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)
|Map of Iran showing peak ground acceleration (m/s2) as a result of the August 11th earthquakes. The city of Tabriz is denoted by the star in the Northwest part of the country. (Click to view larger image.)|
It was Saturday afternoon in Iran's historic city of Tabriz. Located in the Northwestern part of the country, many of the city's 1.4 million inhabitants are Azeris, closely related to the folks in neighboring Azerbaijan. Many people in Tabriz were watching the last events at the London Olympics when shortly before 5 PM local time, the first temblor stuck. Eleven minutes later, the earth violently shook again. The USGS office in Golden, CO, assigned these two quakes magnitudes of 6.4 and 6.3.
Although these two events were rather lightweight on the scale of strong earthquakes, they caused severe destruction mainly in the villages and hamlets around Tabriz and the smaller city of Ahar. At least 250 people were killed and several thousand were injured by the quakes.
Iran belongs to a huge earthquake zone which stretches from Portugal in the West to the Himalaya Mountains in the East. In this zone, three tectonic plates moving northward collide with the large, nearly stationary landmass of Eurasia. In the West, the African Plate pushes against Europe. The eastern section of the earthquake zone is dominated by the collision of the Indian subcontinent with Eurasia. In the central part of this earthquake belt, Eurasia is hit by the Arabian Plate.
The consequences of such a plate collision are similar to a car crash. Just as in a road accident where steel and plastic bend and split, so the Earth's crust deforms heavily where two plates crush into each other. This leads to continuously growing mountain ranges like the Alps in the western part of the belt and the Himalayas in the East. Similar mountains have formed in Iran, for instance the ranges of the Zagros and the Elbrus mountains.
The Arabian plate is drifting northwards with a speed of just over one inch per year. Although extremely slow by everyday standards, this movement is strong enough to cause mountain ranges to be folded and earthquakes to occur. The earthquakes in Turkey, like the one last year in East Anatolia are also a consequence of the tectonic collision between Arabia and Eurasia.
But in few other countries have so many people died in earthquakes as in Iran. According to the Encyclopedia Iranica more than 126,000 people died due to earthquakes in the last century alone. The largest catastrophe ever to strike this region of the world, hit the cities of Damghan and Ardabil in the ninth century, killing more than 350,000 people in two earthquakes. Before Saturday's quakes, the most recent devastating earthquake in Iran occurred in December 2003. The historic city of Bam was leveled then by a 6.6 magnitude quake. An estimated 25000 people were killed in the rubble. (hra079)
This seismogram, recorded at Berkeley's seismic station CMB in the Sierra foothills shows the ground movement associated with the sonic boom generated by the meteor. When the sonic boom reached the station, the ground was slightly pushed down by the pressure wave, hence the strong downward movement in the record. After a few seconds, the ground recovered elastically.
The loud boom, which was heard on Sunday morning around 8 am PDT over large areas of eastern California and Nevada, not only rattled the nerves of many who were rudely awakened by the bang. It also set off house and car alarms, and many frightened people called the police. Astronomers quickly determined that a meteor the size of a washing machine had entered the Earth's atmosphere traveling eastward over the West Coast. It may have been part of the annual Lyrid meteor shower, which peaked over the weekend. Similar to a jet flying at supersonic speed, the bolide's travel generated a sonic boom, which was heard over hundreds of square miles on both sides of the Sierra Nevada. However, the meteor was at least ten times faster than the speediest fighter jets.
The loud bang was also registered by many seismometers in the region, among them the sensitive instruments of the Berkeley Digital Seismic Network and its sister networks operated by the University of Nevada in Reno and by the United States Geological Survey.
How can these seismometers, which are designed to notice the slightest movement of the ground, also pick-up sound? In order to find out why, we need to venture a little bit into the physics of waves. Sound waves and seismic waves are essentially the same. Both belong to a group called elastic waves. A sound wave causes a back-and-forth motion of air molecules. When this vibration reaches our ear drums - or the diaphragm of a microphone - it is sensed or recorded as noise. In a similar fashion, an earthquake wave causes particles in the ground to vibrate, albeit with much lower frequencies than audible sound waves.
