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 Last updated on 18-10-2012

Frequently Asked Questions

Earthquake Measurements Earthquake Physics General queries  Ground Failure Phenomena
 What is a Seismometer

What are P and S waves?

 Can explosions trigger and  earthquake?

What is Surface Faulting?

What is a seismograph?

What are Surface Waves?

Can mining cause earthquake?

What is Liquefaction ?

What is a seismogram ?

What are Free oscillations?

Can mining cause earthquake?

What is Subsidence?

What is a seismological observatory?

What is the magnitude of the earthquake in your town?

Can a reservoir trigger an earthquake? 

What is Collapse?

What is a GPS?

 What is the intensity of this earthquake?

 Can excessive rains cause earthquake?

What are Landslides?

 What is Richter scale?

 Can earthquake be triggered by cutting trees?

 

Differences between seismic signals from underground explosions and earthquakes.

 

 

 

What are microtremors?

What is Tsunami?

What are Seiches?

  What are microseisms?

 

What is a microearthquake?

  What is Tectonic Province?

What is Seismic Gap?

What is Lineament?

 

 

 

What is a seismometer?

Seismometer is an instrument  which senses the ground vibrations, and produces an output in terms of a measurable physical quantity (voltage or current) mathematically related to the amplitude and frequency of the ground motion. The output of the seismometer can be recorded directly on a recording device, or can be used to guide the movement of a pen on a continuously moving recording paper.

What is a seismograph?                                                GoToTop

A combination of the seismometer and the recording device used for recording ground motion for earthquake studies is called seismographs. Along with the output of the seismometer, it is also necessary record accurate time signals (accurate to within 1 in 1000,000 on the records of earthquakes. Thus the timing unit, often called a chronometer, is an important part of a seismograph.

What is a seismogram?                                                  GoToTop

The records of the ground motions along with the time signals is called a seismogram. When the seismogram record contains earthquake induced vibrations, it becomes the seismogram of that particular earthquake.

What is a seismological observatory?                                GoToTop

The place used for recording ground motions (or vibrations) along with the instrumentation used for recording is called a seismological observatory. Seismological signals cover a very wide range of frequencies - from several Hz to several tens of minutes - and a set of instruments is required to cover the entire frequency range. There are short period and long period instruments. Some observatories also deploy instruments to measure very long period movements (tilt meters and strain meters). A timing unit is an important component of a seismological observatory.

What is the magnitude of the earthquake in your town?               GoToTop   

    Magnitude of an earthquake is a measure of the energy released at the earthquake source. It is an absolute quantity, and does not vary from place to place.

What is the intensity of this earthquake?                                      GoToTop

Intensity of an earthquake is assigned by its observed effects in the area of observation. Hence, intensity of an earthquake varies from place to place.                 

 What is Richter scale?                                                                  GoToTop   

Richter scale is conceptual scale on the basis of which a magnitude is assigned to an earthquake. The scale is so designed that a unit increase of magnitude represents nearly 30 times increase in energy released at the earthquake source. For any given magnitude M the energy E released in the earthquake is given as:

log10 E = 11.8 +1.5 M

Can earthquake be triggered by cutting trees?                                   GoToTop   

    For an earthquake to occur a large quantity of mechanical energy should be released from the interior of the earth. Hence, the precondition of earthquake occurrence is that energy must have accumulated. Further, this energy accumulation and release takes place at significant depth. Cutting of the trees leads to many other disadvantages (like soil erosion and climate modification), but it is not connected to earthquake occurrence.

 Can excessive rains cause earthquake?                                            GoToTop

  For an earthquake to occur a large quantity of mechanical energy should be released from the interior of the earth. Hence, the precondition of earthquake occurrence is that energy must have accumulated. Further, this energy accumulation and release takes place at significant depth. Theoretically, in a rare situation, where a geological fault sufficiently exists close enough to the surface so that large quantity of water can seep into it lubricating the fault surfaces making fault movement possible, an earthquake can occur. But that does not, normally, happen.

Can a reservoir trigger an earthquake?                                          GoToTop                       

  For an earthquake to occur a large quantity of mechanical energy should be released from the interior of the earth. Hence, the precondition of earthquake occurrence is that energy must have accumulated.  A large reservoir can trigger an earthquake if the fault was already on the verge of breaking, and only a trigger was required. This trigger can be provided by the reservoir by either water seepage into the fault from the reservoir or additional load of the reservoir on the fault system.

Can mining cause earthquake?                                                           GoToTop

For an earthquake to occur a large quantity of mechanical energy should be released from the interior of the earth. Hence, the precondition of earthquake occurrence is that energy must have accumulated in a fault zone.  Mining of material in very large quantities  can trigger an earthquake if there is a fault in the area,  the fault is  already on the verge of breaking, and only a trigger was required. Mining can disturb the stress balance in the fault zone, and  trigger an earthquake. However, such earthquakes cannot be very significant. 

 Can explosions trigger and  earthquake?                                           GoToTop

For an earthquake to occur a large quantity of mechanical energy should be released from the interior of the earth. Hence, the precondition of earthquake occurrence is that energy must have accumulated. An explosion can trigger an earthquake if the fault is already on the verge of breaking.

