OVER VIEW “A lack of rumbling does not necessary make an Earthquakesharmless. But some of the ‘QUIET’ types could presage devastatingtsunamis or larger, ground shaking shocks”. by Peter Cervalli
SILENT EARTHQUAKES AN OVERVIEWNot all earthquakes shake the ground, it turns out. The so- called Silent types are forcing scientist to rethink their understanding of the way quakes-prone faults behave.In rare instances, silent earthquakes that occur along the flanks of seaside volcanoes may cascade into monstrous landslides that crash into the sea and trigger towering tsunamis.Silent earthquakes that take place within fault zones created by one tectonic plate diving under another may increase the chance of ground shaking shocks.In other locations, however, silent slip may decrease the likelihood of destructive quakes, because they release along faults that might otherwise seem ready to snap.
INTRODUCTION Silent Earthquakes are a tectonic shifts that are slow and may last for several hours to even a week. They usually occur below 25 to 40 km depth under ground barely makes a ripple on the surface. They happen usually along the major fault and subduction zones. They are detected by GPS, since they happen so slow that ground detection is difficult and could not be recorded on a seismogram. Silent earthquakes may result in swarms of micro earthquakes , and hence increasing stress in the fault zones which could be a precursor to a mega quakes.
INTRODUCTION• Silent earthquake – slow moving temblors that displacethe ground without shaking it• Silent earthquake do not generate seismic waves so youcan’t feel them.• silent earthquake occurs at subduction zone plateboundaries around the world.
DESCRIPTION The slow mode vibration along the fault lines can cause large movement along the fault lines but only a small movement is occur which is difficult to detect and can only be detect by GPS. This slow mode vibration cause the silent earthquakes and is detected usually after a long time of its occurance. Green and yellow box is stations, black triangle is direction of plates redline direction of movement. ( inset showing GPS detection). • Silent earthquakes may generate non-volcanic tremor and also produce swarms of micro earthquakes (case of big island of hawaii). • This swarms of earthquakes are clear sign of increasing stress to the fault zones. • This slow-slip event occurs in regions that the fault are not strongly locked up ( case of cascadia fault plane, japan nankai trough, hawaiian islands, costa rica’s). This zones are known as Seismozenic zones
• Also this slow-slip event does not radiate the seismic energy associated with normal earthquakes, and a very sensitive instruments records this events like a lots of Swarms.• Silent earthquakes in subduction zones is associated with non-volcanic tremor (case of Kilauna volcano, hawaii.). This event are estimated to origin from beneath 25 to 40 km depth where mega-thrust of the plate usually happens.• Sometimes this slow-slip is also associated with high frequency earth quakes at the San Andrean Fault which generates a rumbling noise from beneath.• Most silent earthquakes appears to be accompanied by fluid flow and related tremors.• This type of earthquakes are usually observed along subduction zoneslike dip-shallow zones of Japan, Cascadia etc and also on strike-slip fault boundaries such as San-Andrean fault.• The ubiquity of silent earthquakes on fault line both dip-slip and strike- slip suggest that they are a fundamental mode of strain release.
MECHANISM OF SILENT EARTHQUAKES The focal mechanism of silent earthquakes can be interpreted to be low angle thrusting on the plate boundary and hence, seismic activity is not much associated with this type of event. At subduction zones its seemed the heated rocks sticks and slips much like the crust except at a more stately pace. Slow slip tug on the crust more gently than normal earthquakes. They trigger a low frequency seismic pulse that makes no shock waves at all, only faint transient displacement at the surface hence difficult to detect even by GPS also and require a lots of time to observed the phenomena. Also silent earthquakes generates noise but are not creepy, as they begin and end suddenly. That sudden beginning when it takes place in the slope of a volcanic island, creates concern about a possible catastrophic flank event. Red lines shows the motion of GPS station( black triangles), dark circles is Swarms generated due to slow-slip of the two plates.( @ hawaii kiluaea volcano).
