Earthquakes: Processes, Causes And Measurement

Meaning and Definition of Earthquakes

An earthquake is the vibration of the earth caused by shock waves due to the sudden release of energy, that results from sudden displacement along faults or movement or magma or sudden ground subsidence. The energy is abruptly released after a long and slow accumulation of strain along a fault. The adjustments of the earth’s surface after the initial earthquake generate a series of low-intensity earthquakes referred to as aftershocks.

The scientific study of earthquakes is called seismology, derived from the Greek seismos, meaning ‘earthquake’ and logos, meaning ‘reason’ or ‘speech’. Data from seismology have become an integral part of the modern-day scientific understanding of the constitution of the Earth’s interior.

Modern Seismology

The credit for developing the science of seismology goes to a group of British scientists chief amongst them being John Milne who is credited for the creation of the instrument called the seismograph capable of detecting signals from distant earthquakes. The seismograph consists of-

An inertia member,
A transducer, and
A recorder.

The inertia member is a weight suspended by spring so that it acts like a pendulum but is capable of moving in one direction only. The inertia member tends to remain at rest as the earth’s waves pass by.

The transducer is capable of detecting the relative motion between the mass and the ground, which it converts into a recordable form.

A standard recorder is a cylindrical drum with a sheet of recording paper wrapped in it. It rotates at a constant speed, producing a series of parallel lines. When an earthquake strikes a place, these lines move in response.

Seismographs Source: Adopted from ‘Physical Geology: Exploring the Earth’ by Monroe, Wicander, Hazlett, 2007.

Epicentre and Focus of an Earthquake

The point at which the energy is first released is the focus of the earthquake or the hypocentre. Epicentre, on the other hand, is the place on the surface of the earth which is directly above the focus.

For instance, the December 26, 2004 earthquake that triggered the devastating tsunami in the Indian Ocean had an epicentre 160 km off the west coast of northern Sumatra and a focal depth of 30 km. (Fig).

Depending upon the focal depth, three categories of earthquakes can be recognised: shallow focus, intermediate focus and deep focus.

Shallow and intermediate-focus earthquakes have focal depths of less than 70 km and between 70 and 300 km from the surface, respectively. At the same time, those with foci located at a depth of more than 300 km are categorised as deep-focus earthquakes. Shallow-focus earthquakes are the most destructive as the energy has little time to dissipate before reaching the surface.

Earthquake Processes

Various earthquake processes are classified as follows:

Faulting

A fault refers to a crack or a fracture in the earth’s surface, and faulting refers to the process of fault rupture. As the two lithospheric plates move past each other, they are slowed down due to friction along their boundaries, producing strain and deformation in the rocks.

An increase in stress in these rocks over their strength, leads to rupture, forming a fault or a crack and producing an earthquake. Rupturing starts to occur at the focus of the earthquake and continues to move upwards, downwards and laterally. In this process, the stored energy is released, producing shock waves or earthquake waves or seismic waves that vibrate the ground.

Tectonic Creep

Tectonic creep refers to faults displaying gradual displacement and is usually not accompanied by earthquakes. This process, however, can be potentially damaging, causing the deformation of roads and other standing structures.

Slow Earthquake

Slow earthquakes are relatively newly recognized earth processes, also produced due to fault rupture. The rupture is relatively slow and may span over several days and even months. Their magnitude varies between 6 and 7 as they have located over large areas of the rupture.

The amount of slip is generally very small, up to a few centimetres. The continuous geodetic surveys and measurements with GPS (Global Positioning Satellites) have aided in the identification and recognition of these earthquakes.

Types of Seismic Waves

When an earthquake strikes a place, it is the seismic waves that cause strong motion, crack the ground and cause immense damage to standing structures and buildings. The energy released during an earthquake moves mainly in two forms of seismic waves radiating in all directions from the focus – Body waves and Surface waves.

While Body waves travel through the solid body of the earth and are analogous to sound waves, Surface waves travel along the ground and bear similarity to waves on water’s surface.

Body Waves

Two types of Body waves are recognised – P waves and S waves.

P-waves or primary waves are the fastest-moving seismic waves and are felt first. They can travel through solids, liquids and gases. P waves are similar to sound waves and are compressional waves propagating in the same direction as the waves themselves in two and fro motion. The material through which the P waves travel compresses and expands as the waves move through it and come back to their original position once the waves have passed by.

S-waves or secondary waves travel slower than P waves and can travel through solids only. S waves are also called shear waves due to strong side-to-side and up-and-down shearing motion. The movement of the particle is perpendicular to the wave motion. Since liquids are not rigid, they have no shear strength thus, S waves cannot be transmitted through them.

The P and S waves travel at different velocities that are determined by the density and elasticity of the materials through which they travel. The velocity of P waves is greater than S waves in all types of media and thus P waves arrive first at recording seismic stations. The P and S wave travel times are published in time-distance graphs that illustrate the difference between the arrival times of two waves as a function of the distance between the arrivals of the two Primary (P) and Secondary (S) waves.

Since the two waves have varying velocities – P waves travel at about 6 kilometres per second through bedrock while the S waves travel at about 3.5 kilometres – the distance to the earthquake’s focus can be calculated.

Schematic seismogram showing the arrival order of the P, S and L waves. Source: Adopted from ‘Physical Geology: Exploring the Earth’ by Monroe, Wicander and Hazlett, 2007.

Surface Waves

The movement of surface waves is limited to the ground or just below it. These are slower than the body waves and arrive just after the S waves. Two important surface waves are Rayleigh waves and Love waves, named after the British scientist Lord Rayleigh and A.E.H. Love, respectively.

