Earthquake Triggers: The Immediate Precursors
Triggers are distinct from causes; they are specific events or activities that impart the final perturbation to an already critically stressed fault, initiating the sudden rupture and discharge of accumulated energy.
However earthquake triggers do not cause the source stress build-up but rather accelerate or initiate the release of pre-stored stress, which has accumulated due to geological forces over a long time. A most noteworthy result in crustal mechanics is that Earth’s crust as a whole is essentially stressed very close to its failure point even in tectonically quiescent areas.
This unstable state means that even relatively small natural or artificial perturbations of the local stress field are sufficient to cause an earthquake.
Natural Earthquake Triggers
Dynamic Stress Transfer
Large earthquakes can occasionally induce subsequent seismic activity hundreds of kilometers away through dynamic stress transfer. This is because seismic waves from the original earthquake shake energy around the globe, causing a new earthquake.
The stress released from a mainshock can reallocate and concentrate on neighboring faults. A buildup as small as 1% can be enough to advance the timing of the next earthquake on a maximally stressed fault. The 1992 M7.3 Landers earthquake triggered seismicity all over California, for example, and the 2004 M9.1 Sumatra earthquake triggered activity as far away as Alaska.
Slow Slip Events (SSEs)
Slow slip events are “slow motion” earthquakes that build up over days, weeks, or months, unlike typical rapid quakes. These prolonged slips can cause bigger, more devastating earthquakes, such as the 2011 M9.0 Tohoku earthquake in Japan. Large earthquakes can trigger slow slip events, showing an intricate, symbiotic relationship between rapid and slow fault motion.
Volcanic Activity
Even more infrequently, major regional earthquakes (M6+) can cause eruptions or unrest at volcanoes. However this can only occur if the volcano is already primed to erupt with plenty of magma and is unstable. The earthquake in this case acts as a spark, pushing an already unstable volcanic system beyond its eruptive threshold, illustrating how seismic energy has the ability to disrupt magmatic systems.
Human-Induced Triggers
Human activities can cause earthquakes by disturbing the fine balance of stress in the Earth’s crust. Although these activities do not generate the tectonic stress, they can deliver the key “push” needed to release the stored energy. Some of the major human activities that create seismicity are:
Table 1: Common Human-Induced Earthquake Triggers
| Activity | Mechanism of Triggering | Examples |
| Fluid Injection (Wastewater Disposal, Fracking) | Alters pore pressure, reduces friction on faults | Oklahoma, Colorado, Ohio, Arkansas (wastewater); various fracking sites |
| Fluid Withdrawal (Oil/Gas, Groundwater) | Changes pore pressure and stress distribution | Lorca, Spain (groundwater); historical fossil fuel production in Oklahoma/Texas |
| Reservoir Impoundment | Adds an enormous load to the ground, altering the stress | Koyna Dam (India), Oroville Dam (California) |
| Mining Activities | Changes the stress distribution due to material removal | Germany (salt, coal mining) |
|
Geothermal Power Plants |
Extraction/injection of hot, pressurized water alters local stress |
Salton Sea (California) |
Fluid Injection and Withdrawal:
- Injection of fluid into deep wells, e.g., wastewater, can add stress to faults, reducing friction and causing them to slip. Earthquakes in Colorado and Arkansas, for example, stopped after injection was discontinued.
- Hydraulic fracturing (fracking), the injection of pressurized water to create fractures in the rock, has the potential to cause bigger earthquakes if it runs into pre-stressed faults. The more fluid injected, the more likely a major event will happen.
- Conversely, fluid extraction such as oil, gas, or groundwater may change subsurface stress and induce seismicity. The Spanish 2011 M5.1 Lorca earthquake is linked to excessive over-extraction of groundwater.
Reservoir Impoundment
The sheer load of water in large dams and artificial lakes has the potential to exert pressure downward on the crust, re-altering stress on faults and causing earthquakes. India’s M6.5 Koyna Dam earthquake is the largest identified quake induced by reservoir weight alone.
Mining Operations
Moreover, surface and subsurface mining, ranging from coal extraction to salt recovery, can potentially significantly redistribute stress in the Earth’s crust and trigger earthquakes. In Germany, with earthquakes up to M5.6 from salt recovery- mining-induced seismicity.
Conclusion
The distinction between earthquake causes and triggers forms a cornerstone of modern seismology. This differentiation is critical for evaluating and reducing seismic hazard. Understanding the fundamental earthquake triggers and causes is essential for long-term hazard mapping and building resilience in active regions. Furthermore, this knowledge also proves vital for short-term hazard evaluation and developing mitigation techniques for induced seismicity.
Looking ahead, the MENA Earth Science Congress (MESC) will be a vital forum for discussing advanced seismic research. Key topics will include how human activity can trigger earthquakes, new developments in early warning systems, and how to use geophysical data to improve regional safety. For more details and news, follow our LinkedIn showcase.
