All About Earthquakes

All About Earthquakes

Earthquakes can be used to describe any seismic event. Naturally occurring earthquakes can be caused by a sudden release of stored energy within the rocks or faults created by the motion of the tectonic plates or volcanic activity. Whereas human-generated earthquakes can be observed after any sort of human activity, such as mining blasts, gas production, wastewater disposal, or hydraulic fracturing. No matter the source, a seismic event always generates seismic waves that travel through the body and surface of the ground.

An origin of a seismic event in the ground is called hypocentre, while the projected location directly above, on the surface, is called epicentre.

Plate Tectonics

Basics of an Earthquake

Seismic Waves

Earthquake Magnitudes

 
 

Plate Tectonics

The Earth produces energy deep within its interior from the decay of radioactive elements. This energy, combined with the heat that was produced during planetary formation, is redistributed to the cooler surface through both deep-seated melting and the slow circulation (convection) of hot rocks beneath the Earth’s crust. Because the hot mantle material is less dense than the material above it, the hot material is more buoyant and rises to the surface. The hotter material eventually loses heat by conduction, advection, and partial melting. Meanwhile, colder, denser rocks sink back into the mantle.

Tectonic plates overlapping the map of the Earth.

 

Movement of the mantle due to convection, is coupled to the creation and recycling of the crust, which is one of the fundamental processes of plate tectonics. Newly-made oceanic crust is formed at spreading centres (i.e., linear features where magma is extruded from the mantle). These spreading centres divide the crust into 15 major lithospheric plates, comprising relatively cool and buoyant crust and underlying mantle.

A convergent boundary, caused by a downgoing slab on the west coast of Canada, and a divergent boundary observed in the middle of the Atlantic Ocean.

 

The thickness of the oceanic crust varies from as little as 1.6 km to an average of 10–15 km. The continental crust varies in thickness from 10 km to almost 70 km in mountainous regions, with an average of 30–40 km. As the new oceanic crust is created at the divergent boundary, the old crust is shoved aside like a conveyor belt. When an old, dense, and rigid oceanic plate collides with another plate, the denser oceanic crust sinks, or subducts, beneath the more buoyant crust into the mantle. The subducting crust becomes part of a downwelling limb of a mantle convection cell, and forms a convergent boundary.

Over vast stretches of geological time, oceanic crusts are consumed and continents collide with each other, which increases the stress within the crustal elements. The crust is compressed or stretched from these motions, and earthquakes result when a sudden slip releases this stress. The largest earthquakes occur at subduction zones, where an oceanic plate slips beneath a continental plate. An example of this was the magnitude 9 Tohoku earthquake, 70 km east of the Oshika Peninsula on March 11, 2011, which initiated a tsunami.

Seismic waves as observed at stations in Alberta, that were triggered by the magnitude 9 Tohoku earthquake near the Oshika Peninsula.

 

For additional information on Earth structure and plate tectonics, see the U.S Geological Survey (USGS) and the British Geological Survey sites.

 

Basics of an Earthquake

The stress accumulated in rock masses over time can eventually exceed the mechanical strength of the rocks and fracture them in a planar or sheet-like feature known as a fault. The rupture (or slip) on the fault generates an earthquake. Earthquakes occur on existing faults (whether they are known to us or not) or they create new faults.

The direction of movement on the fault depends on the faults orientation with respect to the stress field and to the presence of fluids within the fault.

Fault blocks showing stress field and planar movement for (a) normal fault in which the hanging wall moves downward relative to the foot wall, (b) strike-slip in which the blocks move laterally in opposing directions, and (c) reverse or thrust fault in which the hanging wall moves upward relative to the foot wall. The three orthogonal stress directions are shown for vertical (SV), maximum horizontal (SHmax) and minimum horizontal (SHmin) stresses

 

For additional information on earthquakes and seismic faults, see the U.S Geological Survey (USGS) and the British Geological Survey sites.

