POSTED 3 NOVEMBER 2005
The ground truth
Let's start talking about earthquake prediction by disposing of one bothersome
detail. Nobody can predict earthquakes -- nobody can say a quake of any
given size will happen in a particular place on a particular date.
Even the more general quest -- to anticipate the statistical earthquake hazard at a given location -- remains a tough problem, as Earth continues dishing up surprises. It's becoming clear that places once thought stable actually have this nasty habit of shaking in a big way, every once in a long while.
Socially, economically, even geologically, the American Midwest seems a paragon of stability. But survivors of the New Madrid [Mo.] earthquakes of 1811 and 1812 "reported that the earthquakes caused cracks to open in the earth's surface, the ground to roll in visible waves, and large areas of land to sink or rise."
The New Madrid, Mo., earthquakes of 1811 and 1812, rang church bells in Boston. Notice a similar earthquake in California did not spread the shaking nearly as far. The geology in the Eastern United States transmits shocks more effectively than in the West. Map: USGS
In 1700, a magnitude 9.0 quake shook the Cascadia
Subduction Zone. Experts warn that another giant quake could rattle the
Pacific Northwest. Graphic: USGS
How rare is rare?
Earth science is all about records. Climate studies routinely deal with the relative brevity of climate
records, and a similar shortage has hobbled our understanding of earthquake
hazards. Although the first seismograph was invented in China in AD 132,
detailed records of ground shaking date only to the late 19th century.
So even though the Earth moves according to geologic time -- scales of
thousands to millions of years -- seismologists are forced to understand
by relying on short-term records.
The huge, deadly rupture on the northern section of the Sunda Fault may be quite a rare occurrence, but "rare" translates into "hard to predict." The 2004 quake, says Lay, "taught us that in some regions, the time between events is so long that we are basing the anticipation of future earthquakes on a historical record that is very limited. So we may miss some very large earthquake regions."
If it can be difficult to see earthquakes in hindsight, it's much tougher to predict them in the future. Even in heavily instrumented Parkfield, a location on the San Andreas Fault that may be home to a world record stash of seismological instruments, predictions have been a bit dodgy. Magnitude 6 quakes were thought to have occurred 22 years apart, on average. But when the most recent magnitude 6 quake finally occurred in 2004, it was 16 years behind schedule.
In Parkfield and other shaky locations, scientists have assessed many factors in their quest to predict earthquakes:
Foreshocks -- small shocks that may precedes a larger one
And even though most of these factors have been examined in detail on the San Andreas, there is no indication that anything seemed out of the ordinary before the 2004 quake shook Parkfield. "The lack of anybody jumping up and down about it now tells me what I expected, that efforts to find a predictive sign were a failure," says Thurber. "We are a long way away from making predictions."
Deadly. Unpredictable
After so many years of study, earthquakes remain the natural disaster
that is the most difficult to predict. It's fairly easy to pinpoint locations
where quakes are likely, and often possible to forecast the general level
of future danger. But in terms of saying, "we expect a magnitude X quake
to occur on the Y of July at location Z," don't look for that any time
soon.
Ironically, even as seismologists admit that they can't predict earthquakes, data from seismology plays a crucial role in helping volcanologists predict eruptions -- a science that is a bit further along than earthquake prediction. But earthquakes still amount to a bolt from the blue, and, as we have learned in the past year, they can be the deadliest of surprises.
Further fundamental research is needed into the processes that hold faults together, then allow them to suddenly tear apart, indicates seismologist Thorne Lay. "We have to study, are they even predictable at all? Until we can understand the physics well enough, we can't put into place any reliable predictive scheme."
See what's shaking in our bibliography.
