|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)
In the middle of the 18th century, Lisbon was one of the five largest cities in Europe. As a harbor town, Portugal's capital was an important hub in the trade between the Old World and its colonies. The 275,000 inhabitants of this proud and extremely wealthy city were devoutly Catholic. No wonder, that all of the 40 cathedrals and churches in town were filled to the brim on All Saints Day 1755 - 253 years ago today. During mass, exactly at 9:40am, the city was suddenly violently shaken by seismic waves generated during the biggest earthquake the European continent has seen in historic times. Thirty churches, many palaces and countless houses collapsed. Those people who were not killed outright or trapped in the rubble, ran down to the quays along the Tejo River - only to be swept away by one of the largest tsunamis ever generated in the Atlantic Ocean. Its 20 foot waves destroyed everything in the lower part of the city; fires consumed what was left standing on the hillsides. The casualties of this quake and its aftermaths were never counted, but at least 60,000 people lost their lives. Today we know that this quake had a magnitude of almost 9, stronger than any temblor California has experienced since Spanish settlers arrived here.
Given the strong religious beliefs of the population, it was easy for the survivors to assume God had struck the city with his wrath. Why else would a devastating earthquake occur exactly during mass time on All Saints Day? Only one explanation fit the moral compass of the time: God punished Lisbon for the sinful and immoral lifestyle of its inhabitants. That was indeed the way all natural disasters were explained two and a half centuries ago.
|Immanuel Kant's compendium on the great Lisbon earthquake.|
But the Lisbon quake was about to change that fundamentally. Voltaire, one of the most brilliant figures of the European Period of Enlightment, was shocked by so much devastation. He wrote a poem "Poème sur le désastre de Lisbonne," in which he argued that forces other than God must have been responsible for such a disaster. Within a year, Germany's foremost philosopher, Immanuel Kant, had collected all available accounts of the quake and its effects and published them, together with his comments and conclusions. In his comprehensive report, written in German, he concluded that the source of the earthquake lay in cracks and caverns in the Atlantic Ocean. This impressive scientific compendium (small picture) is considered by many the beginning of the systematic observation of natural phenomena and the start of modern seismological research. Kant was truely the first Seismo Blogger. (hra014)
Imagine a busy intersection with a three-way stop sign at rush hour. It takes only a few irresponsible drivers who don't want to wait their turn and traffic jams and accidents are likely to occur. Something very similar is happening every day in the Earth's crust under Cape Mendocino. There upheaval is caused not by impatient drivers but by tectonic plates, which are crashing into each other with unstoppable momentum. This tectonic three-way intersection is called the Mendocino Triple Junction. It is one of the few places in the world where three of the gigantic plates which continuously drift on the Earth's mantle meet.
Along the length of most of California, the San Andreas Fault defines the boundary between the Pacific Plate to the west and the North American Plate to the east. Along the fault line, the Pacific Plate slides horizontally in a northnorthwesterly direction with respect to North America (see blog October 17, 2008).
Under Cape Mendocino, this trip comes to an abrupt halt, because another plate gets in the way. North of the Cape the off-shore Gorda Plate, a small cousin of the Pacific Plate, glides under the North American Plate in what is called a subduction zone. The boundary line between the Pacific Plate and the Gorda Plate is a fault similiar to the San Andreas, called variably the Mendocino Transform or the Mendocino Fracture Zone. It goes from Cape Mendocino for several hundred miles due west into the Pacific Ocean. Along this fault, the Gorda Plate slides horizontally to the east with respect to the Pacific Plate with a speed of about 2 inches per year.
Because of the different directions and the various rates of movement of the three plates, the area under Cape Mendocino and a region immediately off-shore has to absorb a lot of mechanical strain. That makes this area one of the most seismically active regions in the state. During the last three decades alone, two strong earthquakes with magnitudes above 7 have occured there, one in 1980 and the other one in 1994. Every year at least 80 temblors with magnitudes over three are recorded in this region, like the earthquake duo which shook the area early on Sunday morning.
