There have been growing moves to increase public involvement in scientific issues – from funding priorities and data collection, to policy decisions – particularly on topics with social and environmental importance such as climate change, flooding, energy policy, and genetically modified crops (e.g. Rowe et al., 2005; Parkins and Mitchell, 2005; Horlick-Jones et al., 2007; Nisbet, 2009). This progression brings a new communication challenge, i.e. for scientists, policy makers, and the public to be able to share information, concepts, and ideas, and to make shared decisions, they must be able to understand each other. The truth is that within languages there are subsections that are only accessible to those with technical expertise on the matter at hand. Specific language frameworks and jargon are prevalent within specific disciplines and underpin the explanation of concepts between experts (Montgomery, 1989; Collins, 2011). However, such language can be incomprehensible to those outside the subject area (Leggett and Finlay, 2001; Sharon and Baram-Tsabari, 2014). This creates an “unequal communicative relationship,” whereby laypeople struggle to comprehend the technical language and goals set by experts (Fischer, 2000, p. 18), particularly as many experts are ill-equipped in communicating with members of the public (Simis et al., 2016).

This unequal communicative relationship is likely enhanced in the geosciences where seemingly nontechnical, uncertain, or ambiguous terms are used routinely but assume tacit understanding. As an example, geoscientists may refer to the dip and strike of faults, joints, or cleavage; these are all terms which have specific meanings in geology but also have other meanings in the English language. But tacit understanding is not reliable; loose use of language, ambiguity, and poorly defined technical terms can lead to misunderstanding even amongst experts (van Loon, 2000; Doust, 2010) and between subdisciplines (Collins, 2011).

It is well established that how individuals perceive new information is influenced by factors such as expertise, context, prior knowledge, and the language used (McMahon et al., 2015; Venhuizen et al., 2019). Values and motivation, including affiliations and world view, have particular influence on perceptions of risk and the assessment of any new information (NASEM, 2017; Roberts et al., 2020) and how the information is framed (Pigeon, 2020). Consider the original work on framing by Tverskey and Kahneman (1981). In their example, when disease treatment options were framed positively (lives saved) rather than negatively (lives lost) people chose more risky treatment options. Similar work has found that how geoscience data and information are framed affects decision-making (Taylor et al., 1997; Barclay et al., 2011; Alcalde et al., 2017).

There was a notable shift in the framing of positive and negative arguments around shale gas extraction in the UK. Early arguments adopted local frames, such as concerns about local effects like induced seismicity, traffic, and noise. These arguments were replaced by global frames such as concerns about the climate change implications of developing onshore gas resources (Hilson, 2015) or the changing role of natural gas in the energy transition (Partridge et al., 2017). But, as we show in the remainder of this section, induced seismicity kept a high public and political profile in the UK.

Hydraulic fracturing (often referred to as “fracking”) is the process of fracturing rocks at depth by injecting pressurised fluids. The process locally increases the permeability of the rock formation, which is useful for a range of applications from improving water extraction (Cobbing and Ó Dochartaigh, 2007) and enhancing deep geothermal energy production (Breede et al., 2013) to enabling the recovery of natural gas trapped in rocks with a low permeability, such as “tight gas” or shale gas (Mair et al., 2012). Hydraulic fracturing also occurs in nature, usually where geological processes cause geofluids to become overpressured enough to overcome the rock strength and cause the rock to fracture (e.g. Engelder and Lacazette, 1990; Fall et al., 2015).

For shale gas extraction, hydraulic fracturing is one of several processes that allows the hydrocarbons to be recovered from the low-permeability rocks in which they are trapped (King, 2012). A borehole might be hydraulically fractured as part of shale gas exploration or development, where exploration refers to activities that investigate the commercial viability of a potential shale gas resource and development refers to activities that support the commercial production of the resource.

