|There were two sizable earthquakes just south of Lake Tahoe in the Sierra Nevada Mountains within 24 hours of each other. They are represented as the two blue squares. Click to view a larger image.|
By California standards the Sierra Nevada cannot be considered a seismically very active region. After all, no plate boundary defining fault, like the San Andreas, runs through it. But two earthquakes, one with a magnitude of 4.0, the other a 3.7, which occurred within less than 24 hours and within a few hundred feet of each other are a stark reminder that our famous granitic range is by no means geologically dead. The two quakes occurred in an uninhabited area halfway between Lake Tahoe and Mono Lake just southwest of the point where Highways 89 and 395 meet. The first one jolted the region 21 minutes after midnight on Thursday, the second one occurred about 15 hours later. More than a dozen very small aftershocks were recorded in the intervening time.
Geologically speaking, the Sierra Nevada is a very young mountain range. Its granitic core is a pluton, which was originally a giant blob of magma that eons ago rose from the Earth's interior but never reached the surface. Instead, it got stuck in the Earth's crust, where the magma slowly cooled. Several million years ago, long after the magma had frozen into solid granite, some forces from below pushed this pluton slowly upwards. It began to rise above the surrounding landscape until it reached an elevation of almost three miles. Then erosion slowly began to raze the tops. Still, with Mount Whitney as the highest peak in the conterminous United States, the Sierras are an impressive range.
The uplifting of the pluton, however, did not occur evenly, because its eastern section rose much faster than the western part. The result is a strongly tilted mountain range. This tilt can still be seen today. When you drive east on highway 120 through Yosemite it takes hours before you reach the high point at Tioga Pass. But beyond the culmination, you drop down into Mono Lake Basin within a few dozen minutes while the road hugs some very steep cliffs.
The asymmetry of the upward movement also had other consequences. Along its eastern front a major thrust fault developed, which still causes earthquakes in the Owens Valley (refer to blog October 5th, 2009). In addition, volcanoes erupted during the pluton's uplift. The Long Valley Caldera as well as the Mono and Inyo Craters are clear examples of this volcanism. The youngest of these craters are only 500 to 600 years old - a mere blink of the eye on the geological time scale. And in the Long Valley Caldera near the town of Mammoth Lakes, hot springs and earthquakes are a reminder of its violent past. However, the volcanism is confined to a narrow zone along the eastern margin of the Sierras.
If volcanoes and the thrust fault reign only in the eastern section, how then, you may ask, are earthquakes generated in the rest of this vast, sturdy-looking range? The answer is actually pretty simple: Because the tectonic uplift of the massive pluton was so uneven, it was twisted and bent during its journey. These internal contortions set many parts of the pluton under mechanical stress. It is released today, after millions of years, through earthquakes like the ones which occurred on Thursday. (hra084)
|Damage after the 2009 L'Aquila earthquake (Image: Wikimedia Commons)|
A few days ago, when a court in the Italian town of L'Aquila found six seismologists guilty of manslaughter and sent them to six years in jail each, Earth scientists all over the world were shocked. How can seismologists, who know very well that earthquake prediction is at best an imperfect science, be held responsible for not forecasting an earthquake? What effect will this verdict have on the future of earthquake science, and on the way seismologists interact with the public? Well, the story is not as easy as it seems at first glance.
Here is the background: On April 6, 2009 a moderate earthquake with a magnitude of 6.3 hit the Abruzzo region of Central Italy. More than 300 people were killed in the medieval town of L'Aquila alone and up to 10,000 buildings in the region were damaged or destroyed. During the weeks before, a number of smaller earthquakes had rattled the same area and a committee of eminent Italian earthquake specialists was convened to assess the situation. This National Commission for the Forecast and Prevention of Major Risks met in L'Aquila shortly before the earthquake. It is not known what the group discussed in its closed-door meetings. But in a news conference their members reassured the public that there was nothing to worry about, even though the string of smaller earthquakes continued to rattle the nerves of the people in the region.
The situation was made more complicated by a technician working in a government laboratory located near L'Aquila. He had observed that the concentration of radon in some groundwater wells had changed during the foreshocks. Because he had read somewhere that such a change may be an indicator for an upcoming quake, he went public and predicted a major disaster, even though he was not a seismologist and had not discussed his findings with any experts.
In its verdict the court said, that the members of the Commission had provided ``inaccurate, incomplete and contradictory'' information about the danger heralded by the tremors felt before the 6 April 2009 quake.
