Introduction to engineering seismology

Earthquakes

Definition: An earthquake, also referred to as “quake”, “tremor”, or “seismic excitation”, is the sudden shaking of the surface of the earth.

Cause: The cause of the earthquakes is the relative movement of the earth’s tectonic plates which results in energy accumulation and then sudden release.

The tectonic plates

The earth as a celestial body is not a solid object but consists of different layers. Figure 2.1 gives a schematic view of the interior of the Earth. The surface of the earth comprising the crust (outer layer) and the lithosphere (inner layer) is solid. On the contrary, its interior layers, namely the mantle and the outer core are in a much higher temperature and they are liquid. The inner core is solid due to the high pressure applied to it.

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Figure 2.1. Schematic view of the interior of the Earth.

The tectonic plates

The earth’s crust is not a single uniform surface but consists of a number of pieces called “the tectonic plates”. Because of the movement in the molten interior of the earth, the tectonic plates tend to move as well. The tectonic plates are categorised based on their size to:

• major plates (any plate with an area greater than 20 million km²),
• minor plates (any plate with an area less than 20 million km², but greater than 1 million km²), and
• microplates (any plate with an area less than 1 million km²)

The major tectonic plates are:

• Pacific Plate – 103,300,000 km²
• North American Plate – 75,900,000 km²
• South American Plate – 43,600,000 km²
• Eurasian Plate – 67,800,000 km²
• African Plate – 61,300,000 km²
• Antarctic Plate – 60,900,000 km²
• Indo-Australian Plate – 58,900,000 km²

The Indo-Australian is often considered as two independent plates:

• Australian Plate – 47,000,000 km²
• Indian Plate – 11,900,000 km²

The minor tectonic plates are:

• Somali Plate – 16,700,000 km²
• Nazca Plate – 15,600,000 km²
• Philippine Plate – 5,500,000 km²
• Arabian Plate – 5,000,000 km²
• Caribbean Plate – 3,300,000 km²
• Cocos Plate – 2,900,000 km²
• Caroline Plate – 1,700,000 km²
• Scotia Plate – 1,600,000 km²
• Burma Plate – 1,100,000 km²
• New Hebrides Plate – 1,100,000 km²

Microplates

Microplates are often grouped with an adjacent major plate on a major plate map. Some models identify more minor plates within current orogens (events that lead to a large structural deformation of the Earth's lithosphere) like the Apulian, Explorer, Gorda, and Philippine Mobile Belt plates. There may be scientific consensus as to whether such plates should be considered distinct portions of the crust.

Figure 2.2 shows the major tectonic plates and their relative movement. As can be noticed, the plates’ movement is not in the same direction, so in specific boundaries (referred to in technical literature as “faults”) they push each other (compression), while in others they break apart (tension). There are also boundaries where the relative movement of the tectonic plates is parallel to the boundary line. The main fault types are shown in Figure 2.3.

Scott Nash / Public domain
Figure 2.2. Tectonic plates and relative movement globally.

Wikimedia / CC-BY-SA 

Genesis of the earthquakes

Due to friction between the plates in a fault, strain accumulates on the fault surface. This strain results in the accumulation of potential energy in the fault. However, at some point in time, the accumulated strain is so large that the relates stress exceeds the capacity of the rocks, so rupture occurs. When a rupture occurs, relative movement of the plates takes place and the accumulated energy is released until they reach a new state of equilibrium where relative movement is once again restrained. Figure 2.4 shows the progress of a fault rupture in time for a typical transpressional (strike-slip) fault.

Figure 2.4. Progress of fault rupture in time.

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The energy released from the fault is in the form of seismic waves that propagate through the plates in a radial way. Because of this radial movement of the seismic waves, it is possible to trace back the origin of these waves to a relatively small area, assumed to be a single point, which would be the centre of a fictitious sphere. The vertical projection of this point on the earth’s surface is called the epicentre of the earthquake (see Figure 2.5). The distance between the point of origin of the earthquake and the epicentre is the focus of the earthquake (see Figure 2.5).

