An earthquake is the sudden, rapid release of energy within the Earth’s crust that generates seismic waves. This energy radiates outward from the source, causing the ground to shake. The strength of this shaking determines whether the event is noticeable to humans, often registering as a brief tremor or a violent jolt. What size earthquake can be felt depends on how “size” is defined and the observer’s distance from the energy release.
Defining Earthquake Size: Magnitude vs. Intensity
The size of an earthquake is measured in two distinct ways. Magnitude is a single, objective number that quantifies the total energy released at the earthquake’s source, regardless of where or how the shaking is felt. The standard measure is the Moment Magnitude Scale (MMS). This scale is logarithmic, meaning a whole-number increase represents approximately thirty-two times more energy released. A single magnitude value applies to the entire earthquake.
Intensity, on the other hand, describes the severity of the shaking and its resulting effects at a particular location. It is a subjective measure based on observed effects on people, buildings, and the natural environment. The Modified Mercalli Intensity (MMI) scale quantifies this shaking, ranging from I (not felt) to XII (total destruction). A single seismic event produces a wide range of intensity values across different locations. Intensity diminishes the farther a person is from the epicenter, but local ground conditions also play a significant role.
The Threshold of Human Perception
Earthquakes generally reach the threshold of human perception when they register between Magnitude 2.0 and 2.5. Events below this range are classified as microearthquakes, detectable only by sensitive instruments. A Magnitude 2.0 to 2.5 event occurring nearby can be felt by a small number of people, especially those at rest and indoors. This minimal perception corresponds to a Modified Mercalli Intensity (MMI) of I or II.
MMI Level II is described as “Weak” or “Scarcely Perceptible,” felt only by a few individuals, often on upper floors. At this level, hanging objects, such as lamps or curtains, may swing subtly. When the intensity reaches MMI Level III, the event is felt noticeably indoors. The sensation is often likened to the vibration caused by a heavy truck passing outside, marking a more common perception threshold.
Local Factors That Influence Shaking
The perceived intensity of an earthquake is significantly altered by several local factors, not just its magnitude. One factor is the depth of the earthquake’s hypocenter, the point where the rupture begins beneath the surface. Deeper earthquakes dissipate their energy over a larger volume, making them less intense than a shallower earthquake of the same magnitude.
Distance from the Epicenter
The distance from the epicenter is a major variable, as seismic waves naturally attenuate, or weaken, as they travel through the Earth. A moderate-magnitude earthquake felt strongly at the epicenter may be barely felt tens of miles away.
Local Geology and Amplification
Local geology dramatically affects shaking, a phenomenon known as amplification. Soft sediments, like loose soil or artificial landfill, can amplify the shaking intensity. This causes significantly more movement than a nearby site built on solid bedrock.
Location Within a Structure
A person’s location within a structure also influences perception. Individuals on the higher stories of multi-story buildings often feel the shaking more distinctly due to the building’s natural sway.