7.1 Introduction

It has long been known that humans can cause earthquakes, whether related to dam impoundment, mining activity, geothermal energy development, or hydrocarbon production. These types of earthquakes are commonly referred to as being either induced or triggered. The difference between induced and triggered seismic or earthquake events is considered based on whether the human‐induced stresses are similar to the ambient stress state or are only a small fraction of the ambient level, respectively. In seismology, the term triggered also reflects earthquakes that are caused by previous earthquakes, whether they are naturally occurring or anthropogenic derived.

With the development of shale gas, concern and interest over induced seismicity and identification of the risks associated with well stimulation techniques such as high‐volume hydraulic fracturing have been recognized. Despite this recent attention, anthropogenic‐induced seismicity has been known and reported since at least the 1920s. These events have been associated with a range of activity including controlled explosions associated with the construction and mining industries, impoundments associated with large reservoirs, underground nuclear testing, and certain activities associated with the energy extraction industries (Table 7.1). As of 2017, ~730 anthropogenic projects or projects that have been proposed have induced earthquakes over the past 150 years (Wilson et al. 2017). Recent interest has focused on the extraction of shale gas and heavy oil and CO₂ sequestration operation.

In general, hydraulic fracturing causes a very small percentage of earthquakes that are felt at the ground surface in the United States with most of them a result of subsurface injection of wastewater related to oil and gas production. In contrast, in western Canada, hydraulic fracturing‐induced earthquakes that are felt are more common. Induced seismicity and felt ground shaking associated with the energy industry have been reported in several states including Alabama, Arkansas, California, Colorado, Illinois, Louisiana, Mississippi, Nebraska, Nevada, New Mexico, Ohio, Oklahoma, and Texas. Those specifically associated with hydraulic fracturing operations including associated wastewater injection activities include certain states such as Arkansas, Oklahoma, and Texas.

In a 2013 report published by the US National Research Council of the National Academies titled “Induced Seismicity Potential in Energy Technologies,” three major findings were offered:

1. The process of hydraulic fracturing a well as presently implemented for shale gas recovery does not pose a high risk for inducing felt seismic events.
2. Injection for disposal of wastewater derived from energy technologies into the subsurface does pose some risk for induced seismicity, but very few events have been documented over the past several decades relative to the large number of disposal wells in operation.
3. Carbon capture and recovery (CCS) operations, due to the large net volumes of injected fluids, may have potential for inducing larger seismic events.

Where induced seismicity has been documented, such events can be induced at distances of 10 km or more from the injection point, as well as at significant depths below the injection point. One of the earliest and most documented accounts of induced seismicity related to subsurface injection of fluids occurred at the Rocky Mountain Arsenal. Located in Colorado at a federal reservation where chemical armaments were manufactured, and large quantities of liquid wastes were generated over the years. These liquids required disposal, and from 1962 through 1966, about 165 million gallons of liquid wastes was injected under pressure. The injected fluids increased the pore pressure and reduced the normal stress across a fault, which resulted in earthquakes that could be correlated with the volume and timing of fluid injections. Studies performed at the Rock Mountain Arsenal in the 1960s would report induced seismicity at distances of at least 10 km laterally from the well and at depths of 4 km greater than the depth of injection. More recent studies postulate induced seismicity at 20 km or more from the injection point.

Not all injection wells cause earthquakes, or for that matter earthquakes that are felt. There are tens of thousands of wells that undergo hydraulic fracturing every year in the United States. This does not include the ~35 000 active wastewater injection wells and 80 000 active enhanced oil recovery wells. Of these, only a few dozen of these wells were known to have caused induced seismicity to the level where earthquakes were felt.

After hundreds of thousands of hydraulic fracturing operations, only very few examples of felt seismicity have been documented. Furthermore, the likelihood of inducing felt seismicity by hydraulic fracturing is relatively small compared with other activities and operations associated with mining, oil and gas field depletion, reservoir impoundment, enhanced geothermal system, and wastewater injection. This low potential to be felt at the ground surface is exemplified by the conventional need to deploy sensors using downhole geophone strings a few hundred meters of the hydraulic fracturing area of influence in order to detect fracture growth and fault reactivation. In instances where earthquakes with magnitudes larger than expected for fracture propagation are noted and deemed responsible for the seismic event being felt at the ground surface, they are typically associated with reactivation of a discrete and critically stressed fault.

