杏吧原创

IT WAS ONE of the strangest natural disasters in history. In January 1994 an
earthquake hit Northridge, California, sending many of the city鈥檚 buildings
crashing to the ground. Freeway bridges collapsed and water and gas mains were
ripped open, causing flooding and explosions. As residents began to sift through
the debris they were not surprised to learn that Northridge had been at the
epicentre of a major quake.

But the news came as a shock to the citizens of Santa Monica. Parts of their
coastal city, 21 kilometres south of Northridge and separated from it by a
mountain range, had received shocks just as severe as those felt in Northridge.
Nowhere else so far from the epicentre was so badly hit. Even the experts were
confused. 鈥淲hat we saw didn鈥檛 make any sense to us,鈥� says geotechnical engineer
Jonathan Stewart from the nearby University of California, Los Angeles.

Geologists investigating this strange pattern of destruction have now reached
a startling conclusion. The evidence they have gathered suggests that something
deep underground鈥攕omething like a magnifying glass in the
rock鈥攆ocused the power of the Northridge quake onto the unsuspecting city
of Santa Monica.

It鈥檚 a controversial idea, but the evidence in its favour is growing. Many
seismologists now believe that the same phenomenon occurred a year later in the
Kobe earthquake in Japan. And according to seismologist Robert Graves, who works
for the engineering firm URS, it has happened elsewhere in the world too.
There鈥檚 nothing special about the geology of Santa Monica or Kobe, he says.
Unless you know the structure of the rocks beneath your city, there鈥檚 no telling
whether a fault could trigger a double quake. Around the globe, thousands of
lives could be at risk from the hidden threat of the earthquake magnifier.

A decade ago, engineers would have compared the damage in Santa Monica with
that of nearby districts and concluded that its buildings were poorly
constructed, or that its soil was unstable. And they would probably have left it
at that. But geotechnical engineers are now more thorough in their analysis, and
their investigations of the Northridge quake revealed some oddities.

They examined Santa Monica鈥檚 鈥渞ed-tagged鈥� buildings鈥攖hose so badly
damaged that they would either have to be demolished or undergo major
repairs鈥攂ut found that these were not simply badly built. And when they
laid a map of the damage on top of another showing soil properties, they
discovered that the red tags didn鈥檛 correspond to areas with shaky soils either.
What鈥檚 more, the swathe of red tags ran parallel to a fault line that hadn鈥檛
ruptured. Nothing in standard earthquake theory could explain the excessive
damage.

The seismic data from the earthquake only added to the confusion. An array of
98 seismometer stations, set up within days of the quake, recorded details of
the aftershocks. The data revealed that a rupture in a fault 19 kilometres below
Northridge was to blame for all the devastation. As in all earthquakes, the
seismic waves spread spherically outwards from this rupture, and the power of
the waves should have diminished as they moved away from their source. But by
the time they reached Santa Monica, something strange had happened. Some of the
waves that hit the city were ten times larger than seismologists would have
expected. And at the centre of the worst affected area, the seismic evidence
showed that some waves had converged. Because of this freak occurrence, the
to-and-fro acceleration of the ground was way beyond what it should have been.
鈥淚n Santa Monica, accelerations were similar to values seen in Northridge,鈥� says
Paul Davis, a seismologist at the University of California, Los Angeles.

This trail of puzzling clues has led Davis to suggest the extraordinary
possibility of a 鈥渟eismic lens鈥� under Santa Monica which affected the seismic
waves in the same way that optical lenses can focus light waves.

When a beam of light travelling in air enters a piece of glass it slows down.
If the light hits the glass surface at an oblique angle, the slowing causes the
light beam to bend, a phenomenon known as refraction. Light hitting a curved
surface such as a lens is bent by different amounts depending on where it
enters, which is how lenses focus light. And exactly the same thing can happen
to seismic waves, says Davis. They move faster through hard, dense rock than
they do through soft sediments. When the waves hit a boundary between the two,
they refract and change direction.

If the boundary between the hard rock and the soft sediments is curved in the
right way, it can focus the waves鈥攋ust as the curve of a magnifying glass
focuses light onto a point. Davis believes that deep under Santa Monica there is
a seismic lens capable of magnifying the force of a quake, turning minor tremors
into intense and destructive shocks.

From seismic data, Davis has calculated that this lens is probably a huge
trough in the hard bedrock, 3 kilometres below the surface. Although there鈥檚 not
yet enough geological data to confirm its existence, he believes it lies at the
foot of the Santa Monica Mountains and along the Santa Monica Fault, where the
bedrock meets the softer sediments of the Los Angeles Basin
(see Diagram).

