Two events coincide, according to the Merriam-Webster's dictionary, when they happen at the same time. Nowhere does it say however, whether these two events need to be related or not, or if one can be the consequence of the other. That's why we speak of a "pure coincidence" when two completely unrelated events happen simultaneously. An example could be, when my car gets a flat tire in the Caldecott Tunnel at the same time my daughter in Boston wins the lottery. Some pure coincidences can be very unlikely, say that the A's win the World Series in the same year that the Raiders clinch the Superbowl.
Pure coincidence was at play on Saturday morning when two earthquakes occured that had nothing to do with each other. Within 39 seconds, the earth was shaken by two earthquakes of the same magnitude, 6.4. The epicenters of these two quakes were both in the ocean. The first one occured at 8:01 am PST in the Indian Ocean off the coast of the Indonesian island of Sumatra. The other one happened a little bit more than half a minute later in the South Pacific east of New Caledonia near the Loyalty Islands (see map).
|Map showing locations of 2 earthquakes (red and white "beachball" figures) and station station Matsushiro (MAJO) in Japan (blue square).|
On average, about a hundred temblors with magnitudes between 6 and 6.5 occur worldwide every year (see blog September 14, 2008). If they occured regularly, that means if there were some form of physical relationship between them, we would get about two of them per week. But earthquakes here in California are in no way connected to temblors in Japan or terremotos in Chile. In contrast to many a lore, there is no long-distance force associated with one quake that could trigger another equally strong earthquake on another continent.
|Seismogram at station Matsushiro in Japan from November 22 events. Click to view larger image. Data taken from IRIS datacenter|
Saturday's two epicenters were 4836 miles apart. If such a hypothetical trigger were to exist, it would have had to traverse this distance in 39 seconds, which corresponds to a speed of roughly 450,000 mph. No tectonic force acts with such a high velocity.
The two coincidental earthquakes made for interesting seismograms, because the seismic waves of the two earthquakes were intermingled, making it difficult for seismologists to identify which phases belong to which quake. An example of the recordings of the seismic station Matsushiro in Japan is shown in the figure, labelled with the correct phases. (hra019)
How strong is an earthquake? When you stand in a hallway and the shaking begins, some people may lose their balance and fall, while others simply walk away - albeit with a very heavy Texas swagger. Or, in a different temblor, while some buildings might just suffer cracks in the plaster, others will lose their chimneys or hop off their foundations. These examples show that looking at the effects of earthquakes is certainly not an objective way to measure the strength of a temblor. But how do you go about determining the size of a quake if everything around you is shaken as badly as you are?
Seismometers are the tools of choice. They swing in their own rhythm in reaction to vibrations of the ground. And because their response to different kinds of shaking can be described exactly with mathematical formulas, the output of a seismometer is a good way to determine the strength of the shaking. But there is the crux of the matter: How does the strength of the shaking relate to the size of an earthquake? A medium sized earthquake close by can cause the gound to shake more severely than a stronger temblor farther away. Therefore, the distance between the focus of a quake and the location of the seismometer plays an important role in measuring the strength of a quake.
It was almost 75 years ago that two seismologists found a way to overcome these problems. Charles Richter and German-born Beno Gutenberg were both working at Caltech in Pasadena, and their engineers had set up a unique network of identical seismometers in Southern California. They equipped each site with a device called a Wood-Anderson Seismograph. When Richter and Gutenberg looked at one earthquake which was registered at several sites, they could see how the amplitude of a seismic wave decayed with distance from the earthquake source.
After giving their findings some thought, they came up with a simple definition: If an earthquake caused their seismographs to wiggle 1 millimeter (about 1/25th of an inch) and the temblor was 100 km (60 miles) away, then it had a strength of "three." A wiggle ten times that size was called "four," and a "five" was 100 times as strong as a "three," generating wiggles about 4 inches tall. Because Richter was inspired by the astronomers' scheme to classify the brightness of stars, he called his units magnitude, hence the phrase "magnitude on the Richter Scale." In a future blog, we will explore the limits of this logarithmic scale. (hra018)
|Tectonic setting of the earthquake that generated the devastating December 2004 tsunami. Click to view larger image. (Image courtesey of the USGS)|
It took a disaster of almost biblical proportions, but finally the Indian Ocean got its own tsunami warning system. Earlier this week, when Indonesia's President Yudhoyono pressed a button in the offices of the Meteorological and Geophysical Survey in Jakarta, he inaugurated a system which will be used to warn people in the coastal communities of Indonesia about the menacing monster waves. While for decades several such systems have existed in the Pacific (i.e in Hawaii, Alaska and Japan), there was no way to warn the people along the coast of the Indian Ocean when the biggest earthquake in forty years struck the area in December 2004. A magnitude 9.2 quake off the coast of the Indonesian island of Sumatra created the largest tsunami the world has seen in historical times. Even thousands of miles away from the epicenter, entire coastal communities were swept away by the killer wave. In the end, the death toll reached more than 230,000 and the physical damage to buildings and infrastructure could hardly be measured.
|German Indonesian Tsunami Early Warning System (Image from www.gitews.org)|
In the aftermath of the disaster, many nations generously donated aid to cover the immediate needs of the people affected by the wave. The German government, however, took a different approach. Besides the first aid, it promised the Indonesian government a complete tsunami warning system. Many experts laughed at this proposal, as German science was not known for its expertise in studying these waves. But Berlin put its researchers and engineers to work anyway. At a cost of almost 60 million dollars, a total of 125 scientists put together a complete system in less than four years: they set-up earthquake and GPS stations remote parts of Indonesia. Together with local engineers, they installed tidal gauges and ocean bouys to measure wave heights. In addition, they linked all their sensors to the data center in Jakarta and wrote the programs which calculate the run time and amplitude of a potential tsunami. Finally, they trained local scientists to operate and maintain the system.