Mechanism of Silent earthquakes at subduction zones and volcanicisland• Percolating water may trigger Silent earthquakes if it goes down into the vulnerable faults , as highly pressurised water can push apart the two sides of the fault making it easier to slip past each other. This type of silent slip can occur within subduction zones and volcanic island.• Rain water may sweeps down from the earth surface into shallow faults, such asthose that separate an unstoppable slope from the rest of a volcano.• Here the slipping can be fast or slow, depends on the friction between the twofaults.• Also, chances of catastrophic quakes are high although the slipping may be slow at beginning which can gain its acceleration of slipping which may be a factor of its friction between the faults.• Geologist expect the case of this for the Kiluae silent earthquakes as there was a torrential rainstorm nine days before the observation of silent earthquakes.
SILENT EARTHQUAKES IN FORE CASTING SEISMIC HAZARDS •Silent earthquakes are not hazardous but may be a factor in the happening of large quakes, as this types are usually found in the subduction zones which is where large earthquakes usually occur (M9.2 of sumatra in 2004, taking about 200,000 lives). If there is activity of micro quakes during silent slip events, then an increasing rate of this swarms of quakes will give a way of quantifying how much the hazards has gone up, because each of those small events has some probability of growing into very large ones.• Also this micro-earthquakes could be use as a way of studying the stress on the plate interface in subduction zones.• If silent earthquakes triggers micro earthquakes in subduction zones it can be used as to determine the depth of the silent earthquakes.• If the pattern of silent earthquakes is observed we can predict about the stress distribution in the areas like subduction zones, where big earthquakes is eventually going to nucleate.
Type Of Silent Earthquake •Until now seismologists have been aware of two major type of slow earthquake , deep and Shallow.1. Deep slow earthquake occur about 18 to 24 mile (30 to 40km) underground and last sever minutes to several days. They produce tiny (very small) tremors and each no stronger than 1 or 2 on magnitude moment scale.2. Shallow slow quake happen at depths of about three miles (5km) and can be as strong as magnitude 3.5 to 4
Characteristics of Silent Earthquake Low frequency Earthquakes Nonvolconic tremor Slow slip event Episodic tremor and slip Very low frequncy VLFs have magnitudes of approximately Mw 3–3.5 and haracteristic durations of approximately 20 s All these diversely named henomena occurred at the same time and the same place. Furthermore, all of them occurred y the same mechanism: shear slip. Thus, it is natural to consider them differentmanifestations of a single process.
Case of silent earthquakes at japan( Nankai trough). Small yellow dots represent slip area. Circles represent site of very low frequency earthquakes.
VERY LOW FREQUENCY EARTHQUAKES AT THE NANKAI TROUGH one hour broadband seismogram, 2003/07/06/03hrPic of nankai trough in japan, where phillipinesplate subduct into the eurasian plate.
DISTRIBUTION OF SHORT AND LONG TERM SLOW SLIP EVENT IN JAPAN LONGTERM SLOWSLIPSHORT TERM SLOWSLIP ANDTREMOR G H R OU AIT NK NA TO H O UG A I TR NANK
Tremor triggered by M2.4 micro earthquake TREMORS M 2.4
Epicentral distribution of VLF( very low frequency ) events at nankai troughPink star- 2003Yellow star- 2003Red star- 2004White dots- tremors
Summary of VLF (very low frequency)event1.VLF events generate only surface wave.2.The source depth is very shallow.3.The mechanism is reverse fault type.4.VLF events might occur in the accretionary wedge or the plate boundary.
Conclusions1.Deep tremors occur in a narrow belt at the downdipof the seismogenic zone.2. Slow slip events occur in some parts of thetremor zone.3. The style of the coupling phenomena hasregionaldifferences.4. VLF events are located along the Nankai trough.5. Both events might be related to the subductionof thePhilippine Sea plate.
Selected BibliographySlow Earthquakes and Nonvolcanic Tremor Gregory C. Beroza1 and Satoshi IdeLow frequency earthquake in japan David R. Shelly and Gregory C. Beroza1Earthquake trigered by silent slip event on Kilauea volcano, Hawaii Paul Segall, Emily K. Desmarais , David Shelly