July 29, 2008
Elaine Meinel Supkis
For the last three months, up and down the entire West coast from Panama to Alaska, there have been an increasingly violent series of earthquakes. This is mostly due to the havoc created by the fastest moving continent on earth: Australia. Must be those kangaroos who are at fault, eh? There have been quite a few very violent mass quake events like the recent collapse of part of the lithosphere in Sichuan, China. The Great Boxing Day Quake in Indonesia is perhaps the opening shot in a major planetary rearranging of the tectonic furniture. Today, Los Angeles was shaken up with a medium sized earthquake in one of the hardest to detect or examine upthrust/folds, the Whittier area upthrust region. This all has to do with the basin geology of Los Angeles.
The greater Los Angeles area earthquake of 29 July 2008, 11:42 PDT (18:42 UTC), was centered in the hills south of Diamond Bar and Chino Hills. An early analysis of the earthquake's focal mechanism implies that the shock originated as the result of oblique-reverse faulting, in which a block of crust is thrust up and over the crust on the opposite side of the fault and also moves sideways with respect to the crust on the opposite side of the fault. This style of faulting is intermediate between pure reverse faulting and pure strike-slip faulting. The epicenter is in a region of the greater Los Angeles area in which both significant reverse faulting (the Puente Hills thrust) and right-lateral strike-slip faulting (the Whittier fault) have been documented. The early focal-mechanism is thus consistent with the shock having occurred as the result of stresses similar to those which produced previously mapped faults in the earthquake region. At present, however, the shock has not been associated with a specific geologically mapped fault.
The 29 July 2008 was situated about 30 km east-southeast of the Whittier Narrows earthquake of 1 October 1987, which had a magnitude of 6 and which produced over $358 million in damages. The Whittier Narrows earthquake occurred as the result of thrust-faulting (reverse-faulting on a shallowly dipping plane) on the Puente Hills thrust.
The last six months, the southern half of California has been having many multiple low-level earthquakes. The middle sector of the San Andreas complex system has been totally still. Not so much as the slightest detectible tremor. Like a virgin in a chastity belt, the middle of California is like Sleeping Beauty only when this Princess wakens, she is Kali, the Destroyer. For the last month, the cluster of earthquakes off of Oregon have been rising in velocity and power. A goodly series of violent shakes just this last week is a sign, the Pacific Plate is pushing very hard and something is rather stuck. Instead of a big quake and then smaller and smaller ones, we are seeing bigger and bigger ones. Each session is greater in violence than the earlier sessions. Volcanos, for example, work this way. And most emphatically, Oregon and Washington are both very active volcanic states!
Back to Southern California:
Looking at this chart of today's first quake and the after shakes, one might suggest this is more like a toothpaste tube being squeezed hard. The major forces at work are the Pacific Plate and the North American continent itself. California has a much more unstable geology than say, Japan. The complex slip-faults, folds, tears, collapsed sections, as well as volcanoes and uplift granite mountains, volcanic rifts: California has it all. Japan is being formed by Australia and India which are on one giant plate, shoving very hard on all of Asia, slowly wrapping around Asia. This is causing strings of volcanoes to grow and form archipelagos. The US West Coast is where several non-North American plates are being squeezed like crazy by the huge Pacific Plate and the North American continent. The battle of these behemoths means there are many violent restructuring of the landscape at regular intervals.
Of all places on earth, California is the worst for putting up very, very expensive houses and very, very large cities. The vast expense of this vast expanse means the US is constantly teetering on the edge of financial doom. We saw how Sichuan, China, suffered. The vast landslides as well as the shaking killed many thousands of people and rendered many communities uninhabitable. Sichuan, China is also very interesting because it is a huge basin like the Central Valley of California and the Los Angeles Basin. I notice that where there is huge, constant uplift mountain building, right smack dab in the center there is often these mysterious basins and depressions, some of them quite gigantic. Like the Tibetan Plateau. It is a depression right next to the world's tallest mountain ranges!
Some geologists think that when continental collisions like the one of India slamming into China, cause the lithosphere to get these radiational cracks that go straight down. So perhaps, as the squeeze pushes up a ring of mountains, the center does the exact opposite. It breaks and falls. This is why the foothills surrounding the LA Basin are rising more and more, a detectable rise while the basin itself continues to subside! One very spectacular example of a basin/sink area is the Hudson Bay in Canada. It is nearly totally circular in shape. I suppose this is due to the lack of recent mountain building that can deform it by 'squeezing' it like the Central Valley in Chile and the Central Valley in the US.