As a rock fractures, seismic energy is released (e.g. Tang and Kaiser, 1998) as a seismic event or seismicity. For shale gas hydraulic fracturing, because the fracturing process is caused by human activity, the seismicity is categorised as being human-induced seismicity or, simply, induced seismicity. Many processes induce seismicity, from mining and quarrying and filling and dewatering reservoirs to disposing of wastewaters by injection into rock formations (Westaway and Younger, 2014; Pollyea et al., 2019). However, not all seismic events have any detectable effect in terms of being felt at the surface or even recorded (Kendall et al., 2019).

There are a number of approaches to quantifying, and reporting on, the size of a seismic event. The moment magnitude (Mw) relates to the seismic moment, which is the energy released by the event. The local magnitude (Ml) measures the ground displacement. The two scales of Ml and Mw are fundamentally different, and so the Mw and Ml of a seismic event can diverge, particularly for large (> M 6.0) and small (< M 2.0) events (Clarke et al., 2019; Kendall et al., 2019). Seismologists prefer Mw because it relates to the properties of the fracture (the seismic moment) and because Ml breaks down for events below Ml 2.0 (Kendall et al., 2019). However, Ml is easier to use for real-time reporting, and so it is used to report seismic events and to regulate induced seismicity (Butcher et al., 2017). A variety of terms are used by both experts and laypeople to describe a seismic event, including earthquakes, tremors, and micro-earthquakes. Seismologists have proposed particular terminology based on the property of a seismic event, such as the frequency content or the magnitude (for example, see Bohnhoff et al., 2010; Eaton et al., 2016), but there is no common classification framework. This poses questions such as “How big is a small earthquake?” (Kendall et al., 2019).

Hydraulic fracturing will be accompanied by the release of seismic energy as the rock is fractured by the fluid pressure (Kendall et al., 2019). The energy released by an individual fracture is small, typically representing Ml -1.5 (Mair et al., 2012), but if hydraulic fracturing fluids reach a prestressed fault then larger events can occur (Clarke et al., 2019). Induced seismicity is, thus, inherent in hydraulic fracturing. But there are uncertainties regarding the measurement, forecasting of, and magnitude of these events (Kendall et al., 2019). The nominal detection level for the UK seismic monitoring network (seismograph stations operated by the British Geological Survey) is Ml 2.0 (i.e. events above Ml 2 might be measured at the surface; Kendall et al., 2019), or Ml 2.5 in urban areas due to background noise. Acoustic monitoring systems away from background noise, such as in mines, can record very small seismic events down to magnitude Mw -4 (Kwiatek et al., 2011; Jalali et al., 2018). Whether or not an event is felt at the surface depends on several factors, including the seismic moment, the hypocentral depth and the attenuating properties, the structure of the rocks through which the energy travels, and other local conditions, such as the stiffness of the ground, the background noise, and the time of day (Butcher et al., 2017; Kendall et al., 2019). Furthermore, recorded Ml is dependent on the seismic detection network, including the array density and location distance between the source and the detector (Butcher et al., 2017).

