|Superstorm Sandy (Photo:NASA). (Click to view larger image.)|
As their name suggests, seismometers are there to measure earthquakes. After all "seismos" is the ancient Greek word for quake and the word "meter" derives from another Greek verb for measuring. In reality, however, the name "seismometer" is quite a misnomer for these marvels of electromechanical engineering. In fact, a modern broadband seismometer is a high precision tool, which measures any kind of tilt and vibrations over a wide frequency band and has a large dynamic range. The connection to earthquakes exists, because any temblor inside the earth produces elastic waves, which make the ground vibrate and tilt. These vibrations can sometimes be violent and lead to enormous damage. However, in most cases they are imperceptible because they let ground swing with amplitudes much smaller than the diameter of a hair. Seismometers pick up even those tiny wiggles.
Besides earthquakes, there are myriads of other causes for ground vibrations, from trucks and trains passing on nearby roads and railroads, to underground nuclear explosions, thunder and lightning, and oceans waves crashing against the shore. Even meteors shooting through the atmosphere (Seismo Blog: Meteors on Seismograms) and huge masses of rocks falling off a cliff (Seismo Blog: The Yosemite Rock Fall of July 10, 1996) can rattle the ground. The wind can also make the ground move, either by shaking large trees whose roots transfer their wiggling to the soil, or through the pressure of gusts on the surface. Such wind effects are usually very local.
Now a group of seismologists have reported on continent-wide seismic wiggles, which were generated by giant Superstorm Sandy. This massive storm rattled the East Coast last October. Its hurricane force winds and the associated heavy rains and storm surges wreaked havoc in New York and New Jersey. But Sandy's reach extended far beyond the Big Apple and the Garden State. Keith Koper and Oner Sufri from the Seismographic Station of the University of Utah discovered the superstorm's seismic signature in seismometer recordings almost everywhere in the Lower 48, even in the Pacific Northwest.
Sandy's seismic signal did not look like an earthquake at all. Instead it was an increase in the amplitude of long period rumbling. These so called microseisms are caused by deep ocean waves which pound the continental shelf many dozens of miles away from the coasts. When Sandy approached the East Coast, this pounding became so strong, that it resembled magnitude 3 seismic waves - however not just one temblor, but a continuous series of these small earthquakes going on non-stop for hours and days. Although the swaying was clearly registered by seismometers all across the country, it was not felt by people. The reason: Sandy's waves did not feel like the sudden jolts caused by the impulsive earthquake waves. Instead it was a gentle swaying in slow motion with periods between 5 and 15 seconds. As Koper and Sufri reported during the recent annual meeting of the Seismological Society of America in Salt Lake City, they are now investigating how seismometer recordings can be used to track other strong storms like hurricanes and typhoons. (hra087)
|Following the same trend: The growing world population (solid line) and the number of people who died as a consequence of earthquakes from two different catalogs (From Holzer and Savage, 2013). (Click to view larger image.)|
Sometimes seismologists are --the blogger is afraid to say -- a morbid bunch. When they gather around a computer screen or an old fashioned helicorder to observe incoming seismic waves of a strong earthquake in real-time, they often marvel at the beauty of the signals. In such cases they have to be reminded, that in a far corner of the world hundreds of people may have just lost their livelihood or even their lives in the destruction which this "beautiful" earthquake has caused. Of course, no seismologist wants to see people suffer because of earthquakes and many use their scientific and technical skills to help reduce the seismic risk. This job however is becoming more and more difficult, because the world population is growing rapidly and many megacities are located in seismically active zones or at coastlines prone to be hit by tsunamis.
The mortality due to earthquakes and their after effects seems to have been extremely high during the first decade of this century. Nobody will have forgotten the earthquake in Haiti in 2010, in which at least 220,000 people were killed despite its moderate magnitude of 7.0. (Seismo Blog: Extreme Damage That Didn't Have to Be). A similar number of people vanished in the tsunami, which swept across the Indian Ocean after the magntiude 9.3 Sumatra-Andaman earthquake in late December 2004. Earthquakes in the Pakistani area of Kashmir in 2005 and in the Chinese province of Sichuan in 2008 killed more than 80,000 each (Seismo Blog: Today in Earthquake History: Sichuan 2008). All told, close to 700,000 people died as a consequence of earthquakes from 2001 through 2010 alone, making it one of the seismically deadliest decades ever.
