December 26, 2004 Off the west coast of northern Sumatra earthquake

General Information

Region: Off the west coast of northern Sumatra
Date: 12/26/2004
Time: 00:58:50 (GMT)
Time: 06:58:50 (Local time at epicenter)
Magnitude: Mw 9.0 (HRV)
Latitude: 3.30N
Longitude: 95.78E
Depth: 30 km

Seismograms recorded by LCSN Station PAL (Palisades, NY)

Three-component, long-period seismic records at PAL (Palisades; distance=15,000 km) in Rockland County, NY (about 15 miles North of Manhattan). Notice that the amplitude of the traces are reduced by 100 times compared with other days. PAL is about 134.9 degree from the epicenter, and hence the first arrival P wave is the core phase, PKP(df) that traveled through the Earth's inner core and arrive at 01:18:09, followed by PKiKP at 01:18:11. The visible first arrivals on horizontal component is the SKP(bc) phase which started as S wave at the source, then converted to P wave at the core-mantle boundary and traveled as P wave through the fluid outer core and arrived at PAL as P waves. The SKS(df) arrives at 01:25:20, and the clear SS arrival is at 01:38:38 on both horizontal components, followed by large amplitude surface waves. Among the two types of surface waves that traveled from the epicenter to PAL via the North Pole region, the Love wave (LQ) arrives earlier at around 01:57 on EW component, whereas the Rayleigh wave (LR) arrives later at around 02:07 on both vertical- and NS-component. Back-azimuth is about 15 degree from PAL to epicenter.

Map & Source Mechanism

Epicenters of the December 26, 2004, northern Sumatra earthquake and its large aftershocks are plotted as stars. The Harvard CMT (Centroid Moment Tensor) solutions are plotted as beach-balls which indicate mode of earthquake faulting. The mainshock is predominantly thrust faulting (= reverse-faulting) along fault plane dipping to the northeast. Epicenters of the 45 large aftershocks during Dec. 26-29 and determined by NEIC (National Earthquake Information Center/US Geological Survey, Golden, Colorado) are plotted with circles.

The Harvard CMT (Centroid Moment Tensor) solutions of major earthquakes (Mw > 7) that occurred around the December 26, 2004 Northern Sumatra earthquake epicenter are plotted as beach-balls. There were 8 shallow major earthquakes (Mw > 7) during 1976-2004.

Additional links

Lamont-Doherty Earth Observatory of Columbia University