Incidences of felt seismicity associated with hydraulic fracturing for shale gas in the UK, US, Canada, and China are well documented (Warpinski et al., 2012; Verdon and Bommer, 2021; Schultz et al., 2020), but when shale gas exploration began in the UK circa 2009, this was not the case. Despite many thousands of hydraulic fracturing treatments, there were no recorded or reported incidences of felt seismicity associated with fracking in the shale gas basins first developed in the USA (Verdon and Bommer, 2021). Seismic events that had been felt were due to the geological disposal of hydraulic fracturing wastewater rather than the fracking process itself (e.g. Elsworth, 2013). However, in 2011, a series of seismic events with maximum magnitude (Ml) 2.3 (Clarke et al., 2014) occurred at the Preese Hall shale gas exploration site in Lancashire (northwestern England, UK), suspending operations. These seismic events led shale gas activities to have a high public and political profile (Green et al., 2012; Selley, 2012; Clarke et al., 2014), receiving widespread media coverage and stimulating a wave of public protests against shale gas activities (Matthews and Hansen, 2018; Jaspal and Nerlich, 2014). The UK government introduced a moratorium on hydraulic fracturing for 6 months following the 2011 events. In December 2012, the UK government lifted the moratorium in England and Wales (Alessi and Kuhn, 2012), but in Scotland, moratoria have been applied by Scottish Government. The UK government introduced new regulatory requirements intended to effectively mitigate seismic risks (DECC, 2013a, b), including a traffic light system (Fig. 1) based on the local magnitude (Ml) of induced events. In November 2019, the moratorium was reapplied following publication of the Oil and Gas Authority (OGA)'s report (BEIS, 2019a; OGA, 2019) on a series of seismic events of up to 2.9 Ml that occurred at the Preston New Road shale gas site, also in Lancashire, in August 2019. Since the 2011 events at Preese Hall, many more incidences of felt seismicity related to hydraulic fracturing have been documented in the UK and internationally (Schultz et al., 2020; Verdon and Bommer, 2021). It is now understood that the occurrence of felt seismicity from hydraulic fracturing is highly site specific and depends on the geological and geomechanical conditions of the reservoir and the hydraulic fracturing operation design (Schultz et al., 2020; Verdon and Bommer, 2021), as well as local characteristics (Butcher at al., 2017).

It is with this backdrop that we examine the available evidence of expert and nonexpert perspectives on the risk of hydraulic-fracturing-induced seismicity and the terminology used to describe these risks.

All expert reports that we reviewed, and which examined seismicity risk, concluded that the risks of induced seismicity from hydraulic fracturing in the UK are very low, and that any induced events will be below the threshold of felt seismicity (Table 1). It is, therefore, fair to surmise that there is general agreement amongst expert bodies that the risks of hydraulic-fracturing-induced seismicity are lower than or no different to other types of seismicity caused by human activity. To be clear, agreement on low risks associated with induced seismicity does not reflect agreement on or support for other aspects of shale gas exploration and development, such as the business case for, or environmental ethics of, fracking (Howell, 2018; Van de Graaf et al., 2018).

All studies of public perceptions (nonexpert) around shale gas topics in the UK find that the public associate the risk of induced seismicity with hydraulic fracturing. However, risk of contamination of drinking water is more often of larger concern than induced seismicity. These studies and their findings are summarised in Table 2. Table 2 also illustrates the similarities and differences in the phrases used in these studies to refer to induced seismicity. These differences are typically introduced by researchers either in the research design or the analysis, rather than reflecting the phrasing used by participants. To examine insights from these studies in more detail, we first summarise findings from cross-public surveys before we look to the results of dialogic and deliberative research. In each case, mindful that public views may have been evolving, the studies are presented chronologically in the order in which they were conducted (not the order in which they were published). As before, we are interested in the perceived risks of, and language around, induced seismicity and not the public opinion around fracking for shale gas, though the latter is the primary motivation for many of the studies that we examined.

A number of closed-response surveys have been undertaken to assess UK-wide public attitudes towards shale gas and related topics. The most comprehensive of these in terms of a longitudinal data set is the YouGov survey organised by University of Nottingham. The survey was administered 12 times in the period March 2012–October 2016 (Andersson-Hudson et al., 2016; O'Hara et al., 2016). Following a knowledge question which filtered out participants who did not know what hydraulic fracturing or shale gas was, respondents were then asked questions about multiple aspects of shale gas development. A question asked regarding whether they do or do not associate earthquakes with shale gas, with the option to answer “do not know”. In the period 2012–2014, there is a steady decline in the number of participants who associate shale gas extraction with earthquakes and a corresponding increase in those that do not (Fig. 2). In the three surveys conducted in 2014, the responses appear to have stabilised.

