|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Warning Center in Jakarta (see blog November 14, 2008) issued a tsunami alert immediately after the main shock, but it was revoked within an hour after scientists determined that the quake's epicenter was on land.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)
Many of us still dread our high school days, when the math teacher talked about logarithms. Who understood then and who can remember now the alien concept of grouping numbers not in their natural sequence, but by some funny exponent applied to an arbitrary base. Well, here is a quick refresher on the simplest logarithms, those of base 10. Look at the number 100 and ask how often you have to multiply the number 10, our base, by itself to get 100. The answer is 2 - and that is the logarithm of 100. If you look at the number 1000 and ask again how often you have to multiply our base number 10 by itself to get 1000, the answer is 3. Hence the logarithm of 1000 is 3. You therefore can use logarithms to easily deal with really big numbers, as the logarithm of one million is six and the latest bail-out figure for Wall Street, 100 billion, is reduced to a mere 11 when you determine its logarithm.
Richter scaled his earthquakes using such a logarithmic method (see blog November 18, 2008). A magnitude 3 earthquake therefore has ten times larger amplitudes than a 2; a magnitude 4 is ten times bigger than a 3 and hence one hundred times bigger than a 2. In Richter's day, the seismograph's recordings were on light sensitive paper, which was illuminated by a light beam artfully reflected from the moving mass of a Wood-Anderson seismograph, its mirror. Once the film was developed, the ground movement was represented by a dark line on that paper.
But even with a magnifying glass you were not able to see fine details of the ground movement. As a consequence, with the original Richter scale it was impossible to classify either the really big quakes (see blog December 1, 2008) or the tiniest microtemblors.
With today's electronic seismometers, however, the lower limit of the first Richter scale has been all but eliminated. These instruments are so sensitive that they can register earthquakes a hundred or even a thousand times smaller than an event of magnitude 2. But how do we label such tiny quakes? That is again where the ingenuity of a logarithmic scale comes in. An earthquake ten times smaller than a 2 would have a magnitude of 1; a hundred times smaller would be zero on the logarithmic scale. And if an event is thousand times smaller, its size would be "minus 1" on the Richter scale. And we do indeed measure such nanosized earthquakes regularly, for instance in a borehole, which penetrates the San Andreas Fault deep below Parkfield in Monterey County (see picture). (hra021)
|Recording of a magnitude minus 1 earthquake, measured inside a borehole in Parkfield, CA. (Click to view larger image.)|