M=6.9 earthquake in Chile follows intense seismic swarm

By David Jacobson and Volkan Sevilgen, Temblor

** Read about the seismic swarm that preceded this earthquake **

See earthquakes in Chile

Updated: 9:00 am PDT 25 Apr 2017

santiago-chile-earthquake
A M=6.9 earthquake shook Chile’s capital city of Santiago, just days after an earthquake swarm began offshore. (Photo from: flickr)

 

At just past 5:30 p.m. local time, a M=6.9 earthquake shook central Chile, including the capital city of Santiago (EMS reported the earthquake was a M=6.7). This earthquake’s epicenter was offshore of the port city of Valparaiso, and occurred at a depth of 25 km according to both the USGS and EMS. So far, 89 aftershocks have followed the M=6.9, with the largest being a M=5.4. Based on data from the USGS, severe shaking was recorded close to the epicenter. Despite this, the USGS PAGER system estimates that both economic losses and fatalities will remain low. So far, over 700 people have reported feeling the quake on the USGS website, though the region is home to millions of people.

chile-earthquake-map
This map shows the location of today’s M=6.9 earthquake, in relation to the seismic swarm, which began on Saturday (22 April) and was highlighted by a M=6.0 and M=5.5. In addition to these quakes, there was also a M=5.4 aftershock farther inland. All of these quakes suggest that the sequence is moving inland and getting deeper.

 

This earthquake occurred within an earthquake swarm that started on Saturday, which was highlighted by a M=6.0 Saturday evening. Up to this point, 54 M=3.0+ earthquakes have occurred offshore of Valparaiso in the last 2 days. To read more about the swarm, click here. Based on the initial depth reported by both the USGS and EMS, this quake likely occurred above the megathrust (subduction zone) on a secondary shallowly dipping splay.

This figure shows a time series of the earthquakes offshore of Valparaiso and Santiago Chile in the last 2 days. Two days ago, a seismic swarm began, and was highlighted by a M6.0 Saturday (22 April) night. Now, a M=6.9 has shook the region.
This figure shows a time series of the earthquakes offshore of Valparaiso and Santiago Chile in the last 2 days. Two days ago, a seismic swarm began, and was highlighted by a M6.0 Saturday (22 April) night. Now, a M=6.9 has shook the region.

 

Chile is one of the most seismically active countries on earth, with the large majority of quakes occurring on or near the subduction zone. Based on the Global Earthquake Activity Rate (GEAR) model, which forecasts earthquake magnitudes globally using strain rates and seismicity since 1977, this earthquake should not be considered surprising, as the region is susceptible to M=7.5+ quakes. Nonetheless, this is a major quake and caused buildings to sway in the capital. We will post a more in-depth post on this quake later.

This Temblor map shows the Global Earthquake Activity Rate (GEAR) model, which forecasts the likely earthquake magnitude in your lifetime anywhere on earth. Based on this model, the M=6.9 earthquake which just occurred should not be considered surprising as the region is susceptible to M=7.5 earthquakes.
This Temblor map shows the Global Earthquake Activity Rate (GEAR) model, which forecasts the likely earthquake magnitude in your lifetime anywhere on earth. Based on this model, the M=6.9 earthquake which just occurred should not be considered surprising as the region is susceptible to M=7.5 earthquakes.

 

References
USGS
EMS Catalog

  • Paul Earle

    Nice write-up. The updated USGS location has the mainshock within the foreshock swarm.

    • Ross Stein

      Good point, Paul. Its magnitude, location and depth have all been moving targets. But now the association between the swarm and the M=7 is much stronger. (Paul Earle is a leading USGS seismologist in Golden, Colorado.)

