Seismicity refers to the occurrence of earthquakes, and in the Earth we have two main kinds of Seismicity, these are (1) Natural Seismicity, resulting from geological processes, such as tectonic plate movement or volcanic activity, and (2) Induced Seismicity, often called microseismicity, which is triggered by human activities involving injection/production of fluids into the subsurface (oil & gas), geothermic energy that needs fluid injection, fracking, hydroelectric water reservoir, or even the use of deep boreholes for water exploration (see Figures below from Kivi et al. 2023).
In VERACRUZ We have seismologists specialized in monitoring of Natural and Induced earthquakes. For this, VERACRUZ installs seismic monitoring networks (seismographic stations) to detect and calculate the location of seismic events.
Seismicity monitoring involves installation of seismic sensors (seismometers) to detect natural and induced seismic events from continuous recorded waveforms. Once an event is detected, its hypocenter is evaluated, and the correlation with human activities is assessed. The relationship between seismicity and subsurface operations is analyzed by examining the long-term seismic catalog along with geomechanical characteristics of the rocks and its geological context. Understanding fracture locations and their evolution helps mitigate risks
VERACRUZ can help your company to monitoring any kind of seismic activity, including:
- Induced Seismicity Associated with Enhanced Geothermal Systems
- Earthquakes Induced by Oil and Gas exploration
- Hydraulic fracturing
- Reservoir Induced Seismicity – RIS
For you to understand more about each case, we have shown below a general view about this.
Induced Seismicity Associated with Enhanced Geothermal Systems
In June 2009, the New York Times published an article about the public fear of geothermal development causing earthquakes. The article highlighted a project funded by the U.S. Department of Energy’s (DOE) Geothermal Technologies Program bringing power production at The Geysers back up to capacity using Enhanced Geothermal Systems (EGS) technology. The Geysers geothermal field is located two hours north of San Francisco, California, and therefore, the article drew comparisons to a similar geothermal EGS project in Basel, Switzerland believed to cause a magnitude 3.4 earthquake.
In order to address public concern and gain acceptance from the general public and policymakers for geothermal energy development, specifically EGS, the U.S. Department of Energy started to monitor the seismic activity for EGS, and it was the first initiative in this area in the world, setting an example for the sector.
In fact, geothermal energy is a viable form of alternative energy that is expected to grow significantly in the near and long term. The energy estimated from hydrothermal systems is large, but the total supply from geothermal systems has the potential to become orders of magnitude larger if the energy from geothermal systems can be enhanced — i.e., through Enhanced Geothermal Systems (EGS). EGS is defined as any activities undertaken to increase the permeability in a targeted subsurface volume via injecting and withdrawing fluids into and from the rock formations, intended to result in an increased ability to extract energy from a subsurface heat source. This can be done through such approaches as fluid pressurization, hydrofracture, and chemical stimulation. As with the development of any new technology, some aspects are accepted, while others need clarification and study.
In the case of EGS, fluid injection is used to enhance rock permeability and to recover heat from the rock. During the process of creating an underground heat exchanger by injection or by the subsequent circulation of the system, stress patterns in the rock may change, resulting in seismic events. In almost all cases, these events have been of relatively small magnitude, and by the time the energy reaches the surface, the vast majority are rarely felt (Majer et al. 2007).
Historically, induced seismicity has occurred in many different energy and industrial applications (reservoir impoundment, mining, construction, waste disposal, and oil and gas production). Although certain projects have stopped because of induced seismicity issues, proper study and engineering controls have always been applied to enable the safe and economic implementation of these technologies. Recent publicity surrounding induced seismicity at several geothermal sites points out the need to address and mitigate any potential problems that induced seismicity may cause in geothermal projects (Majer et al. 2007).
In this point is important remember the existence of two types of anthropogenic events:
(1) Induced Micro-seismic Events: These correspond to an inherent part of the injection or production process. They are very small events, considered minor and non-harmful, and require very sensitive monitoring equipment to detect. In this context, microseismic surveys are used to understand the stimulated reservoir volume and/or the shear-enhanced permeability. Seismicity in this case can serve as a valuable resource management tool.
(2) Triggered Earthquakes: These result from injection or production of fluids interacting with existing geological faults, leading to more significant ground accelerations that may be felt at the surface. Such unintended events can often be avoided through careful site selection, injection design, and permanent monitoring. In this case, seismicity must be treated as a risk management tool.
Induced seismicity generally releases a relatively small amount of energy, often not perceptible to humans. However, triggered earthquakes occurring along significant fault planes under specific pre-existing seismo-tectonic conditions can generate larger magnitude events. The characteristics of such maximum earthquakes depend on the local stress conditions (compressional, extensional, etc.) and the existence of potential “capable faults” — pre-existing geologic faults that could slip during a project’s lifetime.
An induced or triggered earthquake cannot exceed the maximum natural earthquake that could occur on the capable faults within the area.