While most humans can hear sound waves with frequencies between a few hundred and about 15,000 Hertz (vibrations per second), earthquakes cause vibrations in the infrasound range and below. In general, these frequencies are too low to be heard by the human ear. However, seismometers are designed to operate best in the infrasound range down to extremely low frequencies of a few milli-Hertz. Hence, when the sound's frequency is low enough, it can be registered by a seismometer.
There are two reasons why a loud sonic boom like the one heard on Sunday morning leaves distinctive traces on the seismic networks. Firstly, a sonic boom consists of sound waves of a myriad of frequencies, among them some in the infrasound range. These low frequency vibrations can directly trigger a seismometer.
The other reason is the sonic boom's intensity. As an aircraft - or a meteor - jets through the air traveling faster than the speed of sound, it creates a series of sound waves in front of it and behind it. These waves are slower than the jet itself, because they cannot travel faster than the speed of sound in air - or in simple terms: They cannot get away from the aircraft fast enough. As the jet keeps producing more and more of these sound waves, they get stuck, are forced together and eventually, they merge into a single shock wave - the sonic boom. When it hits the ground, it not only dazzles our eardrums. It also generates a pressure pulse on the Earth's surface which leads to a good sized vibration. That is what gets picked-up by the seismometer
Seismic signatures of sonic booms were studied extensively when NASA's Space Shuttle fleet was still flying. Every time a Shuttle landed on the dry lake bed of Edwards Air Force Base east of LA, its sonic boom was registered by the seismometer network in southern California. More on these unique seismograms can be found here. (hra078)
The magnitude 8.6 earthquake, which struck off the coast of Sumatra last week, was one of those rare megaquakes. It belongs to the group of the ten or so strongest quakes ever measured. As explained in the previous blog entry, scientists were surprised - and certainly breathed a deep sigh of relief - that this quake did not cause a significant tsunami. In fact, hardly anybody was even seriously hurt when the extremely strong seismic waves caused by this quake hit the Indonesian islands closest to the epicenter. But once the immediate danger of a tidal wave was over, seismologists began to realize that this quake and its aftershock of 8.2 two hours later pose an immensely interesting, multifaceted scientific puzzle. Here is why:
These two quakes occurred in the immediate vicinity of the Sumatra subduction zone, where the Indian Ocean plate dives under fragments of the Eurasian plate. The Great Sumatra Quake of December 26th, 2004 had its origin in the same general area. But while this quake, which produced the devastating tsunami seven and a half years ago, had a focal mechanism expected for subduction zones, last week's quakes did not. They were earthquakes of the California type, that is the two flanks of the rupture slid past each other horizontally instead of slipping vertically against each other as in most subduction temblors. Such horizontal strike slip quakes have almost never been observed on the seaward side of a subduction zone. How and why they occurred off Sumatra is puzzle number 1.
The other unexpected feature of this quake should concern everybody in California. Most quakes along the San Andreas and its sister faults also shift in a strike slip fashion. Seismologists have always assumed that such strike slip events would be able to generate strong temblors, which are likely to cause a lot of damage. However, all the scientific evidence collected here in California and in other strike slip regimes around the world suggests that strike slip quakes cannot grow into really devastating megaquakes with magnitudes of 8 and greater. It seems that the horizontal slip fault zones are too weak to accommodate enough tectonic stress to delay rupturing until the breaking results in a megaquake. Well, since last Wednesday researchers have such an unexpected strike slip megaquake in their collection. And now they are scratching their heads as to how to fit these two unexpected aspects of the puzzle into current earthquake theory.
One possible solution was pointed out at an earthquake conference in Memphis last week by Lori Dengler, a Professor of Geology at Humboldt State University and California's foremost expert on tsunamis. She noted that the Great Sumatra Earthquake released a large amount of tectonic stress along its extremely large rupture area. As a consequence the mechanical stress along the fringes of the rupture area grew and put the surrounding rock under an immense strain, with two forces acting in opposite directions. This pattern is similar to the forces acting on the San Andreas Fault. When the rocks finally gave way last week, the result was a strike slip earthquake. And why was it so large? Dengler thinks that the rocks in the hypocentral area were virgin. They had never been broken in an earthquake before - in contrast to the rocks along the San Andreas fault, which have ruptured many times during the past eons. As a consequence, the oceanic crust off Sumatra was able to accumulate an enormous amount of stress. When the rocks finally gave way, the result was last week's megaquake. (hra077)