|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.)|
|Highway 14 roadcut in Southern California exposes rocks squeezed by the San Andreas Fault at the roadcut's southern end (Photo: Horst Rademacher)|
|The San Andreas Fault at Tejon Pass (Photo: Horst Rademacher)|
Thousands of people drive by every day; millions have passed through without even noticing. There are only very few places in California where the San Andreas Fault, our most famous earthquake line, can be spotted at the Earth's surface. In two of these locations, the fault crosses a major freeway. Both places are in Southern California. The most famous of the two lies just south of Palmdale in the northeastern part of Los Angeles County. Immediately north of its S-Avenue exit, Highway 14 passes through a roadcut in a small ridge. The eastern wall of the road cut is almost 90 feet high and beautifully exposes folded layers of a few million years worth of old rocks containing crystals of gypsum (see picture 1). The rocks are bent and squeezed, because they are wedged between the San Andreas Fault at the southern end of the road cut and a minor fault at the northern end.
Even more drivers pass through the second crossing every day. It lies at the high point where Interstate 5 winds through the Transverse Ranges. A road cut on the west side of the freeway just at the top of Tejon Pass exposes the fault in unsurpassed clarity. There a greyish layer gives way to brownish soil. The two layers are separated by a straight line - the San Andreas Fault itself (see picture 2).
There is some exposure of the fault along Northern California freeways as well, albeit not nearly as clearly as in the south. On the Peninsula west of San Mateo and Burlingame, Interstate 280 runs through a linear valley several miles long. This is actually the expression of the fault. Two elongated lakes, Crystal Springs Reservoir and San Andreas Lake lie in this valley - the latter lends its name to the fault, which runs for 800 miles through our state.
This blog wouldn't be complete without a safety warning: When you pass any of the fault crossings, don't be tempted to stop on the freeway. It is not only illegal, but also very dangerous due to heavy traffic. All locations are near exits and can be accessed by frontage roads. The best spot to view the fault along Interstate 280 is the vista point accessed from the northbound lane. The vista point is dominated by a statue of Father Junipero Serra. (hra022)