Return to Sumatra-Andaman Islands Earthquake Page
Broadband seismometers recorded ground motions from the Sumatran earthquake "on scale" at locations worldwide. At Richter magnitude M=9.0, the Sumatran quake is the largest seismic event in 40 years. The scale of motion is unprecedented in modern seismic recordings. When converted into units of displacement, the peak-to-peak ground motions observed worldwide commonly exceed one centimeter, mainly in the long oscillations (100+ sec) of the Love (G) and Rayleigh (R) surface waves. Motions of the P (pressure) and S (shear) waves, which travel through Earth's deep interior, range from 0.1 to several mm at typical stations.
To appreciate the magnitude of the 26 December 2004 Sumatran earthquake, the lower panels of the graphic compare its P-wave ground motion at an observatory in Europe with motion recorded for the 17 January 1994 Northridge earthquake (M=6.7), which caused an estimated 60 deaths and property damage in excess of $20 billion in the Los Angeles area. Two facts bias this comparison slightly in favor of the Northridge event: (1) GSN station ESK is somewhat closer to the Northridge epicenter (75.1 degrees) than the Sumatran epicenter (92.4 deg) and the Sumatran faulting geometry predicts a relative minimum in amplitude for P waves that radiate toward Europe.
Nevertheless, the 2004 ground motions dwarf the 1994 motions in both amplitude and duration. The bottom panels of the graphic show P-wave motion with frequencies f>1Hz. P waves that reflect at Earth's surface tend to lose high-frequency energy, so the duration of high-frequency signal in a seismogram gives a rough indication of the duration of fault rupture. In contrast to the ~20s of rupture associated with the Northridge earthquake, high-frequency ground motion for the Sumatran earthquake sustains a near constant amplitude for ~500s. The extreme length of primary shaking makes impossible the visual identification of late-arriving P waves in the Sumatra-event seismogram, in contrast to the set of pulses evident in the Northridge record. unlike the distinct short near constant amplitude.
Credits: Jeffrey Park, Yale University