Another tectonically important triple junction is the Afar Triangle, where the Golf of Aden and the Red Sea meet the northern end of the East African Rift Valley. The Galapagos Islands are also located on a triple junction. Under the islands made famous by Darwin's research, three oceanic plates meet: Pacific, Nazca and Cocos. There is however a major difference between the Galapagos Triple Junction and the three-way intersection under Cape Mendocino. While under Northern California the plates crash into one another, the three plates under Galapaos are moving away from each other. (hra013)
It was a balmy morning reminiscent of Indian summer 140 years ago today. Many of the 260,000 people who lived in the Bay Area in the late 1860's were already busily engaging in their day's activities. Their hustle however came to an abrupt halt, when at seven minutes before 8 am, the Earth under a 900 square mile region, from Gilroy and Santa Cruz in the south to Santa Rosa in the north, began to shake violently. The Hayward Fault had gotten loose and shook off its tectonic stress in an earthquake with a magnitude of almost 7.
|Alamada County Courthouse in San Leandro (photo courtesy of Bancroft Library)|
When the shaking was over, nearly every building in Hayward was extensively damaged or completely wrecked, while in San Leandro, the second floor of the Alameda County courthouse collapsed and many other buildings were destroyed. The most famous victim of the quake's fury was Mission San Jose, at that time almost 100 years old. Its adobe walls crumbled and the church was left in ruins.
While Hayward and San Leandro bore the brunt of the seismic shaking, major damage also occured on the other side of the Bay. Indeed, the destruction in San Francisco was so large that the temblor became known as the "Great San Francisco Earthquake." This honorary title was removed four decades later, when a bigger earthquake came along - the disaster on April 18, 1906, when most of the City went up in flames.
During the Hayward quake 140 years ago today, the ground cracked in a straight line that could be traced for nearly 20 miles from San Leandro south to Arroyo Agua Caliente. And unlike the modest cracks in the Hayward parking lots today, this was a huge gash along which the flanks of the fault slipped past each other about six feet.
|Damage to flour mill (drawing courtesy of Bancroft Library)|
Were a similar earthquake to occur today, the consequences in damage and human life would be immensely greater and far more serious than those of Hurricane Katrina. The "1868 Hayward Earthquake Alliance," a private-public consortium of 123 organizations raising awareness about the imminent danger lurking under our feet, has put together studies and scenarios by various researchers and modelers about the potential earthquake's frightening aftermaths: The immediate commercial and residential economic losses, including damage to private buildings and their contents would surpass $120 billion. Thousands of people might be unemployed due to damage to commercial and industrial buidlings, and at least 90,000 residential units would be destroyed, displacing almost 220,000 residents. Major transportation infrastructure, such as Interstates 580 and 880 and the Caldecott Tunnel, are likely to be unusable for months. BART and the major rail lines will be affected and might not run for weeks. Water tunnels and pipelines supplying drinking water from the Sierra to the Bay Area are likely to break and leave residential and commercial spigots dry for days or weeks.
The following statement sounds ridiculous, but bear with the blogger anyway: Assume you could stand on the observation platform on top of Mount Diablo for a few million years. If you looked westwards towards the City and the Pacific Ocean during that time you would see the whole landscape slide by slowly. San Francisco would move to the north, Oakland, the Berkeley Hills and even Walnut Creek, the city right at the foot of Mount Diablo, would move a considerable distance to your right. Now imagine you were to cross the Bay and climb to the top of Mount Tamalpais. Again you camp out there for a million years or so, but during the whole time you look eastwards across the Bay. A similar picture would emerge: Mount Diablo would slide towards the south, the East Bay Hills and all the cities at their foot would also move towards your right.
The cause for the movement is the slipping between the Pacific Plate and its continental counterpart, the North American Plate. They slide past each other horizontally, with the Pacific Plate moving northnorthwest with respect to North America. The direction of the movement depends solely on your point of view. When you stand on Mount Diablo, you are fixed on the North American Plate and only the Pacific Plate seems to slide. However, anchoring yourself on the Farallon Islands, which are on the Pacific Plate, it seems as if the continent would slide by you. But no matter which side you stand on, the other side always slips to your right.
We scientists, of course, are way too impatient to wait for a million years to see the effects of the plates' endless drift. However the same satellite based GPS-System, which helps you navigate around town, enables us to measure the plate motion directly and with high precision. Our GPS instruments are considerably more expensive than the navigation system in your car, but they allow us to pinpoint a location with a precision of less than an inch. In contrast, a typical car nav-system knows its location with a much cruder accuracy of several yards.
By visiting several dozen fixed spots in the Bay Area a few times per year with these precision GPS-tools, or measuring continuously, we can determine the relative positions of these points. The map shows the summary of several years of such measurements. And indeed it turns out, that the various points in the Bay Area move with respect to each another. Look at a black arrows in Livermore or Vacaville and compare them to the arrows at the Farallons or Point Reyes. Each point moves about one inch per year, the East Bay cities to the southwest, while the points in the Pacific move in the opposite direction. How slow is an inch per year? The plate movement in our region has about the same speed which with a healthy fingernail grows. (hra011)