Earth’s crust is in a constant state of motion, resulting in hundreds of thousands of seismic events worldwide every year. Seismological centers locate these events, yet the vast majority of people never feel them. The difference between a recorded tremor and a perceived shake is determined by physics, geology, and human biology. This disconnect highlights how various factors shield us from constant, subtle vibrations.
The Vast Majority Are Too Small
The primary reason most earthquakes go unnoticed is their inherently small magnitude. Earthquakes follow a power law distribution, meaning that for every large event, there are exponentially more smaller events. The vast majority of seismic releases fall below a magnitude of 2.0, which is the threshold for human perception even when standing directly over the hypocenter.
A typical person requires a local magnitude of 2.5 to 3.0 to register the shaking, depending on their proximity to the source. These smaller events are often called micro-earthquakes because the energy they release is minimal. They affect a very limited area and dissipate quickly into the surrounding rock, making them too weak to generate ground motion perceptible to human senses.
Distance and Depth of the Seismic Source
The strength of a seismic wave decreases dramatically as it travels outward from the earthquake’s focus, a process known as attenuation. This energy loss occurs due to geometrical spreading and intrinsic attenuation. As seismic waves expand from the source, the energy spreads over a larger volume, causing the wave amplitude to decay inversely with the distance traveled.
Intrinsic attenuation occurs as wave energy is converted into heat due to friction within the rock layers. This means that an earthquake’s depth, or hypocenter, plays a significant role in surface perception. A magnitude 5.0 earthquake occurring 200 kilometers deep distributes its energy over a massive volume before reaching the surface, resulting in a minor, barely felt tremor. Conversely, a smaller magnitude 3.0 earthquake with a shallow depth of only a few kilometers feels much stronger to nearby observers because the seismic energy has had less distance to travel.
How Local Geology Filters Seismic Waves
The final few kilometers of a seismic wave’s journey significantly alter its felt intensity. The material directly beneath a location, known as the local geology, acts as a filter and amplifier for the incoming waves. Locations built on hard bedrock, such as granite, often experience lower intensity shaking because the rock transmits the energy efficiently without amplifying the wave amplitude.
In contrast, sites built on soft sediments, like alluvial soil or sand, can experience substantially amplified shaking. These less consolidated materials have a lower shear-wave velocity, causing seismic waves to slow down and their energy to concentrate, increasing the amplitude of the ground motion. Soft soils also tend to amplify lower-frequency waves while filtering out the higher-frequency energy that humans are more sensitive to. This filtering effect can dampen the sharp, high-frequency jolt, making the event less noticeable even if the overall ground motion is significant.
Human Sensory Threshold Versus Instrumentation
The ability to detect an earthquake depends on the sensitivity of the observer, where scientific instruments far exceed human capability. Seismographs measure ground motion in the scale of microns, detecting vibrations far below the level of human physiological response. These instruments provide an objective measure of the energy released.
Human perception is subjective and governed by the Modified Mercalli Intensity (MMI) scale, which is based on observable effects. For a person to feel an earthquake, the shaking must reach an MMI of II or III. Level II is felt only by a few people at rest, especially on upper floors, while Level III is felt indoors by many. Factors such as being asleep or outdoors can significantly raise a person’s individual threshold for perception. Being on a building’s upper floor often increases the sensation of motion, making a person more likely to feel a tremor that someone on the ground floor would miss.