Lomax Scientific Software, Mouans-Sartoux, France,
16 Feb 2005
See also: Rapid estimation of faulting extent for large earthquakes by locating the end of rupture: application to the 2004, Mw=9.0 South Asia mega-thrust
This figure shows short period, 1-second Gaussian-filtered seismograms from the
The two closer stations ULHL and NWAO exhibit some short period signal after the first S arrival that could be S energy. However, the farther stations OBN and CASY show little signal after the first S arrival indicating that the short period signal at all stations is primarily P energy radiated directly from the source. The amplitude, duration and timing relations of these signals thus give us important information about the source process.
first P signal arrives at about the same time at the two closer
and NWAO) and at the two farther stations (OBN and CASY), consistent
pair being at the same distance from the hypocenter.
The smaller duration of the stronger signal
at ULHL relative to that at NWAO and at OBN relative to that at CASY is
consistent with rupture propagation from the hypocenter towards the
NNW, i.e. CASY
and NWAO are further than ULHL and OBN respectively from a point of
termination to the NNW of the hypocenter.
The duration of the short period signal on these and other seismograms implies rupture duration of around 8 minutes (also inferred by Jeffrey Park in an earlier contribution to the IRIS Special Report for this event). Taking the point of rupture termination near the northern limit of the early aftershocks (NEIC, 2005) implies a rupture length of around 1200 km (see also: Rapid estimation of faulting extent for large earthquakes). These results imply an average rupture velocity of about 1200 km / 480 sec or about 2.5 km/sec.
short period seismogram from the great 1960 Chile earthquake
shows a duration of
stronger signal of about 4 minutes. If this duration corresponds
to the rupture duration for the 1960 event, this result implies that
the 2004 Sumatra-Andaman earthquake
may be larger than the 1960 Chile earthquake!
Kennett, B.L.N. Engdahl, E.R. & Buland R., (1995). Constraints on seismic velocities in the Earth from travel times, Geophys. J. Int., 122, 108-124
Magnitude 9.0 OFF THE WEST COAST OF NORTHERN
Alberto Michelini first noted the relevance of the 1960 short period seismogram to this analysis. 2004 data provided by the IRIS Global Seismographic Network, and distributed through the IRIS Data Management System.
Anthony Lomax, Anthony Lomax Scientific Software, Mouans-Sartoux, France, email@example.com, www.alomax.net