Earthquake Measurement: Richter, Moment Magnitude & Faults

Every Magnitude Unit Is 32 Times More Energy

The 2011 Tōhoku earthquake measured magnitude 9.1 on the moment magnitude scale — the largest earthquake ever recorded in Japan. The energy released was approximately 600 million times that of the atomic bomb dropped on Hiroshima. A magnitude 6.0 earthquake releases energy equivalent to roughly 15 kilotons of TNT. Stepping from magnitude 5.0 to 6.0 means 32 times more energy released; from 5.0 to 7.0, 1,000 times more. This logarithmic relationship — one of the most consequential in geophysics — explains why a two-unit difference on the scale separates a nuisance tremor from a civilization-altering disaster.

Richter Scale vs. Moment Magnitude Scale

Charles Richter developed the local magnitude scale (ML) in 1935 at Caltech, calibrated to a specific seismograph design (Wood-Anderson torsion seismometer) and designed for earthquakes in Southern California within 600 km. The scale measured the amplitude of seismic waves on that instrument at a standard 100 km distance, with a logarithmic adjustment for distance. It was practical, reproducible, and immediately adopted. Its limitations became apparent as seismology globalized: the Wood-Anderson instrument saturated at magnitudes above about 6.5, producing "magnitude saturation," and the scale poorly characterized earthquakes in other regions and at other depths.

The moment magnitude scale (Mw), developed by Hiroo Kanamori and Thomas Hanks in 1979, solved these problems by measuring the total seismic moment — a physical quantity derived from the fault area that ruptured, the average slip distance, and the shear modulus of the rock. Mw does not saturate, applies globally across all depths and distances, and is the standard used by the USGS for all significant earthquakes today.

Fault Types and Earthquake Generation

Earthquakes occur where tectonic stress accumulated over decades or centuries exceeds the static friction holding a fault locked in place. The fault geometry — how the two sides move relative to each other — determines the style of faulting, which in turn influences ground motion characteristics, tsunami generation potential, and surface deformation.

• Strike-slip faults: The two fault blocks move horizontally past each other with little vertical component. The San Andreas Fault (California) and the North Anatolian Fault (Turkey) are classic examples. Strike-slip earthquakes rarely generate tsunamis but can produce intense horizontal shaking in near-field regions. The 1906 San Francisco earthquake (estimated Mw 7.9) resulted from 4–6 meters of right-lateral slip along the San Andreas.
• Reverse (thrust) faults: One block rides up over the other along a gently inclined fault plane. Occur in compressional settings. Subduction zones — where one tectonic plate descends beneath another — generate the world's largest earthquakes (megathrust events) via this mechanism. The 2004 Indian Ocean earthquake (Mw 9.1–9.3) and 2011 Tōhoku earthquake were both subduction zone megathrusts. Vertical seafloor displacement from thrust faulting is the primary tsunami generator.
• Normal faults: One block drops down relative to the other along an inclined fault plane. Occur in extensional settings (rift zones, basin-and-range provinces). Nevada, the East African Rift, and Iceland experience frequent normal faulting earthquakes. Ground subsidence can be dramatic.

P Waves and S Waves: The Physics of Seismic Motion

Earthquakes radiate energy in two main wave types through the Earth's interior, plus surface waves that travel along the boundary between Earth and atmosphere.

The S wave's inability to propagate through liquid is how seismologists deduced the existence of Earth's liquid outer core in 1906: seismographs on the opposite side of the globe from large earthquakes detected a "S-wave shadow zone" where S waves were absent, indicating a fluid region blocking their path.

Seismograph History

The Chinese polymath Zhang Heng built what is often cited as the first seismoscope in 132 CE — an ornate bronze vessel containing a pendulum mechanism that could indicate the direction of distant earthquakes by dropping a ball into one of eight toad figurines. It reportedly detected a magnitude ~7 earthquake in Gansu Province approximately 500 km away, two days before couriers arrived with the news.

Modern seismometry dates to John Milne's horizontal pendulum seismograph (1880) and the Wiechert inverted pendulum (1900). The Wood-Anderson torsion seismometer (1925) became the calibration standard for Richter's scale. Contemporary broadband seismometers — instruments like the Streckeisen STS-2 used by the IRIS Global Seismographic Network — detect ground motion across a frequency range of 0.008 Hz to 50 Hz with displacement sensitivity below 1 nanometer, capable of recording ocean microseisms from distant storms as well as nuclear tests.

ShakeAlert: Earthquake Early Warning

The U.S. Geological Survey's ShakeAlert system uses a network of more than 1,700 seismic sensors along the West Coast to detect P waves immediately after fault rupture and issue alerts seconds before damaging S waves arrive. The alert time — called "warning time" — depends on the distance from the rupture: cities directly above or adjacent to the fault receive only 2–5 seconds of warning, while cities farther away may receive 30–60 seconds or more.

• ShakeAlert reached public operational status in California in 2019, Oregon in 2021, and Washington in 2021.
• Alerts are disseminated via the Wireless Emergency Alert (WEA) system on cell phones, seismic shutoff valves on gas lines, automatic braking on BART trains, and hospital automation systems.
• A 2019 ShakeAlert publication in Science estimated that the system could prevent 541 deaths in a Hayward Fault scenario earthquake (Mw 7.0) if public response is adequate.
• Japan's similar JMA system, operational since 2007, triggered during the 2011 Tōhoku earthquake and provided up to 80 seconds of warning to Tokyo residents before destructive shaking arrived.