Figure 2.5. Progress of fault rupture in time.

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Seismic waves

The waves created during a fault rupture can travel in different ways:

• through the body of the plates (body waves).
• over the surface of the plates (surface waves).
• reflect on the earth’s core and travel to the surface again.

P-waves [body waves]

Primary waves (P-waves) are compressional waves (see Figure 2.6) that are longitudinal in nature. P-waves are pressure waves that travel faster than other waves through the earth to arrive at seismograph stations first, hence the name "Primary". These waves can travel through any type of material, including fluids, and can travel at nearly 1.7 times faster than the S-waves. In air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1,450 m/s in water and about 5,000 m/s in granite.

Figure 2.6. P-wave propagation.

S-waves [body waves]

Secondary waves (S-waves) are shear waves that are transverse in nature (see Figure 2.7). Following an earthquake event, S-waves arrive at seismograph stations after the faster-moving P-waves and displace the ground perpendicular to the direction of propagation. Depending on the propagational direction, the wave can take on different surface characteristics; for example, in the case of horizontally polarized S-waves, the ground moves alternately to one side and then the other. S-waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material.

Figure 2.7 P-wave propagation.

L-waves [surface waves]

Love waves (L-waves) are horizontally polarized shear waves (SH-waves) existing only in the presence of a semi-infinite medium overlain by an upper layer of finite thickness (see Figure 2.8). They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S-wave velocity, and have the largest amplitude.

Nicoguaro / CC BY

Figure 2.8. L-wave propagation.

Rayleigh waves [surface waves]

Rayleigh waves, also called ground roll, are surface waves that travel as ripples with motions (see Figure 2.9) that are similar to those of waves on the surface of water (note, however, that the associated particle motion at shallow depths is retrograde, and that the restoring force in Rayleigh and in other seismic waves is elastic, not gravitational as for water waves). The existence of these waves was predicted by John William Strutt, Lord Rayleigh, in 1885. They are slower than body waves, roughly 90% of the velocity of S-waves for typical homogeneous elastic media. In the layered medium (like the crust and upper mantle) the velocity of the Rayleigh waves depends on their frequency and wavelength.

Wikimedia / Public domain

Figure 2.9. Rayleigh wave propagation.

Seismic wave travel

Depending on the path seismic waves travel until they reach a seismograph on the earth’s surface, they can be further categorized (see Figure 2.10). Table 2.1 gives the designation of different seismic waves based on their path from the point of origin to the seismograph.

Table 2.1. Seismic wave designation

 Wikimedia / Public domain 

Figure 2.10. Seismic wave designation.

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Reading and Discussion Activities

Natural versus induced seismicity

Seismicity

Seismicity is a term used to describe the probability of occurrence of significant earthquakes in a region. It is difficult to quantify the “significance” of an earthquake, as it is related to its effect on the natural and built environment, as well as our own preparedness against it. However, as a common term, “high seismicity” refers to often occurrence of earthquakes above a specified magnitude in a region, while “low seismicity” refers to the rare occurrence of such events.

There are two types of seismicity, defined by their cause: natural and induced.

Natural seismicity describes the earthquakes that occur due to the natural displacement of the earth’s tectonic plates and their interaction. The majority of these earthquakes take place along the tectonic plate boundaries. Natural seismic activity can occur anywhere in the world but are stronger in regions close to faults and weaker in regions far from them.

Induced seismicity is related to human activity that results in seismic excitations. Human activity such as mining, hydraulic fracturing, geothermal operations, water reservoir dams and construction activities (e.g. foundation laying) can cause minor earthquakes or tremors. In comparison with natural earthquakes, induced earthquakes are typically small in magnitude, while their characteristics differ. Induced earthquakes occur anywhere in the world that such human activity takes place. Hence, induced activity can take place in regions where natural seismicity is relatively low, i.e. regions far from faults.