The Energy and Climate Change Public Attitudes Tracker is a quarterly UK-wide survey conducted by the Department of Business, Energy and Industrial Strategy (BEIS, previously the Department of Energy and Climate Change, DECC), to capture changing public attitudes towards energy and climate change issues. Questions about shale gas were included in the survey from June 2012, and since 2015, the reasons for support, opposition, or no view have been enquired about (Howell, 2018). Of the reasons for opposition to shale gas, one that is consistent across the BEIS surveys is the “risk of earthquakes”, which is ranked fourth out of five common concerns (Bradshaw and Waite, 2017). Opinium Research led two online surveys to explore public attitudes to fracking in 2014 and 2015 (reported in Howell, 2018). The survey did not ask participants about perceived risks. However, questions from Opinium Research were adapted for a different online omnibus survey fielded by YouGov, also in 2015 (Howell, 2018). Howell (2018) found the majority (43.2 %) of respondents who answered a knowledge question about shale gas correctly agreed that “fracking could cause earthquakes and tremors”, whereas 18.8 % disagreed (the remainder answered “do not know”). However, the level of positive response for earthquakes and tremors ranked towards the lowest in the range of negative environmental and social risks (including damage to the local environment, water contamination, negative affect on climate change, and health risks). A one-off online survey in 2014 (Whitmarsh et al., 2015) finds that 40.4 % of participants agreed that they are “concerned about the risks of earthquakes from shale gas fracking”, with 20.8 % reporting that they disagreed, and the remainder undecided. In this survey, the public were marginally less concerned about earthquakes than they were about water contamination.

The UK National Survey of Public Attitudes Towards Shale Gas conducted in April 2019 is the first to seek to understand what the public knows or thinks about specific regulations for shale gas, including the “traffic light system” for monitoring and regulating induced seismicity (Evensen et al., 2019). The majority of participants felt that the traffic light guidance is not stringent enough, and would oppose any changes to raise the threshold to 1.5 Ml, suggesting that concerns around risks of induced seismicity from hydraulic fracturing remain (Evensen et al., 2019).

Overall, these surveys indicate that seismicity induced by hydraulic fracturing is an important issue for the public. However, as is the nature of surveys, to some degree the topics of concern are pre-identified during the survey design and are shaped by the phrasing question (a problem that is well-documented in research methods and risk research; see, for example, Gaskell et al., 2017). For example, the Whitmarsh et al. (2015) survey asked questions in the style “I am concerned about [environmental risk]”; other questions in the same survey were focused on risks around energy security or energy prices, and did not use the words “concern” or “risk”, both of which have negative associations. Similarly, Howell (2018) found the question, “fracking could cause earthquakes and tremors”, is interpreted to be a negative statement about fracking, rather than, say, a factual statement. Furthermore, we note that statements regarding earthquake risk were conditional (“could cause”), whereas all other provided risks except for water contamination were unconditional (“will cause”).

A total of two studies adopted open survey questions. Craig et al. (2019) studied public views towards fracking and how these changed with distance from a region of County Fermanagh with potential shale gas resources and a granted petroleum exploration license. Survey results, which were gathered in 2014, indicated that risk of “increased seismicity” ranked eighth amongst the 10 risks considered to be a concern by survey respondents. All of the identified risks increased with proximity of residence to the licensing area, including the perceived risk of increased seismicity due to hydraulic fracturing. McNally et al. (2018) found seismicity ranked third out of four common disadvantages identified from an open question about advantages and disadvantages of fracking. When the same question was asked about “using hydraulic pressure to extract natural gas”, seismicity was not raised as a disadvantage.

Analysis of qualitative data presented in the public inquiry on planning permission for shale gas development in Lancashire (held in 2016) found that “seismic activity was raised regularly in the public sessions. Several of those who spoke had first-hand experience of seismic activity, having felt the tremors from Cuadrilla's hydraulic fracturing at Preese Hall in 2011” (Bradshaw and Waite, 2017).

