Sumatra Earthquake 12-26-2004

The magnitude 9.0 December 26, 2004, earthquake near Sumatra is the largest earthquake to occur since the 1964 Alaska quake and the fourth largest in this century.

It apparently ruptured the section of the Sumatra subduction zone from northern Sumatra northward for about 1000 kilometers. The rupture area is outlined by aftershocks;

Initiation point of magnitude 9 earthquake (star) and aftershocks (from USGS website

Aftershocks through mid March

The Sumatra subduction zone is where the Indian plate dives beneath the Asian plate along a fault that dips about 20o into the Earth. Because of the low dip angle, earthquakes can rupture along a very large surface area of the fault, producing such large magnitudes. In fact, the 10 largest earthquakes since 1900 have occurred at subduction zones (see

The huge and destructive tsunami was the direct result of the earthquake disturbing the seafloor and displacing an enormous volume of water. The disturbance spreads from the epicenter like ripples in a pond but, unlike circular ripples, is stronger in some directions than others due to the nature of the faulting. An animation of the theoretical spread of the tsunami waves (from the Japanese National Institute of Advanced Industrial Science & Technology) can be viewed at

December 26 Tide Gauge record from Cocos Island in Indian Ocean showing the passage of the tsunami just after 0300 hours (amplitude in centimeters) (from NOAA website).

Expected surface displacements (black arrows) during the Dec 26 earthquake. The colored grid shows a hypothetical distribution of slip (in millimeters) on the fault. Red arrow is one observed displacement (about 20 cm) measured with GPS in Medan by BAKOSURTANAL.

Past research on Sumatra and the subduction zone

Global Positioning System crustal deformation monitoring

RPI: Rob McCaffrey, Colleen Stevens, Peter Zwick
Scripps Institution of Oceanography: Yehuda Bock, Jeff Genrich, Linette Prawirodirdjo
BAKOSURTANAL (Indonesian Mapping Agency): Jakub Rais, Cecep Subarya, Toto Puntodewo

Since 1989, we have been monitoring crustal motion throughout Indonesia with Scripps Inst. of Oceanography and BAKOSURTANAL using the Global Positioning System.

Southeast Asia GPS vectors showing how sections of Indonesia are moving relative to Asia.

Starting in 1989, we established a more dense network of GPS monitoring sites in Northern Sumatra. These show that the subduction zone was steadily squeezing the island of Sumatra, loading the system for the next earthquake.

Sumatra Observed GPS vectors (in blue) showing how sections of Indonesia are moving relative to Asia (orange vectors are predicted results of our simulations). The orange shaded areas are the rupture areas of the 1833 magnitude 8.7 earthquake and the 1861 magnitude 8.4 quake. The 12/26 magnitude 9.0 quake apparently ruptured fom the north edge of the 1861 event all the way north to the northern end of the subduction zone. From Prawirodirdjo et al. (see citation below)

Earthquake and volcano monitoring

RPI: Rob McCaffrey, Dave Wark, Peter Zwick
Badan Meteorologi dan Geofisika (Indonesian Meteorological and Geophysical Agency): Fauzi, Masturyono, Sutardjo
Volcanological Survey of Indonesia: Sukhyar

Additional research at RPI has focused on the earthquakes in the Indonesian region and on subduction zone earthquakes throughout the world.

Earthquakes and subducted lithosphere contours beneath Sumatra from local monitoring network. From Fauzi et al. (see citation below).

Tomography of Toba Caldera in North Sumatra. From Masturyono et al. (see citation below).

This work is supported by the National Science Foundation and UNAVCO.