A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. First detected using long term strain measurements, most slow earthquakes now appear to be accompanied by fluid flow and related tremor, which can be detected and approximately located using seismometer data filtered appropriately (typically in the 1–5 Hz band). That is, they are quiet compared to a regular earthquake, but not "silent" as described in the past. They are also capable of causing enormous tsunamis and damage to cities, just like regular earthquakes.
Earthquakes occur as a consequence of gradual stress increases in a region, and once it reaches the maximum stress that the rocks can withstand a rupture generates and the resulting earthquake motion is related to a drop in the shear stress of the system. Earthquakes generate seismic waves when the rupture in the system occurs, the seismic waves consist of different types of waves that are capable of moving through the Earth like ripples over water. The causes that lead to slow earthquakes have only been theoretically investigated, by the formation of longitudinal shear cracks that were analyzed using mathematical models. The different distributions of initial stress, sliding frictional stress, and specific fracture energy are all taken into account. If the initial stress minus the sliding frictional stress (with respect to the initial crack) is low, and the specific fracture energy or the strength of the crustal material (relative to the amount of stress) is high then slow earthquakes will occur regularly. In other words, slow earthquakes are caused by a variety of stick-slip and creep processes intermediated between asperity-controlled brittle and ductile fracture.[ Asperities are tiny bumps and protrusions along the faces of fractures. They are best documented from intermediate crustal levels of certain subduction zones (especially those that dip shallowly — SW Japan, Cascadia, Chile), but appear to occur on other types of faults as well, notably strike-slip plate boundaries such as the San Andreas fault and "mega-landslide" normal faults on the flanks of volcanos.
Faulting takes place all over Earth; faults can include convergent, divergent, and transform faults, and normally occur on plate margins. As of 2013 some of the locations that have been recently studied for slow earthquakes include: Cascadia, California, Japan, New Zealand, Mexico, and Alaska. The locations of slow earthquakes can provide new insights into the behavior of normal or fast earthquakes. By observing the location of tremors associated with slow-slip and slow earthquakes, seismologists can determine the extension of the system and estimate future earthquakes in the area of study.
Teruyuki Kato identifies various types of slow earthquake:
• low-frequency earthquakes (LFE)
• very low frequency earthquakes (VLF)
• slow-slip events (SSE)
• episodic tremor and slip (ETS)
Episodic tremor and slip
Slow earthquakes can be episodic (relative of plate movement), and therefore somewhat predictable, a phenomenon termed "episodic tremor and slip" or "ETS" in the literature. ETS events can last for weeks as opposed to "normal earthquakes" occur in a matter of seconds. Several slow-earthquake events around the world appear to have triggered major, damaging seismic earthquakes in the shallower crust (e.g., 2001 Nisqually, 1995 Antofagasta). Conversely, major earthquakes trigger "post-seismic creep" in the deeper crust and mantle. Just like regular earthquakes, slow earthquakes can cause devastating tsunamis, such as the Mentawai tsunami in 2010. The earthquake that caused this registered a magnitude of 7.8 and struck offshore the Mentawai islands in western Indonesia, causing more than 400 human casualties. Seismologists characterized it as a slow earthquake due to disproportionately large tsunami waves, rupture duration near 125 seconds, shallow near-trench slip, and deficiencies in energy.
http://www.geodesy.cwu.edu/instruments/ ... n_ETS.html
Slow Earthquakes, ETS, and Cascadia
In 2001, CWU researchers with the continuous GPS network Pacific Northwest Geodetic Array discovered periodic slow-slip across the Cascadia Subduction Zone. Previously undetected by seismic networks, these slip events exhibit regular recurrence intervals thus changing current understanding of earthquake behavior. Since this time, definitions for this newly discovered phenomenon have evolved. At first, the term "silent-earthquake" was employed to illustrate the absence of a seismic signature. Subsequent investigations and recent discoveries have led to a change in characterization. Now these slow-slip events are defined as eposodic tremor and slip (ETS).
