Small tremors frequently raise public concern about a possible larger, destructive earthquake to follow. Seismologists analyze the patterns, locations, and magnitudes of these smaller events to determine their relationship to major hazards. The relationship between a small quake and a large one is complex. It varies significantly depending on whether the small tremor is a foreshock, an aftershock, or an entirely distinct event called a swarm.
How Faults Store and Release Energy
Earthquakes are the result of tectonic plates moving against one another along fractures in the Earth’s crust known as faults. The slow, relentless motion of these plates causes immense stress to build up on the locked sections of faults.
This process is explained by the elastic rebound theory, where rocks on either side of a fault deform and accumulate energy, similar to a stretched rubber band. When the accumulated strain exceeds the strength of the rock and the frictional resistance, the fault ruptures suddenly, releasing the stored energy as seismic waves.
A common misconception is that frequent small earthquakes can relieve enough pressure to prevent a major event. However, earthquake energy is measured on a logarithmic scale, meaning the difference between magnitudes is immense. An increase of one whole number on the moment magnitude scale represents approximately 32 times more energy release. For example, a magnitude 7.0 earthquake releases about 1,000 times more energy than a magnitude 5.0 event, confirming that minor tremors release negligible amounts of the total stored tectonic strain.
Identifying Foreshocks and Aftershocks
Small earthquakes sometimes immediately precede a larger one, but they can only be identified as “foreshocks” after the larger event, called the “mainshock,” has occurred. Foreshocks are relatively smaller earthquakes that happen in the same location as the mainshock. However, not all major earthquakes are preceded by them. In a typical sequence, the mainshock is the largest earthquake in the series, representing the primary release of accumulated elastic energy.
Following the mainshock, a sequence of smaller earthquakes known as aftershocks occurs in the same general area. These aftershocks represent minor readjustments of the crust around the ruptured portion of the fault. The frequency and magnitude of aftershocks generally decrease over time, following predictable statistical decay laws. Aftershocks result from stress transfer, where the main rupture shifts strain onto adjacent sections of the fault, causing them to fail in turn.
A critical aspect of this classification is that the mainshock is simply defined as the largest event in the sequence. If an earthquake initially classified as an aftershock turns out to be larger than the preceding mainshock, the entire sequence is reclassified. The former mainshock is retrospectively designated a foreshock, and the new, larger event becomes the mainshock. This retrospective labeling highlights the challenge in using these small events for immediate warning.
When Small Quakes Cluster: Understanding Swarms
A fundamentally different type of seismic activity is an earthquake swarm, which is a cluster of earthquakes without a clear, dominant mainshock. In a swarm, the earthquakes often have similar magnitudes, and no single event stands out as being significantly larger than the others. Swarms can persist for days, weeks, or even months, featuring thousands of events in a localized area.
This type of clustering is frequently associated with non-tectonic processes, particularly the movement of fluids deep underground. High-pressure water, gas, or magma can migrate through cracks and faults, increasing the fluid pressure within the rock. This increased pore pressure reduces the friction holding the fault together, triggering numerous small earthquakes as the fluid diffuses through the network of fractures.
Swarms are common in geothermal and volcanic regions, such as Yellowstone National Park, where hydrothermal activity is prevalent. The spatial migration of these small quakes can often be tracked, showing patterns consistent with the slow diffusion of high-pressure fluids. While swarms indicate stress changes and crustal instability, they rarely escalate into massive, destructive earthquakes.
Why Scientists Cannot Predict Major Earthquakes
The inability to distinguish a foreshock from an ordinary, random small earthquake is the primary reason why scientists cannot predict major earthquakes. The Earth experiences countless small earthquakes daily, most of which are simply background seismicity and are not followed by a large event. There is currently no observable, measurable physical precursor that reliably signals whether a small tremor is the beginning of a major mainshock sequence.
A meaningful earthquake prediction requires a precise date, time, location, and magnitude for the future event, a level of certainty that remains unattainable. Seismologists lack the ability to directly observe the complex conditions, such as fluid presence or stress state, kilometers beneath the surface where earthquakes begin.
Instead of prediction, the scientific community focuses on long-term forecasting. This calculates the probability that an earthquake of a certain magnitude will occur in a specific region over years or decades. This probabilistic approach, alongside the development of early warning systems, represents the current state of earthquake hazard assessment.