Seismology 1910

The Mechanics of the Earthquake

Harry Fielding Reid

An earthquake is rock springing back from strain it could no longer hold.

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In depth · the introduction

For centuries earthquakes seemed to strike from nowhere. After 1906, Harry Fielding Reid showed they are the ground letting go of a strain it had been building for a hundred years.

The idea, unpacked

Picture two enormous slabs of the Earth's crust sliding slowly past each other, with a fault — a crack — between them. Along most of the fault the two sides are locked together by friction. But the slabs keep moving, so the rock on either side of the stuck zone bends, like a steel ruler pressed from both ends, storing energy as it bends.

It cannot bend forever. When the strain grows stronger than the friction holding the fault shut, the fault breaks and the bent rock snaps back toward its old, straight shape — Reid called this the elastic rebound. All the energy that took a century to store is dumped in seconds, and that violent spring-back is the earthquake.

Where it came from

On 18 April 1906 the San Andreas Fault tore open for nearly 480 kilometres and San Francisco burned. California convened a commission under the geologist Andrew Lawson to study it, and Reid, a physicist at Johns Hopkins, was handed a quieter clue than the ruins: decades of survey measurements. Surveyors had long ago fixed the exact positions of marker points across the fault, and had re-measured them over the years.

Comparing the old surveys with new ones, Reid saw something striking. Distant landmarks on the two sides of the fault had been creeping past each other for fifty years — by several metres — while the markers right at the fault had not moved at all. Then, in the quake, the fault itself jumped sideways to catch up. The earthquake had not made the motion; it had released motion that was already there. From that, he reasoned out the whole mechanism.

Why it mattered

Reid gave earthquakes a cause you could reason about and, in principle, measure. If strain builds steadily and is let go in sudden ruptures, then earthquakes on a fault should repeat, the danger should grow as a fault stays quiet, and the slow bending should be visible to instruments before the break. That single idea — the earthquake cycle — is the foundation of how we judge which faults are dangerous, write building codes, and decide where to look hardest.

Like bending a stick

Hold a green stick at both ends and slowly bend it. For a long time nothing happens but a growing curve and a rising tension in your hands — that is the strain storing in the rock. Bend a little more and the stick suddenly cracks, both halves whipping straight and stinging your palms. The crack is the fault, the whip-back is the elastic rebound, and the sting is the earthquake. The stick took all your slow effort to bend, and let it go in an instant.

Where it sits

Reid had the mechanism of earthquakes decades before anyone knew what drove it. He could see the crust being sheared, but the engine — the slow churn of plate tectonics — was only worked out from the 1910s to the 1960s by Alfred Wegener, Harry Hess and the magnetic-stripe readers Fred Vine and Drummond Matthews (all in this Library). When the plates were finally understood, Reid's locked, straining fault became the boundary where two plates grind past each other, and his elastic rebound became one verse of the larger story. The number that ranks the earthquakes he explained — magnitude — came from Charles Richter a generation later, also here.

The original document

Original source text

Harry Fielding Reid · "The Mechanics of the Earthquake" (Vol. II) · Report of the State Earthquake Investigation Commission · Carnegie Institution of Washington · 1910

A cause, not just a catalogue

After the 1906 San Francisco earthquake, the commission under Andrew Lawson catalogued the damage and traced the fresh fault scar that ran for hundreds of kilometres along the San Andreas. Writing the second volume, Reid asked the deeper question: not how badly the ground shook, but what physical process produced the shock at all. His answer came less from the ruins than from the surveyor's transit.

The evidence: surveys across the fault

The U.S. Coast and Geodetic Survey had fixed the positions of triangulation stations across the fault region in the decades before 1906 — in epochs roughly around 1851–1865 and 1874–1892 — and re-measured them just after the earthquake. Differencing the epochs, Reid found that points far from the fault on its two sides had drifted past each other by several metres over the preceding half-century, gradually, and in the very same right-lateral sense as the sudden offset, while points at the fault trace had scarcely moved. In the earthquake itself the ground along the fault jumped sideways — in places by as much as about 6.4 metres (some 21 feet near the head of Tomales Bay) — as if catching up to a motion the deeper crust had been making all along.

The mechanism: elastic rebound

From this Reid reasoned out his picture. The crust on the two sides of a locked fault is slowly sheared by the larger movements of the Earth, but the fault is stuck; so the rock around it bends elastically and stores strain energy, like a steel spring being wound. When the stress at last exceeds the strength of the fault, the rock ruptures and springs back toward an unstrained shape — the elastic rebound — and the stored energy escapes as the seismic waves we feel. The displacement of an earthquake, he argued, is therefore not created at the instant of the shock: it has been accumulating quietly for decades or centuries, and the rupture merely lets it go.

The cycle it implies

Two consequences follow. First, earthquakes on a fault should recur, as the strain re-accumulates after each release — the seismic cycle. Second, because it is the rock that stores the strain and not the fault surface, the slow build-up could in principle be watched by repeated surveys. Reid hoped this might one day make earthquakes predictable; a century on, that hope is only partly fulfilled — instruments now reveal which faults are loaded, but not the day they will fail.

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The full Volume II — 192 pages with the triangulation tables, fault maps and Reid's analysis — is at the source below; Volume I (the Lawson Report) holds the field descriptions of the 1906 rupture.

Johns Hopkins University, Baltimore · 1910