by John Dvorak
In 1985, a panel of 12 scientists, formally known as the National Earthquake Prediction Evaluation Council, endorsed a Parkfield prediction, saying that there was a 95% chance that a magnitude-6 earthquake would occur along the Parkfield section of the San Andreas Fault by 1993. A dense network of instruments was installed in the hopes of trapping the earthquake, to detect precursory signs that might occur in seismic patterns, in ground movements, in electric or magnetic fields, in radon-gas emission, or in the chemistry or level of water wells. And then people waited.
Twice, an “A”-level alert was issued, on October 19, 1992, and again on November 14, 1993. Both alerts were triggered after felt earthquakes, similar in size to what preceded the 1934 and 1966 events, occurred. Both times there was an increased awareness that the predicted earthquake might occur within the next 72 hours. California state agencies and emergency services were notified. And both times … nothing happened.
The year 1993 came and went, and no earthquake. Then 1994, 1995, and so on. Finally, at 10:15 in the morning on September 28, 2004, a magnitude-6 earthquake ruptured the Parkfield section of the San Andreas Fault. The predicted earthquake had occurred. Or had it?
There were important differences between the events of 1934 and 1966 and the one that occurred in 2004. First, in 1934 and 1966, the ground rupture began north of Parkfield and propagated south. In 2004, it was in the opposite direction: The rupture started south of Parkfield and propagated north. More important, in the dense network of instruments there were no precursors recorded minutes, hours, or days before the event. There was no foreshock or increase of seismic activity before the event. There was no damage to irrigation pipes. There was no measured change in electric or magnetic fields or in chemistry or level of water wells. Most disconcerting, there was no measured ground movement: There was no warping or rise or fall of the ground surface. There was no underground compression or slight expansion of rock—no dilatancy—and this could be measured with great precision.
Five instruments known as borehole strainmeters were installed within a few miles of where the 2004 rupture formed. Essentially, each instrument consists of a fluid-filled bag stuffed deep down a borehole. If the surrounding rock is compressed or stretched by a tiny amount—equivalent to taking a 100-mile-long rigid bar and compressing or stretching one end by the diameter of a human hair—the bag undergoes a small compression or expansion. But no change was recorded for weeks to seconds before the earthquake. As far as anyone can tell, the 2004 Parkfield earthquake was a spontaneous event.
The Parkfield experiment was successful in identifying where an earthquake would occur and how big it would be, though the all-important when was missed; the event came 12 years too late. Which raises the question: Will it ever be possible to predict earthquakes?
The answer, as it is seen today, is: maybe.
The quickest way to start an argument in a room filled with seismologists is to bring up the question of earthquake prediction. Passions can escalate from heated to downright nasty.
An extreme example occurred during the prolonged debate about the Palmdale bulge and whether an earthquake was imminent along that section of the San Andreas Fault. Ross Stein at the United States Geological Survey had argued for years that the bulge was real, but then, after more data was available, he changed his mind. His conversion did not sit well with at least one colleague because twice Stein found a bag of dog excrement in his office mailbox.
Fortunately, such juvenile behavior is rare and should not distract from the serious business of trying to predict earthquakes.
Furthermore, the question of earthquake prediction can be reduced to a more tractable and straightforward question: What triggers a large earthquake?
Imagine this: Initially, an earthquake fault, such as the San Andreas, is relatively quiet. Only a few small earthquakes are occurring, popping off, in a familiar analogy, like kernels of heated popcorn. Then the popping of one earthquake kernel happens to set off more kernels, and those set off more kernels until there is an explosion, or cascade, of kernels popping, a rupture forms, and a large earthquake is produced.
Or imagine this: The lower region of the San Andreas Fault is slowly sliding—without earthquakes—because here the rocks are hot and plastic and are driven to slide smoothly by the slow and ever-constant movement of the Pacific and North American plates. As the slipping region grows, the sliding accelerates until it reaches a critical speed to where a rupture in the brittle overlying rock forms, and a large earthquake is produced.
In the former case—the cascade model of earthquake kernels—the beginning of any large earthquake is no different from the beginnings of countless small ones, which means it is impossible to ever predict large earthquakes.
In the latter case—the pre-slip model—a long process occurs that prepares the San Andreas for a sudden and major slip. In that case, earthquakes might be predicted if we can figure out how to measure the slow sliding and subsequent buildup.
Which idea is true—or whether earthquakes work in some other manner entirely—is still the focus of much research today and is hotly debated. But this much is true: When there is a major earthquake, the probability of another major earthquake happening soon after in the same region goes way up. Once the Earth’s crust starts to adjust to the slow buildup of pressure between the tectonic plates, that pressure may not be relieved simply as a single large event but rather as one—or more—major earthquakes happening in a short time period.
To put this in concrete numbers: History shows that whenever there is a major earthquake in California, say a magnitude-6 event—which can do substantial damage—there is a 1 in 10 chance that another earthquake of equal or greater magnitude will happen in the same general area within the next three days.
