|(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)
|Figure 1: The coastlines of South America and Africa fit together perfectly (figure 18 from Wegener's book "The Origin of Continents and Oceans," 4th edition, 1929, in German)|
When you enter the great hall of the Senckenberg Museum in Frankfurt, Germany, today, you will see dinosaurs, spectacular fossils and other impressive displays which show the dynamics of our ever-changing planet. Had you entered the museum a century ago, you might have been awed by similar exhibits. But nowhere would you have found any hint that the surface of the Earth is in constant motion, or that the innards of our globe are relentlessly moving and churning. At that time, neither scientists and nor laymen knew about moving and colliding tectonic plates, about continents in perpetual motion, or about mid-ocean ridges which constantly produce new crust. This was about to change one hundred years ago today, when a trim-figured young man sporting a mustache entered the museum. Although only 31 years old, this scientist was already a tenured professor of meteorology and astrophysics at the university in the German town of Marburg. He had come to Frankfurt to present his latest research for the first time. For almost an hour, Alfred Wegener spoke to the members of the German Geological Association, who had gathered there for their annual meeting. In painstaking detail, he explained his hypothesis that the surface of the Earth is not a fixed entity, but that continents are in peripatetic motion. He concentrated on the coastlines of South America and Africa. Because they seem to fit together perfectly, Wegener proclaimed that some long time ago, these two continents were one. After Wegener had finished his talk, nobody applauded - instead he was ridiculed and laughed at. Too strong was the belief of the scientific establishment at that time that the Earth is forever stable and cannot move. Nobody of any scientific stature could imagine that continents might drift. Earthquakes and volcanic eruptions were seen as anomalies and not as part of a dynamic process within the Earth's crust and mantle. Even though his ideas were rejected outright, Wegener stuck to his thesis. In 1915 he published his book on "The Origin of Continents and Oceans," which by 1929 had seen four editions in German and had been translated into many languages.
|Alfred Wegener in his later years during an expedition to Greenland|
Wegener's hypothesis of continental drift was never accepted during his lifetime. After World War II, however, scientists in the United States and Britain found more and more evidence that Wegener was right. One of the core arguments for the existence of plates and their movement is the worldwide distribution of earthquakes. The vast majority of them occur along plate boundaries, whether they be spreading centers or collision zones. Today we can use GPS signals to measure precisely how fast the continents drift. In California, our backyard, the Pacific Plate moves at approximately 2 inches per year. Wegener, the brilliant German scientist, died in November 1930 on a scientific expedition in the center of Greenland. (hra072)
|Figure 1: Thirty minute long three component recording of the recent earthquake off Japan|
During the past two weeks the Earth was seismically exceptionally quiet - until Tokyo and parts of Japan's east coast were shaken on New Year's Day. Although high rise buildings in Japanese cities swayed in a gentle fashion, no damage was reported and no tsunami was generated. The cause was an earthquake under the Western Pacific, about 300 miles southwest of Tokyo. The focus of the quake with a magnitude of 6.8 lay in the Earth's upper mantle, almost 220 miles below the ocean surface. Such deep temblors rarely cause damage because most of the waves' energy is absorbed and dissipates before the shaking reaches the Earth's surface.
Nearly 12 minutes after the quake started, the first seismic waves hit California. The recordings of these waves with the broadband seismometers of the Berkeley Digital Seismic Network allows the blogger to make good on a promise he made awhile ago, namely to further explore how to read a seismogram. Figure 1 shows a half hour long record of the seismic waves registered by a three component seismic station in the East Bay Hills. Green is the vertical ground movement, up and down. The red line represents the seismic shaking in the North-South and the blue line in the East-West direction. The most prominent onsets are the P- and S-waves, which arrived approximately 9.5 minutes apart. However, looking at the vertical component one can clearly see more onsets (red circle).
|Figure 2: Four prominent onsets on the vertical component|
When zooming into this area of the seismogram (Figure 2, seven minute window) four prominent peaks stand out. Each of them is called an onset, because it represents the arrival of a distinct type of seismic wave. Over the last decades, seismologists have learned how to distinguish between these various types and how to interpret their differences.
