By David Jacobson, Temblor
At 3:15 a.m. local time, a M=5.5 earthquake struck just north of Chile’s capital city of Santiago. Because the quake occurred at a depth of 88 km, according to the USGS, only light shaking was felt in the capital, while moderate shaking was felt in the port city of Valparaiso. The USGS estimates that economic losses should remain extremely minimal, and so far, there are no reports of damage.
While the majority of earthquake in Chile are compressional in nature, due to the offshore subduction of the Nazca plate beneath the South American plate, this morning’s earthquake was extensional, according to the USGS focal mechanism. Based on the depth of the earthquake, it fell between two compressional regimes. Above it, is the compressional crustal environment that created the San Ramon thrust fault, while below it is the subduction zone. From cross-sections of the area, this earthquake appears to have occurred within a dislodged piece of mantle lithosphere that seems to have unique stress conditions. Due to this quake’s proximity to Santiago, it will be highly sampled by the Chilean seismic network, which will make it very useful for investigating site amplification.
Based on the Global Earthquake Activity Rate (GEAR) model, this M=5.5 earthquake should not be considered surprising. The GEAR model uses global strain rates and seismicity from the last 40 years to determine the likely earthquake magnitude in your lifetime anywhere on earth. In the figure below, one can see that for the area around Santiago, a M=6.75+ is likely in your lifetime. This highlights the seismic hazard in Chile, which is significant because of the offshore subduction zone. It should also be noted that an earthquake with nearly identical characteristics occurred just a few kilometers north of today’s earthquake in 1977. That quake was likely an extensional event as well, given its similar depth.
References
USGS
European-Mediterranean Seismological Centre