A seismogram from 4/22-4/25 at station BKS in Berkeley, CA.
(Click to view larger image.)
The seismic waves generated by last Saturday's earthquake in Nepal were recorded by seismometers all over the globe, including the ones in the network operated by the Berkeley Seismological Laboratory in Northern California and Southern Oregon. However, looking at the seismogram from our station BKS shown here reveals much more than just the seismic waves originating from the hypocenter in the Himalayas.
BKS is located in a short tunnel leading into the Berkeley Hills at Strawberry Canyon near the UC Berkeley Botanical Gardens. The tunnel - or adit - was dug in the late 50's of the last century. Its three letter code BKS is unique to this station and clearly identifies it - just like SFO or LAX are codes used by airlines and pilots to uniquely describe the international airports in San Francisco and Los Angeles. The data collected by the various seismometers in the tunnel are sent to the Seismolab via the internet and stored on various secured data server computers. The computers, located off site, are protected against fire, flood and earthquakes, so that these valuable data don't get lost. Some of these computers are programmed to display the data, and this is how the picture in this blog was generated.
What looks confusing at first can easily be deciphered when you think of reading it line by line, like a page in a book. Each line, read from left to right, shows the motion of the ground caught by the seismometer over exactly one hour, indicated by the 10 minute marks along the top of the picture. The vertical markings depict the actual time in Pacific Daylight time in the 24 hour format. Using these markers one sees that the seismogram begins last Wednesday, April 22, at 3:22pm, as indicated by the blue top line. It ends 72 hours later with the last line on Saturday, April 25, on 3:22 pm. In order to make reading the lines a little bit easier, every fourth line is colored blue, the three lines in between are black.
Looked at through the eyes of a seismologist, this seismogram is actually very unique, because it contains four different kinds of earthquakes, all of them pictured here in a textbook manner. Let's start with the last one (yellow arrow) first: What looks like a black smudge is actually a magnitude 3.3 quake which occurred on Saturday around 9:05 am near the small town of Cobb in The Geysers areas of Lake County. Its epicenter lies about 70 miles north northwest of BKS and it took the first seismic wave about 20 seconds to reach the seismometer. Because of the short distance such a quake is called a local earthquake. These quakes can be spotted on seismogram because they are short, lasting just about a minute.
Now let's look at the quake indicated by the red arrow: It shook the seismometer for more than 12 minutes, starting again with a black smudge, but then after about 2 minutes follow clear wave motions motion. This is the recording of a magnitude 5.5 earthquake, which happened last Thursday at 6:35 pm about 150 miles off the coast of Cape Mendocino. Because this quake's epicenter lies about 350 miles away from BKS, it took the first wave about 2 minutes to reach Berkeley. The greater distance also causes the waves of this quake to be more spread out in time as the local event. Such a quake is commonly called a regional event.
The third set of seismic waves (green arrow) woke up the seismometer last Friday shortly after 7am. They originated from an M6.2 earthquake just south of the Island of Haida Gwaii (formerly known as Queen Charlotte Islands) off the coast of the Canadian Province of British-Columbia. The distance between the epicenter and Berkeley is more than 1050 miles. As a consequence the shaking under BKS lasted more than an hour. You can see the whole blue line wiggle.
The dominant feature in the seismogram is - of course - the Nepal earthquake. The arrival of its first seismic wave is indicated by the pink arrow. It lasts several hours and the waves were strong enough to cause the largest amplitude of ground motion in this seismogram. Our seismometers at BKS are also sensitive enough to record the Nepali seismic waves traveling around the world several times. Both the quake off Canada as well as the quake in Nepal fall under the category of teleseismic events. (hra103).
The earthquake, which wreaked havoc in the Kathmandu Valley of Nepal on Saturday about noon local time, was the strongest quake in the world so far this year. With a magnitude of 7.8 it was felt over a very large area from the Indian capital Delhi 500 miles to the west, to Dhaka, the capital of Bangladesh, about the same distance to the southeast. The large quake triggered a major avalanche on the south slopes of Mt. Everest which devastated the base camp, where hundreds of climbers were waiting for a break in the weather to ascend the world's highest peak. More than a dozen people are feared to have died in the avalanche. Mt. Everest lies 140 miles east of the epicenter.
