Earthquakes usually occur at plate boundaries. Some plate boundaries feature plates that slowly slide past each other over time. When the plates reach points along their boundaries where the rocks become stuck, pressure builds. According to the study, these areas eventually break under enough pressure from the slow movement and release foreshocks. Large, more violent earthquakes are produced when the break in the rock occurs more quickly.
North Pacific Ocean-based earthquakes larger than a magnitude of 6.5 that were generated at depths shallower than 50 kilometers, and occurring between 1999 and 2011, were analyzed. This region was chosen because Japan, Mexico, Taiwan and the United States have many instruments set up in this region designed for earthquake detection. Half of the 62 earthquakes studied occurred at plate boundaries with subduction zones (where one more dense plate slides beneath another less dense plate) and strike-slip faults (where plates slide past each other); the other half of the data came from intraplate earthquakes, which occur far away from plate boundaries.
About 20 days before a major earthquake, the researchers noted that the seismicity (earthquake distribution in a certain location over time) increased due to the presence of foreshocks. This increase in activity continued until a major earthquake occurred. There was no such predictable pattern of activity for intraplate earthquakes, since foreshocks here occurred much less frequently.
Foreshock events such as the ones examined in the Nature Geoscience study were noted prior to a major earthquake in Izmit, Turkey in 1999 that killed 17,000 people and left 500,000 homeless. According to a study published in Science, the magnitude 7.6 earthquake along the North Anatolian fault was preceded by bursts of seismic activity in the form of foreshocks that increased over time. The crust in the region had undergone a period of slow sliding for 44 minutes prior to the earthquake, generating the foreshocks. The sliding accelerated in the two minutes before the major earthquake occurred.
Foreshocks do not create larger earthquakes, but may help scientists to understand the process of earthquake generation. The prolonged activity generated by foreshocks preceding major earthquakes can help us to better prepare for potential earthquake events.
Photo courtesy of Kyodo News/Associated Press. The photo depicts some of the damage incurred from the magnitude 9.0 Tohoku Earthquake and subsequent tsunami near the east coast of Honshu, Japan. Studying seismic zones such as this one for foreshocks could help scientists become more aware of potentially hazardous future earthquakes.