Earthquake Storms
Page 26
In all, nearly 100 miles of the San Andreas Fault can be seen from the top of the Palm Springs Aerial Tramway. It is this, the southernmost segment of the fault, that worries seismologists because this is the only segment that has not ruptured in historical time. And there are additional reasons to be concerned.
Four deep trenches have been dug across this segment, and from a detailed examination of the walls, paleoseismologists have identified five, possibly seven, major ruptures that have occurred during the last 1,100 years, the most recent in 1690.
Moreover, this is one of the most seismically active segments of the San Andreas Fault. More than a dozen moderate earthquakes have occurred since 1935. Such persistent activity could presage a major event—in the same way that a wooden board broken over a knee begins to crack before it breaks.*
To add to the concern, geodetic measurements across this segment of the fault show that points on opposites sides of the fault are sliding slowly and continuously—those on the west side of the fault moving to the north and those on the east side to the south—at an average rate of 1.5 inches a year. This means that, since the last major earthquake in 1690, 27 feet of crustal movement has accumulated on opposites sides of the fault—without any movement yet along this part of the fault. That is a buildup of an enormous amount of seismic energy that has yet to be released.
All in all, the paleoseismic evidence of five or seven major earthquakes in the last 1,100 years—showing that large events are not unusual—and the current high level of seismicity and the steady buildup of seismic energy along the fault point to one thing: A major earthquake will occur soon along the southernmost segment of the San Andreas Fault.
Thomas Jordan, director of the Southern California Earthquake Center at the University of Southern California located in Los Angeles, and a member of the group that produced the 2008 report and an update, has put it bluntly: This segment of “the San Andreas Fault is locked and loaded and ready to rumble.”
But when?
Jordan and others say there is a 59% chance that a magnitude-6.7 or larger earthquake will occur along the Desert Hot Springs–Salton Sea segment of the San Andreas Fault in the next 30 years. That is almost twice the probability for a comparable earthquake happening on the nearby San Jacinto Fault or on the Hayward Fault.
Jordan and others have also considered what they have termed a “doomsday” scenario in which the entire southern half of the San Andreas Fault—from the Salton Sea to Parkfield—ruptures as a single earthquake. Parkfield is considered the northern limit because the rupture of a single event will probably not be able to propagate farther north where the stress on the fault is being relieved constantly by fault creep—evident by the slow pulling apart of the walls of the DeRose Winery—and by the frequent occurrence of magnitude-6 earthquakes, four in the last 100 years with the most recent in 2004.
Such a “wall-to-wall” rupture would involve 350 miles of the fault, considerably more than the 270 miles that ruptured in 1906, and the earthquake would be proportionally much bigger. Jordan and others estimate that such a cataclysmic event would correspond to a magnitude-8.2 earthquake and release about ten times more energy than the one in 1906.
Fortunately, the probability of such an event is low: less than 1% during the next 30 years. But considering that shaking would last more than a minute and be severe both close to the fault and in communities built over sedimentary basins—which would include but not be limited to San Bernardino, Los Angeles, and many towns in Ventura County, where, according to Jordan, the ground will shake “like a bowl of jelly”—there is still reason for concern.
Because small earthquakes are more common than large ones, a more likely scenario is that only the southernmost segment of the San Andreas Fault will rupture, at least at first, relieving some of the built-up seismic energy but not all. To understand what may follow, it is important to return to the North Anatolian and the Xiashuihe Faults and compare them to the San Andreas Fault.
All three are transform faults along plate boundaries. In all three cases, the relative plate motions on either side of the faults are the same, about 1.5 inches a year, so stress is increasing along the faults in all three places at the same rate. All three have significant strands that split off a main strand—in California, the Hayward and the San Jacinto Faults; in China, the Longmenshan; and in Turkey, north and south strands that run west of the city of Düzce and that are responsible for the creation of the sea lane known as the Dardanelles. There is also a segment of the North Anatolian Fault that creeps, just as the San Andreas Fault does north of Parkfield. Whether the Xiashuihe Fault also has a creeping segment is not known; that fault has not been studied as intensely as the other two. And two of these faults have had earthquake storms. By analogy, it seems a third earthquake storm along the San Andreas is possible.
How would such a storm evolve?
Again, from studies of the North Anatolian and the Xiashuihe Faults, it would probably begin at one end, perhaps by a rupture of the southernmost segment of the San Andreas, and proceed along the main strand and some adjacent faults.
The initial rupture would change the stress pattern—just as the 1992 Landers earthquake did and which led to the 1999 Hector Mine earthquake—so that there would be new places where stress was now concentrated. Eventually, because so much stress would be released in southern California, stress concentrations would form in northern California, jumping the 100-mile-long creeping section north of Parkfield. So the northern segment of the San Andreas Fault would also be involved.
Here a new concern arises. As the stress pattern along the San Andreas Fault changes with each successive earthquake, so does the stress pattern change along adjacent faults, causing some of them to rupture out of their previous “pattern.” For example, in 1939, the earthquake that leveled the city of Erzincan broke along the main strand of the North Anatolian Fault, as well as the nearby Sungurlu-Ezinepazari fault. In China in 2008, after decades of earthquakes along the Xiashuihe Fault, a rupture occurred along a nearby parallel fault, the Longmenshan Fault. The same would happen in California.
