Comparison of February 2010 Chile, January 2010 Haiti, and December 2004 Sumatra Earthquakes.

Earthquake Magnitude* Focal Depth Tsunami DeathsChile 8.8 35 km “minor” ~900Haiti 7.0 13 km minor ~230,000Sumatra 9.0 30 km major ~226,000

Energy of earthquake ~32 times with each increase in magnitude.Chile and Sumatra earthquakes are known as “megathrust” earthquakes, that is, earthquakes that occur on major subduction zones . Such a zone , the Cascadia Subduction Zone (or Cascadia “Megathrust”) exists off the coast of northern California through Oregon to southern British Columbia.

*Magnitude = Moment Magnitude (Mw)based on area fault rupture as well as bedrock rigidity and energy released.1960 Chile earthquake = 9.5 Mw; 1964 Alaskan = 9.2Mw; 1906 San Francisco = 7.8Mw.

Subduction beneath a volcanic Island Arc such as in Indonesia (left) and Continental Arc like the Andes (right). Figures from Tarbuck & Lutgens.

Oblique subduction of the Australian-Indian Plate beneath the Eurasian Plate. Note the presence of a right-lateral, strike-slip fault cutting through Sumatra. The location, date, & Richter magnitude for large seismic events are identified on the map. The December 26, 2004 earthquake also occurred in association with the subduction zone off the west coast of Sumatra. Figure from the United States Geological Survey.

Plate Movement: note that the Caribbean Plate moves laterally along the faults which cut through Haiti. Note the direct subduction of the Nazca Plate beneath South America. Also note the oblique convergence of the Australian-Indian Plate into the Java trench. Tarbuck and Lutgens, 2009.

Comparison of Earthquake Magnitude and energy released. Courtesy of USGS.

Haiti

SumatraChile

Samoa

Location of epicenter and related aftershocks. Black line with barbs off the coast represents the location of the subduction zone. Purple triangles represent volcanoes.

Predicted shaking intensity related to substrate. Note that shaking intensity is greatest away from the epicenter (star symbol) where the substrate consists of thicker alluvial material rather than bedrock.

Megathrust (subduction zone)

Long period waves produced in “megathrust” event results in greatest damage to taller buildings (short ones damaged by short period wavelengths).

Most loss of life was due to tsunami in coastal areas (>6m = 18’ high). Maritime radio (Chilean navy) initially said there wouldn’t be a tsunami which contributed to great loss of life.

Seismogram from Umqua Community College. Earthquake P-wave recorded at 6:47:20 UTC (6:34:14 UTC in Chile; wave took 13min 6 sec to reach Roseburg). Roseburg “shook” for ~13 minutes.

ShakeMap: actual shaking during 8.8 earthquake. Note that the intensity of shaking didn’t reach the violent or extreme (red) part of the Modified Mercalli Intensity scale as was the case in the 7.0 Haitian earthquake. The reason was due to the depth of the earthquake plus the type of bedrock.

Earthquake occurred in the “seismic gap” area between the 1906 (M8.2) and 1960 (M9.5) quakes.400 mile long rupture.

Dip of subduction zone as inferred from aftershocks (orange=shallow; blue=deep)

Earthquake Probability map, pre-2010 Haiti earthquake.

Magenta and green lines represent left-lateral strike-slip faults. The southern part of the Caribbean Plate is moving west while the North American Plate to the north is moving east (see red arrows).

Caribbean Plate

North American Plate

Haiti region: Historic Earthquakes, 1990 to January 12, 2010.Magenta and green lines are lateral faults.

Haiti earthquake: Tuesday, January 12, 2010; 4:53 PM; Magnitude 7.0; 13 km depth (shallow); 25 km WSW of Port-Au-Prince; Occurred along a left lateral fault between Caribbean and North American Plate. That fault moves about 7mm/year.

Note “shake intensity” of this M 7.0 ‘quake as compared to the M 8.8 ‘quake in Chile. Much more intensity of shaking in Haiti even though the ‘quake was less intense. Why?

Tsunami! Elevation of the ocean surface measured by satellite 2 hours after the December 26, 2004 earthquake off the coast of Sumatra. Figure from U.S. National Oceanic & Atmospheric Administration & U.S. National Tsunami Hazard Mitigation Program.

Elevation of the ocean surface measured by satellite 7 hours after the December 26, 2004 earthquake off the coast of Sumatra. Figure from U.S. National Oceanic & Atmospheric Administration & U.S. National Tsunami Hazard Mitigation Program.

Around 25-30 minutes notice to coastal areas would be available after a mega-thrust event on the Cascadia subduction zone.

Comparison:Loss of life and damage to property was great for the Sumatra ‘quake because of the magnitude (9.0) of the earthquake, inadequate tsunami warning ,and poor building construction even though people were cognizant of earthquake dangers.

Loss of life and damage to property was extensive in the Port-Au-Prince, Haiti earthquake even though the magnitude (7.0) was less because it had been about 200 years since the last devastating ‘quake, the quake was shallow (13 km) hence earthquake energy wasn’t as dissipated as in deeper ‘quakes, buildings were poorly constructed, and the main city (Port-Au-Prince) was built on unstable ground.

Loss of life and damage was “minimal” in the Chilean ‘quake even though the ‘quake was large (8.8) because Chile is a more affluent country able to construct most buildings according to earthquake standards, earthquakes are frequent (public is attuned to them and most know what to do), the ground is a bit more stable, and the earthquake was deeper than in Haiti. Much of loss of life was from tsunami in the coastal areas as well as collapsed buildings. Chilean government is looking into likely shoddy construction even in buildings constructed according to earthquake standards.