  • Dal Stanley

    Although dangerous for Valparaiso, I don’t see this M6.9 as being anything unique or especially dangerous for Chile. Here is a map of events >M6.5 since 2010, with the M8.3 shown in blue and a M8.8 south of the recent M6.9 noted
    http://image.prntscr.com/image/79c7d222500e4e0cace5dec63f83a3ce.png
    Here is a great article on the structure using the M8.8 location for the profile with 3D modeling.
    http://www.sciencedirect.com/science/article/pii/S0012821X14005342#fg0060

    • Ross Stein

      Just as we did not know if the M=6 would soon be followed by something larger, we do not know if the M=7 would also be succeeded by a larger quake. If the unbroken section of the megathrust between the edges of the 2010 and 2015 shocks were to rupture, a M~8.5 could strike.

      • Dal Stanley

        I totally agree. The curvature between the 8.8 and 8.3 would suggest even more that a M8+ could happen soon enough.

  • Leo Alvarez

    Dal Stanley, what do you mean “not…especially dangerous for Chile”?” Tell that to the people that experience these earthquakes and the panic caused by the tsunami warnings. Chile has a long coast exposed to tsunamis that generate from movements on the subduction zone and has experienced their terrifying consequences.

    • Dal Stanley

      Sorry, my point was basically that the recent event was not unusual for the stretch between the 2010 M8.8 and 2015 M8.3, but of course the location was very scary to Valparaiso. However, a 2010 or 2015 >M8 would have been much worse. The 8.8 killed 525 Chileans.

  • Alejandro Cañas

    For us Chileans, this was a strong tremor, not an earthquake

    • Ross Stein

      I understand that you experienced weak shaking, but that was caused by a distant earthquake.

    • ARon

      One thing that really bugs me about being a foreigner in Chile right now is the attitude Chileans have towards this. That was a full on earthquake and they have this bravado about it. I saw a people laughing at women who were crying. You’re doing no one any favors by acting the way you are

  • ARon

    So, if I’m reading this right from a few of you it’s most likely that a larger quake will occur soon?

    • Ross Stein

      A larger quake will occur, but not necessarily soon, because most M~7 shocks are not followed by M~8 shocks. On the other hand, a M~8 shock is much more likely now than it was before the swarm or before the M=7.

      • Rodrigo Amorim

        Is this a matter of months? Years? Or could be days/weeks?

        • Ross Stein

          We don’t know.

  • KE7IQQ

    One thing I note as a difference with Cascadia is the rate of movement per year.
    79mm per year in Chile vs 7 to 18mm year at GPS stations in Cascadia. With the rates vary by distance from the coast and with the amount of slip back during ETS events. The rate for Chile is still about 5 times what I see in Cascadia. Does this indicate that mager quake in Chile may occur more frequently?

    • Ross Stein

      Yes. All other things being equal, Chile will have 5 times the rate of quakes of all sizes in comparison to Cascadia. And that is consistent with its several-hundred year history. That difference is reflected in the GEAR model we show in Temblor. The largest magnitude in your lifetime off Valparaiso is M~8; off the Oregon coast it is M~7.

  • Dal Stanley

    After looking at the structure details from the excellent paper by Steve Hicks I noted below and replotting the quakes from 2009-2017 with magnitude >M6.8, I am slightly leaning toward the possibility that the M6.9 will be followed by a M8+. This ‘guess’ is based on the gap between the M8.8 in 2010 and the M8.3 in 2015.. But this is a small guess, based mainly on the observation that the M7s between the M8.8 and M8.3 will be regularly spaced with the recent M6.9. Here is a simplified plot which shows this character
    http://image.prntscr.com/image/238b238350aa4115ab4d7477316e0e3e.png
    And going back to 1970 with >7.2 limit, a different view
    http://image.prntscr.com/image/319e95f39737415d88e8afd893c321b5.png

  • Dal Stanley

    Here is a plot I made of the events on 4/23, which was the two M5s and the big event on 4/24, M6.9. Smaller events are also shown to possibly illuminate the structure offshore where these events were gathered
    http://image.prntscr.com/image/10b02480c29942b09a1591da248efc88.png