Since the magnitude 8.2 megaquake, Chile has been rocked by 74 aftershocks of at least 2.5, and that number is growing. The strongest, a 7.6 just before midnight last night, is larger than any earthquake felt in the contiguous United States since San Francisco burned in 1906.

Aftershocks in Chile

Heavy equipment clearing landslides triggered by recent earthquakes in Chile. Photo credit: AP/Luis Hidalgo

While the original magnitude 8.2 megaquake was off-shore by almost a hundred kilometers, producing a tsunami but surprisingly little damage. The 7.6 aftershock was on-land, less than 20 kilometers from the city of Iquique.

Aftershocks in Chile

Seismic waves from the magnitude 8.2 megaquake and the 7.6 aftershock as recorded by a seismograph in Antarctica. Figure via seismologist Steven Gibbons.

Even without the recent megaquake for comparison, a magnitude 7.6 earthquake is a big deal. Nowhere in the contiguous United States has experienced an earthquake this severe since the 1906 magnitude 7.8 earthquake shook San Francisco to rubble. Alaskans regularly feel earthquakes this severe or worse (notably the 1964 megaquake), but California's devastating 1989 Loma Prieta earthquake in San Francisco and 1994 Northridge earthquake in Los Angeles were both smaller.

Although smaller magnitude and at the same depth as the megaquake, the terrestrial location meant the smaller aftershock produced shaking just as intense the original disaster.

Aftershocks in Chile

On the left is the shakemap for magnitude 8.2 megaquake. The offshore location meant that even though a large area of the fault slipped, the on-land shaking intensity was moderate to light in nearby cities. On the right is the shakemap for the magnitude 7.6 aftershock. Although a smaller portion of fault slipped, the closer epicenter meant that the Iquique was shaken just as severely as by the larger earthquake.

The aftershock produced another tsunami, but this one was smaller, with only Chile and Peru put on a tsunami Warning. The buoy network recorded briefly as high as 0.74 meters in Iquique and 0.69 meters in Patache, dropping down 0.19 meters by the time the tsunami propagated to Pisagua. The warning was cancelled a half-hour later when it was clear the tsunami had dissipated to non-hazardous levels.

The sea level buoy at Iquique is part of a tsunami monitoring program. It was knocked offline during the 2010 Chilean earthquake and tsunami, and was briefly unresponsive on the afternoon of April 2nd. The long wave form pattern is the water level changing with the daily tides.

Aftershocks in Chile

The submarine megaquake produced the sudden jump at 00:00 Universal Coordinated Time (UTC) on April 2nd, spiking up to just over 2 meters displacement by the later waves. Within an hour, the bulk of the tsunami had passed, leaving behind chaotic turbulence but no longer posing a threat of inundating the shore. The terrestrial aftershock was far more subdued, immediately drawing down local sea levels by almost a meter in the wave trough, but dissipating before a equally-severe wave crest could come on-land. The waters near the earthquakes remain highly turbulent, with far more activity than evident in the smooth pre-tsunami curve from late night on April 1st.

How does this recent earthquake activity fit into the historic seismicity of the subduction system? Geophysicist Jascha Polet updated a cross-section of recorded earthquakes in Chile. Earthquakes are mapped by their depth, tracing the line of the subducting Nazca plate. The magnitude 8.2 and 7.6 were both at 20 kilometers depth, relatively shallow in the scale of how deep earthquakes in the region can get.

Aftershocks in Chile

The earthquakes are colour-coded by depth, with shallow earthquakes in red running down the rainbow to deep earthquakes in cyan-blue. The size of the marker reflects the earthquake's magnitude. Larger earthquakes are marked with a beach balls, the seismological symbol identifying the focal mechanisms of the earthquakes. Only the megaquake and its two largest aftershocks (the magnitude 7.6, and an immediately-proceeding 6.5) have defined mechanisms describing how the fault moved when it slipped; the rest are all plain circles where no focal mechanism was determined.Outline colours are linked to age, with historical earthquakes outlined in black, the swarm proceeding the megaquake outline in blue, and the megaquake and its aftershocks outlined in green.

The aftershocks in Chile are continuing, with a magnitude 4.9 just after 8 am local time this morning. The earthquakes will continue to redistribute stress on the surrounding faults, dropping off in size and frequency exponentially until settling into the usual occasional rattle that defines normal for region. So far, the earthquakes have produced damage, but very few fatalities. All six reported fatalities are attributed to the initial megaquake: 3 buried in landslides, 2 heart attacks, and 1 crushed by a collapsed wall.

Read more about the geological background of Chile's earthquake zone, and how it differs from California.