In short, an ETS is a discreet time interval (episode) of relative tectonic plate movement (slip) coupled with high frequency seismic energy bursts (tremor). ETS usually last for around a few weeks duration as opposed to regular earthquakes where energy is released within seconds to minutes.
During an ETS relative plate motion occurs within a transition region of a subducting lithospheric plate. This transition delineates an area between the upper-locked and lower-slipping interface of a subduction zone. Stress between these two colliding plates builds since differential movement between the two zones is not entirely compensated from ETS displacement. Quick slip across the upper locked portion of a subduction zone occurs in large megathrust earthquakes when accumulated stresses surpass the upper region's locking threshold.
In contrast, the subtle motion caused by ETS is so "slow" it's difficult to record at the surface. One might say "quiet" or possibly "silent" in nature, but definitely important since these events affect lithospheric plate interactions that are responsible for damaging "fast" earthquakes. Will the size of future large-scale megathrust earthquakes be reduced or will the time interval between these earthquakes increase with an ETS? A process with such imposing consequences is hardly "silent" in terms of relevance. In fact an ETS is not silent at all.
Why not silent?
Due to continuous plate motion, the daily solution of any GPS measurement is recorded as a velocity; reflecting the overall strain accumulating in the upper crust. During an ETS, GPS velocities change direction until the event passes. This minute signal would go unnoticed without extended timeseries. With the proper corrections and long-term stablizations, GPS allows accurate measurements for each day. Since ETS usually last for over 10 days, this provides a nice measurement of offset that could not be recorded on the longest period seismometer.
When slow earthquakes were first described by Dragert and others (2001) as well as Miller and others (2002), the motion of the Juan De Fuca and overriding North American plates was thought to occur in the absence of seismic energy. Since this time, Obara (2002) discovered seismic band energy that correlates with these slip signatures in Japan. This corresponding seismic energy is the tremor of an ETS. Typically between 1-6 vibrations per second, slow earthquake related tremor has since been detected in Cascadia as well. Although separating significant tremor from noise in a seismic dataset proves difficult, once an ETS is delineated by GPS, a clear trend becomes evident.
How an ETS is measured:
Since we cannot measure offsets at depth directly, we infer motion on the fault surface with surface measurements. Before GPS, seismometers were the primary tool to measure earthquake type activity in the earth's crust. Besides the vague tremor associated with an ETS, the weeks-long motion of an ETS goes undetected by any seismometer. Today, with vast GPS arrays, we can now measure events at these time scales. In Cascadia, cumulative measurable surface offsets are less than 0.6cm (Szeliga et al., 2008); well within detectable range of GPS data over an extended time interval. While at the surface this deformation is easily resolved by GPS; unique source location of energy at depth is not.
Tremor allows better determination of source location. In both Japan and Cascadia, the depth of ETS tremor (and presumably slip) locates to 25-40km depth (Kao et al., 2005; McCausland et al., 2005; Obara, 2002), directly within the transition of a subduction zone. These independent tremor measurements substantiate arguments for a tectonic source and fine-tune the timescale of ETS activity. In the region of Washington state's Puget Sound, unlike a normal earthquake, a regular 14.5 month ETS periodicity has been proposed by Miller and others (2002) while a shorter period of 10.9 months seems to exist in northern California (Szeliga, 2004). In other subduction zones apart from Cascadia, ETS events exhibit no discernible periodicity although average horizontal offsets are comparable at around 5mm.
In general, vertical GPS uncertainties are large, and in Cascadia vertical ETS offsets are often small. Vertical offsets, therefore, require independent measurements since fault slip models are highly sensitive to this component. To address this issue, PANGA constructed a Very Long Baseline Tiltmeter Array (VLBT) in collaboration with UC Boulder. Not only does the Cascadia Tiltmeter Array resolve the vertical field to better than 1,000 times that of GPS, these tiltmeters also increase temporal precision with a sampling rate 100 times that of reliable GPS solutions; thus providing tighter constraints for fault models.
Below are 9 years of GPS data from the Cascadia Subduction Zone along the convergent margin from northern California to southwest B.C., Canada. ETS events well recorded are delineated with blue lines and total slip-time is indicated on right plot by brackets. Most events last 3 to 4 weeks with amplitude between 2 and 7mm (Szeliga et al. 2008).
