5.7: Memories of the Future; The Uncertain Art of Earthquake Forecasting

• 5.7.1: A Mix of Science and Astrology

  Predicting the future does not sit well with most earthquake scientists, including Charles Richter, quoted above. Yet if earthquake research is to truly benefit society, it must lead ultimately to prediction, no matter how elusive that goal may be. Society asks specialists to predict many things, not just earthquakes. How will the stock market perform? Will next year be a good crop year? The answers to any of these important questions depend on the complex bits of information.

• 5.7.2: Earthquake Forecasting by Scientists

  Most of the so-called predictors, including those who have been interviewed on national television, will claim that an earthquake prediction is successful if an earthquake of any magnitude occurs in the region. Let’s say that I predict that an earthquake will occur in the Puget Sound region within a two-week period of June of this year. An earthquake occurs, but it is of M 2, not “large” by anyone’s definition.

• 5.7.3: Forecasts Instead of Predictions (The Parkfield Experiment)

  A more sophisticated but more modest forecast was made by the USGS for the San Andreas Fault at Parkfield. Before proceeding, we must distinguish between the term prediction, such as that made by Brady for Peru, in which it is proposed that an earthquake of a specified magnitude will strike a specific region in a certain time window, and the term forecast, in which a specific area is identified as having a higher statistical chance of an earthquake in a time window measured in months or years.

• 5.7.4: The Seismic Gap Theory

  Another idea of the 1970s was the seismic gap theory, designed for subduction zones around the Pacific Rim, but applicable also to the San Andreas Fault. According to theories of plate tectonics, there should be about the same amount of slip over thousands of years along all parts of a subduction zone like the Aleutians or Central America (or central Peru, for that matter, leading Brady toward his prediction). Most of the slip-on these subduction zones should be released as great earthquakes.

• 5.7.5: Have the Chinese Found the Way to Predict Earthquakes?

  Should we write off the possibility of predicting earthquakes as simply wishful thinking? Before we do so, we must first look carefully at earthquake predictions in China, a nation wracked by earthquakes repeatedly throughout its long history. More than eight hundred thousand people lost their lives in an earthquake in north-central China in 1556, and another one hundred eighty thousand died in an earthquake in 1920.

• 5.7.6: A Strange Experience in Greece

  On a pleasant Saturday morning in May 1995, the townspeople of Kozáni and Grevena in northwestern Greece were rattled by a series of small earthquakes that caused people to rush out of their houses. While everyone was outside enjoying the spring weather, an earthquake of M 6.6 struck, causing more than $500 million in damage but no one was killed. Just as at Haicheng, the foreshocks alarmed people and they went outside.

• 5.7.7: Reducing Our Expectations (Forecasts Rather Than Predictions)

  Our lack of success in predicting earthquakes has caused earthquake program managers, even in Japan, to cut back on prediction research and focus on earthquake engineering, the effects of earthquakes, and the faults that are the sources of earthquakes. Yet in a more limited way, we can say something about the future; indeed, we must, because land-use planning, building codes, and insurance underwriting depend on it.

• 5.7.8: The Deterministic Method

  The debate in Chapter 4 about “instant of catastrophe” or “decade of terror” on the Cascadia Subduction Zone—whether the next earthquake will be of magnitude 8 or 9—is in part a deterministic discussion. Nothing is said about when such an earthquake will strike, only that such an earthquake of magnitude 9 is possible, or credible. We have estimated the maximum credible (or considered) earthquake, or MCE, on the Cascadia Subduction Zone.

• 5.7.9: Probabilistic Forecasting

  In the probabilistic forecasting of earthquakes, we use geodesy, geology, paleoseismology, and seismicity to consider the likelihood of a large earthquake in a given region or on a particular fault sometime in the future. A time frame of thirty to fifty years is commonly selected because that is close to the length of a home mortgage and is likely to be within the attention span of political leaders and the general public.

• 5.7.10: Do Faults Talk To Each Other? (Earthquakes Triggered By Other Earthquakes)

  A probability curve for the Cascadia Subduction Zone or the San Andreas Fault treats these features as individual structures, influenced by neither adjacent faults nor other earthquakes. A 1988 probability forecast for the San Francisco Bay Area treated each fault separately. But the 1992 Landers Earthquake in the Mojave Desert appears to have been triggered by earlier earthquakes nearby. The Landers Earthquake also triggered earthquakes hundreds of miles away.

• 5.7.11: Forecasting the 1989 Loma Prieta Earthquake—Close But No Cigar

  Harry Reid of Johns Hopkins University started this forecast in 1910. Repeated surveys of benchmarks on both sides of the San Andreas Fault before the great San Francisco Earthquake of 1906 had shown that the crust deformed elastically before the earthquake, and the elastic strain was released during the earthquake. Reid figured that when the elastic deformation had reached the stage that the next earthquake would release the same amount of strain as in 1906, it would be close at hand.

• 5.7.12: The 1990 Probability Forecast

  The Working Group on California Earthquake Probabilities went back to the drawing boards and a new probability estimate was issued in 1990. Like the earlier estimate, this one was based on the history and slip rate of individual faults, but unlike the earlier estimates, it gave a small amount of weight to interactions among faults. The southern Santa Cruz Mountains segment of the San Andreas Faul was assigned a low probability of an earthquake of M greater than 7 in the next thirty years.

• 5.7.13: The 1999 Bay Area Forecast

  The new ideas of earthquake triggering and stress shadows from the 1906 earthquake, together with much new information about the paleoseismic history of Bay Area faults, led to the formation of a new working group of experts from government, academia, and private industry. This group considered all the major faults of the Bay Area, as well as a “floating earthquake” on a fault the group hadn’t yet identified. A summary of fault slip and paleoseismic data was published by the USGS in 1996.

• 5.7.14: “Predicting” an Earthquake After It Happens

  eismic shock waves travel through the Earth’s crust much more slowly than electrical signals. A great earthquake on the Cascadia Subduction Zone will probably begin offshore, up to three hundred miles away from the major population centers of Vancouver, Seattle, and Portland. Seismographs on the coast recording a large earthquake on the subduction zone could transmit the signal electronically to Seattle and Sidney more than a minute before strong ground shaking began.

• 5.7.15: What Lies Ahead

  In New Zealand, Vere-Jones et al. (1998) proposed that we combine our probabilistic forecasting based on slip rates and estimated return times of earthquakes with the search for earthquake precursors. Geller (1997) has discounted the possibility that earthquakes telegraph their punches, and up to now, the Americans and Japanese have failed to find a “magic bullet” precursor that gives us a warning reliable enough that society can benefit.

• 5.7.16: Why Do We Bother With All This?

  By now, you are probably unimpressed by probabilistic earthquake forecasting techniques. Although probabilistic estimates for well-known structures such as the Cascadia Subduction Zone and the San Andreas Fault are improving, it’s unlikely that we’ll be able to improve probability estimates for faults with low slip rates such as the Seattle and Portland Hills faults, unless we’re more successful with short-term precursors.