Take onsets 1 and 2 for instance: Onset 1 is the most direct wave between the quake's hypocenter and California, commonly labelled as "P" for primary. Eighty-three seconds later, another wave arrives. This wave travelled from the quake's focus initially to the surface of the Earth immediately above the hypocenter. There it was reflected and then followed the original P-wave on its way to California. This type of wave is referred to as "little p - big P" (pP). The sketch in figure 3 shows its idealized path.
|Figure 3: Idealized path of various seismic waves between Japan and California|
Onset 3 is generated by a wave which first travelled as a S-wave to the Earth's surface above the focus. There, upon reflection, it was converted into a P-wave, which followed its two predecessors across the Pacific. Because of the conversion from S to P, this onset is labelled "little s - big P" (sP). Finally, the fourth onset is generated by a wave, which was reflected from the Earth's surface roughly halfway between the hypocenter and California. This wave is referred to as PP and takes more than 100 seconds longer to cross the Pacific than the immediate P-wave. (hra071)
|Figure 1: This relief map of the area around New Madrid shows the low-lying areas of the Mississippi Valley in blue and purple. The higher elevations are depicted in green, yellow and brown. The white dots are the epicenters of recent earthquakes. The Reelfoot Fault is shown as a small red line. (Source: Center for Earthquake Research Information, Memphis, Tenn.)|
The earthquake sequence that struck the central Mississippi Valley two hundred years ago (see most recent blog entry) was remarkable. It occurred far from any tectonic plate boundary, the regions normally marked by high levels of seismic activity. Although temblors in these areas are much less frequent than in California or Alaska, such intra-plate earthquakes are not that unusual. The 5.8 quake that shook Virginia and the capital in August was a reminder that such quakes do happen. But what is it that causes the Earth to tremble thousands of miles from any plate boundary?
Put in simple terms, we can compare the stable interior of a continent with a pane of glass. Virtually every place in the contiguous United States east of the Rocky Mountains is part of this solid platform. Even though the Earth's crust in this area is quite solid when compared to California, it is not totally free of mechanical stress. On the one hand, the continental interior is battered by the actions at the plate boundary in the West. It is also still recovering from the huge load of the ice sheet of the last ice age. During that era, the northern part of the continent was completely blanketed with layers of ice several kilometers thick. It melted only 12,000 years ago, relieving the crust of its load. Another factor is the age of the crust far away from any active tectonic boundary. While the rocks along plate boundaries are usually young, continents can be several hundred million years old. Their age makes them extremely rigid, in contrast to the more ductile crust at the continental fringes.
And this is where the comparison with the pane of glass comes in: When battered at the edges, the mechanical stresses in the center of the pane can build up. Once under stress, a slight puncture at one point is enough to make the entire pane shatter. The area around New Madrid is such a puncture point. There the Reelfoot Fault, an ancient rupture line, crosses the Mississippi Valley (red line in Figure 1). Because of the accumulated stresses and the rigidity of the continental crust, slight movements along this fault can lead to larger ruptures, such as those of two hundred years ago.
|Figure 2: Earthquake hazard map of the United States. The large oval in the eastern half of the continent shows the hazard associated with the New Madrid Seismic Zone. Its center is purple, depicting a hazard similar to the one we have to live with here in California. (Source: USGS)|
Paleoseismic investigations in this area have shown that the earthquake sequence of 1811-1812 was not unique. Similar strong shaking occurred at least three times during prehistoric times - probably around 1450 and 900 A.D. and 2350 B.C. Because other strong temblors cannot be ruled out in the future, scientists from the USGS have determined that the seismic hazard in the area around New Madrid is as high as that along the San Andreas Fault or the Pacific coast of Alaska (see Figure 2). (hra070)