Although induced seismic events have not resulted in loss of life or major damage in the United States, the effects have been felt locally. Because induced or triggered events occur at relatively shallow depths in comparison with deeper tectonic events, ground motions could be greater than those of deeper events of similar magnitudes and epicenter distances. This raises concern among communities that live in areas where energy extraction‐related activities are presently occurring or planned or where such activities occur in environmentally sensitive areas.

7.2 Measuring Earthquake Severity

Conventionally, earthquakes are the result of forces deep within the Earth’s interior that affect the surface of the Earth. The energy from these forces is stored in a variety of ways within the rocks. Energy can be released by a number of mechanisms. The shearing of rocks in the subsurface is one of several mechanisms that allow energy to be released, thus resulting in an earthquake. The release of energy resulting in earthquakes can also be induced. The focus or hypocenter of the earthquake is the area of the fault where the sudden rupture takes place. The epicenter of an earthquake is the point on the Earth’s surface directly above the focus.

Naturally induced earthquakes can occur anywhere between the Earth’s surface and about 700 km below the surface. For general scientific purposes, this depth range can be divided into three zones: shallow, intermediate, and deep. Shallow earthquakes are between 0 and 70 km deep; intermediate earthquakes, 70–300 km deep; and deep earthquakes, 300–700 km deep. The term “deep‐focus earthquakes” is generally applied to earthquakes deeper than 70 km. In addition, all earthquakes whose focus is deeper than 70 km are localized within slabs of shallow lithosphere that are sinking into the Earth’s mantle along subduction zones.

The severity of an earthquake can be expressed in terms of intensity and magnitude. The effect of an earthquake on the Earth’s surface is called the intensity. Intensity is based on the observed effects of ground shaking at the ground surface. This can vary from place to place within the affected area and is largely dependent on the location of the observer with respect to the earthquake epicenter. Magnitude however is related to the amount of seismic energy released at the focus or hypocenter of the earthquake and is based on the amplitude of the earthquake waves that are recorded on instruments that are calibrated similarly. Where intensity values can vary, the magnitude of an earthquake is thus represented by a single, instrumentally determined value.

7.2.1 Seismic Intensity and Magnitude

Although numerous intensity scales have been developed over the last several hundred years to evaluate the effects of earthquakes, in the United States, the modified Mercalli (MM) intensity scale is commonly used. Intensity is based on an earthquake’s local accelerations and how long these persist. The MM intensity scale is descriptive and consists of a series of 12 levels of certain key responses such as people awakening, movement of furniture, damage to chimneys, and if large enough total destruction (Table 7.2).

Intensity scale	Effect
I.	Not felt except by a very few under especially favorable conditions
II.	Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing
III.	Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated
IV.	Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably
V.	Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. Pendulum clocks may stop
VI.	Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight
VII.	Damage negligible in buildings of good design and construction; slight to moderate in well‐built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken
VIII.	Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned
IX.	Damage considerable in specially designed structures; well‐designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations
X.	Some well‐built wooden structures destroyed; most masonry and frame structures destroyed with foundations. Rail bent
XI.	Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly
XII.	Damage total. Lines of sight and level are distorted. Objects thrown into the air

The USGS provides a forecast for ground shaking intensity from natural and induced earthquakes as of 2017 (Figure 7.1). There is a small chance, 1%, that ground shaking intensity will occur at this level or higher, whereas there is a greater chance, 99%, that ground shaking will be lower than what is displayed in these maps.

To illustrate the surface distance from a postulated tectonic earthquake epicenter and from a well, an example is shown in Figures 7.2a and 7.2b. For naturally occurring tectonic earthquakes, the example illustrated in Figure 7.2a shows that only M 5 earthquakes will generate surface ground shaking. However, only M 5 earthquakes will generate surface ground shaking. For shallow‐induced earthquakes as illustrated in Figure 7.2b, an earthquake induced at the bottom of a well would result in shallow depth ground shaking at the surface for all three magnitudes.

