Hidden earthquake discovery challenges tsunami early-warning systems

Seismologists at the University of Liverpool studying the 2011 Chile earthquake have discovered a previously undetected earthquake which took place seconds after the initial rupture. This newly discovered phenomena which they have called a `closely-spaced doublet' presents a challenge to earthquake and tsunami early warning systems as it increases the risk of larger-than-expected tsunamis in the aftermath of a typical subduction earthquake. In a study published in Nature Geoscience , researchers from the University's School of Environmental Sciences analysed in detail the seismic wave recordings from 2 January 2011 when an earthquake of magnitude 7 occurred in Chile along the plate boundary separating the subducting Nazca plate from the South American continent. Undetected rupture They discovered that just 12 seconds later and 30 km further offshore, a second rupture of a similar size, undetected by national and global earthquake monitoring centres, occurred along an extensional (pull-apart) fault in the middle of the South American plate beneath the Pacific Ocean. Liverpool seismologist, Professor Andreas Rietbrock , said: "Real-time global seismic monitoring and early warning events have come a long way and it is possible for a magnitude 5 or greater earthquake to be detected within a matter of minutes. Therefore, it is striking that an earthquake with magnitude close to 7 was effectively hidden from our standard monitoring systems." "Previous doublet events have been documented in subduction zones before, but such instantaneous triggering of large ruptures at close distances has no known precedent.