The magnitude 8.6 earthquake, which struck off the coast of Sumatra last week, was one of those rare megaquakes. It belongs to the group of the ten or so strongest quakes ever measured. As explained in the previous blog entry, scientists were surprised - and certainly breathed a deep sigh of relief - that this quake did not cause a significant tsunami. In fact, hardly anybody was even seriously hurt when the extremely strong seismic waves caused by this quake hit the Indonesian islands closest to the epicenter. But once the immediate danger of a tidal wave was over, seismologists began to realize that this quake and its aftershock of 8.2 two hours later pose an immensely interesting, multifaceted scientific puzzle. Here is why:
These two quakes occurred in the immediate vicinity of the Sumatra subduction zone, where the Indian Ocean plate dives under fragments of the Eurasian plate. The Great Sumatra Quake of December 26th, 2004 had its origin in the same general area. But while this quake, which produced the devastating tsunami seven and a half years ago, had a focal mechanism expected for subduction zones, last week's quakes did not. They were earthquakes of the California type, that is the two flanks of the rupture slid past each other horizontally instead of slipping vertically against each other as in most subduction temblors. Such horizontal strike slip quakes have almost never been observed on the seaward side of a subduction zone. How and why they occurred off Sumatra is puzzle number 1.
The other unexpected feature of this quake should concern everybody in California. Most quakes along the San Andreas and its sister faults also shift in a strike slip fashion. Seismologists have always assumed that such strike slip events would be able to generate strong temblors, which are likely to cause a lot of damage. However, all the scientific evidence collected here in California and in other strike slip regimes around the world suggests that strike slip quakes cannot grow into really devastating megaquakes with magnitudes of 8 and greater. It seems that the horizontal slip fault zones are too weak to accommodate enough tectonic stress to delay rupturing until the breaking results in a megaquake. Well, since last Wednesday researchers have such an unexpected strike slip megaquake in their collection. And now they are scratching their heads as to how to fit these two unexpected aspects of the puzzle into current earthquake theory.
One possible solution was pointed out at an earthquake conference in Memphis last week by Lori Dengler, a Professor of Geology at Humboldt State University and California's foremost expert on tsunamis. She noted that the Great Sumatra Earthquake released a large amount of tectonic stress along its extremely large rupture area. As a consequence the mechanical stress along the fringes of the rupture area grew and put the surrounding rock under an immense strain, with two forces acting in opposite directions. This pattern is similar to the forces acting on the San Andreas Fault. When the rocks finally gave way last week, the result was a strike slip earthquake. And why was it so large? Dengler thinks that the rocks in the hypocentral area were virgin. They had never been broken in an earthquake before - in contrast to the rocks along the San Andreas fault, which have ruptured many times during the past eons. As a consequence, the oceanic crust off Sumatra was able to accumulate an enormous amount of stress. When the rocks finally gave way, the result was last week's megaquake. (hra077)
|Epicenters of last night's earthquakes and the great Sumatra quake of 2004. (Click to view larger image.)|
When their alarm pagers went off around 2 am PDT last night, most seismologists on the West Coast feared the worst. A magnitude 8.6 earthquake had struck the very area off the coast of the Indonesian island of Sumatra that had suffered extreme damage by the now infamous Boxing Day earthquake on December 26, 2004. Then, hundreds of thousands of people were swept away by one of the biggest tsunamis in recent centuries. This monster wave was caused by a giant magnitude 9.2 earthquake. Although somewhat smaller, last night's magnitude 8.6 quake could have set off a similarly strong tsunami. Adding to the anxiety was the fact that about two hours after the first temblor, a second very strong earthquake shook almost the same area. It had a magnitude of 8.2. But after a few hours of anxious waiting, seismologists and emergency responders could relax. Only a small wave less than 3 feet high hit the west coast of Sumatra. It caused no significant damage. Along the shores of the rest of Indian Ocean just a few sensitive tidal stations registered a small disturbance.
Why did these two very strong earthquakes last night spare us a natural disaster similar to those that we witnessed after the 2004 Indonesia earthquake, or after the Tohoku quake, which struck in March of last year along the east coast of Japan, with a magnitude of 9.0? The differences in magnitude can not have been the only reason, because any offshore quake with a magnitude of more than 7.5 is potentially able to cause a tsunami. With their magnitudes of 8.6 and 8.2, last night's quakes were certainly within the tsunamogenic range and each one by itself could have generated a devastating tsunami.
