These giant gashes in the earth are reminders of our planet’s awesome power they are fault lines, fractures in the earth’s crust, where huge slabs of rock slowly grind past each other. Most faults we cannot see, but here in California the San Andreas fault is visible at the surface.
“You cannot have an earthquake without a fault, and I am standing right now on what is perhaps the most famous fault in the world, the San Andreas fault, that extends from Mexico down south way up to Oregon in the north. This is one of the few places in the world where you can stand with one foot on one tectonic plate, and one on another in other words. This ground here is attached to New York and Iceland, the ground here is attached to Hawaii and Japan.”
The San Andreas fault sits on the boundary between the North American and Pacific Plates, it is called a strike-slip fault, a place where two plates collide and try to slip past each other. As these plates move or creep in different directions, sometimes they get stuck the longer they stay stuck, the more tension builds, and the bigger the eventual quake. In other places the plates move smoothly past each other, these areas buckle as the earth shifts.
The changes are subtle Roger Billam keeps track of these changes with his creep-meter, a device that measures movement along the fault. A computer collects data every minute for a year. This jump in the graph tells him the earth shifted about 5 millimetres in a couple of hours. A small shift like this can have huge consequences for us.
“That might not seem much, 5 mm, but it is affecting a long section of the fault, maybe 5 miles long, involving millions of tonnes of rock. Now, it really does not matter here, this is a nice sort of field, there are not any people. But further north, this creep process and the things that are happening beneath the Earth are far more sinister.”
The San Andreas fault and its branches trigger more than 10,000 quakes a year, sometimes the results are devastating – just ask the people of Los Angeles or San Francisco. Over the last two decades, major quakes rocked both of these cities. Life goes on, but the risk remains in an area this active, the next violent tremor could happen at any time.