An earthquake is the sudden shaking of the Earth’s surface resulting from a rapid release of stored energy within the lithosphere. Frequent earthquakes in a specific location indicate that continuous geological processes are actively building stress in the crust. The seismicity of an area, including the frequency, type, and magnitude of quakes, directly reflects underlying geological instability. A high rate of seismic events tells geologists that the ground is actively deforming and that plate movement forces are overcoming the frictional resistance of the rocks.
Tectonic Plate Boundaries
The fundamental reason for an area experiencing frequent earthquakes is its location relative to the planet’s tectonic plates. The lithosphere is broken into about a dozen major plates that are constantly moving. Where these plates meet, immense stress accumulates as their motion is resisted by friction, which ultimately leads to frequent seismic events.
The nature and frequency of quakes depend on how the plates interact at the boundary. At convergent boundaries, where plates collide, one plate often slides beneath the other in a process called subduction, creating the largest and deepest earthquakes. Subduction zones, such as those around the Pacific Ring of Fire, produce abundant seismicity as the descending slab locks and then slips under the overriding plate.
Transform boundaries are characterized by plates sliding horizontally past one another, and these areas have a high frequency of earthquakes. The San Andreas Fault in California is a prime example where frequent, shallow quakes occur as the plates move laterally. The largest quakes along these faults can reach magnitudes up to about 8, though they are shallower than those at subduction zones.
At divergent boundaries, where plates are pulling apart, earthquakes are common but tend to be smaller and shallower due to high rock temperatures. Seismic activity in these zones, such as the Mid-Atlantic Ridge, is concentrated along the transform faults that link the diverging segments. A high frequency of quakes often points toward the continuous, forceful movement characteristic of transform or subduction zones.
Active Fault Systems
While plate boundaries define broad zones of seismicity, frequent earthquakes indicate the presence of localized structures known as active fault systems. A fault is a fracture in the Earth’s crust where blocks of rock have moved relative to one another. An “active” fault has demonstrated movement in recent geological time and is capable of generating future earthquakes.
Frequent smaller earthquakes and micro-seismicity result from the stress-strain cycle on these active faults. Tectonic forces continuously build up stress on the fault plane, which the rock absorbs as elastic strain. Friction and irregularities along the fault surface cause the blocks to “lock,” preventing smooth movement.
When accumulated stress overcomes frictional resistance, the fault slips suddenly, releasing stored elastic strain energy as an earthquake. Frequent quakes suggest a continuous or high rate of strain accumulation, or that the fault is highly segmented, allowing for smaller, more regular stress releases. A cluster of small quakes, known as a swarm, may signal continuous movement or stress loading on a major, locked fault segment.
Non-Tectonic and Human-Induced Seismicity
In some areas, frequent earthquakes arise from localized, non-tectonic processes rather than plate tectonics. Volcanic regions experience frequent tremors and swarms caused by the movement of magma and other fluids beneath the surface. These events are often shallow and related to changes in pressure as molten rock forces its way through the crust.
Other frequent seismic activity is directly attributable to human industrial activities, a phenomenon known as induced seismicity. This activity is localized and involves altering the stresses and strains on existing faults. The most common cause is the injection of large volumes of fluid deep into the ground, often during wastewater disposal from oil and natural gas production.
Injecting wastewater increases the pore pressure within the rock layers, which reduces the friction holding a fault locked, allowing it to slip. The increase in seismicity in Oklahoma in the 2010s was attributed to the injection of wastewater into deep sedimentary rock. Other activities, such as geothermal projects or the impoundment of large reservoirs, can also cause frequent, localized earthquakes by changing the pressure or loading on the crust.
Long-Term Seismic Hazard Assessment
The frequency of earthquakes is the primary data used by geologists to determine the probability of future, potentially damaging events. Frequent seismicity indicates a high seismic hazard, which is the potential for an area to experience ground shaking or other earthquake effects. Scientists use the historical record of quakes to calculate the recurrence interval, the average time between successive earthquakes of a given magnitude on a specific fault.
The frequency of movement is also used to calculate the slip rate, the rate at which the two sides of a fault move past each other over long periods. Faults with high slip rates and short recurrence intervals have a greater probability of generating a large earthquake within a short timeframe. These calculations rely on the assumption that the long-term rate of tectonic strain accumulation is constant.
By analyzing the size, location, and frequency of recorded earthquakes, scientists constrain models that forecast the likelihood of future ground motion. This forecast allows communities to differentiate between seismic hazard and seismic risk, the latter being the potential for damage and loss of life. Frequent earthquakes indicate the area is subject to high strain accumulation that must be accounted for in building codes and emergency planning.