Archives for: April 2013
|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)