At a cursory glance, the verdict seems to punish scientists for not predicting the earthquake. Unfortunately, many media and much of the public reaction did not take a deeper look at what the court really said. The judges actually chided the commission not for their failure to predict an earthquake – but actually for just doing the opposite. In their public statements the commission did not focus – as they should have - on the well known seismic hazard in the Abruzzo area and the associated high risk given the many unreinforced, medieval buildings in L'Aquila. The court found the members guilty, because of their reassuring, risk-denying public statements. This gave the populace, according to the verdict, a false sense of security that led many people to stop expecting and preparing for a major earthquake.
The reaction of seismologists all over the world to this verdict was by no means unanimous: It is clear that at the current state of Earth science, earthquakes cannot be predicted with any degree of reliability. In some cases, radon levels have changed before an imminent quake; in many others, the radon concentration in groundwater did not change at all. The same is true for all other indicators which have been investigated as possibly having predictive value for earthquakes: None of them have worked reliably.
But no seismologist should use this lack of predictability to downplay the risk associated with earthquakes. For the very reason that destructive temblors can happen anytime in seismically active regions – be it in Japan, California or in Italy's Abbruzi - serious Earth scientists will always have a cautionary message for the public: Even though we can't tell you exactly when a quake will happen, you should not panic, but you must be prepared for a big one to strike at any time. The happy-go-lucky attitude apparently presented by the members of the Italian commission is not the right response. One can discuss whether a verdict of six years in jail is too harsh a term, but there is no doubt, that the commissioners acted irresponsibly in their public statements before the L'Aquila earthquake. (hra083)
|Kenzo Ito, anchorman at NHK, providing earthquake early warning information and response instructions to viewers on March 11, 2011 (courtesy wsj.com) View full video.|
One expects TV anchormen to stay cool under fire, whether in a heated political interview on live television, or when they report from a war zone. But the action of Kenzo Ito, an anchorman on the Japanese network, NHK, during the largest earthquake to strike Japan in modern times was outright übercool.
During the afternoon debate in the Japanese parliament, known as the Diet, which NHK carried live on March 11, 2011, the giant magnitude 9.0 Tohoku Earthquake hit. Because the epicenter was a few hundred miles from the Parliament Building in Tokyo, the members of the Diet were not aware of the strong shaking in the northern part of Honshu which started when the initial earthquake waves arrived at the coast.
However, several dozen seconds before the seismic waves were due to hit Tokyo, the NHK studios and their anchorman received a nationwide automatic earthquake warning, which had been issued by the computers of the Japanese Meteorological Agency, the official earthquake monitor in Japan. At first Ito's voice overrides the parliamentary debate, stating that strong shaking is expected in the northern prefectures of Honshu. Then the cameras switch from the Diet to the studio. In a calm voice he tells the TV viewers repeatedly to turn off gas stoves and take cover. Then, as a low rumble is heard in the background, the anchorman proclaims, still utterly calm and composed, that shaking has reached the TV studios in Tokyo. One can see how he holds on to his desk, still advising the viewers to stay calm and seek cover from falling objects. Finally, the producers switch to cameras outside of the studio which rattle due to the seismic waves, giving a wild impression of the shaking of buildings throughout Tokyo.
The warning went not only to the TV studios, but millions of Japanese received the alert as well, on their cell phones, on billboards, via the radio and if properly equipped, on networked computers. While the Japanese system is by far the most advanced early warning system, similar Earthquake Early Warning (EEW) systems have existed for years in Taiwan and in more rudimentary forms in Turkey, Mexico and Romania (The map below shows EEW around the world).
|Earthquake early warning systems throughout the world (courtesy R. Allen, BSL). (Click to view larger image.)|
Why is California, a state with a large potential for being struck by a devastating quake, lagging so far behind? After all, it was only in late September that Shake Alert, the EEW system for our State, was officially introduced and only as a demonstration system.
The answer to why other countries are so far ahead, is very simple: We do not have more than a prototype because of a lack of political will on the Federal and State levels. Many attempts over several years have been made by seismologists from different institutions to push for an EEW system but so far, their pleas have fallen largely on deaf ears in Washington and Sacramento. Although there is no doubt about the earthquake risk for all states along the West Coast, at this point, no influential politician has taken up the cause to establish an EEW.