Germans and Indonesians tested the system together in many simulated sessions, and most international experts agree that its warnings will be valid. Once it's fully funtional, the system will be upgraded to cover the whole Indian Ocean basin. See a detailed description of the system. (hra017)
Can something that happens in Bombay Beach, that tiny hamlet at one end of the San Andreas Fault (see blog Nov 4, 2008), have an impact on the bustling megacity of Los Angeles more than 150 miles away? A sandstorm, perhaps? Or a flood in the Salton Sea? Well, if you believe more than a dozen seismologists from the Southland, when certain things happen in Bombay Beach, Tinseltown and all its suburbs will be shaken up rather badly. Coordinated by Lucille Jones from the US Geological Survey's Pasadena office, these scientists have looked at the most likely scenario for a really big earthquake in Southern California. They found that the San Andreas Fault could start breaking at its southern end near Bombay Beach. So much tectonic energy is currently stored in this section of the fault that once the rupture starts it might not stop for more than 2 minutes. During that time, the rupture front of the earthquake will travel at the incredible speed of 2 miles per second, emitting seismic waves all along the way. Destructive waves in front of the rupture's head will build up like the bow wave of a boat.
When the rupture finally stops, the ground in the immediate vicinity of a 120 mile long stretch of the San Andreas Fault will have shifted by up to 44 feet. Dozens of freeways, railway lines and utility tunnels will be destroyed where they are offset. All over the Southland, the strong shaking associated with the seismic waves of this magnitude 7.8 earthquake will have done more than just rattle the 22 million people living in the region. Countless houses, overpasses and dams will have be badly damaged or destroyed. Power and water will be out for weeks. Without a doubt, this earthquake will be the deadliest and costliest natural disaster ever to hit the United States.
A scenario like this is more than just an academic exercise. The probablity that a temblor of this magnitude will happen in Southern California in the next 30 years is calculated to be almost 100 percent. And what is being done to deal with the inevitable? This Thursday at exactly 10 am PST, hundreds of government agencies, schools, private businesses, hospitals, nursing homes, and many residents will join the biggest earthquake drill this country has ever seen. While thousands of people "duck, cover and hold on" as part of "The Great Southern California ShakeOut", the state's Office of Emergency Services and other public and private entities will practice their response to a scenario based on the M 7.8 earthquake described above. The lessons learned down south will not only apply to Southern California. Although a big earthquake in the Bay Area is currently considered to be less likely than in LA (see blog Oct 10, 2008), our region will - without a doubt - someday be shaken at least as badly. While such an earthquake is inevitable, damage to you and your family is not. Learn about how to protect yourself and your home by following the tips on the ShakeOut website. (hra016)
|Mud pots like this may define the southern end of the San Andreas Fault (Photo: Horst Rademacher)|
For 800 miles - give or take a few - the San Andreas Fault runs through almost the whole length of our Golden State. On the one end, it stops at Cape Mendocino, where the Gorda Plate prohibits its extension to the north (see Seismo Blog from October 27, 2008). The other end is also exactly known: All geologic evidence for the fault trace at the surface vanishes about 2.5 miles northwest of Bombay Beach, an isolated hamlet with a population of less than 400 on the east shore of the Salton Sea in Imperial County. To the northwest of this point, the fault is not only visible on the surface, but also defined by thousands of microearthquakes at depth. Southeast of the vanishing point, however, the seismicity along the strike of the San Andreas Fault stops abruptly, only to jump several miles west to the Imperial Fault, which connects California tectonically south to the Sea of Cortez.
|The southern end of the San Andreas Fault system. MP marks the location of the mud pots in the Imperial Wildlife Area. SAF - San Andreas Fault; BSZ - Brawley Seismic Zone; IF - Imperial Fault. Map adapted from SCEC|
That at least was the generally accepted view until a few months ago, when two earth scientists from the US Geological Survey office in Pasadena published their study on a very murky subject. For several years, David Lynch and Kenneth Hudnut had endured the sizzling heat and the choking sand storms in the desolate Imperial Wildlife Area along the southeastern shore of the Salton Sea, way south of Bombay Beach. This place is probably most famous among bird watchers, who flock here when migratory waterfowl use the region as a resting place on their Pacific flyway. But the area also contains hundreds of circular holes in the ground, none of which is man made. Instead, gases from the Earth's interior, mostly carbon dioxide and water vapor, hiss from the center of the holes or bubble through pits filled with muddy ground water. Mud pots or mud volcanoes, as these features are known in geologic terms, are in fact so abundant in this out of the way place that the CO2 eminating from them was once captured industrially for the production of dry ice.
The two researchers took a detailed inventory of all mud pots in the wildlife area, recorded their gases, and observed the way they bubbled and ejected their gas-filled liquid mud. They also measured the exact location of each mud pot with a GPS receiver. When Lynch and Hudnut finally plotted all their mud pots on a map, they noticed something peculiar: Most of them lined up as straight as a ruler. Their alignment was also exactly an extension of the San Andreas Fault, whose currently known end lies almost 20 miles to the northwest of the wildlife area. It may very well be, concluded the researchers, that the most famous earthquake fault in the world ends unspectacularly in a field of bubbly mud, too weak to generate any significant temblors in that area. (hra015)