One possible explanation for the tameness of the most recent quakes is their location. Both temblors occurred under the sea floor more than 60 miles to the southwest of the epicenter of the 2004 quake. This epicenter lay very close to the deep ocean trench, which represents the boundary line between the Indian Ocean and the Eurasian plates. Last night's quakes were much further from the trench and thereby further from Sumatra's coastline. But as we have seen again and again, distance is no protector from the devastating effects of a tsunami. These waves can cross an ocean basin with the speed of a jetliner and are able to wreak havoc thousands of miles away from their source.
The actual reason why the last two quakes did not generate any tsunami is their focal mechanism. Seismologists use this term to describe the relative direction in which the two rock masses actually move when they spring past each other in an earthquake. Take the temblors on our doorstep for example: Their mechanism is called strike slip, because our two home plates, the North American and the Pacific plates, scrape past each other horizontally. This results in quakes in which the two sides of the fault slip past each other without much vertical movement.
In contrast, the Great Sumatra Quake of 2004 had a completely different focal mechanism. It was a thrust event, in which almost all movement was vertical: One flank of the fault sprang upwards. This upwards thrust of the sea floor hit the water column above very hard, generating the monstrous killer wave. Last night's earthquakes were not of this dangerous thrust type, because their mechanisms were California style: The two flanks of the undersea fault slipped past each other horizontally. (hra076)
The series of small earthquakes, which shook the Bay Area two weeks ago occurred right under one of the more striking golf courses in the region. The Mira Vista Golf and Country Club in the hills of El Cerrito is not only known for its spectacular views of the Bay - hence its name. It is also one of the few golfing spots in the world, where tectonics and birdies meet. The course sits right on top of the northern Hayward Fault. If you strike a hole-in-one on the second hole, you may not necessarily claim only your talent or pure luck. Plate tectonics may have played a role, as the second fairway is essentially a section of the Hayward Fault, its direction determined more by the boundary of two lithospheric plates than anything else.
Because the golf course is tectonically so exposed, it is favored not only by golfers, but also by Earth scientists. In fact, much of what we know about the northern Hayward Fault stems from research conducted along and under Mira Vista's second fairway. One of the most important projects was a pair of trenches, dug by a group of researchers across the immaculate green. The goal of this big dig: Finding out how often in recent Earth history strong earthquakes had struck along the northern Hayward Fault.
If we want to know something about today's seismicity, we rely on digital recordings of seismometers. We learn about historic earthquakes from old newspaper articles, entries in the records of the California missions or reports by early explorers. However, in contrast to China or Japan, two seismically active countries with written histories going back millenia, the Golden State has only a very short tally. The oldest records in the Bay Area date back a mere 236 years to 1776, when Mission Dolores was founded in what is now San Francisco.
In earthquake history however, this time span of not even two and a half centuries is just a blink of an eye. The reason: Really big earthquakes of magnitude 7.5 or greater occur at any given spot along a faultline only every several hundred years. This long repeat interval poses a dilemma for "young" states like California. In order to estimate the probabilities of a strong shaker occurring, researchers need to know when the last few big ones ruptured certain sections of a fault line.
This is where paleoseismologists and their trenches come in. By exposing several meters of soil layers right on the fault trace, these researchers can count the number of previous earthquakes by analyzing characteristic patterns of disturbances within the layers left behind by big temblors. Fragments of charcoal or other dead organic matter buried within these disturbance allow them to be dated.
The results for Mira Vista: At least seven big quakes left their traces in the soil of the golf course. The last big quake shook the area sometime between 1640 and 1776. Given the typical movement of the fault of a few centimeters per year, another big quake seems imminent. This is one of the reasons that the northern Hayward Fault has the highest probability of all faults in the Bay Area that a strong quake will strike within the next 30 years. For a detailed description of the trench across the golf course, go here. (hra075)
|(Click to view larger image.)|
This morning the Bay Area literally woke up to a flurry of earthquakes. Between 5:30 and 6:30 am four temblors struck in the area around El Cerrito and Richmond Heights along the northern section of the Hayward Fault. The strongest of these quakes had a magnitude 4.0. This shaker rattled nerves and it even caused some very light damage, when glass and porcelain fell off shelves in a few homes near the epicenter. Despite the early morning hour, more than 13,000 people reported to the website "Did you feel it?", how they experienced the strongest quake of this series in their respective locations.