Earthquakes Induced by Hydraulic Fracturing
Hydraulic fracturing can induce seismicity through fluid injection and disturbance of subsurface stress in tight reservoirs. Most seismic events associated with hydraulic fracturing exhibit magnitudes of Mw ≤ 3 and are referred to as microseismicity, while a few larger-magnitude earthquakes (Mw > 3) may occur due to reactivation of pre-existing faults. Induced seismicity provides valuable information about subsurface characteristics such as rock failure potential and seismogenic zones. Microseismic monitoring is therefore essential for reservoir characterization, fracture geometry delineation, and geomechanical analysis.
Over the last decade, shale gas development has expanded globally — transforming energy structures in countries such as the United States, China, the United Kingdom, Germany, France, and Poland.
Hydraulic fracturing (HF), also known as “fracking” or “stimulation,” is a key technology that enhances production and recovery rates in tight reservoirs (shale oil, gas, coal bed methane). It increases reservoir permeability by creating or extending fracture networks via high-pressure fluid injection. These fractures can extend tens to hundreds of meters away from the wellbore, connecting natural fractures and improving permeability.
The sequences of induced seismicity are commonly observed in proximity to the wellbore, which is caused by the direct pressure transmission from the stimulation zone (wellbore) to surrounding areas. However, pressure perturbations can also reach distant areas through paths of high permeability, and result in seismicity hundreds or even thousands of meters away from the wellbore, though it is less likely to happen in shales with very low permeability. Hydraulic fracturing is carried out to fracture the rock and is naturally accompanied with seismicity. Thus, induced seismicity monitoring is essential in monitoring the development of hydraulic fracture networks and managing/mitigating related potential geohazards.
It is already well known that many human activities have the potential to induce seismicity. Most human-induced seismicity involves rock fracturing, which results from various underground operations, such as mining, CO2 sequestration, hydraulic stimulation of geothermal fields, wastewater injection, and hydraulic fracturing for tight oil and gas exploitation, etc.
High-pressure fluid injection causes rock fracturing, accompanied by induced seismicity, most of which are microseismic events (moment magnitude (Mw) ≤ 3). When pre-existing faults are reactivated, higher-magnitude seismicity (Mw > 3) may occur. The expected magnitude of microseismic events around the fractures is typically below Mw 0, and most induced seismicity on or near pre-existing faults is typically below Mw 3, only a few events are large enough to be felt by individuals at the Earth’s surface.
In the past thirty years, significant progress has been made in exploring and understanding induced seismicity associated with human activities.
In oil and gas and geothermal wells, monitoring ensures the safe production of resources while minimizing risks of unwanted seismicity.
Reservoir Induced Seismicity – RIS
For the last 70 years, over 80 cases of reservoir induced seismicity (RIS) have been reported. Induced seismicity has been expressed through earthquakes with magnitudes up to 6.3 on Richter scale. Temporal distribution of induced seismicity following the filling of large reservoirs shows two types of response: (1) at some reservoirs, seismicity begins almost immediately after filling of the reservoir; (2) at others, increases in seismicity is observed after a number of seasonal filling cycles. These differences in response may correspond to two fundamentally different mechanisms of RIS (see below the map of RIS in the World by http://www.inducedearthquakes.org).
In fact, the reservoir induced seismicity (RIS) is a very complicated phenomenon, a result of complex, not entirely known mechanisms, that are very different in various cases. The investigations of this phenomenon have so far pointed to the existence of a number of different factors that control such a seismicity and that it is necessary to provide a large amount of data by installing instruments for recording the seismic activity of the terrain starting from the beginning of the construction of a dam.
In most cases of induced seismicity, the earthquake magnitudes have been small and have not represented a threat to the structural integrity of the dam and the surrounding structures.
In the case of occurrence of induced seismicity, two types of earthquakes can be defined:
(1) Earthquakes which are not of tectonic nature, have shallow hypocentres and are mainly associated with the adaptation of the stresses in the foundation rock, the collapse of karst holes or mines, and landslides. Characterized by relatively small magnitudes, they often occur soon after the filling of the reservoir and follow the abrupt change of the water level in the reservoir.
(2) Earthquakes of tectonic nature that are caused by displacement of seismically active faults that cut or run through the reservoir region. The initial stress state is usually very close to the failure point so that even small changes of the strength characteristics at the fault plane, which result from the action of the reservoir, may induce seismic activity. The epicentres of the foreshocks with small magnitudes are usually located around the faults; the earthquake magnitudes gradually decrease until the major (the strongest) shock occurs. The aftershocks that follow the main shock may last for a certain time. Since the process of infiltration of water into the rock masses lasts for a longer time, there is usually a time interval between the achievement of the maximum water level and the occurrence of the main shock.
These two types of earthquakes are indicated as endogenic and exogenic induced earthquakes.
The occurrence of induced earthquakes at a reservoir location is conditioned by certain geological conditions. The surrounding of the reservoir must contain either seismogene structures that may generate tectonic earthquakes and, at the same time, such hydrological conditions that enable infiltration of water deep into the rock masses, or fractured rock masses and karst pits which lead to the occurrence of non-tectonic earthquakes.