The worst damage reported so far occurred in the Nepalese capital Kathmandu, where many buildings collapsed and at least 524 people lost their lives. Police sources spoke of 1000 more deaths in the rural areas of Nepal. 50 more people were killed in northeastern India, and China also reported at least twelve fatalities as a consequence of the quake.
|The earthquake caused huge cracks to open in the streets of Kathmandu. The Nepalese capital lies in a sediment filled basin, which tends to amplify the seismic ground shaking.Source: EMSC|
One of the reasons for the quake's destructive power was its shallow depth. Calculations by all major earthquake monitoring agencies in the US and Europe put the depth between 7 and 10 miles. Contributing to the devastation was the fact that Kathmandu lies in a basin filled with about 2000 feet of sediments from a former lake. During the shaking of an earthquake, such sedimentary basins tend to amplify the ground motion and thereby increase the destruction. The worst historic effect of this kind led to the huge destruction in Mexico City during an earthquake in 1985.
The cause for Saturday's earthquake is the northward movement of the Indian Plate. It crawls with a speed of about 2 inches per year, roughly the same rate with which the Pacific Plate slides past North American in our area. However, the major tectonic difference between California and the Indo-Eurasian plate boundary is the consequence of the plate encounter. When India meets the huge Eurasian Plate, it is forced to dive into the Earth's mantle. This head-on collision also thrusts upwards the Himalayas, making it the highest mountain chain in the world.
This plate collision also causes some of the most devastating earthquakes in the world. In the last 125 years, this eastern section of the Himalayan Front alone has seen at least three quakes of magnitude 8 or greater. In 1897 and in 1950 the Indian state of Assam and neighboring Tibet were shaken by quakes with magnitudes of 8.1 and 8.6, respectively. The total death toll for both quakes exceeded 5000. In 1934 Nepal and the neighboring Indian state of Bihar suffered severe damage when an 8.2 quake struck. Almost 20,000 people perished in this quake, most of them in Nepal. This quake destroyed a quarter of all buildings in Kathmandu.
In 1998 an international group of seismologists and experts on disaster preparedness got together and evaluated the seismic risk for the Kathmandu Valley. The report they published one year later on the likely earthquake scenario makes very eerie reading, because it describes pretty much in detail what happened when the real temblor struck this Saturday. (hra102)
Texas scientists found that injecting waste water into an aquifer more than 7000 feet beneath the town of Azle, TX, reactivated ancient faults (thick black lines) and caused earthquakes.
Source: Nature Communications/SMU
(Click to view larger image.)
As seismologists, we are often asked if fracking, the brute force technology for the advanced recovery of hydrocarbons and geothermal energy from underground reservoirs, causes earthquakes. The answer is a clear yes, because, simply put, fracking is the very definition of an earthquake. The sole purpose of fracking is to break the rock underground, so that fluids and gases can flow more easily. It is this rock breaking, which in Earth science is commonly defined as an earthquake. There are, however, many major differences between a frack quake and the kind of temblors we are experiencing here in California all the time.
Given the tectonic situation in this state, most of our quakes are of a sort called shear fractures. In such a quake the two flanks of a fault rumble past each other, but they do not move away from each other. In contrast, in a frack quake the two flanks of a fault become separated and show only minimal sliding or shear movement. Such quakes, which not only occur during fracking but also when magma rises in a volcano, are called tensional fractures. Given the differences of physical movement in a fault, tensional fractures are a separate class of earthquakes from shear fractures.
The other major difference between fracks and tectonic quakes are the forces involved. While it looks impressive and sounds deafening when huge pumps and compressors are moved to a drill site and begin pumping water into a borehole, the pressures they generate are tiny compared to what our own Earth is capable of when the tectonic plates shift. It is for a good reason that the power of tectonic earthquakes is often compared to the energy released in the detonation of a nuclear weapons. You can bring all the pump trucks of the western hemisphere to a drill site and even if they could access the bore hole simultaneously, they would not be able to generate enough force to cause even a medium sized shaker. Bill Ellsworth, a seismologist at the USGS office in Menlo Park summarized in the journal "Science" in 2013: "More than 100,000 wells have been subjected to fracking in recent years, and the largest induced earthquake was magnitude 3.6, which is too small to pose a serious risk." The bottom line: Frack quakes are small and only very few of them are even felt by humans.
