|Picture 1: The first X-ray photograph|
It is often said that seismologists use earthquake waves to illuminate the interior of the Earth like a radiologist uses X-rays to study the insides of the human body. There is indeed some similarity, as both of these branches of physics are of the same age. The German researcher Wilhelm Conrad Röntgen took his first X-ray photograph in 1896 (see picture 1). Roughly at the same time, the first seismographs were installed and earthquake researchers learned to read the "trembling of the rock" (see blog January 9, 2009).
In a strict physical sense, however, major differences exist between X-ray pictures and seismograms. Through the different shades of grey in the X-ray picture one can clearly distinguish between the wedding ring, the bones, and the soft tissue in the hand. This is due to the fact that metal, the calcium structure of the bone, and the soft tissue absorb electromagnetic waves differently. Metals swallow X-rays almost completely, but the calcium in the bones is not quite as opaque as metal, while the soft tissue lets most of the rays pass.
Although seismic waves experience some absorption in the rocks of the Earth, it is by no means the dominant feature in a seismogram. Look at picture 2. It is a recording of the magnitude 7.6 earthquake in eastern Indonesia on January 4, 2009 (see blog January 5, 2009). The blue line represents 40 minutes of ground movement at our seismic station "CMB" in Columbia in Tuolumne County. Starting from the left, it begins as an almost straight line. Then, roughly at 19:57 UTC, the ground starts to wiggle. There is another shaking at minute 20:02 UTC, a bigger one at 20:12, and an even larger one at 20:17, with the biggest one at 20:31. Each of the arrows marks one of these "onsets" or "phases," as seismologists call them.
What are these phases? As the seismic waves travel from the earthquake focus through the earth, they hit major boundary layers in its interior. At each of such "discontinuities" the waves are reflected, refracted or diffracted. Each of these processes alters the path of the waves, which in turn makes them travel longer until they reach CMB. In addition, different kinds of waves travel through the Earth with different velocities. The P-waves (red arrows) are faster than the S-Waves (green arrows) (see also blog September 10, 2008). In one of the next blogs, we will explore, how these phases can be used to "X-ray" the Earth's interior. (hra027)
|Picture 2: Seismogram showing M 7.6 earthquake in West Irian Jaya.|
|Map illustrating the distribution of BDSN stations in Northern and Central California.|
|Inscription on the entrance to the Göttingen University earthquake station (Photo by Horst Rademacher)|
Recording earthquakes is one of the main jobs of the folks at the Berkeley Seismological Laboratory (BSL), the host of this blog. But measuring those tiny wiggles of the ground caused by temblors near and far is by no means trivial. Compared to reading an electrical current off an ampere meter or the temperature from a thermometer, running a network of seismometers is a rather complex undertaking. BSL operates a network of earthquake stations all over Northern California, from Fresno County in the south to the Oregon border (see map).
Undoubtedly the most important items in this network are the seismometers. Most of these sensors are handmade masterpieces of mechanical engineering. Their function is to pick up even the slightest movement of the ground. Modern feedback sensors are easily capable of registering a millionth of an inch or less. Even the tiny footsteps of a mouse on the concrete floor inside a station can be noticed. These seismometers convert the ground vibrations into electrical voltages. They in turn are tranformed into computer readable bits by a digitizer, the second most important item in a seismic station. The bits are then electronically packaged into groups, which in turn are transmitted to BSL's data center on UCB's campus. Sometimes, the data packets follow circuitous paths before they reach Berkeley. Some travel by satellite; others by phone lines, by radio or simply over the internet.
Once they reach the BSL, the data are checked, analyzed and stored forever on specially secured computer disks. BSL's archive of earthquake recordings is one of the most complete collections in the world, dating back almost 100 years. All of these data are accesible to all researchers free of charge.
And what do scientists do with Berkeley's data? There is indeed a wealth of information stored in those seismograms. Some seismologists use them to study the actual physical processes during an earthquake. Others compute how strongly the ground shook in response to the seismic waves, thereby laying the ground work for the planning by civil engineers. But one of the most important aspects of seismology is the unravelling of the Earth's interior. One of the first professors of seismology in the world, Göttingen's Emil Wiechert, once described this scientific endeavour in very poetic terms: "The trembling rocks bear tidings from afar. Learn to read their meaning."
This sentence, in its original German, is cut into the stone above the entrance to the earthquake station at Göttingen University (see picture). In one of the next blogs, we will see what scientists do to read the meaning from the "trembling rocks." (hra026)
|Map showing Sunday's two large earthquakes on the Bird's Head Penninsula in West Irian Jaya.|
Earthquakes, of course, do not recognize religious or other holidays. Neither do they take notice of the - from a geophysical perspective - completely arbitrary choice of the date of the beginning of the new year. Nevertheless, during the last two weeks, the world's seismicity was exceptionally low, which allowed the blogger to take a few days off. The relative quiescence, however, ended with a bang over the weekend, when Indonesia was struck by two major earthquakes within less than three hours of each other. According to local news reports, at least four people were killed and dozens injured; several buildings collapsed.
The first quake happened shortly before 5 am on Sunday morning (local time) and had a magnitude of 7.6. It was followed by dozens of aftershocks, the strongest of which occured nearly three hours after the main shock with a magnitude of 7.4. On the long term average, only about ten earthquakes of such strength happen worldwide every year r 14, 2008). The latest earthquake sequence originated under Indonesia's eastern province, West Irian Jaya. Geologically this province belongs to the island of New Guinea. The epicenters were located beneath the island's westernmost "Bird's Head Peninsula", which derives its name from its unmistakable shape. The casualties and building collapses were reported from the provincial capital Manokwari, more than 1800 miles east of Indonesia's federal capital Jakarta.
