A magnitude 8.2 (M8.2) earthquake struck off the coast of Chile on 1 April, following considerable seismic activity in the region over the past few weeks.
News reports indicate that at least five people died as a result of the Chile ‘quake, with many others injured. The tremor caused power cuts and damage to buildings, and the authorities evacuated many thousands of people following a tsunami warning.
The M8.2 Earthquake of 1 April
This Chile earthquake was the largest to strike in the world this year. It occurred at a depth of 20km, around 100km offshore from the city of Iquique.
At this point the Nazca tectonic plate is moving eastward against the south American continent at a rate of around 65mm per year. The dense oceanic crust dips beneath the continental South American plate and the tremor occurred at the interface between these plates (the so-called ‘megathrust’). The relatively shallow depth of the tremor can be explained by the shallow angle of subduction (attributed to the subduction of several submarine ridges along this length of the boundary).
Megathrust earthquakes often involve a degree of vertical movement. This movement, if the quake is large enough and occurs offshore, is capable of generating tsunamis. The 1 April tremor satisfied these conditions, and a tsunami measuring up to 2.1m struck locally around 12 minutes after the tremor itself.
Waves struck further along the coast and extended into the Pacific as far as the Galapagos Islands in the north and Juan Fernandez in the south. The latest from the Tsunami Warning Center is as follows:
“BASED ON ALL AVAILABLE DATA A MAJOR TSUNAMI IS NOT EXPECTED TO STRIKE THE STATE OF HAWAII. HOWEVER…SEA LEVEL CHANGES AND STRONG CURRENTS MAY OCCUR ALONG ALL COASTS THAT COULD BE A HAZARD TO SWIMMERS AND BOATERS AS WELL AS TO PERSONS NEAR THE SHORE AT BEACHES AND IN HARBORS AND MARINAS. THE THREAT MAY CONTINUE FOR SEVERAL HOURS AFTER THE INITIAL WAVE ARRIVAL.
THE ESTIMATED TIME OF ARRIVAL OF THE INITIAL WAVE IS: 0324 AM HST WED 02 APR 2014″
The M8.2 Iquique Earthquake and The Seismic Gap
Although the western coast of South America is known for its large earthquakes (the most recent is one of 17 with magnitudes of at least M8 to strike the South America-Nazca plate boundary since 1900), the April 1 tremor is the largest to have occurred on this particular stretch. This area is what the USGS describes as “a region of historic seismic quiescence – termed the northern Chile or Iquique seismic gap”.
Seismic gaps are areas where the mechanisms for generating large earthquakes are in place, but where no major tremor has occurred for a significant period of time. (Another such is the Cascadia subduction zone off Oregon/Washington.) In the case of Iquique, the USGS database includes no earthquakes (since 1900) over a distance of around 1000km.
Although there have been no large tremors, there has been plenty of recent activity in the immediate area. An earthquake of M6.5 on 16 March, and the smaller tremors which followed, appeared at the time to be a mainshock-aftershock sequence. The April 1 event recasts these as foreshocks (since the mainshock is, by definition, the largest in a sequence and therefore cannot be properly identified until the series of tremors is complete).
In total, at the time of writing the Iquique earthquake sequence comprises 89 tremors of at least M4.5 – a number that will almost certainly increase.
Chile Earthquake Sequence
This earthquake sequence demonstrates two key points in seismology. The first is that seismologists cannot say for certain whether a large shock will be followed by a series of smaller ones – or an even larger one. the second is that a region that is quiet, seismically speaking, for many years may still be capable of generating a large earthquake.