Transform boundaries

The San Andreas fault is about 1,300 km long, and for 10 million years the land west of it has been sliding north-west at about 5 cm a year.

Sliding side by side

At a transform boundary two plates slide horizontally past each other. No new crust is made, as at a ridge, and none is destroyed, as at a trench, which is why BGS also calls it a conservative boundary. The idea came from the Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults connect two spreading ridges, or less often two trenches. Most transform faults are on the ocean floor, where they cut the mid-ocean ridges into zig-zag segments and are marked by shallow earthquakes. A few run across land, and the most famous is the San Andreas fault zone in California.

The San Andreas is about 1,300 km long and in places tens of kilometres wide. It links the East Pacific Rise, a spreading ridge to the south, with another set of ridges off the coast of Oregon and Washington to the north. Land on the west side belongs to the Pacific Plate and is moving north-west relative to land on the east side, on the North American Plate. Stand on either side and look across: the other side moves to your right. Geologists call that right-lateral motion, and the fence in the model moves the same way.

Stick, bend, snap

If the plates slid smoothly, there would be no big earthquakes, only steady creep. Most of the time, though, friction locks the fault while the plates on either side keep moving. The rock near the fault bends like a ruler held at one end, storing energy. After the great San Francisco earthquake of 1906, the geologist Henry Fielding Reid explained what happens next: when the strain becomes greater than the friction can hold, the fault slips suddenly, the bent rock springs back, and the stored energy leaves as seismic waves. The rocks return to their original shape but are now offset across the fault. This is called elastic rebound.

Fences, roads, streams and rows of trees that cross an active fault show this offset, and after a large earthquake they can be shifted by several metres in a moment. Over long times the numbers add up. At 5 cm per year, a fence would be offset by 5 m in a century. BGS notes that transform boundaries typically produce large earthquakes at shallow depth, which is why they are dangerous to cities built close to them, even though they lack the deep earthquakes of subduction zones. Another example on land is the North Anatolian fault in Turkey.

On the ocean floor, transform faults leave long scars called fracture zones. Some are hundreds to thousands of kilometres long and as deep as 8 km, such as the Clarion, Molokai and Pioneer fracture zones off California and Mexico. Their old parts are inactive now, but the magnetic stripes on either side are offset along them, which records where the transform fault used to slide.