Rayleigh-waves are slow waves and behave like water waves wherein the individual particles of the material move in an elliptical path within a vertical plane oriented in the direction of wave movement.

Love-waves are similar to an S wave, and the individual particles move only back and forth in a horizontal plane perpendicular to the direction of wave travel.

Causes of Earthquakes

Most earthquakes are caused by the deforming forces in the Earth, and the immediate cause is the sudden rupture of the earth’s materials distorted beyond the limit of their strength.

Elastic Rebound Theory

After the 1906 San Francisco earthquake, H.F. Reid, a scientist at John Hopkins University, conducted a study to understand the mechanics of earthquakes. The field study showed that the Pacific Plate had moved past the adjacent North American plate by as much as 4.7 meters during this earthquake.

According to Reid, tectonic stress builds up over hundreds of years, deforming rocks on both sides of a fault. When rocks experience continuous stress, they bend and store elastic energy. As soon as the strength of the rock is exceeded, it ‘snaps’ back and rebounds to its original position and shape. Since the rocks behave like elastically like stretched rubber bands, Reid termed it an ‘elastic rebound’ (fig.).

In this process of rocks snapping back to the original unstrained position, vibrations are generated that are felt as tremors or earthquakes. During the San Francisco earthquake along the San Andreas fault, straight-line fences and roads that crossed the fault gradually bent as the rocks on both sides moved relative to each other. (Fig.).

Earthquakes Induced by Anthropogenic Activities

Human activities, too, are responsible for inducing large earthquakes, causing widespread havoc and destruction. Three ways in which human activities trigger earthquakes include:

A load of Dam or Reservoir on the crust
Waste Disposal in deep underground wells
Nuclear explosions.

Reservoir and Seismicity

Reservoir-induced seismicity (RIS) has triggered hundreds of earthquakes. The construction of Zipingpu Dam induced the 7.9 magnitude Sichuan earthquake in May 2008, in which more than 80,000 people lost their lives.

As the dams are constructed, extra water pressure is created in the micro-cracks and fissures in the ground and near the reservoir. The increased water pressure in rocks lubricates faults already under tectonic strain but have remained in place due to friction of rock surfaces.

Deep Waste Disposal

Injecting waste into deep disposal wells, like in the disposal of produced waters from oil and natural gas wells, has been known to cause earthquakes. The 2011 Oklahoma earthquake of magnitude 5.6 is believed to be the strongest earthquake induced by the injection of waste materials.

Nuclear Explosions

Earthquakes with large magnitudes like M5 to M6.3 have been known to be triggered off by underground nuclear explosions. This was because the explosions caused a simultaneous release of natural tectonic strain.

Mining and Quarrying

Mining generally alters the balance of forces in the rock, often causing rock bursts.

Measuring the Earthquakes

Earthquakes are measured based on their Magnitude and Intensity.

Magnitude

The amount of energy released during an earthquake at the source is quantified through magnitude. It is measured with the help of seismographs, which plot the earthquake waves travelling through the earth. Two measures for earthquake magnitude are the Richter Scale and Moment magnitude.

Richter Magnitude

Using seismic records, Charles Richter, in 1935 from the California Institute of Technology, developed the first magnitude scale. The amount of ground displacement and shaking produced at the epicentre is measured on the Richter scale.

It is a logarithmic scale, meaning each successive magnitude is 10 times stronger than the previous one. Magnitude 2 means that the energy released was 10 times more than that released during an earthquake of magnitude 1 and in magnitude 3, 100 times more energy is released.

The scale calculates the amplitude of the seismic waves (P, S and surface) as they are recorded by the seismograph. The epicentre of the earthquake and the distance between the various seismograms are calculated. Accordingly, adjustments are made in the measurements obtained on the seismographs, as there is variation in the time the seismic waves take to travel through different rock types.

Although earthquakes of a smaller magnitude occur each day on the lithosphere, the most devastating are the ones with more than magnitude 8. It is shown by the symbol ML where M is for magnitude, and L is for local.

The merit of the Richter scale is the ease of describing the size of an earthquake by a single number that is calculated from a seismogram. Furthermore, unlike the intensity scales, which are popular for understanding earthquakes in densely populated areas of the globe, the magnitude of earthquakes in the remotest regions can be measured by the Richter scale.

Also, the events that occur in ocean basins can be measured by the same. Despite its usefulness, the Richter scale cannot be used to describe earthquakes of very high magnitude. For instance, the 1906 San Francisco and 1964 Alaskan earthquakes had roughly the same Richter magnitudes.

However, based on the relative size of the affected area and the associated tectonic changes, the Alaskan earthquake released a considerably large amount of energy. Thus, the Richter scale cannot distinguish between major earthquakes.

Moment Magnitude

An improvement over the shortcomings of the Richter scale the moment magnitude scale has been developed to measure the relative energy release. It is denoted by Mw. It is calculated by measuring the average slip on the fault, the area of the fault surface that slipped, and the strength of the faulted rock.

This method is especially useful for measuring large earthquakes. The largest earthquake, which took place in Chile in 1960, had a Richter magnitude of 8.9 but an estimated moment magnitude of 9.5. All magnitude scales are logarithmic in nature.

Intensity

The Intensity scale is devised to interpret the qualitative measure of damage caused by the earthquake. News headlines vividly discuss the aftermath when an earthquake strikes a place. It gives us information about the death toll, the destruction and other such tragic details of the damage caused, all of which tell us how intense the earthquake had been.

Modified Mercalli Intensity Scale has values ranging between I and XII, where I mean that the earthquake is Not felt except by a few, and XII is total damage to the structures (Fig.).

Earthquakes: Processes, Causes and Measurement

Meaning and Definition of Earthquakes

Modern Seismology

Epicentre and Focus of an Earthquake