In particular, in southern California, the Cucamonga Fault, which runs west from Cajon Pass and along the southern base of the San Gabriel Mountains, could rupture simultaneously with or soon after a major earthquake along the San Andreas Fault. And that would lead to stress changes along the Raymond Fault—which is at the western end of the Cucamonga Fault—and from that to other faults in the Los Angeles region.
In northern California, the Calaveras Fault splits off the main strand of the San Andreas just south of San Juan Bautista. So a rupture of the northern San Andreas Fault could lead to a rupture of the Calaveras—or the Hayward or the Greenville or the San Gregorio Fault.
All this is to emphasize an important point: The exact sequence of future ruptures, and hence major earthquakes, along the San Andreas and its many adjacent faults cannot be predicted—which is why Jordan and others issued probabilities in their reports. The series of quakes would not disseminate out in a necessarily coherent direction.
But one thing is certain: The last 100 years in California—which happen to correspond to a period of rapid urban growth—have been a period of seismic calm. That cannot continue.
The damaging earthquakes that have occurred—1925 Santa Barbara, 1933 Long Beach, 1952 Long Beach, 1952 south of Bakersfield, 1971 San Fernando, 1989 Loma Prieta, 1992 Landers, and 1994 Northridge—were moderate events, seismically speaking; they released only a minuscule amount of the stress that has built up between the North American and Pacific plates. This enormous amount of stress and, thus, seismic energy can only be relieved one way: as a series of large earthquakes.
And that could occur as an earthquake storm.
*As often happens in science, several people may have the same idea at about the same time. In this case, the idea that stress transfer could trigger a series of major
earthquakes was proposed by several scientists. The successful forecast made in 1997 for additional earthquakes in north Turkey was made by a trio of researchers—Ross Stein, Aykut Barka, and James Dieterich.
*Here it must be pointed out that, though a specific fault might rupture only once every few thousand years, the fact that there are more than 700 active faults in California means that some “unlikely” earthquakes are expected to occur during a person’s lifetime.
*To emphasize a point made in a previous chapter: Contrary to popular opinion, when a major earthquake happens, the chance of another major event happening soon after does not decrease, but increases dramatically. For example, for any three-day period, the chance of a major earthquake occurring somewhere in California is 1 in 100,000; however, if a major earthquake has just happened, then the chance of another earthquake of equal or greater magnitude striking in the next three days is 1 in 10, a sobering statistic and one that needs to be known by anyone involved in rescue operations after a major seismic event.
*More than a dozen moderate earthquakes occurred in the San Francisco area during the 70 years before the 1906 earthquake, but only one during the 70 years after the event.
Epilogue
Bodega Bay
What we want is a story that starts with an earthquake
and works its way up to a climax.
—Samuel Goldwyn, Hollywood movie producer
There are dozens of places where I go to sit next to the San Andreas Fault. My favorite is on the rocky headland at Bodega Bay, 60 miles north of San Francisco.
Here the fault runs close to the shoreline, making its way onto land near the marina at the north end of the bay. There is cinematographic history here: In 1963, Alfred Hitchcock filmed one of his most famous movies, The Birds, at the village of Bodega Bay.
Early in the film, actress Tippi Hedren is alone in a powered skiff crossing from the far side of the bay. Just before she reaches the marina, she is violently attacked by a large bird. It is the first such encounter in the movie. If one pays careful attention to the editing, one sees that the attack occurs exactly when she is over the San Andreas Fault. One can only imagine that this was a coincidence. Hitchcock, though a master of suspense and intrigue and irony, intended the village of Bodega Bay to be a seaside town in New England, where earthquakes are rare and no one ever speaks of active faults.
There is another oddity relevant to the story of earthquakes at Bodega Bay. In driving from the village to the rocky headland, one passes a deep, huge hole that was blasted into the hard granite. This hole was to be the beginning of the construction of the nation’s first commercial nuclear reactor. The site is only one mile west of where the San Andreas Fault shifted in 1906. Fortunately, enough was understood about the San Andreas Fault and about earthquakes in the 1960s to halt the construction.
The white granite of the headland is comprised of large flakes of brown mica and black biotite and crossed through with dikes of pink feldspars. This granite is just one of many blocks torn out of the Earth’s crust south of the Sierra Nevada Mountains and transported, in stuttering steps, hundreds of miles. These blocks now lie scattered along the California coast. Each step was a possible catastrophe, the product of an ancient earthquake.
I first visited Bodega Bay in a winter month when no one was around. So I took it upon myself to pick up the heaviest piece of loose granite that I could lift, and I carried it 15 feet to the northwest. That is about how much it will move when the 1906 earthquake is repeated. In short, I gave that boulder a head start. It has been my only real contribution to being a tectonic force.
I visit that boulder frequently. When I do, I imagine the journey it has been on—and the journey it is yet to take.
