Kansas Geological Survey Public Information Circular 36 • April 2014 Induced Seismicity: The Potential for Triggered Earthquakes in Kansas Rex C. Buchanan, K. David Newell, Catherine S. Evans, and Richard D. Miller, Kansas Geological Survey Introduction Earthquake activity in the Earth’s crust is known as seismicity. When linked to human activities, it is commonly referred to as “induced seismicity.” Industries that have been associated with induced seismicity include oil and gas production, mining, geothermal energy production, construction, underground nuclear testing, and impoundment of large reservoirs (National Research Council, 2012). Nearly all instances of induced seismicity are not felt on the surface and do not cause damage. In the early 2000s, concern began to grow over an increase in the number of earthquakes in the vicinity of a few oil and gas exploration and production operations, particularly in Oklahoma, Arkansas, Ohio, Colorado, and Texas. Figure 1—Earthquake hazard maps show the probability that ground shaking, or motion, will Horizontal drilling in conjunction exceed a certain level, over a 50-year period. The low-hazard areas on this map have a 2% chance with hydraulic fracturing has often of exceeding a low level of shaking and the high-hazard areas have a 2% chance of topping a much been singled out for blame in the greater level of shaking (modified from USGS, 2008). public discourse. Hydraulic fracturing, popularly called “fracking,” does of wells currently in operation have recorded near disposal wells starting cause extremely low-level seismicity, been suspected of inducing earthquakes in September 2013, about three years too small to be felt, as do explosions large enough to be felt or cause damage after horizontal drilling activities in the associated with quarrying, mining, dam (National Research Council, 2012). Mississippian limestone play—and building, and other industrial activities. Most often, detected seismic activity associated water disposal—had crossed Although the actual process of associated with oil and gas operations over the state line into Kansas from hydraulic fracturing has been suspected is thought to be triggered when Oklahoma. However, the region also of inducing larger earthquakes a few wastewater is injected into a disposal experienced several small historical times worldwide, the U.S. Geological well. In the disposal process, waste earthquakes long before the increased oil Survey has found no evidence to products—such as saltwater produced activity, making it difficult to determine suggest that it has contributed much with oil and gas and recovered the cause of the recent seismic events. to increases in the rate of earthquakes hydraulic fracturing fluids—are injected Although some areas of Kansas are at (Hayes, 2012). into deep and confined porous rock. greater risk of seismicity than others, Felt earthquakes associated with any Identifying a link between whether natural or induced, none of oil and gas production activities are earthquakes and human activities the state is in a high-hazard earthquake rare. In the United States, only a small is difficult. Complex subsurface zone (fig. 1). fraction of the hundreds of thousands geology and limited data about that Scientists continue to monitor and geology make it hard to pinpoint evaluate possible instances of induced Terms in bold are defined in the glossary. the cause of many seismic events seismicity. In states with significant in the midcontinent, particularly in increases in seismic activity, including regions historically prone to naturally Oklahoma, monitoring has increased in occurring low-level seismic activity. localized areas where unusually high In south-central Kansas, for example, rates of seismicity have occurred near several small earthquakes have been oil and gas production activities. To help reduce the potential for induced Measuring Earthquake Magnitude precisely comparable, they can, in seismicity related to the rate of injection and Intensity general, be correlated when intensity of wastewater into disposal wells, Earthquakes can be measured in two measurements nearest the epicenter are scientists and others have developed different ways. One method is based used in the comparison (fig. 2; Steeples some best-practice recommendations on magnitude—the amount of energy and Brosius, 1996). Seismologists that would help prevent activation of released at the earthquake source. categorize earthquakes by their stressed faults. The other is based on intensity—how magnitude, not by their perceived much the ground shakes at a specific intensity. Natural vs. Induced Seismicity location. Although several scales Earthquakes and the Potential Most seismic activity occurs when have been developed over the