The quantitative measure of an earthquake’s size at its source is its magnitude. There are a number of ways to measure the magnitude of an earthquake. The first widely used method, the Richter scale, was developed by Charles F. Richter in 1935 for the purpose of measuring the total energy released during an earthquake, and a formula based on amplitude of the largest wave recorded on a specific type of seismometer and the distance between the earthquake and the seismometer is used. Specific to California earthquakes, other scales, based on wave amplitudes and total earthquake duration, would later be developed for use in other situations and designed to be consistent with Richter’s scale. The amount of energy that is radiated by an earthquake is a measure of the potential for damage to man‐made structures. In relation to magnitude, for every increase in magnitude by 1 unit, the associated seismic energy increases by about 32 times.

To alleviate some of the limitations using Richter's scale, the moment magnitude scale, abbreviated M_w, is preferred because it works over a wider range of earthquake sizes and is applicable globally. Developed in the 1970s to succeed the 1930s‐era Richter magnitude scale (M_L), the moment magnitude scale (MMS) (denoted as M_w or M) is used by seismologists to measure the size of earthquakes. The MMS is based on the total moment release of the earthquake. With moment as a product of the distance a fault moved and the force required to move it, it is derived from modeling recordings of the earthquake at multiple stations.

Magnitudes are based on a logarithmic scale (base 10). What this means is that for each whole number you go up on the magnitude scale, the amplitude of the ground motion recorded by a seismograph goes up 10 times. Using this scale, a magnitude 5 earthquake would result in 10 times the level of ground shaking as a magnitude 4 earthquake, with a corresponding 32 times as much energy released. Magnitude scales can be used to describe earthquakes so small that they are expressed in negative numbers. The scale also has no upper limit, so it can describe earthquakes of unimaginable and unexperienced intensity, such as magnitude 10.0 and beyond.

The size of the area over which the shaking is felt provides another measure of the relative strength of an earthquake, thus making size affected particularly useful in estimating the relative severity of historic shocks that were not recorded by seismographs or occurred in less populated areas. A well‐known example that exemplifies high severity was the 1811 and 1812 New Madrid earthquake, situated near New Madrid, Missouri. Both were felt over the entire eastern US region. With so few people populating the region west of New Madrid at the time, it is not known how far these events were felt in that direction. The other is the 1886 earthquake near Charleston, South Carolina, which was also felt over a region of about 2 million square miles and included most of the eastern United States.

The Richter scale was long viewed as the standard for measuring the intensity of earthquakes, particularly in comparing one quake to another. In recent years, however, the Richter scale has been superseded by the MMS. Both scales measure the magnitude of a quake by the seismic energy released, and the readings generated by the two can be very similar, but the MMS uses newer technologies to produce a more accurate measurement of a specific earthquake event, and only the MMS is capable of accurately measuring magnitude 8 (M 8) or greater.

Today, news reports about earthquakes often omit references to either scale and indicate the magnitude number only. The truth remains in that the higher the number, the more severe the quake. Earthquakes can also be categorized in various classes ranging from minor to great based on their respective magnitude (Table 7.3).

Class	Magnitude
Great	8 or more
Major	7–7.9
Strong	6–6.9
Moderate	5–5.9
Light	4–4.9
Minor	3–3.9

Relative to magnitude, the increase in the degree of surface shaking (intensity) for each unit increase of magnitude of a shallow crustal earthquake is unknown. Intensity and magnitude both depend on many variables that include exactly how rock breaks and how energy travels from an earthquake to some point.

7.3 Anthropogenic‐Induced Earthquakes

The amount of energy released during a seismic event is conventionally referred to as the magnitude. Since all hydraulic fracturing activities induce fractures in the rock, thus earthquakes, many if not most induced earthquakes are simply not felt or are too small to detect. These are typically referred to as microearthquakes and are typically smaller than M 0. These intentionally induced events are typically on the order of −3.0 ≤ M ≤ 0. To be felt at the ground surface, a magnitude of about M 3 or greater is needed. To induce earthquakes of this magnitude, a number of factors come into play including:

• Faults large enough to produce earthquakes of M 3 or greater.
• Stresses that are large enough to produce earthquakes of M 3 or greater.
• The presence of fluid pathways from the injection point to the fault.
• Fluid pressures large enough to induce earthquakes of M 3 or greater.