Note how some of these big basins are nearly circular. Eventually, Japan will encroach on the edges of the Sea of Japan basin and cut it off from the Pacific Ocean. It will become first like that huge basin we call the Mediterranean Sea or the somewhat smaller Black Sea. Periodically in geological history, the Mediterranean Sea has dried up and it becomes a huge Death Valley like...Death Valley or the Dead Sea. If Japan does cut off the ocean from the Japan Sea Basin, it too, would dry up and become very similar to Death Valley. Note how Death Valley, by the way, it sinking and eventually will become part of the Gulf of California. Less than 60 million years ago, there was no Gulf of California! Eventually, all of the Central Valley will be part of the Gulf of California as it continues to subside.
ABS Consulting estimates that a Mw » 7 earthquake on the Puente Hills Thrust would cause in excess of $100 billion in damage. This is over double the damage in the 1994 Northridge (Mw 6.7) earthquake. Such an event, if credible, should not be ignored, especially if it can occur every 250 to 2000 years as postulated by Shaw and Shearer.
Given the evidence presented by Shaw and Shearer, the Puente Hills Thrust clearly exists. However, the authors have to make many assumptions to postulate its downward extent and its potential "seismogenic" (earthquake-producing) capability. The fault is only defined to a depth of about 5 to 7 kilometers in the seismic profiles. At this point, the fault is confined to upper Tertiary-age sediments that are not believed to be seismogenic based on past earthquakes. They must extrapolate this fault to the depth of the Whittier Narrows earthquake to project it into harder rock which is believed to be capable of generating a significant earthquake. Furthermore, only part of the Sante Fe Springs segment is defined to this depth, making the projection of the other segments to seismogenic depth even more speculative. Given such a limited seismogenic rupture surface, the potential magnitude that this fault system could generate, even if extrapolated to the depth of the Whittier Narrows earthquake, would be smaller than the authors indicate.
The quantification of earthquake risk requires that the frequency as well as the size of a potential earthquake be known. If a fault generates an earthquake once every 100,000 years, then it does not pose much of a risk. The recurrence intervals suggested by Shaw and Shearer for the Puente Hills Thrust of 250 to 2000 years are short enough to make the fault a significant risk. However, these estimates are based on fault slip rates that are inferred from crustal deformations observed on the surface and could be quite inaccurate. Given the still speculative extent and seismogenic potential of the Puente Hills Thrust, ABS Consulting scientists believe that it would be irresponsible to include it in its earthquake risk model at this time. However, we will carefully monitor the developing information on this fault and will include it in our model as soon as the scientific community has reached a consensus on its geographic extent, seismogenic potential, and recurrence frequency.
It is always a gamble, insuring housing in California. That state has an amazing talent for trouble. Fire, water, earth and air: all seem to switch from friendly and happy to super-nasty and in a flash, too. There seems to not be some slow build up to a disaster like say, with a hurricane. Instead, these freakish, violent alterations in reality come often with little warning. We just know they come, they must come and they will come.
Insurance companies bet that they can make a big profit in between these many disasters. The problem with this philosophy is, California can have epic fires, massive floods AND a massive earthquake all in one year! It is all up to Mother Nature to decide. Indeed, the concept of a massive fire, water problems and earthquakes coinciding is not only possible but very likely. As we saw in the Great San Francisco Quake 100 years ago.
May 22, 2008
The U.S. Geological Survey (USGS) and the California Geological Survey (CGS) have jointly published a report detailing a hypothetical scenario the impact a 7.8 magnitude earthquake — similar to the recent earthquake in China — would have in the Southern California region.
Scientists said an earthquake of that magnitude would cause loss of lives and massive damage to infrastructure, including critical transportation, power and water systems. They predicted the earthquake would kill 1,800 people, injure 50,000, cause $200 billion in damage, and have long-lasting social and economic consequences. Although imaginary, the Shakeout Scenario is based on USGS' and CGS' best predictions of what would actually occur during and after a major earthquake on the San Andreas Fault, the organizations said.
Among their predictions:
•The strongest shaking and greatest damage would occur near the stretch of the San Andreas Fault that extends through the fastest growing areas of Southern California, including the Coachella Valley, Inland Empire and Antelope Valley.
•At least 10 million people will be exposed to heavy shaking. California's efforts at mitigation have concentrated on life safety and have been largely successful. Thus, in spite of the large numbers of people in highly shaken areas, deaths are estimated at only 1,800.