I then sit quietly and look for a long time to the southeast, wondering if a segment of the San Andreas Fault might have just ruptured. And, if it has, whether seismic waves, strong enough to hurl down a house and devastate a city, might be racing at me at unbelievable speeds.
Acknowledgments
I owe thanks to many people who gave unselfishly of their time to educate me about earthquakes.
James Dieterich has been a stable influence in my life for nearly four decades and gave me my first opportunity to examine the San Andreas Fault in detail. Clarence Allen was a quiet motivator, who, when I was a student, showed me field evidence that crustal blocks had slid great distances in California. Clarence is always ready with a bag of one-liners to entertain students.
Douglas Morton can be shown a rock fragment from anywhere in southern California and tell you where the rock originated and the journey that rock has taken. I spent a pleasant day listening to him describe the endless subtleties of California geology. Ross Stein sees earthquakes in mathematical form, an ability that I envy. Ray Weldon has dug into the San Andreas Fault at several places. It was he who pointed me to the Triassic monzogranite in the San Gabriel and San Bernardino Mountains. James Dolan’s enthusiasm for sleuthing ancient earthquakes is addictive. A large chunk of fault gouge and drilling mud sits on his desk.
Carol Prentice is a human encyclopedia of the 1906 earthquake and of earlier events that have occurred along the San Andreas Fault. Tom Parsons examines patterns of earthquakes, both near California and far away. Amos Nur had an insight 20 years ago at Mycenae that led to a new understanding of when and where major earthquakes occur.
My first trip to Moonstone Beach was under the guidance of Robert Sharp. Eugene Shoemaker took me into the Mojave Desert and showed me Rainbow Basin. I can still hear his infectious laugh echo off the walls. I can also recall the high-pitched raspy voice of Jerry Eaton as he explains how he successfully challenged the building of a nuclear reactor at Bodega Bay. These three men have passed away. They are greatly missed.
Marcia McNutt and I shared an office soon after each of us completed graduate school. We talked about physics and Stoicism and whether stray cats could be used to predict earthquakes.
Also in the office was John Langbein. He and Malcolm Johnston kept the Parkfield experiment alive after the prediction window had closed.
Tom Heaton and I played basketball almost every afternoon when we were students. It was Tom who told me the story of sharing a radio interview with Charles Richter.
Special thanks are owed to Rebekah Kim and Yolanda Bustos, who opened the doors of the archives at the California Academy of Sciences. It was within those archives that I read the datebook diaries of Alice Eastwood.
Robert Tilling and David Hill have allowed me to engage them for many years in discussions about earthquakes and volcanoes and how best to communicate science to the public. Wilfred Tanigawa showed me how it is possible to glance at a seismograph and decipher the physical parameters of an earthquake.
This book has benefitted greatly from several other books. California Earthquakes by Carl-Henry Geschwind recounts the history of earthquake studies in the United States. Richter’s Scale by Susan Elizabeth Hough is a passionate telling of the complex life of Charles Richter. Chapter 11 of this book is a postscript to Assembling California by John McPhee. Plate Tectonics: An Insider’s History of the Modern Theory of the Earth by Naomi Oreskes tells the personal stories that led to the development of the theory of plate tectonics. And Apocalypse by Amos Nur shows how one person can bring a new perspective to a problem and thereby challenge established scientific thought.
Permissions to reprint photographs were provided by Susan Snyder of the Bancroft Library at the University of California, Berkeley; Christopher Crosby of the Open Topography Project at unavco; John Nakata and Charles Meyer of Sight and Sound Productions in Palo Alto, California; and Andrew Selkirk, a Fellow of the Society of Antiquaries of London.
My literary agent, Laura Wood, was able to take a long-struggling author and turn him into a published one. My editor, Jessica Case, provided clarity and direction to an early manuscript that was often muddled. Both
contributed immeasurably to this book.
Tom Peek has mentored my writing and has been a close friend for many years. He was living in Santa Cruz during the 1989 Loma Prieta earthquake and, at my urging, has repeated his firsthand account of what it is like to experience strong seismic shaking.
Finally, we all must acknowledge the tireless work being performed by those who are preparing California for future seismic disasters. Such disasters will certainly come.
Image Gallery
Andrew Cowper Lawson, who discovered the San Andreas Fault. Photo courtesy of The Bancroft Library, University of California, Berkeley.
Passenger train derailed at Point Reyes Station, California, by the 1906 San Francisco earthquake. Photo courtesy of the United States Geological Survey/Gilbert.
View down Sacramento Street, San Francisco, April 18, 1906. Notice the collapsed walls on the buildings at the right. Photo courtesy of The Bancroft Library, University of California, Berkeley.
View down Sacramento Street, San Francisco, 2006. Photo by the author.
Alice Eastwood standing along the “mole track” formed by the 1906 earthquake. Photo courtesy of the United States Geological Survey/Gilbert.
The Lawson house in the Berkeley Hills. Photo by the author.
The Hayward Fault passes directly beneath the football stadium at the University of California at Berkeley. Photo courtesy Google/Seismological Laboratory, Berkeley.