Abstract:
Earthquakes are naturally occurring phenomena that are unpredictable and have a significant destructive potential. Human activity such as oil and gas production and wastewater injection can cause earthquakes by changing the stress levels in rock masses. This induced seismicity differs from naturally occurring seismic activity, which is due to sudden slips, fault formations, or movements of tectonic plates. We investigated the correlation between two factors (1. Oil and gas production; 2. Earthquake occurrence) and discussed the implication of wastewater injection in causing induced seismicity. We concluded that human activities have an impact on the physical world and that it is important to understand the causality in order to mitigate and prevent damages.
Introduction:
Earthquakes occur more frequently than we think. According to the United States Geological Survey (USGS), we can only feel 100,000 earthquakes of the 500,000 detectable ones occurring each year in the world [1]. However, most of the destruction is caused by only a few major earthquakes characterized by magnitudes upwards of 7.0. The damage comes from the earth’s violent shaking, which can rupture its surface, destroy properties, cause landslides, and may lead to other geological disasters [2]. As a recent example, in 2011, the magnitude 9.0-9.1 Tohoku earthquake off of Japan’s East coast generated a tsunami that killed 20,000 people and caused $360 billion in economic losses. This is just an example of the destructive potential that earthquakes possess.
What makes earthquakes much more dangerous is that they’re very unpredictable. Predicting an earthquake involves determining (1) the date and time, (2) the location, and (3) the magnitude [3]. So far, none of the major earthquakes have been accurately predicted based on the three aforementioned criteria. This unpredictability hinders prevention and preparation in the event of an earthquake, such as evacuating a region, having disaster supplies on hand, or developing an emergency communication and rescue plan. In other words, earthquakes have enormous destructive potential and are hard to predict.
One recent, additional danger has to do with induced earthquakes. Induced seismicity usually involves low magnitude tremors (< M3.0) caused by human activity, such as mining, hydraulic fracking, waste disposal wells, and hydrocarbon extraction. However, studies have found that these activities can cause significant earthquakes with higher magnitudes than previously thought. For example, in California, an average of 17 earthquakes of M3.0 or higher occur each year due to geothermal plants in the region [4]. This is also the case in Oklahoma, where research suggests that the oil industry triggered an M5.7 earthquake in 1952 [5]. Thus, it is important to understand the origins of induced seismicity because the increase in human activity in the past years has meant more induced earthquakes of higher magnitudes [6]. This means that induced earthquakes present a larger potential of destruction and a higher endangerment of populations nearby affected regions, such as California.
This investigation aims to assess the correlation between earthquake occurrence and two activities that induce earthquakes, namely (1) oil and gas production and (2) wastewater injection. We will examine whether the variables are related and in what way they are related through the analysis of data from the Los Angeles Basin in California.
Background:
Tectonic Earthquakes
Earthquakes occur when a sudden release of energy in the lithosphere causes seismic waves to propagate and to shake the Earth’s surface. Tectonic earthquakes refer to those that occur naturally anywhere in the world, where elastic energy is stored and released, producing a fracture propagation along the fault plane. There exists many types of faults, which lead to different potentials to produce major earthquakes. Earthquakes occur mostly near plate boundaries, where a lot of stress is accumulated and suddenly released due to tectonic plate movement over time. Albeit much rarer, earthquakes may occur far away from plate boundaries, anywhere where sufficient accumulated stress drives ruptures in rock masses.
Stress
Stress refers to the force applied to rock masses in the lithosphere. There are 3 types of stress, depending on the plate movement: (1) compression due to convergent boundaries, (2) tension due to divergent boundaries, and (3) shear due to transform boundaries, where plates slide sideways against each other.
Strain
Strain refers to the deformation and the change in shape of the rock. There are 3 types of strain: (1) elastic deformation if the rock is able to return to its original shape when the stress is removed, (2) inelastic deformation if the rock is unable to return to its original shape, and (3) fracture when the rock breaks.
Faults
Fractures occur when the rock is under too much stress. A fracture becomes a fault when there is movement on either side. Earthquakes tend to occur near faults where the movement of rock masses releases energy rapidly. There are no earthquakes when there is no fracturing.
Tectonic earthquakes occurrence varies depending on certain conditions. First, rock that is closer to the surface is usually more brittle and breaks easier than rock deeper in the crust, where high pressure and temperature cause rock to deform plastically. In addition, strain rate may determine the conditions for an earthquake to occur. Sudden stress is more likely to make rock break, thus causing fracturing. On the other hand, if the strain rate is relatively low and if stress is applied over a longer period of time, rock masses often tend to undergo plastic deformation.
Induced Earthquakes
As previously mentioned, induced earthquakes refer to those that are triggered by human activity. While many factors are at play, generally, induced seismicity occurs when humans alter the stress levels in the lithosphere instead of plate tectonics. According to the USGS, the number of significant earthquakes (M3.0 or higher) in the central United States has increased dramatically over the past 10 years, at an average rate of 25 earthquakes year over year [7]. The question remained of the nature of these earthquakes, whether they were natural or man-made. Keranen et al. investigated the case in Oklahoma and demonstrated the very high likelihood that the seismic activity was related to wastewater injection [8]. Another question relates to what should be done in the future to reduce risks and damage associated with induced earthquakes. The first step in answering this question is understanding how human activities such as mining and water injection correlate to earthquakes.
Overview of Variables
The Los Angeles basin holds sands that are very saturated in oil, such that the region is abundant with oil.
