A Positron Named Priscilla Scientific Discovery at the Frontier (1994) / Chapter Skim
Currently Skimming:
6 Clocks in the Earth? The Science of Earthquake Prediction
6 Clocks in the Earth? The Science of Earthquake Prediction
Pages 154-195
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 154... ...
In America this epithet usually evokes images of western (California, where the San Andreas fault perhaps our ~ most notorious geological feature and probably the most intensely studied by scientists-appears for most of its length as the thinnest of lines in the dirt. The fault was indubitably the cause, however, of the earthquake and consequent raging fire in 1906 that devastated San Francisco (see Figure 6.i)
|
From page 155... ...
Geological Survey (USGS) -run project nearly a decade in the running: The Parkfield Earthquake Prediction Experiment.
|
From page 156... ...
This famous photo- important information about the San Andreas fault around San Frangraph by Arnold Genthe shows Sacramento Street cisco, but Landers provided a message to Los Angelenos, whose city's and the approaching fire in fate rests upon a network of underground faults that share with North the distance. {Photograph em California only the fundamental fact that all are part of the major courtesy of the Fine Arts fault system defining the boundary between the Pacific and North .
|
From page 157... ...
A more accurate image than a river, suggests Ellsworth, is a boiling pot of water, where the sea of bursting bubbles on the surface indicates that convection cells have developed in the pot. For the same reasons of basic physics convection-hotter rock in the earth's deeper mantle 157
|
From page 158... ...
Loma Prieta released 30 times as much energy as Parkfield.
|
From page 159... ...
Masonry D-Weak materials, such as adobe; poor mortar; low standards of workmanship; weak horizontally. FIGURE 6.2 Moclifiecl Mercalli Intensity Scale {1956 version from Richter, 1 958, pp.
|
From page 160... ...
But those on the very edge of a plate, including millions of Californians, experience plate tectonics viscerally, as patches of rock at the western edge of the North American plate undergo stress, accumulate elastic strain, and eventually and inevitably break loose from rock of the adjacent Pacific plate that is being propelled in a different direction. All such underground shifts are earthquakes, though most occur at levels undetectable except by instruments.
|
From page 161... ...
They simply don't contain that information. "Very clever people," says Ellsworth, "working with very sparse information, cracked the plate tectonics puzzle in the early 1960s, and the dramatic advances in seismology since then rest on better data gathering through advances in tools and technology, the enhanced power to compute complex numerical solutions, and advances in the theory." What Agnew and Ellsworth call the first "modern" theory of earthquake prediction, by G
|
From page 162... ...
153~. Tectonic forces develop from convective recycling of the earth's interior, heat conducted to the crust that is converted to energy stored in rocks, under strain, which eventually fracture: An earthquake.
|
From page 163... ...
up in fault zones, the time-predictable model could one day lead to a more precise means of forecasting. THE LANGUAGE OF EARTHQUAKE PREDICTION What is an earthquake prediction?
|
From page 164... ...
With the fault terrain thus delineated, a legitimate earthquake prediction must locate where on a given fault an event is expected. Most large earthquakes that occur at plate boundaries are categorized as shallow-focus earthquakes, occurring less than 70 kilometers deep.
|
From page 165... ...
Though the Japanese have a much larger billiondollar experiment in place southwest of Tokyo, the most dedicated American effort ever marshaled to answer this question is currently under way at a small town in central California. STALKING ELUSIVE PATTERNS The Characteristic Earthquake Hypothesis If the time-predictable model is to prevail and provide planners with information upon which useful precautions can be based, a discernible pattern should emerge from the sequence of ruptures at a given point on a particular fault.
|
From page 166... ...
of 1988. That this window has now closed and the Parkfield segment has not yet ruptured casts doubt on Bakun and McEvilly's model, and seismologists have joined a debate about the characteristic earthquake hypothesis.
|
From page 167... ...
each time, what initiates the rupture, and just how the rupture proceeds along the fault." There has been sufficient data gathered about such events, he believes, to state that the characteristic earthquake model "is quite strong for some parts of the San Andreas fault," notwithstanding dissenting opinions about Parkfield. As evidence, he relies less on the Parkfield sequence and the timing of the next rupture there than on the extant record of smaller earthquakes (M 1-5)
|
From page 168... ...
