65) occurred in the San Fernando Valley. So, about 25 major earthquakes happened in California, of varying degrees of significance with respect to property damage and loss of life, since the earliest influx of immigrants associated with the Gold Rush, which is how California got permanently and meaningfully populated by Europeans.
Right after the Sylmar earthquake, a law was passed that required that earthquake hazard be considered as part of the approval process for new development.
One hundred and six years of time during which a significant earthquake occurred about every four years, passed before the first meaningful response by the civilization living on top of these active faults. Civilization does, indeed, have its faults. As it were.
Will Seattle and Portland Suffer Cataclysmic Earthquakes Any Time Soon?
Meanwhile, to the north, in British Columbia, Washington State, Oregon and parts of northern California, earthquakes were not recognized as a problem. They hardly ever happened. Buildings, homes, bridges, gas-lines, and other infrastructure were deployed without consideration of earthquake hazard for decades.
However, the earthquake hazard in that region is probably much greater in some ways than the earthquake hazard around Los Angeles and San Francisco, which are regularly rocked by fault-line activity. Here, the great plates that make up our planet’s surface do something different than they do in the southern California.
In southern California, the plates are mainly grinding past each other. Fragments of the plates separated by fault lines are squishing past each other like an eraser rubbing against paper. It is not a smooth process, but rather one in which pressure builds up and is released at numerous locations, with each of those release events resulting in some sort of earthquake.
To the north, the main interaction between the plates is the subduction of one plate beneath the other. The subducting (going under) plate moves steadily under the continent, with little fanfare other than slowly elevating that part of the continent, tilting of the land upward to the west and downward to the east (relatively speaking). Then, every now and then, there is an adjustment. The top plate drops all at once, causing a major change in elevation that results in coastal areas being suddenly under the sea, and also resulting in a major earthquake, perhaps magnitude 9.
(Remember, each whole number on the scale used to measure earthquakes is one order of magnitude, so a magnitude 9 earthquake is 100 times stronger than a magnitude 7 earthquake).
It appears that the nearly 700 mile long zone of subduction has suffered 19 “subduction zone earthquakes” over the last 10,000 years, with many more affecting a smaller length of this zone. So, long term, a major earthquake affecting an area hundreds of miles long and who knows how wide, and by major earthquake I mean as never seen before by living humans in the region, and hardly ever observed in recent times anywhere on the planet, affects an area larger than many countries.
Can earthquakes be predicted?
It is said that earthquakes can’t be predicted, but from the point of view of regular humans (as opposed, say, to geologists or statisticians) they can be. Many people think weather can be predicted, right? Well, not really. We can make long term predictions of months or even years about overall changes in the climate, and we can predict what the weather will be like in several hours from now. But anything in between is largely guess work except in a few rare cases (the track of hurricanes can sometimes be predicted pretty well several days out, even before they exist, at least roughly).
Same with earthquakes. Sort of. The short term with earthquakes is, unfortunately very very short. We know when an earthquake starts that there will be an earthquake over the next several seconds or minutes. That is a little like predicting that it is going to be raining over the next little while when the first drops fall from the sky. You’ve heard of predicting earthquakes longer term, like over days. Every now and then someone observes something that seems to be associated with the geological processes that produce earthquakes, then there is an earthquake, and bingo, we’ve got a method of prediction. But so far every time this has happened, that method of prediction has been invalidated by reality, when it fails to predict subsequent quakes, or produces false positives.
(An interesting example of this happened just yesterday when a scientist — but not a geologist — happen to observe the presence of huge amounts of various gasses appearing along the coast of California, and thought this might be the indicator of an impending earthquake. This prediction was supported by a several years old research project that suggested that gas outflows might predict earthquakes. I’m pretty sure the gas outflow idea has not developed. And, it turns out that the scientist who observed the California gas was simply looking at a common meteorological phenomenon that involved normal human pollution combined with certain atmospheric conditions. Nothing to see here!)
However, long term, earthquakes can be “predicted” using the term “predicted” in modern vernacular parlance. What I mean by that is that the earthquake hazard for a given region can be estimated over longish periods of time with reasonable certainty. We can say, for example, that there is a 63% probability of there being one or more earthquakes of 6.7 magnitude or greater between the years of 2007 and 2036 in certain clearly defined parts of California around San Francisco. This is based on a combination of empirical observation of earthquake frequency and an understanding of how earthquakes happen. According to one study, there is about a one in three chance of a Cascadia subduction zone earthquake (magnitude 8 or 9 or so) over the next fifty years.