More than that number died in Hurricane Katrina. The quake predictions for the San Andreas is totally false. The Whittier quakes might be this great, but if the middle of the San Andreas blows, it hasn't moved in several hundred years and is under extreme tension! That quake has a very good chance of being a +9 mag event. This is much, much, much more powerful than a mere 7.8 event. In China, much of the problem was due to whole mountainsides suddenly shifting from above to down into any and all valleys. In California, the hills surrounding LA are very steep, very unstable, very loose which is why it was so easy to build houses one above the other there and....guess what?
It just has to rain slightly too much in an El Nino winter and these stupid, ill-anchored houses go crashing down the hillsides! No earthquakes needed! IF we have earthquakes as violent as the recent group of quakes in China, whole sectors of suburbia will be jumbled under a mountain of dirt down on top of highways, malls and other houses! If this were to happen, and since LA is a basin exactly like Sichuan, China, the chances of this killing many, many, many people is VERY HIGH. Not low! My family has lived in California since the Gold Rush. I have climbed all over the place in my childhood.
Those hillsides and mountains covered with houses to day were, even in my youth, naked. My grandfather used to shake his head in dismay as he watched houses climb these steep canyons. 'They are all doomed,' he would inform me. Geeze, guess why I sound so doom-saying all the time! Heh.
The center of LA is right smack dab in the center of this basin that is falling. If the Big One blows out the middle of the state, I would not be very surprised if there are side quakes in this basin region. After all, Sichuan saw a series of +6 mag quakes run like a zipper northwards in a series of very violent sub-quakes!
California can't plan for just mere 7.8 quakes, they must be realistic about a +9 quake. The people who have been lured there should know the consequences. The real estate industry is totally and completely against this just like they refuse to follow fault line maps to avoid building directly on fault lines! I once lived in Berkeley and traced many of these major fault lines in the San Francisco BASIN...ah! It is like the LA Basin only it fell so far, it became part of the ocean, didn't it?
In Oakland, Berkeley, and further south: zillions of businesses, highways and houses are right smack dab on top of a major, very destructive fault line! This is PAINFULLY OBVIOUS. One fault line ends at the End Zone of the University of California's football stadium! Ye gads! 99% of the campus has been built since the last major blowout of this particular fault line. Everyone hopes Mother Nature won't come in, shrieking, ripping all these lovely buildings apart.
By the way, the West dearly loves to make fun of the Chinese all the time. 'Why, look at those schools that fell!' we taunt them. Well, a number of grade schools, high schools and a major university is just as vulnerable as any Chinese school! We should condemn all buildings built on major fault lines. I would love to see this and we will NEVER see this. Mother Nature gets to do all the demolition, with everyone inside, of course.
The good news, Dolan explained in interviews with local media, is that these findings show that it is several thousand years between devastating earthquakes on the Puente Hills fault. As just one of a few dozen major faults in and around the Los Angeles metropolitan area, it doesn't add much to the overall seismic hazard of Los Angeles. The findings of Dolan's group will, however, help refine the seismic hazard assessment for the area; knowing the threat is the first step in managing it. Another plus: the methods developed during this research may be applied to other, similar areas of seismic hazard that are difficult to study using more conventional methods.
The bad news, Dolan continued, is that when the Puente Hills thrust fault ruptures in an earthquake, it tends to do so in a very big way. Evidence from the new study, the first ever to probe the behavior of the Puente Hills fault, indicates that the four past earthquakes discovered along the fault had magnitudes between MW7.2 and 7.5 -- 10 to 15 times larger than the 1994 Northridge earthquake (MW6.7), the costliest earthquake disaster in U.S. history. In a magnitude 7 earthquake, not only would more energy be released, causing a larger area to experience damaging levels of shaking, but the shaking would last longer and more of the lower frequency seismic waves that affect tall buildings and other large structures would be generated.
"So, obviously, very large earthquakes this close to the high-rise district are of concern," Dolan said.
Worrisome, too, are the changes that would result from ground uplift during such an earthquake. According to Dolan, in a single magnitude 7 earthquake along the Puente Hills thrust fault, "USC would go up a meter or two relative to the area just south of the Los Angeles Coliseum. Downtown, and everything north of USC, would also be lifted by a meter or two." Adding to the bad news is the fact that the Puente Hills fault extends through the Los Angeles basin, a highly populated area made up of soft sediments that amplify shaking.
"This fault is in one of the worst places you could think of to put a fault of this size and geometry," Dolan said.
Some of these sediments, however, were the key to the study released this week. The Puente Hills thrust fault is "blind" because it never breaks through at the surface, instead producing folds in the rocks above it, which may sometimes be expressed as chain of low hills, raised up by successive earthquakes along the fault below. Each year, the Los Angeles basin is squeezed a few millimeters from north to south as tectonic plates of the Earth's crust carrying North America and part of the Pacific Ocean floor converge. This squeezing is accommodated by the many faults in and around the basin, which shift to release the energy of this compression as earthquakes.