Williams et al. (2017) report on deliberative focus group discussions on shale gas development. The groups were held in northern England in 2013, and Williams et al. (2017) reported that explicit concern about induced seismicity was not expressed, although some groups did express “worst-case scenario” thinking around a number of potential risk and impact pathways (Williams et al., 2017). Similarly, a series of 1 d deliberations in the UK and the US, held in 2014, found that participants did not express particular concern about induced seismicity (Thomas et al., 2017a). In deliberative interviews held in Wales in 2013–2014 the risk of earthquakes or tremors was ranked 13th out of 19 pre-identified risks in a card-sorting exercise (Whitmarsh et al., 2014). In 2016, a Citizens' Jury (a format for public deliberation) was held in Preston, Lancashire (northwestern England) approximately 15 km from the Preese Hall shale gas development. Transcriptions from the proceedings show that while participants raise questions around earthquake risks from shale gas extraction (and geological CO2 storage), concerns about induced seismicity are not reported to be a dominant issue (Bryant, 2016).

As Jaspal and Nerlich (2014) reflect, terms such as “earthquakes” evoke imagery of destruction and disaster, whereas phrases like “seismic activity” or “tremors” are less threatening. Since language is not a neutral tool, the choice of words used by experts, social researchers, and public participants might be carefully chosen to communicate particular meaning.

Experts use a range of terms to describe induced seismicity (Table 1). The seismic events themselves might be referred to as “micro-seismic events”, “seismicity”, and “earthquakes”. A distinction is made between natural and induced earthquakes and the events that may occur from hydraulic fracturing or other human-caused activities are described as being “induced” by or “triggered” by these activities, where induced can mean solely due to fracking and triggered can mean that the occurrence was accelerated by fracking but might have occurred naturally. The authors use qualifiers such as “minor”, “low”, and “small” to indicate the magnitude of seismicity associated with fracking. Finally, while the consequences of seismicity are sometimes referred to in terms of “vibrations” or “tremors” and more often there is a distinction between “felt” and “not felt” events.

In some cases, the language around seismicity in policy reports is inconsistent and confusing. For example, a DECC (2013c) report lays out regulatory requirements designed “to ensure that seismic risks are effectively mitigated” (p. 6) and “to prevent any more earthquakes being triggered by fracking” (p. 19). But the regulations allowed induced seismic events of magnitude (Ml) < 0.5 (“green light”), implying that these events are not considered to be earthquakes, although no definition of the term is provided. On the next page (p. 20), an additional qualifier is added which works around this contradiction; the regulations are “designed to prevent any more perceptible earthquakes being triggered by fracturing”. The 2019 OGA report (which summarised a series of studies commissioned by the OGA to understand and learn from the induced seismicity observed at the Preston New Road development in 2018) concluded that rules based on the current understanding of induced seismicity cannot be “reliably applied to eliminate or mitigate induced seismicity” (OGA, 2019). The authors of this OGA report do not define what is meant by induced seismicity (i.e. what magnitude will not be reliably mitigated). As outlined in Sect. 2.1, it is not possible to eliminate risks of all magnitudes of induced seismicity from the hydraulic fracturing process.

In comparison, the terminology to describe the induced seismicity reported in public perception studies is much less varied (Table 2). However, in many cases, the phrases are selected by the researchers, either when designing the survey question or when reporting on the research outcomes. For example, four of the five closed-question surveys about induced seismicity refer to risk of “earthquakes”. The researchers designing closed-question surveys might have opted to use the term “earthquake”, since it is commonplace and widely understood, whereas “seismic activity” might be considered to be jargon. Results from the only survey to add a size qualifier, asking about “earthquakes or tremors” (Howell, 2018), are very similar to the results of surveys which simply asked about “earthquakes”.