This oil is trapped by underground structures such as anticlines. The oil fields in the basin began to see activity starting from the end of the 19th century. The first reported oil-producing well dates to 1892. While produced and extracted amounts were not recorded at first, more data was taken over time. Earthquake detection began in California in 1932. Over the years, the network was expanded and updated to include more sophisticated recording methods. While oil and gas production began more than 30 years ago, earthquakes were recorded through observational data [9].
Water injection is variable that is not being mainly investigated in this paper, but worth mentioning. Water injection occurs for many reasons, including wastewater disposal, fracking, oil recovery, pressure balancing, etc. Water injection has been linked to induced earthquake occurrence and swarms in certain cases [10].
Current Findings
Hough and Bilham (2018) found strong evidence supporting a correlation between earthquake occurrence and oil and gas production [11]. Based on industry data and computation methods, they concluded that the activity due to the oil boom in the 1930s could have already significantly affected stress levels and caused moderate earthquakes. They also present preventive measures such as water injection to minimize the change in stress levels and mitigate earthquake risks.
Hough and Page (2016) addresses papers proposing that there is no evidence for significant induced seismic activity in the LA region between 1935 and the present [12]. They that the correlation between oil and gas production and earthquake occurrence (between 1915 and 1932) is believed to be attributed to industry practices that are no longer in use. This goes to show that there is a correlation between said variables.
Methodology:
For this investigation, the aim is to assess the correlation between oil and gas production and earthquake occurrence in the area. The analysis is performed by evaluating the case in the Los Angeles Basin.
To compute earthquake activity levels over time and see how they correlate to oil and gas production, I used online data from the database of the Southern California Earthquake Data Center (SCEDC) [13]. Specifically, I fetched the data from the earthquake catalogs separated by year of occurrence. I loaded the data into a data frame using pandas.DataFrame on a Jupiter Notebook. I kept the relevant data columns, namely longitude, latitude, time, and magnitude. Then, I plotted earthquakes according to their longitude and latitude. I included color for occurrence to see if there is a progression in time. Earthquakes are plotted according to circles, where the size is proportional to the magnitude.
To plot for the location of oil fields and wells, I retrieved data from the Department of Conservation, Geologic Energy Management Division (CalGEM) website [14]. I followed the same process as for the earthquake activity data. However, wells were not plotted according to time due to the complications in the plot. It would be overly difficult to include spud date, fill date, and whether the wells are active and inactive. Based on geographic location, we can establish a basic correlation to easily validate current findings.
Table 1. Sample data frame used to plot for earthquake occurrence.
Table 2. Sample data frame used to plot for oil and gas production through well locations
Results:
Plot 1 and plot 2 show geographical data of earthquake occurrence and well locations. While overlaying the graphs clutters the space and doesn’t provide significant results, we can observe a basic geographical correlation between the two plots. For instance, we can observe a general trend of earthquake occurrence towards the north following the coast of California. This corresponds indeed to oil fields located in the Los Angeles basin and well locations on those fields.
Plot 3 shows the magnitude of earthquakes over time. We can see that earthquakes that occur are mostly small to moderate with a few outliers with magnitudes over 5.
Plot 1. Geographical location of earthquake occurrence of different magnitudes over time
Plot 2. Geographical location of wells drilled for oil and gas production
Plot 3. Magnitude of observed earthquakes over time
Discussion:
Based on the plots, we were able to see a correlation between oil and gas production and earthquake occurrence. This is likely due to a causality relationship between said variables. Drilling and producing oil and gas affect the stress state of surrounding rock mass, causing seismic activity when the stress threshold is exceeded, and energy is released in waves through the Earth’s crust. Earthquakes occur as well due to a change in pore pressure. Extracting oil and gas or injecting water and wastewater also alter stress levels in rock masses. Thus, human activity changes the underground stress and strain, which would normally only change due to sudden slips, faults, and the slow movement of tectonic plates. This leads to induced earthquakes rather than naturally occurring ones, which could explain the relationship between oil and gas production and earthquakes.
In California, the large number of earthquakes occurring in the region has long been attributed to the boundary of two tectonic plates: the Pacific Plate (Pacific Ocean floor and California coastline) and the North American Plate (American continent and parts of the ocean floor). Seismic activity in the region is caused by the northwestward movement of the Pacific Plate, whose movement grinds on the North American Plate at a rate of around 2 inches per year. In addition, the tectonic plate boundary leads to the presence of multiple faults, of which the most significant one is the San Andreas fault. A large number of earthquakes can be attributed to the regional fault system that passes through much of the state of California.
Through calculations of rock stresses created by oil production, scientists have matched the location of larger earthquakes to areas of great rock stress due to human activity. Thus, researchers concluded that the number of natural earthquakes in the region is lower than previously thought. Since the 1960s, oil companies began injecting water and wastewater into their wells to counterbalance the change in rock stress. Thus, researchers noticed a reduced number of triggered earthquakes since then. By decreasing the anthropological influence on rock masses, it is now more difficult to differentiate naturally occurring earthquakes from induced earthquakes in the data.
This constitutes a limitation to this investigation, as it is difficult to separate the tectonic, naturally occurring earthquakes from human-induced earthquakes. Another limitation comes from extrapolating the correlation between investigated variables to all cases, as this investigation was performed based on data from the Los Angeles basin only.
Conclusion:
Through this investigation, a geographic correlation was found between oil and gas production and earthquake occurrence, which supports current findings on this matter. Broadly speaking, we can conclude that human activities (including wastewater injection) have a significant impact on the physical world and have the potential to cause seismic activity. Earthquakes are unpredictable and can cause major damage and loss to cities and populated areas. Thus, it is important to acknowledge the impact of our actions and search for ways to mitigate them.
Acknowledgements
Mr. Josimar Alves da Silva Jr, PhD.
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