Hitting the Trail into Past Time As any scientist will readily concede, your conclusions are only as good as your data. And for models as heavily reliant on the particularities of data as are the seismic cycle, elastic rebound, and characteristic earthquake theories, interevent times for a particular fault shine like diamonds in a coal mine.
|
From page 169... ...
geological era coincides approximately with the appearance and rise of mammals some 65 million years ago, but geologists searching the record for evidence of prehistoric earthquakes are content for now to rummage around in the Holocene epoch, approximately the last 10,000 years. One of the lead scouts on this expedition into the past is Kerry Sieh, who modestly explains away his first National Science Foundation grant (awarded before he had even begun graduate school at Stanford)
|
From page 170... ...
(see Figure 6.4 for a history of earthquakes on the San Andreas fault.) There had been thirteen significant earthquakes on a fault that, though it had not broken for well over a century, now lay within 55 kilometers of Los Angeles, the second-largest city in the country.
|
From page 171... ...
allows the fossil to be dated. Kerry Sieh of Caltech, following the lead of Stanford's late Richard [ahns, was a trailblazer with his work at Pallett Creek (known as the Rosetta Stone of paleoseismology)
|
From page 172... ...
At a study site on the San Andreas fault northeast of Los Angeles, earthquakes have occurred in clusters of two or three events spanning several decades separated by dormant periods of two or three centuries. Precision radiocarbon dating of buried peat horizons in a faulted sequence of marsh and stream deposits has yielded age ranges (shaded barsJ for eight earthquakes prior to the 18 12 and 1857 shocks, which are documented by historical accounts.
|
From page 173... ...
When it gets so far behind that fracture finally occurs, an earthquake is the likely result, and-at least in transform fault zones like the one where the Pacific and North American plates meet in California measurable offsets will be carved into the earth. Take one specific fault strand, such as Pallett Creek, and a stratigraphic map will reveal a history of such offsets for many centuries.
|
From page 174... ...
When the slip rates for other faults were determined, few matched the underlying annual slip rate of 4.8 centimeters per year for the Pacific plate itself, and thus other major fault zones are known to be accumulating some of the stress. This piece of data permitted slip rate calculations, based on offset distance and interevent time, to be compared throughout the San Andreas fault zone.
|
From page 175... ...
The group decided that the Landers earthquake increased the stress toward the failure limit on parts of the southern San Andreas fault, in particular on the San Bernardino Mountains segment. Contrary to some popular reports, Landers and Big Bear do not likely represent a major underground shift of stress accumulation and release to the Mojave zone.
|
From page 176... ...
With each new spring a tree begins a new ring and thus develops a putative birth certificate. When Sieh realized that many trees shaken by prehistoric earthquakes had been damaged but not destroyed, he began to look to their tree rings to determine the precise year of the earthquakes.
|
From page 177... ...
The significance of this possibility, like the work at Wallace and Pallett creeks, is what it says about historical seismicity in Southern California and the fate of Los Angeles and other major urban centers. With his work on the 1812 quake, Sieh had moved into a more recent area of inquiry, using nineteenth-century historical records.
|
From page 178... ...
Because their studies are primarily limited to earthquakes of large magnitudes, which have interevent times of many decades, even centuries, scientists have no choice but to invent complementary ways, like paleoseismology, to look back in time, the farther and more accurately the better. But Ellsworth's conviction about the importance of testing the characteristic earthquake hypothesis has led him to focus on the more numerous smaller-magnitude earthquakes that can be studied in the seismographic record.
|
From page 179... ...
These statistics rate it as one of America's most serious natural disasters. As such, it raises questions about how long-term forecasting fits into the American political infrastructure, since in the context of work done by Ellsworth, Agnew, Sieh, and many others, the USGS in the Science article classified Loma Prieta as "an anticipated event." Californians have felt thousands of earthquakes over the decades, FIGURE 6.5 Schematic diagram showing inferred motion on the San Andreas fault during the Loma Prieta earthquake.
|
From page 180... ...
In the first report, seismologists had calculated the probability that each of California's major faults would rupture in the next 30 years, expressed as a fraction. The highest probability in Northern California was assigned to the southern Santa Cruz Mountains segment, .3 (see Figure 6.6~.
|
From page 181... ...
