An earthquake is the ground shaking caused by a sudden slip on a fault, a fracture in the Earth’s crust where blocks of rock slide past one another. This abrupt movement releases stored energy in the form of seismic waves that travel through the Earth. The experience of an earthquake varies widely, depending on the event’s power and a person’s location relative to the fault. Understanding how these forces manifest as physical sensations and environmental changes helps prepare for this often unsettling natural phenomenon.
The Immediate Sensory Experience
The onset of an earthquake often begins with a subtle sensation that quickly escalates. The initial feeling is frequently described as a sharp bump or jolt, similar to a heavy object striking the building or a large truck rumbling past. This is followed by the main shaking event, which can feel like a violent side-to-side motion, a deep rolling, or a combination of both.
During the shaking, the feeling of losing balance is immediate and profound, making it difficult or impossible to stand upright. Objects fall from shelves, pictures swing wildly, and unsecured furniture may slide across the floor. The visual distortion, where solid structures appear to wobble and move, contributes to a sense of unreality for many people.
The event is also a loud auditory one. A deep rumbling or roaring sound, often compared to a freight train passing nearby, accompanies the ground movement. This noise is caused by the seismic waves and the rattling, banging, and crashing of the surrounding environment, including breaking glass and the creaking of building structures. The unsettling feeling of being on an unstable surface can persist briefly after the shaking stops, an effect called self-motion sensation.
How Magnitude and Location Change the Feeling
The intensity of the shaking felt depends on the total energy released and the distance from the earthquake’s origin. The Moment Magnitude Scale quantifies the overall energy released at the source. A more relatable measure is the Modified Mercalli Intensity (MMI) Scale, which describes the actual effects felt at a specific location, based on observations from people and the damage to structures.
The MMI scale ranges from I (Instrumental), which is not felt by people but is recorded by instruments, to XII (Catastrophic). An MMI of II or III is a subtle movement felt only by a few people at rest. By MMI V, the shaking is felt by nearly everyone, awakening some from sleep and overturning unstable objects. An MMI VII means all people feel the shaking, heavy furniture is moved, and damage is slight to moderate in ordinary structures.
The physical nature of the seismic waves determines the initial sensation and the main shaking. The faster-traveling compressional waves (P-waves) arrive first and cause the initial, brief vertical jolt or thump. P-waves are typically less damaging and can act as a natural early warning. The slower-moving, more destructive shear waves (S-waves) arrive next and cause the main rolling, side-to-side, and often violent shaking that causes most damage.
A person’s location within a structure modifies the feeling of the shaking. On the ground floor, the motion tends to be a sharp, violent jolting that closely matches the ground movement. Conversely, people in high-rise buildings often feel a long, slow, swaying motion, especially at the upper levels. The building acts like an inverted pendulum, and the structure’s flexibility can amplify the movement higher up, leading to motion sickness.
Immediate Response: Safety During the Shaking
The moment the ground begins to shake, the immediate action is the “Drop, Cover, and Hold On” procedure. Instantly drop to your hands and knees to prevent being knocked over. Dropping allows a person to remain low, while covering their head and neck or seeking shelter under a sturdy table or desk. Hold on to the shelter until the shaking completely stops, moving with the furniture if it shifts.
Running outside or moving to another room during the shaking is strongly discouraged because people are most often injured by falling debris, glass, or unsecured objects.
If driving, pull over to a clear area away from bridges, overpasses, and power lines, set the parking brake, and stay inside the vehicle until the movement ceases. If outdoors in an open area, move away from buildings, streetlights, and utility wires, then drop to the ground and cover your head.