In contrast, of the phrasing chosen by researchers to communicate outcomes from qualitative methods, only one study refers to “earthquakes” (Thomas et al., 2017a). Instead, researchers reporting qualitative methods use terms such as “seismic activity”, “seismicity”, or “minor earthquakes”. These terms might have been selected to reflect the level of risk perceived by participants. The phrases that the public themselves adopted are not reported in these studies, except for in the report on the Citizens' Jury on fracking where, in their questions, participants wanted to come to grips with whether the 2011 Preese Hall seismic events had been “real” or “genuine” (i.e. caused by hydraulic fracturing) or a “natural tremor” (i.e. background seismicity) (Bryant, 2016, pp. 14).

While dialogic or deliberative studies in the UK find that risks of induced seismicity tend not to take precedence in the public discussions, that is not to say that the risks are acceptable. Thomas et al. (2017a) report that deliberative groups in the UK and the US felt that, if shale gas development were to cause earthquakes, however small, then development should not be pursued. Similarly, Williams et al. (2017) reports how one deliberative group reflected that public tolerances to industrial activities which induce seismicity may have changed such that activities that were acceptable in the past are no longer acceptable to the public. Finally, early results from a recent investigation into public attitudes to the UK government's traffic light system to regulate induced seismicity suggest that participants support stringent monitoring of induced seismicity (Evensen et al., 2019). These insights imply that the public's risk tolerance to induced seismicity from shale gas production is low.

The physical process of hydraulic fracturing will, by definition, release seismic energy – whether the release of this energy is detectable as an event or not. Accordingly, the expert reports that we reviewed conclude that there is risk of induced seismicity from hydraulic fracturing, albeit low. Depending on how the term earthquake is defined (e.g. “How big is a small earthquake?”; Kendall et al., 2019), it could be argued that assertions used to gauge public views such as “shale gas development is associated with earthquakes” are factual. Might the questions indicate level of knowledge of the association, rather than indicate the level of perceived risk? Howell (2018) finds that respondents who correctly answer a knowledge question about shale gas are more likely to agree with the statement “fracking could cause earthquakes and tremors” (43.2 %) than to answer that they do not know (38.0 %) or to disagree (18.8 %). Furthermore, Andersson-Hudson et al. (2019) find that laypeople who are more knowledgeable about shale gas have more unified views. Indeed, all cross-public surveys studied here find that motivations determine public responses: associating fracking with earthquakes negatively correlates with support for the technology and relates to demographic variables, including political views and gender (Andersson-Hudson et al., 2016, 2019; Howell, 2018; O'Hara et al., 2016; Evensen, 2017). These findings align with similar studies in Europe (Lis et al., 2015; Evensen, 2018), the US (Boudet et al., 2014; Graham et al., 2015), and Canada (Thomas et al., 2017b).

In summary, through our review and analysis of previous surveys, reports, and papers, we have revealed uncertainties in the perceived risk of seismicity induced by hydraulic fracturing for shale gas. There is broad agreement amongst experts that, while induced seismicity is associated with hydraulic fracturing, the likelihood of felt seismicity is dependent on context-specific technical factors. All the expert reviews concluded that the risk presented by such seismicity is low. Generally, these reports distinguish between felt and not felt seismic events, but there is no systematic use of terminology to describe seismicity or the risk it presents. We find that associations between induced seismicity and shale gas are common across nearly all public studies that we reviewed. Perceived risk is not ubiquitous amongst all members of the public, and often, other reported environment or social risks take prevalence. However, the level of perceived risk of induced seismicity and understanding around the topic is difficult to compare due to differences in research approaches and the language used to elicit and report on public views. Given the ambiguities in terminology around hydraulic-fracturing-induced seismicity, it is interesting to consider whether questions around the risk of earthquakes might be understood or interpreted differently according to, say, the participants' views about shale gas or understanding of the hydraulic fracturing process. And are ambiguous terms, such as earthquake or tremor, potentially loaded or leading?

In the next section, we explore whether or not knowledge levels affect whether seismicity is associated with shale gas, and how the language used in the questions asked affects the answers provided.