While studying a 7.1-magnitude intracontinental earthquake that occurred in Pakistan in 1997, assistant professor of geophysics Edwin Nissen, along with other researchers, discovered that earthquakes can “jump” between faults, a concept that was not previously thought possible.
“Remote sensing and seismological observations of a recent earthquake in Pakistan highlighted an unexpected incidence of an earthquake rupture ‘jumping’ across a large gap between two faults,” said Nissen. “This has important implications for how we anticipate the size of future earthquakes, a significant question for the half the U.S. population (and for several hundred million people worldwide) who are at risk from damaging seismic shaking.”
The earthquake was listed in published seismic catalogs as a single event, but surface deformation mapped using “before” and “after” satellite radar images revealed two large zones of uplift, generated by slip on two distinct faults. Meanwhile, seismograms show two large pulses of seismic energy, spaced just 20 seconds apart, in the same location as the radar uplift zones.
“The size of an earthquake depends on the length of the faulting involved, and modern earthquake hazard assessments are based on the premise that ruptures are restricted to closely spaced segments and cannot jump between widely spaced ones. Our observations indicate that earthquakes can do just this. In this example, the earthquake doublet greatly increased the duration and area of maximum shaking and probably contributed to the relatively large death toll due to the earthquake,” Nissen said.