Natural seismicity in the UK

Natural seismicity in the UK is low compared to other countries in the world because the UK is relatively far from major faults (boundaries of the tectonic plates). As illustrated in Figure 2.2 (see p4 Microplates), countries like Spain, Italy and Greece in the Mediterranean Sea, or Iceland in the Atlantic Ocean are directly on those boundaries, so earthquakes are more often and stronger in these regions.

Natural seismicity in the UK is relatively low. However, significant earthquakes have taken place. On average, 20 to 30 earthquakes each year are felt by people in the UK.  Figure 2.11 (see next page) shows the locations of the epicentre of significant earthquakes (M_w ≥ 3) in the UK. Two main observations can be made based on this figure: (a) the epicentre of the earthquake can be virtually anywhere and (b) there are regions which can be defined as “hotspots”, i.e. a large number of seismic events originated from these areas. On average, earthquakes with a magnitude above 5, which can be felt by everyone nearby occur in the UK every 20 years,  while events measuring 4 (felt by many people) occur every three to four years. Based on a search conducted by the British Geological Survey on the 27th April 2020, there were 22 recorded earthquakes in the previous 50 days.

While this is a particularly rare occurrence for the UK, this is relatively common for other European countries such as Greece. Figure 2.12 shows the epicentres of all significant earthquakes (M_w ≥ 4) that took place in the Balkan peninsula with a focus on Greece. Comparison between Figure 2.11 and Figure 2.12 (see next page) is indicative of the difference in the frequency of seismic activity and the magnitude of the earthquakes occurring. While in the UK a large number of earthquakes with M_w = 4 ± 0.5 has taken place historically, this is not a common occurrence. On the contrary, in Greece, such earthquakes are so frequent that virtually the whole map is covered with them, while smaller magnitude events (M_w = 3 ± 0.5) are not even depicted.

A list of the most significant earthquakes in the UK retrieved from the British Geological Survey website is shown in Table 2.2. The strongest earthquake in the UK recorded has a magnitude of 6.1 and took place in the North Sea in 1931. For comparison purposes, the most significant earthquakes in Greece are given in Table 2.3. As can be seen, the strongest earthquake recorded in the 20th century had a magnitude of 7.7, while historically, earthquakes with magnitudes estimated up to 8.5 have taken place.

Research and discussion activity

When it comes to seismic events, it is a common misconception to assume the scales used to be linear. For example, most people would assume that a M_w = 5 earthquake is 25% “stronger” than a M_w = 4 earthquake. Using the formulas for local magnitude and moment magnitude, compare the strongest earthquake for the 20th century in the UK (i.e. < M_w = 6.1) with the strongest earthquake for the 20th century in Greece (i.e. < M_w = 7.7). Discuss how this is compared with the common conception?

Induced seismicity in the UK

As explained before, various different human activities can cause earthquakes. In the UK, it is estimated that at least 21% of seismic events between 1970 and 2012 with local magnitude M_L ≥ 1.5 are anthropogenic (Willson et al., 2015), while only 40% of them are confirmed to be of a natural cause. Considering the proportion of earthquakes of undetermined origin (i.e. 39%), one can see that a significant proportion of earthquakes in the UK is the result of human activity.

Induced seismicity events are monitored on a global scale. In the HiQuake (Human-Induced Quake) database, Foulger et al. (2018) present a global overview of human-induced earthquakes between 1868 and 2016. Based on the HiQuake database, to date 1174 such events have been confirmed (Foulger et al., 2018).

Hydraulic fracturing

Hydraulic fracturing also referred to as fracking (see Figure 2.13) is a process used to increase the permeability of reservoir formations and stimulate the recovery of hydrocarbons (see videos below for a brief introduction to hydraulic fracturing). This process is typically related to microseismicity, i.e. earthquakes with magnitude below 2.0. The mechanism of this earthquake generation is relatively well understood. Initially, the injection of fluids under high pressure creates new small cracks and fractures in a previously solid rock mass. These cracks aim to release the gas from the rock into the fluid so that it can be retrieved. While these cracks grow and spread, brittle failure of the rock occurs which results in microseismic events (often referred to as "fracked" events). The energy released by these "fracked" events is determined by the energy of the injection process: the larger the input, the larger the expected output. When this process takes place close to natural faults, both the presence of high-pressure fluid and the stress accumulation caused by the fluid can change the effective stress on pre-existing faults, causing rupture of the fault (see Figure 2.14). Such events are often also called "triggered" events. Unlike the microseismic events mentioned above, the energy released in this case depends on the accumulated energy in the fault and the additional energy due to fracking. Hence, in this case, the magnitude of the earthquake depends mainly on the capacity of the fault.

