Earthquake questions, answered

Most earthquakes happen when stress built up along a fault - a fracture between two blocks of rock - is suddenly released and the rocks slip past each other. That stress comes from the slow motion of Earth's tectonic plates. The sudden slip radiates energy as seismic waves, which is the shaking you feel.

Magnitude is a measure of the energy released at the source. It is logarithmic: each whole number is about 32 times more energy, so a magnitude 6 releases roughly 1,000 times more energy than a magnitude 4. That is why small differences in magnitude matter a lot.

The old Richter scale worked well for small, local quakes but saturated (stopped increasing) for very large ones. Modern agencies use the moment magnitude scale (Mw), based on the physical size of the rupture and the rock strength, which stays accurate for the largest earthquakes. News still loosely calls it "the Richter scale", but it is almost always Mw today.

Magnitude is a single number for the whole earthquake - the energy at the source. Intensity describes how strongly the shaking was felt at a particular place, so it varies with distance, depth and ground type. The same quake can be intensity "violent" near the epicenter and "weak" a hundred kilometres away.

A 4.8 is a light-to-moderate earthquake. It is widely felt and can rattle windows, move small objects and occasionally cause minor cracks, but it rarely causes serious structural damage to well-built buildings. Damage depends heavily on depth, distance and local construction.

A lot, and it climbs fast. A magnitude 4.8 releases on the order of tens of kilotonnes of TNT equivalent in seismic energy; a magnitude 6 is in the megatonne range; the largest recorded quakes rival many gigatonnes. The key is the logarithmic scale - going up one magnitude multiplies the energy by about 32.

A depth of "10 km" is frequently a default placeholder used in the first automatic location, before analysts have enough data to pin the true depth. Many real quakes are genuinely shallow, but if you see exactly 10.0 km on a brand-new event, treat it as preliminary - it is often revised later.

Yes. Injecting wastewater deep underground, large-scale fracking, filling big reservoirs behind dams, geothermal energy and mining can all trigger earthquakes - usually by changing the fluid pressure or load on existing faults. Most induced quakes are small, but some have reached magnitude 5+ (notably in Oklahoma).

Fracking itself usually causes only tiny tremors. The bigger driver of human-induced quakes in places like Oklahoma is the deep disposal of wastewater (a byproduct of oil and gas), which raises pressure on faults. Both are forms of induced seismicity, but wastewater injection has produced the larger events.

Only an enormous one, and even then the comparison is loose. As one widely-shared answer put it, you would need on the order of 25,000 tonnes of well-placed TNT to match the energy of a ~M4.8. Explosions also shake the ground differently from natural quakes (see the next question), so seismologists can usually tell them apart.

By the seismograms. Explosions are mostly compressive, so they produce strong P waves and weak S waves; natural earthquakes are shear ruptures with strong S waves. Analysts (and automatic systems) compare the wave types, depth and frequency content to distinguish a quake from a blast - which is also how nuclear tests are detected.

Yes - it is called reservoir-induced seismicity. The weight of the water and, more importantly, the increased water pressure seeping into the rock can unclamp nearby faults. Most events are small, though a few large reservoirs have been linked to significant earthquakes.

Not in the "magnitude X at place Y next Tuesday" sense - no reliable short-term prediction method exists, despite many claims. Scientists can estimate long-term probabilities for a region and detect quakes the instant they start, but pinpoint prediction remains out of reach.

Early warning is not prediction. It detects an earthquake that has already started and races a digital alert ahead of the slower-moving shaking, giving you seconds to tens of seconds of notice. That is enough time to drop and cover or for systems to slow trains and stop machinery.

There are many anecdotes of unusual animal behaviour before quakes, but no consistent, scientifically reliable signal has been confirmed. Animals may react to early P waves or small foreshocks a moment before the strong shaking, but they are not proven earthquake predictors.

Foreshocks are smaller quakes that sometimes precede a larger "mainshock"; you usually only know they were foreshocks after the big one hits. Aftershocks follow the mainshock as the crust readjusts - they are most frequent right after and taper off over days to years, and can still be damaging.

Drop, Cover and Hold On: drop to your hands and knees, take cover under a sturdy table (or next to an interior wall, away from windows), and hold on until the shaking stops. Do not run outside mid-shaking - most injuries come from falling objects and moving around.

That is an outdated myth from old adobe houses. In modern buildings a doorway is no stronger than the rest of the structure, and you are exposed to a swinging door and flying debris. Get under sturdy furniture and cover your head and neck instead.

Water (about 4 litres per person per day for several days), non-perishable food, a flashlight, spare batteries or a power bank, a first-aid kit, essential medications, copies of important documents, cash, sturdy shoes and a whistle. Keep some supplies at home, at work and in the car.

Official government early-warning systems exist in some regions, and apps fill the gaps elsewhere. QuakeMate (free on iOS) sends real-time push notifications and lets you set custom rules by magnitude, region and distance, so you only hear about the quakes you care about. You can grab it at quakemate.app/download.

Strong quakes can destroy habitat, trigger landslides and, offshore, generate tsunamis that reshape coastlines. Animals may flee shaking ground or react to early waves, and large events can disrupt nesting, migration and food sources - though many ecosystems recover over time.

Shaking makes structures sway; if the motion exceeds what they were built to handle, walls crack, soft storeys collapse and unbraced contents fall. Secondary effects do a lot of damage too: ground liquefaction, landslides, fires from broken gas lines, and failures of bridges, pipes and power lines. Modern building codes are designed to keep people alive even when a building is damaged.

Usually when a large (roughly M7+) and shallow quake under the ocean suddenly lifts or drops the seafloor, displacing the water above it. Strike-slip quakes (mostly sideways motion) rarely make big tsunamis; vertical subduction-zone ruptures are the dangerous ones. QuakeMate flags tsunami-capable events and shows active warnings at quakemate.app/tsunamis.

The 1960 Valdivia earthquake in Chile, at magnitude 9.5 - the strongest instrumentally recorded quake in history. It generated a Pacific-wide tsunami that caused damage as far away as Japan and Hawaii.

By raw count, Japan, Indonesia and the United States (especially Alaska and California) record huge numbers of events thanks to their position on plate boundaries. Globally, the Pacific "Ring of Fire" produces the majority of the world's large earthquakes.

The Ring of Fire is a horseshoe-shaped belt around the Pacific Ocean where many tectonic plates meet. About 90% of the world's earthquakes and most of its active volcanoes occur along it, from New Zealand up through Japan and across to the Americas.

Because they trace plate boundaries and active faults that are permanently under stress. Once a fault slips, plate motion immediately starts reloading it, so the same regions rupture again and again over years to centuries.

QuakeMate shows a live worldwide feed and an interactive map of recent earthquakes. See the latest events at quakemate.app/now or explore the map with magnitude, time and source filters at quakemate.app/explore.

Yes. QuakeMate has a live page for every country (and every US state) with recent activity, exact 7-day and 30-day counts, the strongest and most recent event, and a map. Start at quakemate.app/earthquakes-in.

Did You Feel It (DYFI) lets you report what you felt during a quake - the intensity, what moved, any damage - and see how your neighbours experienced the same event. Those community reports help everyone understand how an earthquake was felt across a region. You can report from the QuakeMate app.

QuakeMate aggregates real-time feeds from public seismic agencies including the USGS, EMSC and regional networks, plus volcano data from the Smithsonian Global Volcanism Program and tsunami alerts from official warning centres. Magnitudes and locations are preliminary at first and may be revised as agencies refine them.