So, when planning development or putting together emergency systems, it is possible to know two things. One, what kinds of earthquakes are going to happen (in terms of location, overall range, and magnitude, etc.) and what is the chance of something like this happening.
How do we adapt to earthquakes?
From this emerges something rather counter-intuitive. It turns out that the magnitude of the largest likely quake is more important than the likelihood that it will happen during any medium length time period. It does not matter if a magnitude 9 earthquake is 10% or 1% likely to happen over the next 20 years when you are building a major interstate highway bridge or a skyscraper. What matters is that you build the thing to handle a magnitude 9 earthquake (or, I suppose, prepare yourselves for total destruction of the thing, and have a backup plan of some kind). Development in southern California has to deal with magnitude 7-point-something quakes during the lifespan of a major long-lived structure, while development in Washington and Oregon has to deal with magnitude 8 or 9 quakes during the lifespan of a major long-lived structure. The truth is, your highway bridge near San Francisco has a good chance of being shaken by a magnitude 7 quake, while a highway bridge near Seattle may well outlive its usefulness and be replaced or retrofitted before the once in 500 year trans-Cascadia 9+ quake hits. But you still have to build it to handle the quake because you don’t want to be that guy. (Who didn’t, and then everyone died, and it was your fault.)
There is an interesting historical pattern in the recognition of, and in addressing, earthquakes both in the US an around the world. That century plus time period between what should have been a clue that San Francisco was a quake zone and the first meaningful safety conscious zoning regulation happened initially because developers covered up the first few quakes. They pretended they didn’t happen, downplayed, lied, etc. The 1906 quake was too big to really cover up, of course. Covering up switched to lobbying and lobbying kept regulations off the table for many more decades. Then several dozen suburbanites, voters, taxpayers, whatever got wiped out by a quake that really wasn’t all that bad compared to some of the earlier ones, and a law got passed. So this part of the pattern is denial, followed by different kinds of denial, then some more denial.
Denial of what? Science, of course.
The second part of the historical pattern is science progressing. While most early and mid 20th century construction went along blind to earthquake hazard in southern California because people were being willfully stupid, earthquake unsafe construction proceeded in the northern regions because science had not yet figured it out. Then the denial vs. science thing happened, and is still going on. Decisions have been made at various levels of government in the Cascade subduction zone area that will doom people of the future (one year from now, one century from now, we can’t say) to disaster.
A great new book on earthquakes: “Earthquake Time Bomb” by Robert Yeats
Do you find any of this interesting or important? Then you need to read Earthquake Time Bombs
by Robert Yeats.
Yeats explains what earthquakes are. Then he discussed the development of earthquake science, and the politics, cultural response, and technological response to earthquakes, starting with the examples I gave above plus the Haiti earthquake. Then he goes around the world to most of the major earthquake zones and examines the same processes — the geology, the geological science, the engineering and political responses, etc. — in each area.
Yeats is an expert on this, and in fact, has been involved in what he refers to, I think correctly, as the “paradigm shift” in understanding earthquake hazard and risk. This is a shift that happens both within the science and the regulatory and social systems that necessarily address the hazards and risks. He also explains the difference between hazards and risks. Yeats is the go to guy when you want to find out about what to do about earthquakes.
How do we know about the 19 subduction zone earthquakes in the Pacific Northeast that happened over thousands of years? What went wrong at Fukushima, and how do the Japanese deal with earthquakes? What about that New Madrid fault in the middle of the US? What about the Rift Valleys of Africa (where I worked)? What are we doing to do next, what is undone, and how do we do it? These are all addressed in the book.
I came away from Yeats book feeling better about earthquakes. I already knew about the Cascadia quakes and a bunch of other stuff, having done research that required an understanding of tectonic processes myself (though this is not my area). What made me feel better is the simple fact that we can adapt to earthquake hazards by first understanding what they are locally, then applying the proper technology and other systems.
The problem is bad, of course, in regions where earthquake hazard is high, and pre-adaptation is not done for any of a number of reasons, including political or economic ones. Yeats contrasts Japan, the most earthquake ready country in the world, with Haiti, one of the least.
Geology is fun. Earthquakes are one place where the rubber hits the road in geology. This book is a great overview and an important analysis of earthquake hazard and risk worldwide. I highly recommend Earthquake Time Bombs by Robert Yeats.