US Environmental Protection Agency, / Public domain

Figure 2.13. Activities related to hydraulic fracturing.

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Mikenorton / CC BY-SA  

Figure 2.14. Hydraulic fracturing near an existing fault.

Hydraulic fracturing 2

The common consensus among scientists is that the process of hydraulic fracturing used in order to recover shale gas does not pose a high risk for induction of significant earthquakes (felt, damaging or destructive). In the U.S., where thousands of stimulations have taken place, the magnitudes of the induced earthquakes are typically less than Mw = 1. Typically, such earthquakes cannot be felt by humans, so appropriate instrumentation is required to monitor them. However, most of these sites do not have the required instrumentation to appropriately monitor microseismic events, so earthquakes with a magnitude below Mw = 2.5, might be undetected, unless local seismographs can pick them up.

On the other hand, there are recorded examples of earthquakes with magnitudes larger than two which have been related to hydraulic fracturing for shale gas exploration and/or recovery.

• In the Etsho and Tattoo fields in Horn River, Canada, 216 earthquakes were detected during between 2009 and 2011 (BC Oil and Gas Commission, 2012). Twenty-one of these earthquakes had magnitudes of 3.0 or greater, and the largest event had a magnitude of 3.8 ML.
• In the Eola Field, Garvin County, Oklahoma, 86 earthquakes were detected during hydraulic fracturing in 2011 with magnitudes up to 2.9 ML (Holland, 2011).
• In July and August 2014, earthquakes with magnitudes of 4.0 and 4.2 occurred near Fort St. John, British Columbia, Canada. Both earthquakes are considered to have been induced by hydraulic fracturing activities in the region (Atkinson et al, 2015).
• In Lancashire, UK, 58 earthquakes were linked to fluid injection during hydraulic fracturing at the Preese Hall well in 2011 (de Pater and Baisch, 2011). The largest had a magnitude of 2.3 ML and was felt locally.

Activities

References

Atkinson, G., Assatourians, K., Cheadle, B. and Greig, W. 2015. Ground Motions from Three Recent Earthquakes in Western Alberta and Northeastern British Columbia and Their Implications for Induced Seismicity Hazard in Eastern Regions, Seismological Research Letters, 86, 3, 1-10

BC Oil and Gas Commission (2012). Investigation of Observed Seismicity in the Horn River Basin. BC Oil and Gas Commission: British Columbia.

British Geological Survey (2007). Eurocode 8 seismic hazard zoning maps for the UK Seismology and Geomagnetism Programme. [online] Available at: http://www.earthquakes.bgs.ac.uk/hazard/UK_seismic_hazard_report.pdf [Accessed 9 Jun. 2020].

‌de Pater, H. & Baisch, S. 2011. Geomechanical Study of Bowland Shale Seismicity, Synthesis Report

Foulger, G. R., Wilson, M. P., Gluyas, J. G., Julian, B. R., & Davies, R. J. (2018). Global review of human-induced earthquakes. Earth-Science Reviews, 178, 438-514.

Holland, A. (2011). Examination of Possibly Induced Seismicity from Hydraulic Fracturing in the Eola Field, Garvin County, Oklahoma. Oklahoma Geological Survey Open File Report OF1, 2011.

Wilson, M. P., Davies, R. J., Foulger, G. R., Julian, B. R., Styles, P., Gluyas, J. G., & Almond, S. (2015). Anthropogenic earthquakes in the UK: A national baseline prior to shale exploitation. Marine and Petroleum Geology, 68, 1-17.