years, for Induced Seismicity in Kansas stress within the Earth’s crust causes the two commonly used today in the a fault or faults in subsurface rocks United States are the Modified Mercalli The majority of seismic events are to slip and release enough energy to scale, which measures intensity, and microearthquakes, too small to be generate tremors. The vast majority of the moment magnitude scale, which felt or to cause damage. The largest earthquakes are instigated naturally measures magnitude (M), or size. documented earthquake in Kansas, and generally happen where the Earth’s The moment magnitude scale is now centered near Wamego east of tectonic plates interact. For the United preferred to the older, more familiar Manhattan in 1867, rocked buildings, States, that means most seismic activity Richter scale because it overcomes some cracked walls, stopped clocks, broke windows, and reportedly caused ground is on the west coast along the boundary of the limitations of the Richter scale between the Pacific and North American to sink and endanger the bank of a canal (USGS, 2014). plates. near Carthage, Ohio (Parker, 1868). That Measurements on the moment Away from plate boundaries, earthquake was likely associated with magnitude scale are determined using earthquakes are most often triggered the Nemaha Ridge, a 300-million-year- a complex mathematical formula when geological processes, such as the old buried mountain range extending to convert motion recorded with deposition and erosion of surface rock, roughly from Omaha to Oklahoma City. a seismometer into a number that alter the balance of opposing stresses The Humboldt fault zone on the eastern represents the amount of energy on subsurface rocks. Change in stress boundary of the Nemaha Ridge is still released during an earthquake. Energy increases or decreases strain—the slightly active (Steeples and Brosius, released for each whole number amount of rock deformation brought 1996). Based on damage and reports, the measurement is about 31 times greater about by stress—which weakens the Wamego earthquake was estimated to than that released by the whole number stability of faults confined by the have a magnitude of 5.2 (Niemi et al., before (USGS, 2014). The smallest stress. Faults may then slip and release 2004). Smaller faults and fault systems earthquakes recorded today have pent-up energy, which rolls in waves also have been identified in the state, negative magnitudes (e.g., M -2.0) on through the Earth’s crust (Ellsworth, mainly during oil and gas exploration, 2013). the moment magnitude scale because but none have been connected with large The U.S. Geological Survey estimates the scale’s range is based on that of the earthquakes. several million earthquakes occur Richter scale, developed in the 1930s At least 25 earthquakes in Kansas around the world each year, although when monitoring equipment was less were documented in newspaper many small ones go undetected (USGS, sensitive. Scientists are now able to accounts and other sources between 2014). Seismic events too small to detect earthquakes smaller in magnitude 1867 and 1976. A few of the later ones be felt on the surface are known as than the “0” used as the Richter scale were recorded with seismic equipment. microearthquakes, or microseisms. baseline. Between 1977 and 1989, the Kansas The term “induced seismicity” Measurements of intensity on the Geological Survey recorded more is popularly used for any seismic Modified Mercalli scale range from I than 200 small earthquakes with a activity linked to human activity. Some to XII and are based solely on damage temporary seismic network as part of researchers, however, more narrowly assessment and eyewitness accounts. a study to identify seismic risk in the define “induced seismicity” as seismicity Intensity measurements near the source state (fig. 3). The monitoring equipment caused solely by human activity and use of an earthquake are generally higher was sensitive enough to detect artillery the term “triggered seismicity” to define than those at a distance. They can also fire at Fort Riley from 30 miles (50 km) human activity that sets off a small remain high in the direction the waves away and large earthquakes as far away transient event, which then instigates of energy travel and may be magnified as Japan (Steeples and Brosius, 1996). or contributes to a larger earthquake in areas underlain by loose gravels and Today, two seismic monitoring stations, controlled by natural stresses (Cesca, unconsolidated sediments. Determining operated by the U.S. Geological 2012). Because the amount of influence a intensity can be difficult in sparsely Survey, are located in Kansas. One is human activity has on a seismic event is populated areas with few buildings at