Why is it then that fracking - or hydraulic fracturing as it is technically called - has such a bad reputation? While the blogger can't comment on how fracking could affect groundwater quality, one thing is clear: There won't be any man made frack quakes which will rock the whole planet like the megaquakes we have had during the past few years. Nevertheless, there is an important aftermath of fracking, which can cause sizable earthquakes. It has to do with the thousands of gallons of water used during the fracturing process. This water contains additives to help the fracking and while underground, it also dissolves minerals from the deep rock formations - in short this water is contaminated. Hence it cannot simply be discharged into a river, used as irrigation water or even sent to a sewage treatment plant. Instead this water is pumped back into the ground, deep below the groundwater horizons. This process is called injection and has been a regulator approved standard operating procedure in the oil and gas industry for decades. According to some sources, there are more than 500,000 such injection wells in the US.
Most of them cause no problems at all, but a few dozen of them are associated with significant earthquakes. Here a two examples: The largest quake ever measured in Oklahoma, a 5.6 shaker which struck 40 miles east of Oklahoma City in 2011, was caused by waste water injection.
Yesterday, a group of researchers led by Matthew Hornbach from the Earth Science Department of the Southern Methodist University (SMU) in Dallas, TX, published a study about the seismicity in their own neighborhood. They write in the journal "Nature Communications" that a series of small quakes near the town of Azle, northwest of Dallas-Fort Worth, was clearly caused by wastewater injection.
If you want to learn more about fracking, injection wells and the risk of earthquakes associated with both, come to the Berkeley campus next Wednesday, April 29. Greg Beroza, an Earth scientist from Stanford, will give BSL's annual Lawson Lecture under the title "Induced Earthquakes in the 21st Century". The talk will start at 5:30 pm in the Banatao Auditorium in Sutardja Dai Hall (Citris-Building). (hra101)
|UCERF Bay Area map (Click to view larger image.)|
Where is the most hazardous place to live in the Bay Area? That's a difficult question to answer, because it all depends on the type of risk one is taking into account. A map showing the likelihood of becoming a victim of violent crime will look very different from a map of all fatal traffic accidents or a plot of the geographical distribution of heart attacks that happen while the victim is jogging. Thanks to the efforts of dozens of Earth scientists from many institutions, we now have a much better map of another danger lurking beneath the Bay Area - the hazard associated with earthquake ruptures. This new map clearly shows a distinct distribution of hot spots and low risk areas.
As described in several previous blogs, these regional details are part of a much larger effort, the "Uniform California Earthquake Rupture Forecast" (UCERF-3), which was published last month. For this forecast, researchers computed the probabilities that potentially damaging earthquakes - those with magnitudes larger than 5 - will strike various parts of California within the next 30 years. For the Bay Area, scientists divided the known faults of our region into approximately 100 smaller sections and looked at the likelihood that each of them might rupture in a quake.
The accompanying map - compiled by Jack Boatwright from the USGS office in Menlo Park based on the UCERF-3 modelling - shows the major faults in the Bay Area in different colors. All faults in blue and green have a very low hazard of rupture, below 3 percent. Yellow faults have a risk that is up to twice as high. But still, a 6 percent chance translates into a rather unlikely event. However, once the colors are range and red, the probabilities become significant. Three fault segments with the reddest colors - hence the greatest likelihoods of rupture - stand out. In the far north it is the Maacama Fault, which runs parallel to the San Andreas Fault far inland in Mendocino County. It has an almost 25 percent chance of generating a large quake in the coming three decades. The San Andreas Fault to the south beyond Hollister has an even slightly higher likelihood of rupture. Fortunately, both of these fault segments cross areas with relatively sparse population. Since "risk" is defined as the product of "hazard" times "vulnerability", the risk is lower in rural areas.