What is actually needed to push Shake Alert beyond its prototype stage and roll it out as a fully operational system for the West Coast? The seismic networks must have more densely installed instrumentation and the telecommunication lines between the field stations and the data centers in Berkeley, Menlo Park and Pasadena must be hardened against possible disruptions in the event of an earthquake. The cost for these improvements is estimated to be roughly $150 million. This is peanuts given that fact that an EEW could save thousands of lives and help to protect people living under constant seismic risk. (hra082)
Every once in a while some computers in the offices of the Berkeley Seismological Laboratory (BSL) seem to take on a life of their own. A window with a map of California suddenly pops up, from the speakers blares a nasty siren and a computer generated voice proclaims: Shaking expected in 30 seconds. In the meantime two concentric circles, one in yellow and one in red, slowly expand over the whole map (see figure). Whenever this happens, BSL scientists get excited, because their prototype earthquake early warning system has detected another potentially damaging tremor.
|Scenario earthquake alert on the User Display for the Loma Prieta earthquake. The yellow circle shows the progress of the P-wave and the red circle shows the S-wave.|
Developed jointly by researchers at Berkeley, Caltech in Pasadena and the Swiss Federal Institute of Technology in Zürich, this fully automated system, dubbed Shake Alert, is designed to give people up to few dozen seconds of warning time before the seismic waves rock one's location. In most cases, this warning time is long enough for people to take protective measures (drop, cover and hold on) or even leave the building before the shaking starts. Currently a few organizations like BART use the warning to automatically slow down trains to a safer speed.
As mentioned in the previous blog, the warning is possible when seismic stations very close to the quake's epicenter detect a temblor and relay the recording with the speed of light to the data center. As seismic waves are much slower than light, the data outraces the shaking. The further away the quake, the longer the warning time.
While Shake Alert seems to work reasonably well on a lab scale and for a few customers, it is by no means ready to be rolled out as a public warning system. On the one hand, there are too few earthquake stations in critical areas like northern or central California. This is where a truly great earthquake along the San Andreas Fault will most likely initiate. Another reason is that the data transmission network is not nearly as robust as BSL scientists want it to be. In many cases, the data transmission from the station to the data center has no redundancy and is prone to failure in a strong quake. "Technically, these bottlenecks can easily be solved," says BSL's director Richard Allen. "What's lacking are the funds to improve the network." In order to make Shake Alert a reliable public system for all of the West Coast (Washington and Oregon can have strong earthquakes too), an investment of about $150 million would be necessary.
And what happens when an alert goes public? Californians need to be trained and understand, what to do in these critical seconds between the time the warning is issued and the shaking starts. "We need a major public education campaign," says Peggy Hellweg, BSL's operations manager and one of the principal investigators for Shake Alert. "To start and execute such an endeavor goes far beyond the capacity of our small lab," says Hellweg.
To watch a simulated warning of how Shake Alert would have worked during the 1989 Loma Prieta quake click here. (hra081)
|BART train in a station. Photo courtesy Richard Allen|
|BART train control center (Click to view larger image.) Photo courtesy of Richard Allen|
No matter how often you ride BART, at one point you will have asked yourself what will happen to these slick blue and silver trains when a major earthquake strikes. For years, the BART system has been equipped with accelerometers distributed throughout the network. Once these sensors register strong ground shaking, a signal is automatically sent to each train to slow down. Since early August, the transit system has taken these precautionary measures to a completely new level. BART has teamed up with the Berkeley Seismological Laboratory (BSL) and other institutions in using an earthquake early warning system to slow down the trains before the shaking actually starts.
If this sounds like magic to you, consider the physics of propagating seismic waves. P-waves race through the upper layers of the Earth with a speed of roughly 3.5 miles per second, or 12,600 miles per hour. Although S-Waves are somewhat slower, at 7,600 miles per hour they still beat the fastest fighter jets. However, compared to the speed of light (approximately 700 million miles per hour) the velocity of seismic waves is just a crawl. The secret of Earthquake Early Warning (EEW) is to use this considerable difference in speed to detect a tremor before the shaking starts.
Here is how it works: The BSL, Caltech and the US Geological Survey operate a dense network of seismic stations throughout California. All those stations transmit their recordings over cables, radio or satellite telemetry in real time to data centers in Berkeley, Menlo Park and Pasadena. This data transfer happens electronically, that is with the speed of light. Now let's say a strong earthquake starts to rupture the northern section of the San Andreas Fault. The seismic sensors along the North Coast detect the shaking associated with this rupture and instantaneously report it to the data centers. There, dedicated computers immediately calculate the location of the earthquake, its strength and the peak values of ground motion. Although this whole process might take several seconds, there is still ample time before potentially destructive S-waves from this quake hit the Bay Area and cause damage. The reason: It takes more than one minute for S-waves to travel the distance between Cape Mendocino and downtown San Francisco.
The BSL data center is linked via the internet to computers in BART's operations center in Oakland. As soon as BART receives the warning about possible strong shaking, their computers automatically send messages to all trains to slow down either to a complete stop or at least to a "safe" speed of 26 miles per hour. The next time, you are in a BART train and it doesn't go as fast as you are used to, there maybe an earthquake coming.
In one of the next blogs, we will explain more about "Shake Alert" California's new earthquake early warning system. (hra080)