|Figure 1. This animation shows the earthquakes, which occurred offshore of Northern Chile in the two weeks preceeding the M8.2. The sequence culminated in Monday's event. (animation courtesy of GFZ Potsdam)|
The great magnitude 8.2 earthquake, which struck Northern Chile in the early evening hours on Monday, April 1, 2014, was the strongest temblor in the world since late May last year, when a magnitude 8.3 event shook the Sea of Okhotsk in the Russian far east. Although the magnitudes of the two events are almost the same, there are hardly any similarities between these earthquakes. Last year the earthquake occurred more than 380 miles below the surface and belonged to a class of very deep earthquakes. Although such quakes have been known and recorded for almost a century, the actual physical processes which lead to rupture at this enormous depth are not yet fully understood. However, because they occur so deep underground, such quakes usually do not cause much damage on the surface.
Monday's quake however originated less than 20 miles below the surface, about 60 miles offshore of the Chilean harbor town of Iquique close to the border with Peru. It belongs to a class of shallow depth, thrust earthquakes in which one flank of the rupture plane rapidly slides upward with respect to the opposite flank. Most of the strongest earthquakes on the planet belong to this class of events, among them the Good Friday earthquake, which struck Alaska fifty years ago last week. Because of this rapid upward movement, such thrust earthquakes can generate tsunamis, when they occur offshore. Monday's temblor was no exception.
|Figure 2. Strong earthquakes along the coast of Chile line up on this map like a string of chain links. The location of Monday's quake is marked in red. The last big quake in Chile had a magnitude of 8.8 and occurred in 2010 farther south (Read more from past blogs on March 1st and March 9th 2010). The largest earthquake ever recorded, a magnitude 9.5, can be seen at the bottom of the map. (figure courtesy of GFZ Potsdam)|
However with a maximum height of about seven feet along the the sparsely populated north Chilean coast, its tsunami did not cause any significant damage. Although at least six people died as a result of the shaking by the seismic waves originating from this quake, the damage in the coastal towns close to the epicenter was very limited.
Seismologist had long assumed, that a strong earthquake would strike this region of Chile. The first indication that something was brewing offshore of Iquique came on March 16, when a magnitude 6.7 event struck in exactly the same region. Since then hundreds of quakes with magnitudes larger than 4 were recorded, culminating in the strong event on Monday. While the National Earthquake Information Center operated by the USGS in Golden, Co., determined its magnitude to be 8.2, other world-wide quake monitoring organizations like Geofon, run by the Deutsches Geoforschungszentrum (GFZ) in Potsdam, Germany, computed a slightly lower magnitude of 8.1 The location of the epicenter is marked as red dot on figure 2.
The last time a significant earthquake occurred in this region was in 1877, when a magnitude 8.8 event struck. The extent of its rupture is indicated by the orange oval in figure 2. In the time since that event, the tectonic stress generated by the subduction of the Nazca Plate under the huge South American plate has been accumulating in this area without being released by earthquakes. Seismologists call this build-up of stress along a tectonic boundary the development of a "seismic gap". On Monday, after accumulating for more than 130 years, this stress has been released and the last seismic gap along the coast of Chile has been filled. (hra091)
|The epicenters of dozens of earthquakes with magnitudes greater than 6 dot this map of the tectonic situation near Cape Mendocino. The location of Sunday night's temblor is shown as an orange star. The earthquake labelled with a blue '2010' was described in a previous blog. (Map courtesy of Ross Stein, USGS) (Click to view larger image.)|
Another strong earthquake occurred late Sunday night in the far northern regions of the State. The offshore magnitude 6.9 temblor was widely felt from Roseburg and Medford in southern Oregon all the way to the Bay Area and beyond. Although no significant damage was reported on land within the first several hours of the quake, residents in Humboldt County felt the shaking for dozens of seconds. The quake struck at 10:18 pm. Its hypocenter was almost exactly 50 miles west of Eureka on what is known as the Gorda Plate.
This small cousin of the Pacific Plate glides under the North American Plate in the Cascadia subduction zone. This very seismically active area reaches from Northern California all the way to the state of Washington. The boundary line between the Pacific Plate and the Gorda Plate west of Cape Mendocino is a fault similar to the San Andreas, variably called the Mendocino Transform Fault or the Mendocino Fracture Zone. It points from Cape Mendocino for several hundred miles straight west into the Pacific Ocean. Along this fault, the Gorda Plate slides horizontally to the east with respect to the Pacific Plate with a speed of about 2 inches per year.
Sunday night's quake was located just north of this fracture zone inside the Gorda Plate. This area has to absorb a lot of mechanical strain, which makes it one of the most seismically active regions in the state. Since 1980 there have been at least ten strong earthquakes with magnitudes of 6.5 and above, including the one which shook the region on Sunday night. Taking all of these larger earthquakes and the thousands of smaller ones together, the region around Cape Mendocino has a very clear distinction among seismologists. When measured by the release of seismic energy, it is in fact the most active region of California, beating by far the active faults of the Bay Area and the seismically notorious Los Angeles Basin and its surrounding mountains. (hra090)
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)