While the magnitude 4 shaker was felt all over the Bay Area, the smallest quake in the series was noticed only by a few people. It had a magnitude of just 1.1. Because the magnitude scale is defined as a logarithmic yardstick, a jump of one magnitude unit means that the amplitude of the earthquake's shaking measured by a seismometer increases by a factor of ten. As the difference in magnitude between the smallest and the largest of today's quakes is almost 3, the ground moved about one thousand times more in the 4.0 temblor than the little 1.1 quake. That is the reason, that the smallest quake was felt by only a few.
To the hundreds of seismometers deployed all over Northern California, however, this difference did not matter. Most of them are so sensitive, that they recorded all four earthquakes equally well. The figure shows the recordings of the four quakes by a broadband seismometer located in Orinda, less than 8 miles southeast of the epicenter. The trace on the top is the 4.0 event, the bottom trace shows the smallest quake. Each record is about 4 seconds long.
In each of the traces, the arrivals of the P- and S-waves are very clear. It took just 3.4 seconds for the P-waves of each quake to reach the seismometer in Orinda (first vertical black line in figure). As S-waves travel more slowly through the Earth's crust, they arrive about 2.2 seconds later (second line). The fact that the differences in travel time between P and S is the same for all four quakes is an indication that they all occurred at the same focus.
When looking at the waveforms of the traces, it seems as if the strongest event generated a gentle ground motion. The smaller the magnitudes get, the more wiggly the recording seems to be. This is a typical observation, because generally larger earthquakes radiate their seismic energy in a lower frequency than smaller quakes.
By now, the observant reader will have noticed, that there is something wrong with the figure. Didn't we say earlier, that the magnitude 4 event was about a thousand times stronger than the small 1.1 quake? How come then, that all four traces in the figure have roughly the same amplitudes? Well, the blogger digitally amplified the records of the smaller quakes, so that their amplitude "on paper" would match that the big event. If they had been depicted without that trick and had we shown the amplitude of the smallest event to be one inch, the waveforms of the large event would have been 83 feet tall - almost exactly the length of a typical high school basketball court. (hra074)
|Map of Cape Mendocino region showing today's M 5.6 quake and the M 6.5 quake of January 2010.|
The magnitude 5.6 earthquake that rattled a remote part of Humboldt County shortly after 1 pm today was a stark reminder about the dangers lurking in the northern part of our state. Even though nobody seems to have been injured by the latest temblor and the damage appears to be confined to a few broken cups, glasses and windows, this is no reason to take the shaking lightly. As pointed out several times in earlier entries in this blog, the area between Cape Mendocino and the Oregon border is the most active region of the state when measured by the release of seismic energy. It is also the area where one can expect the strongest earthquakes on the west coast.
This statement, however, is in strong contrast to public perception. We in the Bay Area, as well as our Southern California neighbors in the LA region, tend to focus on the seismic hazards hidden in our own back yards. Given human nature, such a point of view is completely understandable. Many of us have seen the devastating effects the Loma Prieta earthquake had on our infrastructure in 1989. After all, the construction of the new Bay Bridge is a direct consequence of this magnitude 6.9 earthquake. In the southland, people will remember the destruction caused by similarly sized earthquakes in Northridge (1994) and San Fernando (1971). But with all due respect to anybody affected by these quakes, their strengths pale in comparison to what can be expected in the very north of California.
The earthquake regime south of Cape Mendocino is dominated by the San Andreas Fault. Even though this vertical strike slip fault takes up the complete northwestwards movement of the Pacific Plate, it is horizontally segmented. Locked sections alternate with creeping regions. Most seismologists think that this segmentation limits the size of the biggest earthquakes that can occur along the San Andreas. For really big quakes of magnitudes 8.5 and up, a fault has to rupture for several hundred miles - and none of the segments of the San Andreas Fault are that long.
The tectonic layout north of Cape Mendocino is completely different. There we have a subduction zone in which the Pacific dips beneath the American continent and dives into the Earth's mantle. Such zones are capable of generating the strongest earthquakes in the world, like last March's 9.0 in Japan or the 2004 earthquake off the coast of Sumatra, which had a magnitude of 9.1. The Cascadia subduction zone, which stretches between Cape Mendocino and the coast of the Canadian province of British Columbia, has also generated such megaquakes. The last one occurred around 1700. Were such a giant quake to happen today, its effects would not be limited to the sparsely populated Humboldt County or to Oregon and Washington. Its seismic waves would be strong enough to cause damage even in the Bay Area, although Cape Mendocino is 200 miles away. (hra073)