Anybody who has ever felt an earthquake will remember distinct jolts during the shaking. In strong temblors these sudden jolts can be sharp enough to throw a person off balance. In seismological terms they represent the arrivals of unique wave trains, like P- or S-waves, which are generated by the rupture in the earthquake source. Measuring and interpreting these onsets, as scientists call the jolts in a rather benign fashion, has been the bread and butter for studying the physics of earthquake sources and the structure of the Earth's interior for a very long time (see blog from January 1st 2012). That is why a discovery just over a decade ago surprised many seismologists. Japanese researchers found signals in the recordings of their dense sensor networks which had no onsets at all. Instead of producing a series of sharp peaks - the jolts - these recordings showed a gentle beginning and then rumbled on for minutes or even hours. In the end, the signal tapered off as sneakily as it began. Nowhere in those recordings could the researchers identify clear arrivals of distinct wave trains.
|Local earthquakes and tremor recorded at Tremorscope station TCHL. The top trace shows two small earthquakes (M2.1 and M1.8) which occurred just north of Parkfield, about 33 km from the station. The bottom trace (bottom trace) shows a 40 minute interval of tremor closer to the station but at a depth of 31 km. The data processing for both traces is the same.|
Similar signals were observed almost 90 years ago on Japanese volcanoes. There they were called volcanic tremor. Their cause: The movement of a fluid within the volcanic edifice, be it magma, hot water or steam, generates a murmur similar to the gurgling of a creek. But the new discovery had nothing to do with volcanoes. The murmur came from deep within the subduction zone east of Japan, where the Pacific plates dives into the Earth's mantle. Hence this type of signal got the name "non-volcanic" or "seismic" tremor. In the years that followed similar signals were observed in the Cascadia subduction zone off the coasts of Oregon and Washington. In 2005 two researchers at the Berkeley Seismological Laboratory, Bob Nadeau and David Dolenc, were the first to discover such seismic tremor in a strike slip fault. They identified the strange signal coming from deep along the San Andreas Fault just south of the tiny hamlet of Cholame in Central California.
In contrast to its volcanic sibling, the origin of the seismic tremor is by no means understood. It is possible that fluid deep below the brittle zone of an earthquake fault may play a role. But it was also observed that tremor episodes can be triggered by passing waves from other earthquakes. Looking through the vast archive of seismic recordings at the BSL, researchers found tremor episodes under Cholame which coincided with passing waves from strong local earthquakes, like the M 6.5 San Simeon quake of 2003 or the Parkfield earthquake of 2004 (M=6). Similarly, such tremor has been triggered by the waves of the strong M 9.0 Tohoku Earthquake, which struck Japan in March 2011 (see blog March 11, 2011 and the following).
Using the various sensors which are being installed in the Tremorscope network - described in the last blog entry - researchers hope to shed more light on these elusive rumbles and murmurs of the deep sections of the San Andreas Fault. (hra089)
|The drilling rig at work. (photo courtesy of Horst Rademacher)|
Drilling holes into the ground has never been a clean business. But operating a multi-ton drill rig on a windy ridge far from the amenities of everyday life takes special dedication. Grey drill-mud spatters out of the well. The wind scatters the mud all over the place and covers the drillers and their machines in thick, mucky goo. The noise of the machinery is deafening and the water to keep the mud circulating in the well has to be trucked in from miles away. The hole on very remote ranch land in northeastern San Luis Obispo County, that a Livermore based company has just started to drill, is the beginning of a new phase for studying the seismicity along the San Andreas Fault in Central California.
Under a contract from the BSL, the company will drill four 1000 ft. deep holes along the fault. These holes are part of a major research program called Tremorscope. Financed mainly by a grant from the Gordon and Betty Moore Foundation, Tremorscope will allow BSL scientists to investigate a different and hitherto poorly understood type of rumbling of the Earth called seismic tremor. When completed, each hole will be equipped with an array of sensors, all capable of measuring the slightest movement of the ground in various frequency ranges.
Most seismic sensors are put into action by installing them in small concrete vaults at the Earth's surface. Hundreds of these tiny vaults dot the landscape of California, especially along the major fault lines. But even far from any signs of civilization, like roads, railways, farms and factories, these surface sensors are so sensitive that they can still pick up movement of the ground unrelated to earthquakes and faults. One cause of what we call seismic noise is the lovely soughing of the wind in trees. It disturbs seismic measurements because the trees transmit the rustling of the wind through their roots into the ground, causing the Earth to shake ever so slightly. Seismometers easily pick-up this surface noise, which can mask the slight shaking of the ground caused by activity on a fault.
|As remote as you can get in Central California... (photo courtesy of Horst Rademacher)|
Going deep underground with the seismic sensors reduces such surface noise and as a consequence, their recordings of seismic signals are clearer. In addition, the deeper one drills into the Earth, the more solid the rock becomes. Seismic waves are much less attenuated and scattered by competent rock than by the loose sediments generally found near the surface. This also makes for a cleaner seismic signal and hence a better chance to study seismic activity.
Within the framework of Tremorscope, the four new boreholes will augment a network of four modern seismic stations, which were recently installed in surface vaults along both sides of the San Andreas Fault south of the tiny hamlet of Cholame. Analyzing the recordings of the eight stations together with the data from a multitude of existing sensors in the area, scientists hope to shed more light on seismic tremor. In our next blog, we will describe what is already known about this unique and elusive seismic signal. (hra088)
|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)