Like any ordinary earthquake, an ETS has a measure of energy released during the event. This is calculated as moment magnitude (Mw). Cascadia ETS events average 6.7Mw (almost equivalent to the 2001 6.8Mw Nisqually Earthquake). This would represent around 2-3cm of slip across the plates at depth if measured ETS surface deformations are in fact caused by integrated slips at depth and tremor is simply the artifact of each individual slip of two portions of lithospheric plates in this subduction zone (Szeliga et al. 2008).
Slow Slip Events
http://www.gns.cri.nz/Home/Learning/Sci ... lip-Events
More than a dozen slow slip events (also known as "silent" earthquakes) have been recorded in New Zealand between 2002 and 2012. Scientists have only been able to detect them recently due to the advent of global positioning system (GPS) equipment which can detect sub-centimetre changes in land movements. As part of the GeoNet project in New Zealand, continuously operating GPS have been installed throughout the country. The GeoNet cGPS data show that these silent earthquakes occurring deep under New Zealand are changing the shape of parts of the North Island over time periods of weeks to years.
The Pacific plate descends westward beneath the eastern North Island, but for most of the time the descending Pacific plate and the over-riding North Island are stuck together at their interface, which causes large parts of the eastern North Island to be pushed to the west.
The silent earthquakes are a sign that some of this tectonic stress is being relieved by ‘slow slip’ occurring on parts of the plate interface at 15 – 50 km depth beneath the North Island. These slow slip events tend to occur just below the area of the plate interface that is “stuck” and building up stress to be relieved in future earthquakes.
Offshore the Gisborne and Hawke’s Bay regions, slow slip events are very shallow (<15 km depth), and occur over a period of around 2 weeks approximately every two years. Beneath the Manawatu and Kapiti regions, much deeper slow slip events (60-25 km depth) are observed over periods of 1-1.5 years. The most recent events beneath the Manawatu, Gisborne, and Hawke’s Bay regions occurred in 2010/2011, while the last slow slip event beneath the Kapiti area and the Marlborough sounds was in 2008. If the plate boundary slip that occurred during the Manawatu 2010/2011 event occurred suddenly (rather than over a year), it would have been equivalent to a magnitude 7.0 earthquake.
In the New Zealand slow slip events, large areas of land have been observed to move eastward by up to 30mm over days, weeks, or months. Some scientists believe that these movements can shift stress within the Earth's crust and trigger earthquakes, so they are not necessarily benign events.
This 'time lapse' graphic animation shows the slow slip events that occurred around the North Island between 2009 and 2012. Small red circles represent earthquake epicentres.
Researchers reproduce mechanism of slow earthquakes
http://news.psu.edu/story/400913/2016/0 ... arthquakes
By A'ndrea Elyse Messer
March 31, 2016
UNIVERSITY PARK, Pa. -
- Up until now catching lightning in a bottle has been easier than reproducing a range of earthquakes in the laboratory, according to a team of seismologists who can now duplicate the range of fault slip modes found during earthquakes, quiet periods and slow earthquakes.
"We were never able to make slow stick slip happen in the laboratory," said Christopher Marone, professor of geosciences, Penn State. "Our ability to systematically control stick velocity starts with this paper."
The research, led by John Leeman, Ph.D. candidate in geoscience and including Marone, Demian Saffer, professor of geosciences at Penn State and Marco Scuderi, a former Ph.D. student in geosciences now at Sapienza Università di Roma, Italy, recreated the forces and motion required to generate slow earthquakes in the laboratory using ground quartz and a machine that can apply pressure on the materials altering stresses and other parameters to understand frictional processes.
"While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood," the researchers report in today's (Mar. 30) issue of Nature Communications.
Catastrophic earthquakes, the kind that destroy buildings and send people scurrying for doorways and safe locations, are caused when two tectonic plates that are sliding in opposite directions stick and then slip suddenly, releasing a large amount of energy, creating tremors and sometimes causing destruction. Along regions of faults that do not produce earthquakes, the two sides of the fault slowly slip past each other in a stable fashion. Slow earthquakes occur somewhere between the stable regime and fast stick slip.
