How Earthquakes Are Measured

The seismometer

A seismometer measures ground motion. At its heart is a mass that, because of its inertia, tends to stay still while the ground and the instrument frame move around it. The relative motion between the mass and the frame is recorded; the paper or digital trace it produces is a seismogram.

Early instruments used a physical pen on a rotating drum. Modern broadband seismometers use an electronic feedback loop to hold the mass in place and measure the force needed to do so, which lets a single instrument record everything from a passing truck to the sway of the whole planet after a great earthquake. Thousands of them stream data continuously to data centres around the world.

Seismic waves

An earthquake radiates several kinds of wave, and they arrive at a distant station in a fixed order because they travel at different speeds:

• P waves (primary) are compressional — the rock is pushed and pulled in the direction the wave travels. They are the fastest (about 6 km/s in the upper crust) and arrive first. They pass through solids and liquids.
• S waves (secondary) are shear — the rock moves side to side. They travel at roughly 60% of the P-wave speed and cannot pass through liquid. Their failure to cross Earth’s outer core is the main evidence that the outer core is molten.
• Surface waves travel along the Earth’s surface, arrive last, and usually have the largest amplitude. They cause most of the shaking damage in a large earthquake.

Locating the earthquake

Because P waves outrun S waves by a predictable amount, the gap between the P and S arrivals at a station tells you how far away the earthquake was — a longer gap means a more distant source. One station gives a distance but not a direction. With three or more stations, the intersecting distance circles pin down the epicenter; modern software fits the arrival times at dozens or hundreds of stations to a model of how fast waves travel through the Earth.

Depth is harder from distant stations alone. It is refined using waves that bounce off the surface directly above the source (the pP and sP phases): the delay between the direct wave and its surface reflection depends on how deep the rupture was. A nearby station directly over the earthquake constrains depth best of all.

Magnitude and mechanism

Magnitude is computed from the amplitude of the recorded waves, corrected for distance. For moment magnitude, seismologists invert the shape of the whole waveform to recover the seismic moment and the orientation of the fault — the result is often drawn as a “beachball” diagram showing whether the fault slipped by thrusting, normal (extensional), or strike-slip motion.

For a large earthquake the first automatic magnitude, available within minutes, is often revised over the following hours and days as more data arrive and analysts review it. This is normal, and it is why early news reports of a big earthquake’s magnitude sometimes change.

The global network

No single country monitors the whole planet. Seismograph networks in more than 150 countries share data in real time through the International Federation of Digital Seismograph Networks (FDSN). In the United States, the USGS National Earthquake Information Center in Golden, Colorado, operates around the clock and publishes locations for roughly 20,000 earthquakes a year worldwide. The International Seismological Centre in the UK later compiles the definitive long-term global catalogue.

The earthquakes on this map come from the USGS catalogue built on that shared data.

Earthquakes before seismometers

The first useful seismographs date from the 1890s, and global coverage is only reliable from the mid-20th century. Earlier earthquakes are reconstructed from other evidence:

• Historical accounts — diaries, newspapers and official reports are converted to intensity values and mapped, giving an approximate location and size.
• Paleoseismology — digging trenches across a fault exposes layers offset by past ruptures, which can be dated with radiocarbon.
• Tsunami and turbidite deposits — sand layers left inland by tsunamis, and underwater landslide deposits shaken loose by strong shaking, record prehistoric earthquakes.