This leads to a practical concern. After a major earthquake, people should brace themselves for an equal or larger event. Emergency services, such as fire and police stations and hospitals, need to prepare for additional injuries and for the disruption of still more roads and utilities. And those who are attempting to rescue people who are already trapped under debris should be aware that a larger earthquake could strike and a greater catastrophe could happen.
*The largest was the magnitude-9.5 earthquake in Chile on May 22, 1960.
*These rocks are easy to find in the New York area. The Manhattan Schist can be seen exposed in J. Hood Wright Park, and the Fordham Gneiss (one of the oldest rock formations in the world, dated to have formed 1.1 billion years ago) is exposed at Inwood Hill Park, both in northern Manhattan. The choice of where to build skyscrapers is limited by where these rocks are close to the surface. The Palisades Sill is a diabase that intruded about 200 million years ago and that forms the cliffs along Henry Hudson Drive in New Jersey directly west of Manhattan Island, best seen just north of the interstate approach to the George Washington Bridge.
Chapter 10
Ancient Tremors
We can’t, of course, know exactly what the San Andreas Fault
has in store for us.
—Kerry Sieh, on future earthquakes, 1981
One of the enduring mysteries of southern California is the annual return of swallows to the old Spanish mission at San Juan Capistrano. The return is part of a 15,000-mile migration the cliff swallow makes from the west coast of South America to the west coast of North America and back again. The birds, of course, congregate and nest at many sites in North America, though San Juan Capistrano is the most famous, memorialized in song and in a movie and illustrated and told in countless tourist brochures. But one wonders: Why San Juan Capistrano?
The cliff swallow, Petrochelidon pyrrhonota, needs three things to thrive. It needs an ample source of insects to eat and a source of mud to build a nest. Both can be found in many places in California. But the third requirement is a challenge. The cliff swallow prefers to build nests under cliff overhangs or at cave entrances
. Neither can be found near San Juan Capistrano, but there is something just as good. There are high walls and arched insets of a great stone church—a church that was ruined by an earthquake.
Work was completed on the great church at San Juan Capistrano on September 8, 1806. At the time, it was the only church in Spanish California not constructed out of adobe. Instead, blocks of cut sandstone were used for the walls and vaulted ceilings and for a high bell tower that stood next to the main entrance. It was said the tower could be seen from distances as great as 10 miles, and that the sound of its bells could be heard from even farther. This marvelous structure, which took nine years to build, collapsed in less than a minute on the morning of December 8, 1812.
On that date, the ground shook twice. The first shock caused the bell tower to sway. Parishioners who were inside the church, mostly Native American women and their children who were there for the first Mass of the day, felt the shaking and heard the bell ring. They ran for the main entrance, but the shaking had jammed the door. A second shock, stronger than the first, amplified the swaying of the bell tower and caused it to fall onto the church through the stone roof. Forty people were killed. Eventually the debris was cleared away and the bodies recovered and given burials, but the walls and the arches that still stood were thought to be too precarious to tear down. So the ruined walls and arches were left standing. And the swallows took up annual residence.
The same earthquake also damaged the church missions at San Gabriel, San Fernando, and San Buenaventura, the last one 100 miles west of San Juan Capistrano, which indicates this was felt over a large area and therefore was a major event. But where was the earthquake located? What was its epicenter?
Such a complex lattice of faults runs through southern California that it might seem impossible to ever know exactly which fault had slipped on December 8, 1812, and damaged and caused fatalities at the great stone church at San Juan Capistrano. But in 1975, an examination of tree rings revealed the source.
The fact that large earthquakes can affect the growth of trees is well documented. Severe ground shaking can bend a tree far over, breaking many limbs. Ground rupture can sever roots, retarding growth. In 1906, near the northern end of the San Andreas Fault near Fort Ross, several large trees were actually split apart by the ground rupture, but survived and lived to show the ordeal to future scientists.
In 1975, cores were drilled into 65 Jeffrey pines standing close to the San Andreas Fault northwest of Cajon Pass. The purpose was to see if the rings of any of the trees indicated retarded growth after the 1857 earthquake. The cores of nine of the trees did show a narrowing of tree rings after 1857. And there was more.
There was another narrowing of tree rings that began between the end of the growing season in the fall of 1812 and before renewed growth in the spring of 1814. Could this be evidence that the San Andreas Fault, where it passes through Cajon Pass, had been the source of the December 8, 1812, earthquake? If so, there should be additional evidence of the earthquake elsewhere along the fault. But how might it be possible to cut into and examine a fault and, in all the complexity, see the effects of a single earthquake?
Imagine you are a baker. A customer comes into your store and orders a six-layer wedding cake. You are known to be methodical and to always begin baking at noon. You are also poor and have only one cake pan.
You prepare the batter for the first layer, bake it, and pull it out of the oven at precisely 1:00 P.M. and set it on a plate. You prepare the next layer, pull it out at 2:00 P.M., set it atop the first layer, then prepare the third layer, pull it out at 3:00 P.M., and so forth.