 

Seismic Waves

Earthquakes result from the sudden slip of rock along a fault. This event sends seismic waves through the surrounding rock, some of which travel to the surface of the Earth. An analogy is a pebble thrown into a pond, creating ripples that travel through the water in all directions. The atoms within the rock respond to the shockwave in a similar way to the water in the pond, by moving back and forth parallel to the direction the wave is travelling (primary or compressional waves “P”) and up and down or side to side in a direction perpendicular to wave propagation (secondary or shear waves “S”). Energy is transferred through the rocks, moving away from the focus of the earthquake and, like the ripples in water, decay with distance.

 

Particle movement of seismic waves.

Shallow earthquakes can generate other types of seismic waves known as surface waves, which are trapped in the surface layers of the crust. These are trapped shear waves (Love waves) and combinations of shear and compressional waves (Rayleigh waves). It is the shearing and rolling action of surface waves and, to a lesser degree, the secondary waves travelling up through the crust that cause the most damage to structures.

For additional information on seismic waves, see the British Geological Survey, and  the U.S Geological Survey (USGS).

 

Earthquake Magnitudes

The magnitude or size of an earthquake is a measure of the energy released during an earthquake. The amount of energy released is directly related to the size of the fault (specifically the planar area) and the amount of movement on the fault (called slippage). This means that large faults can generate large earthquakes. An early magnitude scale developed by Charles Richter in 1935, from measurements of the largest peak of ground displacement shown on the vertical trace of the seismogram. The peak displacement of the shear wave is normalized by the time to complete one cycle, corrected for the distance the wave travelled to the seismometer and scaled to mimic the results that would have been recorded on an instrument similar to that used by Richter. Richter designed the units on the magnitude scale so that each unit marked a tenfold increase in size (meaning that each unit is 10 times larger than the previous unit – this is called logarithmic), in order to address the large range of values. Although the Richter scale is popular in the media, it is generally restricted to small local earthquakes.

Traditionally, for large shallow earthquakes (5 ML or greater), the surface wave peak displacement was used to calculate the magnitude (MS), and for large deep earthquakes, the displacement of the primary body wave was used to calculate the magnitude (Mb). These magnitudes, however, tend to saturate (reach a maximum value) at values over 8. For this reason, the moment magnitude (MW) is now preferred for earthquakes larger than M = 3.5 ML and at any depth. The MW is calculated from the seismic moment (MO), which is based on the product of the slippage area, the total displacement of the fault, and the rigidity of the rocks. Seismic events that are reported by the AGS use local magnitudes (ML), in order to stay consistent with media reported events, and due to the small magnitudes of the earthquakes in Alberta.

In general, the closer you are to the source of an earthquake, the more likely you are to feel the earth move beneath your feet. Humans often feel ML larger than about 3.  Earthquakes larger than an ML  5, in a populated area, can cause minor damage to structures. An exception to this is the case when ground conditions affect the release of energy. Because seismic waves travel more slowly through loose sediments than bedrock, when the wave moves from bedrock to sediment, the energy of the seismic wave is diverted to displacing the loose material instead of moving the seismic wave forward, causing stronger ground movement.

Magnitude (ML)

Earthquake Class

Observable Effects

Method of Recording

5 to 5.9

Moderate

Felt by everyone at further distances. Exact structural damage is unknown

Can be observed on distant near surface seismographs

4 to 4.9

Light

Felt by everyone. Structural damage can occur; some dishes and windows can break, and unstable objects overturned

Can be observed on regional near-surface seismographs

3 to 3.9

Minor

Will be felt at close distances but will feel very similar to the vibrations caused by a passing truck

Can be observed on regional near-surface seismographs

0 to 2.9

Very Minor

Rarely felt. Will be most noticeable on the top floors of the buildings

Can be observed on local near-surface seismographs

<0

Micro

Not felt

Can be observed on the instruments within the well

*Observable effects can vary throughout the province.

 

For a detailed explanation of different magnitude types and their calculation, see the U.S Geological Survey (USGS) and the British Geological Survey