=1 A Los Angeles Rather than try to unveil what may be seen as the esoteric mysteries of such statistical phenomena, recent experience suggests it is better to look broadly at the results. Using the current models, the consensus 30year probability of a rupture for the Santa Cruz segment was .3.
|
From page 182... ...
159~. By contrast, the seismic cycle does not represent a coherent explanation of known phenomena with fully identifiable causes; rather it seems to
|
From page 183... ...
But the general pattern that arises from the conjunction of the earthquake cycle generally accepted and the elastic rebound theory does not mean that all earthquakes share enough underlying aspects to be encompassed by a descriptive model like the characteristic earthquake hypothesis. The USGS does not evoke the specter of a monolithic bureaucracy.
|
From page 184... ...
stresses of at least one half of the confining pressure, before it yields.... Faults obviously yield during earthquakes," he reasons, and thus "we expect the shear stress to be on the order of 1 to 2 kilobars," that is, 1000 to 2000 times the atmospheric pressure at the surface.
|
From page 185... ...
The point is, that although earthquakes seem violent to humans they actually involve stress changes that are small (typically about 15 bars) compared with the confining stress at the depth of earthquakes." The measurements of accumulating strain in the crust over several decades also support this premise, says Heaton: "The rate at which shear strain builds on the San Andreas fault is about 2 x 10-7 per year, or less than 1 bar of shear stress per decade." This is the first part of the dilemma: There would appear to be much more stress at the depths where earthquakes originate than is released during the event.
|
From page 186... ...
Second, his own work and that of others on the physics of dynamic rupture suggest that earthquakes can occur at relatively low stresses. Early in the careers of many of these scientists, only about a decade after plate tectonics began to take hold, seismology appeared to be in the grip of another revolution, based on the dilatancy-fluid diffusion model.
|
From page 187... ...
But these experiments are hounded by three crucial issues of scaling: Whether simulations can actually match, or at least transfer to, dynamic subterranean conditions; whether the compressed time of laboratory experiments distorts processes occurring over much longer periods; and also whether the lower range of seismic phenomena, about which there is more (and more accessible) data, is qualitatively different from what may be happening in M 6 or larger earthquakes.
|
From page 188... ...
The stress dilemma reminds us that the coefficient of friction for a given material is a function of its confining pressure, but in the local environment of the rupture-this confining pressure is not known for certain, nor are its complexities fully understood by geophysicists, though Terry Tullis from Brown University has thrown much light on this area. Heaton also believes that the slipping friction may actually be a complex function of the slip history itself.
|
From page 189... ...
The implications of dynamic friction unpredictability as indicated by heterogeneous slip behavior pose major problems for the characteristic earthquake model, concludes Heaton: "If these earthquakes were to repeat themselves over several recurrence cycles, then large slips would accumulate in some regions, but other regions would have very little slip. This is clearly an untenable situation.
|
From page 190... ...
And thus it should be emphasized that the science going on at Parkfield, which after years of development-now might fairly be regarded as a National Earthquake Prediction Laboratory, is not overly reliant on when the next Parkfield quake will arrive. To be sure, the extensive attention to measuring the rupture will provide irrefutable "facts" that the characteristic hypothesis must be able to accommodate.
|
From page 191... ...
However, given the strongly focused nature and high sensitivity of the Parkfield monitoring networks, there can be little doubt that new and unexpected features of the earthquake mechanism will be uncovered, and that significant constraints will be placed on the mechanics of the precursory process" (Thatcher, 1988, p.
|
From page 192... ...
But in present time the dilemma makes even the most seasoned seismologist nervous because only time will tell whether, say, an M 4.7 event is an isolated quake or among an escalating series of shocks that could grow into a much larger earthquake, even "The Big One." In October it turned out to be part of a smaller series, and the long-awaited Parkfield characteristic earthquake remains a no-show. To the pack of
|
From page 193... ...
Greater subtlety in the relationship of wave structure to fault behavior has been developed from the experiment and recognized to be operative on other faults in California. The USGS pamphlet distributed to millions after Loma Prieta was modeled on a mailing that had been sent to Parkfield residents.
|
From page 194... ...
1991. Fault stress states, pore pressure distributions, and the weakness of the San Andreas fault.
|
From page 195... ...
1988. Scientific goals of the Parkfield Earthquake Prediction Experiment.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.