The RIS cases that have happened worldwide show that the strongest earthquakes occurred after a number of foreshocks and were followed by a series of aftershocks. In certain cases, such activity lasted for several years. The existence of a series of foreshocks shows how necessary seismic monitoring of reservoir locations is in the period prior to and during construction. Another important data point is the time interval between the first filling of the reservoir and the strongest earthquake — this can be as short as several days (Kremasta, Greece), weeks (Kariba, Zimbabwe), or one or several years (Hiengfengijang, Koyna). These differences are essential for defining the seismic triggering mechanism. The postponed seismic activity points out that the seismic trigger is more likely due to the increase of water pressure rather than stresses in hard rock.
Mine-Induced Earthquakes and Rock Bursts
Mine-induced seismicity (including rockbursts) is a problem in most mining areas where there are deep mines and local populations. In general, the micro-seismicity is composed of small earthquakes (magnitude <0.5), and is not important on the surface (with some exceptions). On the other hand, these small events can cause the collapse of mine workings, kill miners, and damage structures on the surface, making it important to understand the evolution of seismic activity in any mining area.
The spatio-temporal correlation of micro-earthquakes occurring in a mining-induced seismic system can be investigated with VERACRUZ service. For seismicity systems, the fact that clustering is observed both in space and time should make us aware of the possible existence of a stress diffusion phenomenon, corresponding to a propagation of stress away from the initial earthquake, at time scales much larger than those involved in seismic wave propagation. Such a diffusion phenomenon has been reported or predicted in many papers.
Microseismic monitoring is a critical tool to improve production and mitigate risk in underground operations. Seismicity is common in mining operations, as the rock shifts in order to redistribute stress in the rock mass. Typically, induced seismicity is measured on a micro-scale at levels equivalent to very small earthquakes measuring -3 Mw to +1 Mw in magnitude. However, on occasion, microseismic systems may be combined with strong ground motion sensors on the surface to record larger-magnitude seismicity (up to +4 Mw).
The hypocenter determination to characterize the location and size of these seismic events and evaluating seismicity temporally and spatially as it relates to mining operations serves as an essential tool to quantify and understand stress-induced rock-mass behavior. Since microseismic monitoring reveals continuous information about what is happening behind walls and in areas not accessible to mine workers, it can operate as an early-warning system for potential hazards caused by changing rock conditions.
The microseismic monitoring done by VERACRUZ is probably better known in the mining industry as a diagnostic and safety tool for underground operations in hard-rock mines.
Seismic records contain information about the effect of the source as well as the effect of wave propagation through the rock mass. The effects of wave propagation are usually not well known as only simplified models of geological structures are available. Therefore, the information about the source retrieved by inverting seismograms may include errors due to incomplete knowledge of the rock mass along the propagation path, which in turn causes a distortion in the calculated moment tensor.
Below are shown some important parts of microseismic monitoring for underground mining and tunnels:
- Monitor, interpret and analyze seismicity, including hypocenter determination (latitude, longitude, and depth of energy focus) and size magnitude of earthquakes.
- Make 3D seismicity maps to identify dangerous situations and take remedial action before persons are injured or works are damaged.
- Evaluate all face layout positions to determine any significant risks relating to rockfalls or rockbursts created by or likely to be created by mining operations.
- Identify, review, and make recommendations to the employer regarding systems, procedures, and techniques used or to be used by the employer to eliminate, control, or minimize rockfall or rockburst hazards.
- Establish monitoring, recording, and reporting systems and procedures that ensure that relevant information related to rockfalls and rockbursts is timely provided to those persons involved in the planning and operating of mining activities.
- Approve plans for mining sequences to ensure that the probability of seismic events/rockbursts is minimized; the factors affecting the stability of off-reef excavations are taken into account; support systems accommodate current and anticipated rock conditions; and all precautions necessary for remnant mining are stipulated.
- Inspect all major rockbursts and large or serious falls of ground and submit a report making recommendations to the employer.
Other Kinds of Induced Seismicity
Regardless of the type of induced seismicity, VERACRUZ can perform all types of monitoring, as it uses the same equipment and methodologies. Other less-discussed cases of induced seismicity include:
- Induced Seismicity by oil and gas exploration
- Induced Seismicity by water wells in specific geological conditions
- Induced Seismicity by large buildings (like the seismicity induced by Taipei 101)
The Importance of the Monitoring of Natural Seismicity
While companies and communities are more likely to be interested in monitoring induced seismicity, natural seismicity monitoring is important for:
- Nuclear Power Plants
- Public Authorities who need to provide accurate responses to the public when natural seismic activity begins in a city or state
As previously mentioned, VERACRUZ is ready to serve your company or community by monitoring any type of natural or induced seismic activity.
Reference
Kivi, I.R.; Boyet, A.; Wu, H.; Walter, L.; Hanson-Hedgecock, S.; Parisio, F. and Vilarrasa, V. (2023), Global physics-based database of injection-induced seismicity, Earth System Science Data, 15(7), 3163–3182.