why do earthquakes spin

The Deep Dive

When a fault ruptures underground, it releases energy in the form of seismic waves radiating outward from the epicenter. Scientists classify these into body waves, which travel through the Earth's interior, and surface waves, which ripple along the planet's crust. Body waves include P-waves that push and pull material in the direction of travel and S-waves that move the ground perpendicular to their path. Surface waves come in two main varieties: Love waves, which shift the ground side to side, and Rayleigh waves, which roll the ground in an elliptical motion similar to ocean waves. The rotational component of ground motion arises when these different wave types interact with one another and with the heterogeneous geology beneath our feet. Variations in rock density, fault geometry, and the angle of wave incidence all contribute to twisting motions at the surface. Rotational seismology, a relatively young field, now uses specialized instruments called ring laser gyroscopes and arrays of seismometers to measure these subtle spin components. Research has shown that rotational motions can amplify structural damage, particularly to tall buildings and bridges, because twisting forces stress materials in ways that simple back-and-forth shaking does not. The 2011 Tōhoku earthquake in Japan produced measurable rotational ground motions that contributed to the complex destruction patterns engineers observed afterward.

Why It Matters

Understanding rotational ground motions is critical for earthquake engineering and building safety. Engineers who design skyscrapers, bridges, and nuclear facilities must account for twisting forces, not just horizontal shaking. Traditional seismic hazard models that ignore rotation may underestimate the actual forces structures experience during a major quake. This knowledge also improves early warning systems and helps seismologists create more accurate ground motion predictions. For communities in earthquake-prone regions, better models translate directly into stronger buildings, smarter infrastructure, and ultimately saved lives.

Common Misconceptions

A widespread myth is that earthquakes cause the ground to spin like a merry-go-round. In reality, rotational motions are subtle, measured in microradians, and far too small to feel as a distinct spinning sensation. Another misconception is that rotational ground motion is a newly discovered phenomenon. While the field of rotational seismology is young, engineers have long observed that buildings twist during earthquakes. The difference is that modern instruments now allow scientists to directly measure and quantify these motions rather than inferring them from structural damage alone.

Fun Facts

• The rotational component of the 2011 Tōhoku earthquake was so powerful that it slightly shifted Earth's axis by about 10 centimeters.
• Some ancient pagodas in Japan survived centuries of earthquakes partly because their flexible wooden joints allowed them to absorb rotational forces that destroy rigid modern structures.