Since around 2009, injection or hydraulically fractured induced earthquakes are generally based on consideration of recorded event hypocenters relative to the hydraulically fractured or injection wells and associated injection rates or volumes. In consideration of the timing of the shale revolution, comparable data is not available for the period prior to 2009. Historically, induced seismicity or the ability to measure earthquakes from man‐induced activities was in the 1960s and associated with wastewater injection at the Rocky Mountain Arsenal in the 1960s. In the context of hydraulic fracturing, however, the first reported instance occurred near Blackpool in the United Kingdom in 2011. The largest induced event from long‐term wastewater disposal was the November 2011 M_w 5.6 earthquake near Prague, Oklahoma.

The largest event associated with the hydraulic fracturing stage of shale gas, likely the result of reactivation of preexisting faults within their respective area of influence, was in 2015 where two M 4.4 events in Fox Creek, central west Alberta, and northeast British Columbia were reported. In these cases, relatively high volumes of fluids on the order of 630 000 barrels or 100 000 m³ were injected. Oklahoma has experienced a 160‐fold increase in earthquakes, and in 2014, the measureable earthquake rate was reported to surpass California’s (Figure 7.3). In the United States, Texas, for example, has experienced a sixfold increase in earthquakes relative to historic levels (Figures 7.3 and 7.4). This study by Frohlich et al. (2018) attempted to distinguished natural earthquakes from induced earthquakes by looking at timing, spatial correlation, depth existence or inferred presence of faults, and published analysis linking seismicity to production or injection operations. The investigators also attempted to distinguish between tectonic and possibly, probably, and almost certainly induced events.

Induced seismicity has been recognized with subsurface fluid injection technologies related to long‐term wastewater disposal, enhanced oil recovery, and CO₂ capture and storage. Notably, wastewater generated as a by‐product of hydrocarbon extraction via both traditional oil extractions as well as during enhanced recovery processes, including hydraulic fracturing, is the primary focus. Wastewater is often disposed of via injection wells into deep (1–2 km) wells, which has been shown, for a small percentage of wells, to induce seismicity in Oklahoma, as well as the broader central and eastern United States since 2009. The recent increase in injection‐induced seismicity accompanied a corresponding increase in wastewater disposal in the central United States. The earthquake rate increase in Oklahoma, where the vast majority of the increase has occurred (585 of 688 M ≥ 3 earthquakes in the central United States in 2014), was noted to correspond to a doubling of the wastewater disposal rate in the state from 1999 to 2013, where injection increased by factors of 5–10. Currently, Oklahoma has more M 3+ earthquakes per year than California.

Oklahoma is not alone, and other areas of increased rates of induced earthquakes that also experienced sudden increases in wastewater disposal have been reported. A summary of select induced seismic events with greater than M_w is provided in Table 7.4.

7.4 Mechanics of Anthropogenic‐Induced Earthquakes

The causes for induced seismicity are varied and complex (Figure 7.5). Historically, anthropogenic‐induced earthquakes have been documented for such activities as reservoir impoundment, fluid injection, fluid extraction, mining, and construction. Regardless of the activity, the mechanics behind inducing earthquakes is a reflection of a change in the stress conditions of faults within the sphere of influence. Studies have shown that subsurface oil and gas enhancement strategies such as hydraulic fracturing and steam and wastewater injection can induce changes in the impacted area by changing local pore pressures and in situ stresses. This results in brittle failure in the rocks and in some cases causes additional slip and shearing in naturally fractured rocks. In the case of fluid injection, induced earthquakes can occur in four different ways:

• Injection of fluids resulting in an increase in pore pressure within a fault.
• Injection of fluids that in turn fills and compresses fluids within pore spaces resulting in deformation (i.e. poroelastic effects).
• Injection of fluids that is relatively colder than the rock into which it is being injected resulting in thermoelastic deformation.
• Injection of fluids in a manner that reduces the effective normal stress. In this case, the fluids act to hold the fault closed (the normal stress). The frictional resistance to slip is lower, and the fault is more prone to slip (Figure 7.6).

