Question 1

What causes earthquakes, and how is magnitude measured?

Accepted Answer

The sudden release of **elastic strain** accumulated in rock, and magnitude is measured on a **logarithmic** scale — which is why the numbers behave in ways people consistently misjudge.

**The mechanism: elastic rebound.** Tectonic plates move continuously, but faults are locked by friction. Strain builds in the surrounding rock, deforming it elastically, until the stress exceeds the friction holding the fault. The fault then **slips suddenly**, releasing the accumulated energy as seismic waves, and the rock springs back toward its undeformed shape.

So earthquakes are not the Earth cracking open; they are the sudden failure of an already-existing fault that was stuck.

**The focus** is where rupture begins, at depth; the **epicentre** is the point on the surface directly above it. Shallow earthquakes cause far more damage than deep ones of the same magnitude.

**Magnitude scales.** The **Richter scale** is widely cited and largely obsolete for large events — it was designed for local Californian earthquakes with particular instruments and **saturates**, giving similar values for very different large earthquakes.

**Moment magnitude (Mw)** is the modern standard, calculated from the **seismic moment** — the rigidity of the rock, the fault area that slipped, and the average slip distance. It does not saturate and is physically meaningful.

**How the scale actually works, which is the part routinely misunderstood:**

**Each whole number is ten times the amplitude** of ground motion.

**But roughly 32 times the energy released.**

So a magnitude 7 releases about **1,000 times** the energy of a magnitude 5 — not twice as much, and not 100 times. The difference between a damaging earthquake and a catastrophic one is far larger than the numbers suggest.

**Magnitude is not the same as shaking.** **Intensity** scales — such as the Modified Mercalli scale — describe observed effects at a location, and vary across the affected area for one earthquake, depending on distance, depth, local geology and building construction. **Soft sediment amplifies shaking** considerably, which is why damage is frequently patchy.

**Aftershocks** follow a predictable statistical decay, and **prediction of specific earthquakes remains impossible** — forecasting is probabilistic.

Question 2

What is plate tectonics?

Accepted Answer

The theory that the Earth's outer shell is divided into rigid plates that move relative to one another over a weaker layer beneath — explaining earthquakes, volcanoes, mountain ranges and the

Question 3

How do we know what is inside the Earth?

Accepted Answer

Almost entirely from earthquakes. Seismic waves travel through the planet and arrive at stations around the world subtly changed by everything they passed through, which makes the Earth its own

Question 4

How do mountains form, and why do they stop growing?

Accepted Answer

Mostly through tectonic collision thickening the crust, and they stop growing because erosion and gravity eventually balance uplift — so a mountain range's height reflects an equilibrium rather than

Question 5

How does the geological time scale work?

Accepted Answer

A hierarchy of named intervals dividing Earth's history, defined by changes in the rock and fossil record rather than by round numbers — which is why the boundaries fall at irregular dates. The

Question 6

Why do some volcanoes erupt explosively and others flow gently?

Accepted Answer

Because of the viscosity of the magma and how much dissolved gas it contains — and both are determined largely by silica content, which in turn depends on the tectonic setting. The mechanism. Magma

Question 7

What are the SI units actually based on now?

Accepted Answer

Since a redefinition that took effect in 2019, all seven base units are defined by fixing the numerical values of physical constants — not by physical objects, and not by properties of materials. It

Related Questions