An earthquake is commonly understood as a sudden, violent shaking of the ground that occurs along a fault line, primarily where the Earth’s tectonic plates meet. While the vast majority of earthquakes happen in these seismically active zones, the idea that they only occur there is incorrect. Stress can accumulate in locations far removed from major plate edges, leading to significant exceptions. These exceptions are caused by both natural forces deep within continents and, increasingly, by human actions, resulting in seismic activity in unexpected places.
Earthquakes at Tectonic Plate Boundaries
The Earth’s lithosphere is fragmented into tectonic plates, and the boundaries between them are the primary locations for global seismicity. These boundaries are categorized by how the plates interact: convergent, divergent, or transform.
Convergent boundaries, where plates collide, often create the largest and deepest earthquakes, such as along the subduction zones of the Pacific Ring of Fire. Here, one plate slides beneath the other, generating immense friction and stress that can be released in powerful seismic events down to depths of 700 kilometers.
At divergent boundaries, plates move away from each other, like along the Mid-Atlantic Ridge, resulting in common but typically shallow earthquakes, usually less than 30 kilometers deep. Transform boundaries, such as the San Andreas Fault, involve plates sliding past each other horizontally. Fault lines are the fractures in the Earth’s crust where tectonic stress is released in a sudden slip.
Intraplate Earthquakes: The Natural Exceptions
Some large earthquakes occur far from the edges of tectonic plates, deep within continental interiors, and these are known as intraplate earthquakes. These events happen on ancient, pre-existing zones of weakness, often called relict faults, created by tectonic activity millions of years ago. Current plate movements apply stress across the entire plate, which can reactivate these old, buried fault systems.
The mechanism for these quakes involves stress transfer, where forces from distant plate interactions are channeled into these weak zones. For instance, compression from the spreading of the Atlantic Ocean and the collision of the Pacific plate contributes to the stress budget in the central United States.
A classic example is the New Madrid Seismic Zone, located in the center of the North American Plate, which produced a sequence of three large earthquakes (magnitude 7.5 to 8.3) in 1811 and 1812. These earthquakes occur along reactivated faults within a buried geological feature known as the Reelfoot Rift.
Intraplate events are relatively infrequent, making it challenging to measure the slow rate of strain accumulation. The recurrence interval for large events in a region like the New Madrid Seismic Zone is estimated to be between 250 to 4,000 years. Although strain rates are very low, the accumulated stress is eventually released on these faults with significant power.
Seismicity Driven by Human Activity
A completely separate category of seismic events are those triggered directly by human intervention, known as induced seismicity. These earthquakes occur on existing, often small, faults that are already close to failure, but the human activity provides the final push. The most widely recognized cause is deep underground fluid injection, primarily for wastewater disposal from oil and gas production. Injecting large volumes of fluid into deep wells increases the pore pressure within the rock formation and along pre-existing faults.
This increased pressure acts as a lubricant, reducing the effective normal stress holding the fault surfaces together, thereby allowing them to slip. This mechanism has led to a dramatic increase in seismicity in regions like Oklahoma, where the rate of earthquakes above magnitude three grew tenfold over a decade due to large-volume wastewater disposal. Other human activities can also induce seismicity by altering the natural stress state of the crust.
Large reservoirs created by dams can cause earthquakes through reservoir loading, where the immense weight of the water changes the stress on underlying faults. Mining and quarrying operations also induce localized seismicity by removing large masses of rock, which shifts the stress on surrounding faults. In all these cases, the earthquakes are not generated by natural tectonic forces, but rather by the change in the fluid pressure or rock mass that directly triggers a slip on an existing fault.