Davis believes that as the seismic waves spread out from the rupture below
Northridge and hit this curved interface, it focused them towards the surface.
And there, sitting at the lens鈥檚 focal point, were the unlucky residents of
Santa Monica. The uneven geometry of the rock may have affected the focusing,
giving as many as three different focal points within Santa Monica. That, Davis
says, would explain the pattern of damaged and undamaged areas in the city.

Some Japanese seismologists studying Kobe鈥檚 1995 quake suspect a similar
phenomenon was at work there. A deep sediment basin similar to that on the Santa
Monica Fault runs along Kobe鈥檚 Uemachi Fault, and the pattern of destruction in
Kobe was strikingly similar to what happened in Santa Monica. In 1996,
encouraged by the Kobe patterns, Davis and his team published a preliminary
paper on the idea of seismic lensing in the Bulletin of the Seismological
Society of America.

The paper sparked a controversy: few seismologists believed that the lens
could cause enough magnification to explain the damage in Santa Monica. But in
December last year, Davis published further research demonstrating how a seismic
lens could easily amplify low-frequency seismic waves by a factor of at least
six, and by even more for higher frequencies (Science, vol 289, p
1746).

But Davis鈥檚 group has not been content just to do a few calculations. In
1999, armed with high explosives, they headed back to Northridge to test their
ideas in real rocks. The team set up a dense array of seismometers around the
Northridge region. Then they detonated dynamite charges and pounded the earth
with vibrating 鈥渢humper鈥� trucks to send measurable seismic waves out across the
region. The idea was to listen for seismic echoes from any deep structures that
might have focused the quake on Santa Monica.

鈥淭he data are still coming in on this,鈥� Davis says. Even so, there is already
evidence that seismic lensing can occur. But the seismic experiments also
support an alternative explanation put forward by Graves. Like Davis, Graves
believes that seismic waves will be refracted when they hit the Santa Monica
Fault. But he thinks interference is more important than lensing. Two waves
hitting the fault from slightly different directions could be refracted towards
the same point, he argues. Where they meet, they could interfere with each
other: if the waves are out of phase鈥攖he peak of one coinciding with the
trough of the other鈥攖hey will cancel each other out. But if they鈥檙e in
phase, they will reinforce each other, creating one huge, destructive wave.

This interference effect would account for the distinctive pattern of
destruction seen in Santa Monica, Graves says. Moving south through the city,
the pattern of damage is exactly what would be expected: a line of badly
affected areas parallel to the Santa Monica Fault, cheek by jowl with relatively
unscathed regions. The interference either amplifies or cancels out the seismic
waves, giving this banded pattern of damage. Graves points out that his
explanation has the added advantage of doing away with the need to find a curved
lens of rock.

No one is yet sure of the exact mechanisms behind the amplification and
focusing. Advocates of both theories admit it鈥檚 too early to rule either of them
out, and Graves believes they may both turn out to be correct. Both rely on
local geological features and the refraction of seismic waves. 鈥淭hese hypotheses
are not mutually exclusive and both phenomena are probably taking place,鈥� he
says. 鈥淭he question is which, if either, is the dominant factor.鈥�

This is more than just an academic debate: understanding the quake magnifier
could save lives, according to seismologist Harold Magistrale of California
State University, San Diego. A better knowledge of what happened below Santa
Monica would help seismologists identify high-risk zones in California and
other, geologically similar areas around the world. 鈥淭here are deep sediment
basins all over the place,鈥� he says.

Finding these basins and understanding their effects on seismic waves means
making 3D maps of the geological structures beneath the Earth鈥檚
surface鈥攏ot an easy task. Californian geologists are luckier than most,
because they have access to large scientific and oil-drilling archives for the
region. But even these records aren鈥檛 detailed enough. Peering through the deep
sediments requires expensive, high-tech gravimetric techniques鈥攗sing the
strength of gravity to probe the density of the Earth鈥檚 crust鈥攁nd seismic
reflection studies like the ones Davis and his team carried out in Northridge.
This may still not be enough to pinpoint basin structures, however, because it
leaves too many gaps in the data. 鈥淭here鈥檚 a lot of extrapolation and
guesswork,鈥� says geophysicist Tom Brocher of the US Geological Survey in Menlo
Park, California.

Although progress is slow, seismologists are edging closer to solving the
Santa Monica puzzle, and are now looking for the threat of a similar occurrence
elsewhere. Because they have realised that quakes can reach out and strike
cities that may appear to be safe, seismologists believe there鈥檚 no room for
complacency near a quake zone. Wherever you are, a deadly earthquake magnifier
could be focused on the ground beneath your feet.