Because two colleagues from the USGS in Menlo Park, Tom Holzer and James Savage, were intrigued by this tragic record, they looked at long term trends in seismic mortality in the world. They analyzed the entries in several independent earthquake catalogs, which list the number of fatalities associated with seismic events. After running these data through various statistical algorithms and taking the growth of the world population into account, Holzer and Savage project a dim future. As they report in the latest issue of the technical journal "Earthquake Spectra" (Vol. 29, pg. 155), the number of deadly earthquakes will rise sharply during this century. Compared to the 20th century, the number of earthquakes with more than 50,000 fatalities will almost triple to about 20. The number of megaquakes with 100,000 or more fatalities will double to about nine. According to their projections, Holzer and Savage estimate that worldwide between 2.5 and 3.5 million people will perish in earthquakes and tsunamis during this century. (hra086)
|The locations of the three nuclear tests by North Korea lie within less than two miles of each other. The blue star depicts the location of Monday's test (Photo: CTBTO Vienna)|
Even though North Korea was warned by the international community not to do it, on Monday evening (PST) the country conducted its third nuclear test - a necessary requirement for building a functioning atomic weapon. As with its two predecessors in 2006 and 2009 (see blog May 25, 2009), the latest test was again recorded by seismometers all around the world. Using common rules to calculate the strength of the blast, scientists at the USGS National Earthquake Information Center (NEIC) in Golden, Colo., determined that the latest detonation was equivalent to a tectonic earthquake with magnitude of 5.1. That makes it by far the strongest of the three tests which North Korea has conducted. The previous two detonations had magnitudes of 4.3 and 4.7, respectively.
During the cold war, when mainly the US and the Soviet Union conducted hundreds of nuclear detonations at their respective test sites, seismologists found an empirical relationship between the calculated seismic magnitude and the yield of a nuclear device.
|Seismograms of the last three North Korean nuclear tests recorded on station MDJ, near Mudanjiang, China, about 370 km north-northeast of the test site. One can see them getting progressively bigger.|
According to this scale, an event with a magnitude of 5.1 corresponds to a nuclear weapon with a yield between 5 and 7 kilotons. These values are between one third and half the yield of the atomic bomb, which destroyed the Japanese city of Hiroshima in August 1945. Given the increase in magnitude of the three successive North Korean nuclear tests, it seems that the yields of their atomic devices have consistently gotten stronger.
Further evidence that Monday's blast was indeed a nuclear test came from calculations by scientists from both the NEIC and the international organization in the Austrian capital Vienna, which is tasked with monitoring for clandestine nuclear explosions. The location of the latest seismic event was within two miles of the locations of the previous tests and lay well within the perimeter of the Punggye Ri test-site, about 240 miles East of North Korea's capital Pyongyang. The South Korean Earthquake Research Center in Daejeon, which operates a dense seismic network in South Korea and China, confirmed this location. "This area is free of any tectonic earthquakes", says Chi Heon-Cheol, the director of the center. (hra085)
|There were two sizable earthquakes just south of Lake Tahoe in the Sierra Nevada Mountains within 24 hours of each other. They are represented as the two blue squares. Click to view a larger image.|
By California standards the Sierra Nevada cannot be considered a seismically very active region. After all, no plate boundary defining fault, like the San Andreas, runs through it. But two earthquakes, one with a magnitude of 4.0, the other a 3.7, which occurred within less than 24 hours and within a few hundred feet of each other are a stark reminder that our famous granitic range is by no means geologically dead. The two quakes occurred in an uninhabited area halfway between Lake Tahoe and Mono Lake just southwest of the point where Highways 89 and 395 meet. The first one jolted the region 21 minutes after midnight on Thursday, the second one occurred about 15 hours later. More than a dozen very small aftershocks were recorded in the intervening time.
Geologically speaking, the Sierra Nevada is a very young mountain range. Its granitic core is a pluton, which was originally a giant blob of magma that eons ago rose from the Earth's interior but never reached the surface. Instead, it got stuck in the Earth's crust, where the magma slowly cooled. Several million years ago, long after the magma had frozen into solid granite, some forces from below pushed this pluton slowly upwards. It began to rise above the surrounding landscape until it reached an elevation of almost three miles. Then erosion slowly began to raze the tops. Still, with Mount Whitney as the highest peak in the conterminous United States, the Sierras are an impressive range.
The uplifting of the pluton, however, did not occur evenly, because its eastern section rose much faster than the western part. The result is a strongly tilted mountain range. This tilt can still be seen today. When you drive east on highway 120 through Yosemite it takes hours before you reach the high point at Tioga Pass. But beyond the culmination, you drop down into Mono Lake Basin within a few dozen minutes while the road hugs some very steep cliffs.