Regular earthquakes take place rapidly, while slow earthquakes occur on time scales that may range up to months. They can be as large as magnitude 7 or more and may be precursors to regular earthquakes. However, slow earthquakes propagate slowly and do not produce high-frequency seismic energy. They exist in the regime between stable slipping and regular earthquakes.
The researchers applied stress perpendicular to the direction of shear and then applied forces to shear the ground quartz. By altering the amount of stress placed in the perpendicular direction, they could achieve the audible crack of a regular earthquake, stable slippage and a wide range of slip-stick behaviors including slow earthquake.
"What's really cool about this is that nobody has been able to systematically produce a slow earthquake, stable sticking, the whole range between a slow and fast earthquake," said Marone.
For years scientists have known about slow earthquakes—temblors that last days, weeks, or even months.
But do these creeping quakes increase or decrease the threat of "regular" earthquakes—the big, quick kind? A new study may hold a key to the answer. (Until now seismologists have been aware of two major types of slow earthquake, deep and shallow. (Deep slow earthquakes occur about 18 to 24 miles (30 to 40 kilometers) underground and last several minutes to several days. They produce tiny tremors, each no stronger than 1 or 2 on the magnitude moment scale, which measures earthquakes based on the seismic-wave energy they release.
Shallow slow quakes happen at depths of about three miles (five kilometers) and can be as strong as magnitude 3.5 or 4.
Like other earthquakes, these slow quakes release built-up tension in shifting rock layers. Once the layers release that energy, they can "relax" for a while, making additional earthquakes less likely in the near future
Seismic Mystery in the Middle
What happens in the gap between the deep and shallow quakes, though, has long been unknown, according to geophysicist Chris Goldfinger.
Do slow quakes release tension overall in the region? Or do the deep and shallow quakes transfer tension to the middle, "loading" that section for future big ruptures?
To get to the bottom of the slow-quake mystery, study author Hitoshi Hirose and colleagues studied seismic readings of slow earthquakes in an area around the Bungo Suido, a channel in southwest Japan. Slow earthquakes have been recorded here about every six years since 1997.
Deep and shallow earthquakes measured by the team occurred in tandem, indicating that the deeper and shallower motions are linked, perhaps with one set triggering the other.
This, in turn, suggested that the gap was also moving. But since middle-layer quakes have never registered on seismographs, Hirose's team turned to high-precision GPS instruments—which measure general movement, rather than seismic waves—to confirm the middle slow quakes.
Sure enough, the GPS data revealed slow earthquakes in the gap—really slow.
The movements were "too slow to radiate any seismic waves. Slip on the fault lasts for several days to years," said Hirose, of Japan's National Research Institute for Earth Science and Disaster Prevention.
And though the mid-level quakes were slow, they weren't exactly weak, he added.
Each temblor was about a magnitude 7, roughly the same as Haiti's disastrous 2010 earthquake—the quakes only seemed like nothing because all that energy was released over months, rather than seconds or minutes.
Slow Earthquakes May Signal Buffer Zones
The new find is useful, because it shows that slow earthquakes can release the tension that would otherwise result in damaging earthquakes, said Goldfinger, of Oregon State University, who wasn't involved in the new study.
In the fault zone around the Bungo Suido, at least, the entire segment is releasing a lot of strain, but without knocking down buildings, he added.
It's also important because it means that "the area of slow earthquakes might act as a barrier" to the spread of big quakes.
Similar slow earthquakes have been seen in other parts of the world, suggesting that these quakes also might indicate zones that can help limit big quakes, Hirose said.
"But," he noted, "not all regions have all kinds of slow earthquakes"—deep, mid-level, and shallow—at least as far as we know. If stress is not relieved on all levels of slow-earthquake zones, they might not be effective at reducing strain.
On the other hand, he added hopefully, since the quake-buffering zones are tough to detect, the world may be secretly brimming with them.
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