What are these P and S waves?                                                            GoToTop 

    At the of earthquake occurrence, adjacent surfaces of a geological fault move relative to each other, the fault breaks and energy is released. A large part of this energy release is dissipated in crushing and heating the rock in the area of the fault break. A small fraction of the released energy escapes as elastic energy, produces compression and shear stresses in the medium, thereby producing compression and shear waves. The compression waves are called P-waves and the Shear waves are called S-wave. P-waves travel faster than the S-waves in the interior of the earth, and these are the first ones to reach a seismological observatory. For this reason P-waves are called Primary waves and S-Waves are called Secondary waves. P wave vibrations have a frequency range of 1-2 Hz at thousand kilometers or more while S-wave frequencies  are lower than this. In addition to the P and S waves earthquakes also produce what are called surface waves.

Surface Waves.                                                                                   GoToTop    

Surface waves travel along the surface of the earth, and from very strong earthquakes they can do several rounds of the earth surface. It is surface waves which cause maximum damage to objects on the earth's surface. Periods of the ground motion in surface waves is in the range of 10-30 seconds.

Free oscillations?                                                                                  GoToTop  

At times, very large earthquakes have caused the entire earth to vibrate as one single body. Such vibrations. Periods as high as 57 seconds were observed during the great Lisbon.

What is a microearthquake?                                                                   GoToTop 

    Earthquakes of magnitude less than 3 are called microearthquakes.

What are microtremors?                                                                         GoToTop 

Because of the sources of disturbance and movements within and on the surface of the earth, it is in a state of continuous vibratory motion. These vibrations are characteristics of individual locations on the surface because of the influence of local geological conditions. When these vibrations are recorded, they also show characteristic periods and distribution of energy as a function of frequency. These continuous vibrations are called microtremors.

What are microseisms?                                                                          GoToTop  

A typical pattern of vibrations is injected into the earth by a combination of external sources, which include ocean waves hitting the coasts and  wind vibrations which in turn cause large trees and other tree like structures to vibrate. These vibrations are called microseisms. These also have a characteristic period, close to about 6 seconds.

What is a GPS?                                                                                          GoToTop

Global Positioning System (GPS) is space-based radio-navigation system  consisting of 24 satellites and ground support for getting   accurate information on position and velocity, as well as the time, anywhere in the world and in all weather conditions. A GPS determines location by computing the difference between the time that a signal is sent and the time it is received. GPS satellites carry atomic clocks that provide accurate time.  The time information is sent back to the receiver, and it can continuously determine the time the signal was broadcast. The signal contains data that a receiver uses to compute the locations of the satellites and to make other adjustments needed for accurate positioning. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the distance, or range, from the receiver to the satellite. The receiver must account for propagation delays, or decreases in the signal's speed caused by the ionosphere and the troposphere. With information about the ranges to three satellites and the location of the satellite when the signal was sent, the receiver can compute its own three-dimensional position (latitude, longitude, altitude). An atomic clock synchronized to GPS is required in order to compute ranges from these three signals. This need is avoided by including measurements from a fourth satellite. the receiver avoids the need for an atomic clock. 

What is Surface Faulting?                                                            GoToTop     

Surface faulting is displacement that reaches the earth's surface during slip along a fault. Surface faulting commonly occurs with shallow earthquakes, those with an epicenter less than 20 km.

What is Liquefaction ?                                                                  GoToTop

Liquefaction describes a phenomenon whereby a soil substantially loses strength and stiffness in response to an applied stress, usually arising from earthquake shaking or other rapid loading (force), causing it to behave like a liquid.The phenomenon is most often observed in loose sandy soils. This is because a loose sand has a tendency to compress when a load is applied, dense sands by contrast tend to expand in volume or 'dilate'. 

What is Subsidence?                                                                       GoToTop    

 Lowering of a portion of a land area is termed subsidence.  Land subsidence can occur naturally or through human activity.  Earthquakes can  cause subsidence of the land because of the movement of  faults, fractures in the earth's crust in which the rock on one side of the fracture moves in relation to the rock on the other side.  

What is Collapse?                                                                              GoToTop

Collapse is a phenomenon where the Earth's material falls suddenly to close the empty space in the adjacent area, by falling or shrinking together. 

What is Landslide?                                                                                GoToTop

Landslide  includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows.  Whereas gravity is the driving force for a landslide,  there are specific sub-surface affecting slope  stability  that make the area/slope prone to failure, and  the actual landslides are  often  triggered by external agents like an earthquake.

What is Seismic Gap?                                                                             GoToTop

A seismic gap is a segment of an active fault that has not slipped in an unusually long time when compared with other segments along the same structure. A section of a fault that has produced earthquakes in the past but is now quiet is also considered a seismic gap. For some seismic gaps, no earthquakes have been observed historically, but it is believed that the fault segment may produce earthquakes in future. 

What is Lineament?                                                                                GoToTop

A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will comprise a fault-aligned valley, a series of fault or fold-aligned hills, a straight coastline or indeed a combination of these features. Fracture zones, shear zones and igneous intrusions such as dykes can also give rise to lineaments. Lineaments are often apparent in geological or topographic maps and can appear obvious on aerial or satellite photographs.   

What is Tectonic Province?                                                                      GoToTop

A tectonic province is  a specific contiguous geographic area in which the tectonic nature is identical.  

What is Tsunamis?                                                                                       GoToTop

A tsunami (also called or tidal wave) is a series of water waves  caused by the displacement of a large  body of water, usually an ocean or a large lake.  Due to the immense volumes of water and energy involved, tsunamis can devastate coastal regions. Large earthquakes and other disturbances above or below water  have the potential to generate a tsunami.

What are Seiches?                                                                                             GoToTop

A free or standing-wave oscillation of the surface of water in an enclosed or semi-enclosed basin (as a lake, bay, or harbor) that varies in a period, depending on the physical dimensions of the basin, from a few minutes to several hours, and in height from several centimeters to a few meters.