Sometime while the fifth layer is baking an earthquake happens and the unfinished four-layer cake falls to the floor. Fortunately it does not crumble, but it splits into two parts. You scoop it up and set it back on the plate, adding the fifth layer at 5:00 P.M. and the sixth layer an hour later. You then cover the whole cake with frosting and deliver it to the wedding, hoping that no one will be able to figure out what happened.
You are unaware that the bride is a geologist.
When she cuts the cake, she notices a crack that extends through all the lower four layers but not into the upper two. She also knows of your methodical baking habits and easily figures out that whatever calamity happened to the lower four layers to split the cake must have occurred between 4:00 P.M. and 5:00 P.M. That, in essence, is the foundation of paleoseismology.
Paleoseismology is the study of the geologic effects of past earthquakes. Such effects include the fracturing, warping, folding, or sliding of sedimentary layers that were laid down at the bottom of swamps, rivers, or lakes. The key is finding a place where the layers were deposited continuously for many, many years and where they lie over an active fault. In principle, the work of deciphering these layers is straightforward—as the cake analogy demonstrates—but in practice, it is tedious and can best be described as trying to produce a highly detailed geologic map of an area that can be covered with a bed sheet. It requires distinguishing the limits of myriad mud, sand, and gravel deposits, then tracing hairline cracks through them. It also requires that layers of organic material, usually peat, be present so that radiometric dating of the carbon can be done to determine when the various hairline cracks formed. It is meticulous, highly frustrating work. At times, it seems impossible—and that’s because it almost is.
Two early attempts at paleoseismology were tried immediately after two moderate earthquakes in southern California, after the 1968 Borrego Mountain earthquake west of the Salton Sea and after the 1971 San Fernando earthquake. Ruptures of both tremors broke the surface, and in both cases bulldozers were used to dig trenches across each break. The results were disappointing.
The problem, as one investigator put it, was that in the exposed walls of the trenches, it was “extremely difficult to see” where the fault breaks were because they passed through “massive unbedded material ranging from silt to clay to coarse bouldery sand and gravel.” What was needed, recommended the same investigator, was someone who was willing to devote years to digging, scraping, and carefully brushing the walls of such trenches to reveal the barely discernible cracks and offsets of mud and sand layers that were caused by recent earthquakes.
Enter Kerry Sieh.
Sieh’s fascination with the San Andreas Fault began when he was a student at Stanford University in the 1970s. He decided he wanted to know more about the last great earthquake to occur along the San Andreas Fault before 1906: the 1857 Fort Tejon earthquake. Surprisingly little was known about the earthquake when Sieh started his work, the shaking having occurred when the population of the greater Los Angeles area was just a few thousand people.
Sieh tracked down more than 60 personal accounts of the earthquake, mostly found as brief newspaper reports, as well as several personal letters and a few unpublished memoirs that described the event. Then he set off and within a year had hiked or bicycled most of the 200 miles of the rupture, looking for indicators of where the fault had slid, focusing much of his attention on where stream channels had been offset. Next, he decided he needed to dig to determine exactly where the fault trace was located and exactly how much the fault had moved in 1857. At that notion, his Stanford professors balked.
It was doubtful, so they said, that the rupture of an earthquake, even one as large as in 1857, would leave a readable record after more than 100 years. Besides, they said somewhat quietly, Sieh was proposing to spend years examining just the thin surface layer that most geologists ignore, instead of delving deeper into bedrock, where the clues to the fundamental problems of the growth of mountains, the origin of continents, and the history of ocean basins would be found.
But Sieh, with the stubbornness of a brilliant student, was undeterred. He started his project armed at first with only a shovel. But where should he dig? Here his yearlong reconnaissance of the San Andreas Fault paid off. He decided on Pallett Creek, a few miles west of Valyermo.
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Pallett Creek is an ephemeral stream that flows down the north side of the San Gabriel Mountains and onto the Mojave Desert. It is at the base of those mountains that the creek crosses the San Andreas Fault, a setting favorable for the repeated deposition of mud and gravel and the development of the occasional layer of peat. Taking advantage of a 30-foot-deep erosional trench made by the creek—and, eventually, with his work augmented by use of a backhoe and a bulldozer—Sieh dug into the ground and exposed a profile of the very upper reaches of the San Andreas Fault.
About two feet below the ground surface, he uncovered a former surface layer that had a broad warp in it. By carbon age dating of decomposed plants that once grew on the former surface, Sieh was able to determine that the warping had been caused by the 1857 earthquake. A foot deeper there was another disruption, this one caused by the 1812 earthquake—confirming that the earthquake that damaged the church at San Juan Capistrano had occurred along the San Andreas Fault. And greater surprises lay beneath.
By tracing disrupted layers, Sieh identified the occurrence of seven more earthquakes, determining the timing of these events also by carbon dating. The next older event before 1812 had occurred in about 1500. The earliest one he exposed had occurred in about 730 a.d. It was a spectacular discovery because the record of individual earthquakes for the San Andreas Fault had been pushed back from the 200 years of historical records to a paleoseismic record that covered nearly 1,500 years.
