BACKGROUND: Many earthquakes in the deep ocean are much smaller in magnitude than expected. Geophysicists from the Woods Hole Oceanographic Institute (WHOI) have found new evidence that the fragmented structure of seafloor faults, along with previously unrecognized volcanic activity, may be dampening the effects of these quakes.
ABOUT THE STUDY: The WHOI scientists examined data on ocean transform faults collected by ships and satellites from 19 locations in the Atlantic, Pacific and Indian oceans, augmented with bathymetry maps. Ocean transform faults are fractures in the rock of the ocean floor along which horizontal motion occurs. They cut across the mid-ocean ridge system, a 40,000 mile long mountainous seam in the Earth's crust that marks the edges of the planet's tectonic plates. The mid-ocean ridges are like the seams on a baseball, and the transform faults are like the red stitching, lying perpendicular to the ridge. These faults help accommodate the motion of the tectonic plates, cracking at the edges as the different pieces of rocky crust slip past each other. Along some plates, new crust is formed, while along others, old crust is driven back down into the earth.
WHAT THEY FOUND: The largest quakes at mid-ocean ridges tend to occur at transform faults, yet the WHOI scientists found that earthquakes along the seafloor faults on the East Pacific Rise were not as large in magnitude, or resonating with as much energy as they ought to, considering the length of these faults. Conventional wisdom has held that transform faults should contain rocks that are colder, denser and heavier than the new crust being formed at the mid-ocean ridge. In contrast, the mid-ocean ridge should have a lower density because the crust is lighter than the underlying mantle rocks and thicker along the ridge, and the newer molten rock is less dense. But the WHOI scientists found that, surprisingly, the faults were not more dense -- rather, many fault zones seemed to have lighter rock within and beneath the faults.
WHAT'S GOING ON? The WHOI scientists believe that many of the transform fault lines on the ocean floor are not as continuous as they first appear from looking at low-resolution maps. Instead, they are fragmented into smaller pieces, making the length of any given earthquake rupture on the seafloor shorter -- so the earthquake travels less distance along the surface. It is also possible that magma (molten rock) from inside the earth is rising up beneath the faults. Molten rock is less brittle and more malleable, and could be dampening the strains and jolts as the plate crusts rub together, serving as a geological lubricant. Both phenomena appear to prevent earthquakes from spreading across the seafloor, thus reducing their magnitude and impact.
WHAT CAUSES EARTHQUAKES? An earthquake is the result of a sudden release of stored energy in the Earth's crust triggered by shifting tectonic plates. The Earth's lithosphere is an elaborate network of interconnected plates that move constantly -- far too slow for us to be aware of them, but moving, nonetheless. Occasionally they lock up at the boundaries, and this creates frictional stress. When that gets to be too large a strain, the rocks give way and break and slide along fault lines. This can give rise to a violent displacement of the Earth's crust, which we feel as vibrations or tremors as the pent-up energy is released. However, only 10% or so of the total energy is released in the seismic waves. However, the rest is converted into heat, used to crush and deform rock, or released as friction.