The asymmetry of the upward movement also had other consequences. Along its eastern front a major thrust fault developed, which still causes earthquakes in the Owens Valley (refer to blog October 5th, 2009). In addition, volcanoes erupted during the pluton's uplift. The Long Valley Caldera as well as the Mono and Inyo Craters are clear examples of this volcanism. The youngest of these craters are only 500 to 600 years old - a mere blink of the eye on the geological time scale. And in the Long Valley Caldera near the town of Mammoth Lakes, hot springs and earthquakes are a reminder of its violent past. However, the volcanism is confined to a narrow zone along the eastern margin of the Sierras.
If volcanoes and the thrust fault reign only in the eastern section, how then, you may ask, are earthquakes generated in the rest of this vast, sturdy-looking range? The answer is actually pretty simple: Because the tectonic uplift of the massive pluton was so uneven, it was twisted and bent during its journey. These internal contortions set many parts of the pluton under mechanical stress. It is released today, after millions of years, through earthquakes like the ones which occurred on Thursday. (hra084)
|Damage after the 2009 L'Aquila earthquake (Image: Wikimedia Commons)|
A few days ago, when a court in the Italian town of L'Aquila found six seismologists guilty of manslaughter and sent them to six years in jail each, Earth scientists all over the world were shocked. How can seismologists, who know very well that earthquake prediction is at best an imperfect science, be held responsible for not forecasting an earthquake? What effect will this verdict have on the future of earthquake science, and on the way seismologists interact with the public? Well, the story is not as easy as it seems at first glance.
Here is the background: On April 6, 2009 a moderate earthquake with a magnitude of 6.3 hit the Abruzzo region of Central Italy. More than 300 people were killed in the medieval town of L'Aquila alone and up to 10,000 buildings in the region were damaged or destroyed. During the weeks before, a number of smaller earthquakes had rattled the same area and a committee of eminent Italian earthquake specialists was convened to assess the situation. This National Commission for the Forecast and Prevention of Major Risks met in L'Aquila shortly before the earthquake. It is not known what the group discussed in its closed-door meetings. But in a news conference their members reassured the public that there was nothing to worry about, even though the string of smaller earthquakes continued to rattle the nerves of the people in the region.
The situation was made more complicated by a technician working in a government laboratory located near L'Aquila. He had observed that the concentration of radon in some groundwater wells had changed during the foreshocks. Because he had read somewhere that such a change may be an indicator for an upcoming quake, he went public and predicted a major disaster, even though he was not a seismologist and had not discussed his findings with any experts.
In its verdict the court said, that the members of the Commission had provided ``inaccurate, incomplete and contradictory'' information about the danger heralded by the tremors felt before the 6 April 2009 quake.
At a cursory glance, the verdict seems to punish scientists for not predicting the earthquake. Unfortunately, many media and much of the public reaction did not take a deeper look at what the court really said. The judges actually chided the commission not for their failure to predict an earthquake – but actually for just doing the opposite. In their public statements the commission did not focus – as they should have - on the well known seismic hazard in the Abruzzo area and the associated high risk given the many unreinforced, medieval buildings in L'Aquila. The court found the members guilty, because of their reassuring, risk-denying public statements. This gave the populace, according to the verdict, a false sense of security that led many people to stop expecting and preparing for a major earthquake.
The reaction of seismologists all over the world to this verdict was by no means unanimous: It is clear that at the current state of Earth science, earthquakes cannot be predicted with any degree of reliability. In some cases, radon levels have changed before an imminent quake; in many others, the radon concentration in groundwater did not change at all. The same is true for all other indicators which have been investigated as possibly having predictive value for earthquakes: None of them have worked reliably.
But no seismologist should use this lack of predictability to downplay the risk associated with earthquakes. For the very reason that destructive temblors can happen anytime in seismically active regions – be it in Japan, California or in Italy's Abbruzi - serious Earth scientists will always have a cautionary message for the public: Even though we can't tell you exactly when a quake will happen, you should not panic, but you must be prepared for a big one to strike at any time. The happy-go-lucky attitude apparently presented by the members of the Italian commission is not the right response. One can discuss whether a verdict of six years in jail is too harsh a term, but there is no doubt, that the commissioners acted irresponsibly in their public statements before the L'Aquila earthquake. (hra083)