An Improved Framework for Discriminating Seismicity Induced by Industrial Activities from Natural Earthquakes

Abstract

Heightened concerns regarding induced seismicity necessitate robust methods to assess whether detected earthquakes near to industrial sites are natural, or induced by the industrial activity. These assessments are required rapidly, which often precludes detailed modeling of fluid pressures and the geomechanical response of the reservoir and nearby faults. Simple question-based assessment schemes in current use are a useful tool but suffer from several shortcomings: they do not specifically address questions regarding whether available evidence supports the case for natural seismicity; they give all questions equal weighting regardless of the relative influence of different factors; they are not formulated to account for ambiguous or uncertain evidence; and the final outcomes can be difficult to interpret. We propose a new framework that addresses these shortcomings by assigning numerical scores to each question, with positive values for answers that support induced seismicity and negative values for responses favoring natural seismicity. The score values available for each question reflect the relative importance of the different questions, and for each question the absolute value of the score is modulated according to the degree of uncertainty. The final outcome is a score, the Induced Assessment Ratio (IAR), either positive or negative (or zero), that reflects whether events were induced or natural. A second score, the Evidence Strength Ratio (ESR), is assigned that characterizes the strength of the available evidence, expressed as the ratio of the maximum score possible with the available evidence relative to the maximum score that could be obtained if all desired data were available at a site. We demonstrate this approach by application to two case studies in the UK, one widely regarded as a case of induced seismicity, the other more likely to be a series of tectonic earthquakes.

A Shallow Earthquake Swarm Close to Hydrocarbon Activities: Discriminating between Natural and Induced Causes for the 2018–2019 Surrey, United Kingdom, Earthquake Sequence

“…This network sparsity made it difficult to initially constrain depth and faulting mechanisms, which can help to discriminate between natural and induced causes (Frohlich et al, Peer-reviewed post-print published in Seismological Research Letters on 28/08/2019Link to publisher's version of article: http://dx.doi.org/10.1785/0220190125 5 2016. The strong macroseismic intensity observations supported early shallow depth estimates (initially fixed to 5.0 km), raising further suspicions over possible induced seismicity (Verdon et al, 2019). Figure 1.…”

“…Using the Frohlich et al (2016) criteria, uncertainty arises from testing the questions posed, as we could classify the Newdigate sequence as either possibly induced or probably induced. However, this approach is likely an oversimplification as it does not consider uncertainties in the arising answers, detailed source parameters, or knowledge of fluid pressure or pathways (Verdon et al, 2019). We therefore assess the seismic source parameters as to whether they indicate an induced or natural cause for the events.…”

“…Peer-reviewed post-print published in Seismological Research Letters on 28/08/2019Link to publisher's version of article: http://dx.doi.org/10.1785 Criteria to discriminate induced versus natural seismicity include answering qualitative questions (Davis and Frohlich, 1993;Verdon et al, 2019), and more quantitative analyses such as earthquake source studies, numerical modeling, and statistical tests (Grigoli et al, 2017).…”

A database of damaging small-to-medium magnitude earthquakes

“…The anthropogenic origin of earthquakes classified as induced or triggered is often very clear and without controversy, but in other cases, the distinction between natural and induced seismicity can be much more ambiguous and an issue of contention. Approaches for determining whether seismic activity was related to anthropogenic activities range from mainly qualitative assessments based on series of simple questions (e.g., Davis and Frohlich 1993;Verdon et al 2019) to detailed physics-based calculations of probabilities (e.g., Dahm et al 2015). Whenever possible, suspected anthropogenic origins have been indicated in the database by means of the "Induced Flag" field, relying mainly on existing evaluations rather than attempting to make such assessments as part of this work.…”

A novel approach to assessing nuisance risk from seismicity induced by UK shale gas development, with implications for future policy design

“…Gupta and Baker (2019) developed a probabilistic framework for estimating regional risk due to induced seismicity related to wastewater injection in Oklahoma that extends conventional probabilistic seismic hazard analysis to account for spatiotemporally varying seismicity rates. Walters et al (2015) developed a qualitative risk assessment framework for triggered seismicity related to saltwater disposal and hydraulic G. Cremen and M. J. Werner: Assessing nuisance risk from seismicity induced by UK shale gas development fracturing that included risk tolerance matrices to be considered by different stakeholders.…”