Dual approach gets to heart of earthquakes

Earth scientists have made a huge leap forward in understanding how earthquakes are formed using state-of-the-art technology to monitor the movement of the Earth

American scientists are tracking the Earth’s surface using GPS, with some surprising results, while researchers in Bristol have been studying how the Earth’s core moves.

Researchers from Purdue University used global positioning system equipment and radar interferometry to measure how the ground moved during January’s earthquake in Haiti. Their computer model showed the quake wasn’t caused by a fault on the Enriquillo fault – instead it was caused by a previously unmapped fault they’ve named Léogâne.

“It was a big surprise that we couldn’t find a surface rupture anywhere,” said professor of earth and atmospheric sciences, Andrew Freed, “We did find other physical changes that we expected after an earthquake of that magnitude, but in entirely the wrong location to have come from the Enriquillo fault.”

Freed explained that only portions of the fault are affected during any given quake, the length of which is relative to the magnitude. Many faults show scars from previous quakes, but the Léogâne – which runs parallel to the Enriquillo fault – doesn’t.

The shifting of the Earth’s crust along the Léogâne fault may have added to or reduced the stresses building up along the Enriquillo fault – meaning it may have advanced or delayed the timing of the next earthquake along that fault.

“The Enriquillo fault is still capable of producing large earthquakes, and Haiti has to adapt to this seismic hazard,” said Eric Calais, professor of geophysics and science advisor for the United Nations development program in Haiti. “The fault system is more complex than we thought, and we don’t yet know how the January earthquake impacted the other faults,” he continued, “We need to investigate the fault system further to be able to determine where the next earthquakes might occur and how large they might be.”

The team will continue to take measurements of the post-seismic process to help them understand how changing stresses within the Earth’s crust over time could point to areas of increasing seismic hazard. They also plan to create models to better understand the fault systems, their behaviour and why they exist at these particular locations.

Meanwhile, researchers from the University of Bristol have been studying how the Earth’s deep interior slowly moves around and have developed what they call a seismological speed gun.

Mantle motion controls the location of the continents and oceans, and where tectonic plates collide and shift to form mountains, volcanoes and earthquakes. Scientists know how the material moves when it reaches the top of the mantle, but little about what goes on at the bottom.

“The only way to measure the inside of the Earth at such huge depths is with seismic waves,” said Andy Nowacki from the school of Earth sciences. “When a large earthquake occurs and waves travel through the Earth, they are affected in different ways, and we can examine their properties to work out what is happening thousands of miles beneath our feet, a region where we can never go.”

The study focuses on the mysterious layer where the mantle meets the core – a sphere of iron at the centre of the Earth 7,000km across. Researchers have dubbed this area D’’.

Researchers hope that by examining the wave’s properties as they pass through this area they can understand what’s going on thousands of miles below the surface.

Scientists believe D’’ – dee-double-prime – consists of crystals which line up in a certain orientation when the mantle flows.  By measuring how they line up they can see which way the mantle is moving.

“This part of the Earth is incredibly important,” said Professor Mike Kendall, “It drives the motion of the plates on the Earth’s surface, which build mountains, feed volcanoes and cause earthquakes. Measuring the flow in the lowermost mantle is vital to understanding the long term evolution of the Earth.”