Um, if the University of Southern California shoots upwards by three feet, I would strongly suggest the dirt on the surface will do what things do when they are jerked upwards faster than a speeding bullet: they have avalanches of dirt, houses, schools and cars. And trees. Everything slides downhill. This is because the earth doesn't move three feet like in a slow heave like in the most dangerous volcanic caldera on earth, Yellowstone as it snorts and snores itching to waken and devour several states.
No, this moves like the crack of a bullet from a gun: one second, you are listening to a boring lecture and texting your friends about what to eat at lunch, the next minute the ceiling is falling, the building is buckling as it slides towards the center of LA and glass windows of all the towers in LA are flying down onto the streets below, killing anyone caught out in the open or running out of violently shaking buildings...I have been in earthquakes. They certainly startle everyone and in general, people freeze in place. If they try to run, it is very difficult. In a major shake where say, USC jumps three feet, everyone will be falling down, not jumping along.
Borehole data from young sediments folded above the Puente Hills blind thrust fault beneath Los Angeles reveal that the folding extends to the surface as a discrete zone (145 meters wide). Buried fold scarps within an upward- narrowing zone of deformation, which extends from the upward termination of the thrust ramp at 3 kilometers depth to the surface, document the occurrence of at least four large (moment-magnitude 7.2 to 7.5) earthquakes on this fault during the past 11,000 years. Future events of this type pose a seismic hazard to metropolitan Los Angeles. Moreover, the methods developed in this study can be used to refine seismic hazard assessments of blind thrusts in other metropolitan regions.
Since LA is a classic basin caused by vast mountain building activity where two or more plates grind against each other, worse, since it is part of a multi-junction of plates not just pushing but sliding against each other, I would figure there are a host of unknown upthrust situations throughout this basin and one way to discover them is for there to be some major blow out event whereby a section of the San Andreas suddenly moves forwards, northwards, by over 30 feet in one split second. That would be on par with the Great Boxing Day quake that saw an even greater shift of the planetary surface.
The major boundary zone between the Pacific and North America plates continues southward from the San Andreas fault system into the Gulf of California. Across this boundary, extension and strike-slip faulting is causing the Baja California peninsula to separate away from mainland Mexico, thereby opening the Gulf of California. This is one example of how a small fragment of crust (Baja California) may get transferred from one plate (North America) to another plate (Pacific), a process that has happened many other times in geologic history.
Our research has focused on trying to understand the early history of this rift system, using a variety of approaches: 1) geological field study of faults within the Gulf Extensional Province (surrounding the Gulf of California) to document geometries, timing, and amounts of fault motion within this region; 2) volcanic stratigraphy of the Puertecitos Volcanic Province, a major Miocene-Pliocene rhyolite field that was dismembered by phases of extensional and strike-slip faulting related to the evolution of the Gulf of California; 3) argon geochronology to establish ages of the various volcanic units; 4) paleomagnetic work, to establish magnetic reversal stratigraphy of both volcanic sequences and marine sequences, and to look for tectonic rotation, about vertical axes, during the faulting; 5) processing of Landsat-TM data , to distinguish the main volcanic packages based on their reflectivities, and to identify active faults in the region. Results of this work so far indicate that the westernmost fault systems have been active more than 6 million years, that is, before the time that is commonly assumed that major plate motion in the Gulf really began. The eastern fault systems have evolved through time, with high percentages of early (between 6 Ma and 3 Ma) extension, and rather low percentages of post-3 Ma extension. We have identified a major discontinuity called the "Matomi Accommodation Zone" south of which the deformation may be younger (Pliocene), due to a jump of the Gulf spreading centers. We have found new tie points in Miocene and Pliocene rocks on both sides of the Gulf of California (Upper Delfin Basin segment) which allow us to constrain the total amount of slip in this segment of the Gulf since late Miocene time.
Interesting: the Gulf of California is very, very new, geologically speaking. And there was this 'jump' within the centers which were spreading the Gulf which back then, was a Central Valley. The ocean came flooding in.
The nature of all this is obvious: the Sea of Japan will, over the eons, turn into a Death Valley. Death Valley will turn into the Gulf of California. Quite a few basins have turned into watery zones, often great places to find oil, naturally. And other basins, if they are too far from water, turn into super-hot deserts. The uplift in Asia caused by India has raised the Tibetan basin to a very high altitude. But who knows? It may continue to fall relative to the mountains being built and reach sea level? This unstable earth is most interesting.