In contrast, the third fault with a similarly high probability cuts though the heart of a very densely inhabited part of the Bay Area. The southern Hayward Fault, which extends along the East Bay foothills between south of Fremont and San Leandro, has the highest likelihood of giving way. One of the reasons for its high probability is the fact, that this segment has not ruptured in a significant quake since 1868. Since then tectonic stress has accumulated and according to the latest UCERF model there is a 26 percent chance it will give way in the next 30 years. A detailed scenario of what's is in store for us, when we experience a repeat of the 1868 earthquake, is described in a previous blog - and the news is not good.
However, there are some pieces of positive information in the UCERF map. The San Andreas Fault, the mother of all earthquake faults in California, shows a much lower probability of rupturing than the more hazardous faults further inland. Particularly along the Peninsula and further south, the chances of a large quake are only 9 percent. The reason lies in the not so distant past. In 1989 the Loma Prieta Quake relieved the fault of a good amount of tectonic stress. Even though that quake occurred more than 25 years ago, its relaxing effect on the Earth's crust is still being felt. (hra100)
Alternating layers of tidal mud (grey) and soil (brown), here pointed out in the Girdwood Marsh, show the repeated cycle of subsidence and uplift caused by the megathrust earthquakes in Alaska. (Foto: Horst Rademacher) (Click to view larger.)
Although seismologists have made enormous progress in the last few decades in understanding the causes and effects of earthquakes, many mysteries still remain. Now compare our knowledge today to the state of seismology in the year 1964: Coordinated worldwide seismic networks were still in their infancy. Plate tectonics, today's standard in explaining the restless surface of the earth with its volcanoes and earthquakes, was still a very controversial hypothesis. Computers, which could be programmed to model the processes inside our planet, were few and far between and in addition they were excruciatingly slow. In short: Five decades ago, almost every major earthquake posed a great new challenge to researchers, forcing them to throw out old notions and raise completely new questions.
This was especially true for those giant quakes with magnitudes of 8.5 and above, which we now call megathrust earthquakes. This lack of understanding of the really giant quakes was about to change, when in the late afternoon today 51 years ago Alaska was rocked and rolled by a truly strong quake. With a magnitude of 9.2 this Great Alaska Earthquake of 1964 remains today the second biggest earthquake ever recorded. The largest, with a magnitude of 9.5, had occurred four years earlier in Chile.
This entry into the "Giant Temblor Hall of Fame" is not the only reason why the blogger thinks that this quake, which wreaked havoc in Anchorage and whose tsunami cost hundreds of lives along the southern cost of the northernmost State, deserves a look back on its anniversary. The main reason is that by studying this quake, scientists were for the first time able to convincingly connect plate tectonics and earthquakes.
This connection was mainly the work of one Earth scientist, who wasn't even an expert in seismology. A day after the quake, the USGS sent George Plafker to Alaska, who was then a young geologist. His mission was to tally the devastation the quake had caused, and also to note how the effects of the temblor had changed the Alaskan landscape. After a few days in the field, he noticed a curious pattern of subsidence and uplift over a large area of Alaska. Along the southern coast the land was uplifted by 30 feet or more, further inland, in a sweeping arc encompassing the Kenai Peninsula, Kodiak Island and the area around Anchorage, the land had subsided by up to 7 feet.
At first, this pattern didn't make sense to Plafker, but a few weeks after the quake he found a convincing explanation. Plate tectonics predicted that the Pacific Plate moves to the Northwest and dives under the North-American continent along the Alaska coast. Because this process is neither smooth nor nicely lubricated, the movement would cause the two plates to stick to another. During its dive the Pacific would grab pieces of the North American plate and drag them along into the abyss of the Earth's mantle. As a consequence, over the years the land along the immediate coast would subside at a substantial rate, while at the same time the countryside further inland bulged upward. The earthquake finally releases the two interlocked plates and the segments of earth crust jump back into their original positions: Hence the observed uplift long the coast and subsidence further inland.
It is this jumping up of the land which scientists now call a thrust - and if the jump is big enough, it gets the name megathrust. As the pattern measured by Plafker showed, the Alaska earthquake of 51 years ago today, was indeed the first megathrust ever measured. The devastating quakes of the past decade, like the Sumatra earthquake of 2004 or the Tohuku quake of 2011 also belong to the same category. (hra099)