two studies by researchers from the us, India and Japan have thrown new light on the extremely challenging science of earthquake forecasting.
The first has to do with silent earthquakes -- tremors that can last weeks and occur up to 40 km underground. Seismologists believe that they foreshadow strong tremors capable of wiping out cities. However, they have been difficult to detect because they displace the ground without shaking it. They register on global positioning systems long after they occur.
Now researchers say they might have discovered a way to monitor these slow-moving tremblors. Seismologists from Stanford University, California, and the University of Tokyo claim that silent quakes give rise to non-volcanic tremors -- faint vibrations that originate deep inside active fault zones -- which can be picked up by sensitive equipment (Nature, Vol 442, No 7099, July 13, 2006).
"It's a small step towards earthquake prediction," says David R Shelly, the lead author of the study. Earlier observations have shown "that you tend to get non-volcanic tremors the same time as the silent earthquakes are detected by global positioning monitors, but this is the first time that the phenomena have been linked directly," he adds.
Japanese seismologists discovered non-volcanic tremors four years ago in the Nankai trough, an active fault zone in southwest Japan. To explore the link between non-volcanic tremors and silent earthquakes, the research team focused on low-frequency earthquakes (lfes) of magnitude two or less. lfes are routinely accompanied by non-volcanic tremors. Shelly analysed more than 1,000 lfe seismograms recorded in the Nankai trough between June 2002 and July 2005 and nailed down their locations. His analysis revealed that lfes and non-volcanic tremors were occurring simultaneously in a section of the fault.
"We believe that as the pressure and temperature increase here, minerals in the plate become more densely packed and start giving off water," explains co-author Gregory C Beroza. "As these fluids are liberated, they generate lfes and tremors and also lubricate the plate interface, which allows silent earthquakes to happen." Silent earthquakes could increase the stress on faults. Thus knowing when a silent quake has occurred could help predict massive surface quakes.
Increases in chlorophyll, specifically, chlorophyll-a, concentration are due to sudden blooms of phytoplankton (microscopic plants) that are triggered by the release of thermal energy prior to an earthquake.
Here's how it happens Thermal energy causes the surface temperature of the sea to rise, which increases evaporation. This in turn increases upwelling -- a rising of cold seawater from the depths to the surface. This nutrient-rich cold water is ideal for phytoplankton, which use chlorophyll-a to absorb energy from the sun and synthesise foods from carbon dioxide and water. The phytoplankton blooms can be observed via satellite images as increased concentrations of chlorophyll-a.
The researchers analysed sea temperatures and satellite images from coastal areas near the epicentres of four earthquakes -- in Gujarat (2001), Algeria (2002), the Andaman Islands (2002) and Bam, Iran (2003) -- and found a link between increased chlorophyll- a concentrations and impending earthquakes in the region.
They found the period of advance notice depended on the proximity to the epicentre of the quake, magnitude of the impending quake (directly proportional to the amount of energy released) and the depth of the ocean (lesser the depth, more advanced the notice). Abnormal chlorophyll concentrations were found between three (Gujarat) to nine (Bam) days prior to the quakes.
Ramesh Singh of the Indian Institute of Technology, Kanpur, who co-authored the report, first noticed this phenomenon after the 2001 Gujarat earthquake. He said he'd also observed similar increases in chlorophyll-a concentrations prior to hurricanes and cyclones.