When considering natural or induced fracturing, there are two distinct types of failure by rupture: brittle fracture, which takes place in the course of elastic deformation, and ductile fracture, which takes place in the course of ductile deformation. From a dynamic perspective, there are two basic types of resulting fracture surfaces: extension fractures and shear fractures. To understand the initiation of shear slip on fault planes, the Mohr–Coulomb model provides an explanation. Regardless of operational differences such as wastewater injection or hydraulic fracturing, the Mohr–Coulomb model also helps us understand the mechanism of fracture and effect of pore/fluid pressure in a compressive medium. Regardless of the operational differences between wastewater injection and hydraulic fracturing, initiation of shear slip on fault planes can be explained by the Mohr–Coulomb model (Figure 7.7).

Other parameters and factors can also influence whether or not injection will induce earthquakes. Some of these factors include injection parameters such as cumulative injection volume, injection rate, injection pressure, fluid temperature, and injection depth. Reservoir conditions and parameters such as pore pressure, temperature, permeability, and rock strength can also be factors. Geologic and hydrogeologic conditions are also important. Notably, the presence and orientation of preexisting faults relative to the overall local stress field of significant and abrupt changes in geologic conditions including significant permeability contrasts are areas where slip could occur under certain conditions. Understanding what specific conditions may result in induced earthquakes during or after injection is difficult since many of the parameters are variable and not well known within a specific system or are unknown. Because of the nature of deep‐seated faults that do not daylight the ground surface or were not characterized during exploratory efforts, only become known once slippage along them occurs.

7.5 Induced Microseismicity and Microseismic Monitoring

In the context of hydraulic fracturing, microseismic events are basically very small earthquakes characterized by generally negative moment magnitude. Over the past decade, a significant interest in microseismic monitoring has accompanied the increase in the development of unconventional resources associated with shale gas and enhanced oil plays.

With the increased interest in induced seismicity and human‐caused earthquakes associated with shale gas and heavy oil plays, and related activities associated with injection practices, a significant increase in microseismic monitoring has been reported in Canada, the United States, Eastern Europe, and the United Kingdom. Our understanding of these terrains where induced microseismicity has been reported or encountered varies widely and is challenging since it requires specific knowledge of the geology of the area (i.e. preexisting faults), reservoir permeability, injection parameter, ambient stress field, and the mechanisms associated with fault‐slip triggering, among others.

As previously noted, activities such as hydraulic fracturing, steam injection, and wastewater injection can cause changes in the local pore pressure regime and in situ stresses, resulting in brittle fracturing of intact rock and additional slip or shearing of naturally fractured rock. During rock failure or slip, an acoustic emission referred to as a microseismic event occurs. The resultant microseismic cloud presents a volumetric map of the extent of induced fracturing and slippage or shearing, opening, and closing. The value of microseismic monitoring is that it can provide pertinent information on the in situ reservoir deformation due to fluid stimulation, thus facilitating reservation drainage.

Produced water from depleted oil and gas wells supplies a large portion of the waste fluids, which are currently disposed through deep injection wells. In the last few years, a growing percent of the waste fluids that require injection are derived from unconventional oil and gas wells. USGS uses computer modeling to simulating fluid flow to see how the fluid pressure in the subsurface changes in response to injection. In their studies (USGS 2017), fluid pressure increases within faults, which are believed to be the main cause of induced earthquakes. The modeling was based on a scenario similar to actual injections in Oklahoma and shows a detectable rise in fluid pressure out to 5 mi (8 km) away from the well. USGS noted that after one decade, that same pore pressure change can be seen at nearly 15 mi (24 km) (Figure 7.8).

7.6 Exercises

1. 7.1 What were the three major findings of the National Research Council of the National Academies in their 2013 report titled “Induced Seismicity in Energy Technologies”?
2. 7.2 What are some of the maximum distances and depths reported where induced seismicity has been documented?
3. 7.3 What is the difference between earthquake intensity and earthquake magnitude?
4. 7.4 What is the moment magnitude scale, and why is it preferred over other magnitude scales?
5. 7.5 What are some of the examples of anthropogenic‐induced earthquakes?
6. 7.6 What is the major cause of induced seismicity associated with hydraulic fracturing operations, and where is it most prominent?
7. 7.7 What is the correlation between wastewater injection and induced seismicity? Where have such circumstances been reported and documented?
8. 7.8 What are some of the mechanisms for induced seismicity associated with hydraulic fracturing operations to occur?
9. 7.9 What are the two basic types of fracture surfaces? Describe them in respect to the Mohr–Coulomb model.
10. 7.10 What are microseismic events?