Earthquakes

Beneath our feet, the Earth’s crust is alive with 1motion. Massive slabs, known as tectonic plates, drift constantly, shaping the very ground we stand on. These silent travelers lie both under continents and oceans, creating majestic mountains, sparking volcanic eruptions, and occasionally triggering the catastrophic tremors we know as earthquakes.

Delve deeper, and the Earth reveals its layered secrets. At its heart is a solid inner core enveloped by a molten outer core. Above this, the mantle—a thick, predominantly solid layer—claims about 84% of our planet’s volume. Lastly, a relatively thin crust blankets the Earth, with its thickness varying from 3 to 30 miles (5 to 50 km).

This crust isn’t a single solid sheet. It’s segmented into colossal tectonic plates that are perpetually on the move. Where these plates meet and grind against one another along their edges, in regions called fault zones, tension builds over time. When this tension eventually surpasses the friction holding the plates together, the crust might rupture or shift, unleashing powerful seismic waves.

Imagine a stone tossed into a pond. The initial splash sends ripples radiating outward. Similarly, when the Earth’s crust breaks, the originating point, known as the hypocentre or focus, sends shock waves in every direction. The spot on the surface directly above this focus is the earthquake’s epicenter. This explosive release of energy is what we recognize as the shaking of an earthquake.

Earthquakes come with a symphony of energy waves. “P waves” or “primary waves” lead the pack, compressing and expanding as they traverse rock and fluid. They’re succeeded by “S waves” or “secondary waves”, which move exclusively through rock, swaying side to side. The final act belongs to surface waves which, true to their name, roll along the Earth’s surface, wreaking the most havoc. Among these are the swift Love waves, shaking the ground sideways, and the undulating Rayleigh waves, resembling ocean waves.

If a P wave’s frequency falls within our audible range (20-20,000 Hertz), it might produce a distinct rumble. However, most earthquake waves vibrate below 20 Hz, making the earthquake itself almost silent. The familiar rumbles during quakes usually come from shifting buildings and their contents.

Interestingly, not all earthquakes have a tectonic origin. There are volcanic quakes linked to volcanic activities, collapse quakes from underground voids, and explosion quakes triggered by underground detonations.

Annually, the Earth experiences an estimated 500,000 earthquakes. Without seismographs, many would go undetected, as they occur deep within the crust. Of these, only 20% are perceptible to humans, and a mere 100 cause notable damage.

The sensation of an earthquake hinges on its magnitude, a measure of its energy. A magnitude of 8 or above is considered immense. This magnitude remains consistent, regardless of one’s location, while the intensity—or the level of shaking—varies depending on proximity.

Earthquakes map onto specific global belts. The notorious “Ring of Fire” encircling the Pacific Ocean stands out, responsible for over 80% of major quakes, a result of the Pacific plate interacting with neighboring plates.

The most powerful recorded earthquake struck Chile in 1960, registering a magnitude of 9.5. Such mammoth events can minutely shift Earth’s axis, subtly altering the length of our days. For instance, the 2004 Sumatran earthquake trimmed 6.8 microseconds from a day.

Despite scientific advancements, predicting an exact earthquake remains elusive. While we can identify risky fault zones, pinpointing the “when” evades us. Nevertheless, progress offers hope. Innovations like ShakeAlert, developed by experts at Caltech, can provide precious seconds of warning by detecting an earthquake’s initial waves, giving people a brief but potentially life-saving moment to brace for impact.