The tectonic situation in this part of the world is complex. Three major lithospheric plates converge in this region and break up into half a dozen microplates, which tend to wiggle under the onslaught of tectonic forces. Under the Bird's Head Peninsula, the Pacific plate is moving southwest with respect to the Australia plate. Compared to its speed in California of about 2 inches per year, the Pacific plate races under New Guinea, moving with a velocity of almost 5 inches per year. According to the USGS, the focal-mechanism of Sunday's earthquakes are "broadly consistent with Pacific plate lithosphere being subducted beneath Australia plate lithosphere." This subduction zone along the northwest coast of New Guinea is characterized by an offshore oceanic trench, the New Guinea trench.
The Tsunami Warning Center in Jakarta (see blog November 14, 2008) issued a tsunami alert immediately after the main shock, but it was revoked within an hour after scientists determined that the quake's epicenter was on land. (hra025)
The Claremont Tunnel through the Berkeley Hills is a major artery of the East Bay Municipal Utility District's water supply system (EBMUD). It delivers drinking water to more than 800,000 people living in East Bay communities from Oakland to Richmond. Damage to the tunnel by a major earthquake on the northern section of the Hayward Fault would cause more than "just" economic losses of $1.9 billion (see blog December 15, 2008). A study commissioned by EBMUD more than 10 years ago concluded that a quake would disrupt water delivery for weeks, reduce fire fighting capabilities, and lead to severe water rationing for up to six months during tunnel repairs. To avoid service disruptions, EBMUD, the tunnel's owner, decided to make the tunnel safe and keep water flowing even if an earthquake of magnitude 7 hits the area.
Unbeknownst to most of us, miners and engineers have been digging through the Berkeley Hills near the landmark Claremont Hotel for the past two years. In June 2006, they started the project with a 480 ft long access tunnel into the fault zone. When that was complete, they added a bypass tunnel parallel to the old tunnel. This 1600 ft long bypass is 10 ft across except in the 100 ft long section where it crosses the fault zone. There it is 17 ft in diameter. This section is also specially reinforced. It has a concrete liner more than 2 ft thick, and more than 20 engineered breaking points along the tunnel which are designed to break and shift during a major earthquake. Here the water is carried through an 85 ft long steel pipe with a diameter of 6 ft and a wall thickness of 3 inches. It rests on pipe guides. (See "before" picture.)
In constructing the bypass tunnel, the engineers assumed the maximum offset during the earthquake would be 8.5 ft or less. If the tunnel lining and the pipe guides shift that much during the quake, the pipe will stay intact (see "after" picture). It will continue to deliver up to 130 million gallons of pure drinking water to users. Above ground, meanwhile, the quake might have wreaked havoc. EBMUD finished the project this summer and its customers west of the Berkeley Hills now have a reliable water supply.
Another water agency, San Francisco Public Utility Commission (SFPUC), which supplies drinking water to the city from Hetch Hetchy Reservoir, will also engage in a major upgrade project. Having completed many smaller improvements to their system, they will open bids next spring for a new, 5 mile long water tunnel under the bay near the Dumbarton Bridge. (hra024)
|The design of the new EBMUD Claremont Tunnel in the Hayward Fault zone. (Picture courtesy of D. Lee, EBMUD.)|
This weekend's rain brought delight to skiers and snowboarders. What fell as droplets in the Bay Area metamorphosed into snow flakes in the Sierra Nevada and its foothills. The snow brought smiles to more faces than just the outdoor enthusiasts'. Officials of the various agencies supplying drinking water to the Bay Area rejoiced in the renewal of the snow pack - and thus also of our water supply. Because most of us drink, shower and cook with melted snow.
Earthquakes pose a major risk to that water supply. The pipelines and tunnels carrying clean snow melt from the Sierras to our houses cross major faults in the East Bay. Even a moderate quake on one of those faults can wreak havoc with these lifelines - and experts predict that your faucets may remain dry for weeks after a really big quake. All the local water agencies are therefore engaged in major seismic upgrades of their infrastructure, be it pipelines, tunnels, or water treatment plants. We all bear the costs for these upgrades through surcharges on our water bills, like the $1.18 per month that is added to the blogger's bill.
Take the example of the East Bay Municipal Utilities District (EBMUD), which supplies water to 1.2 million customers in Alameda and Contra Costa Counties. It spent more than 35 million of its surcharge dollars to upgrade the Claremont Tunnel through the Berkeley Hills. Built in 1929, this 3.4 mi long, 9 ft wide tunnel connects the treatment plant in Orinda with EBMUD's pipe network west of the hills. At peak demand, it can carry 175 million gallons of water per day. The tunnel itself is a sturdy piece of engineering. It would continue to serve well, if it were not for the Hayward fault, which it crosses at an almost right angle 850 ft from its western portal. During the 79 years of the tunnel's existence, the creeping of the fault has caused 13 inches of offset of the reinforced tunnel lining (see red arrows in figure).
However creepy this movement may make the engineers feel, they are even more worried about a major earthquake along this section of the Hayward Fault. The reason: There is a one in three chance that the fault will break in a quake of magnitude 6.7 or greater during the next 30 years (see blog October 10, 2008). In 1994, experts estimated that a quake-caused disruption of the water supply through the Claremont Tunnel would result in economic losses of about $1.9 billion. Read more about how EBMUD made the tunnel safer in the next blog. (hra023)
|Evidence of fault creep in EBMUD's Claremont Water Tunnel. (Picture courtesy of D. Lee, EBMUD.)|