Research in the Earthquake Source Process
This Nisqually earthquake occurred on February 28, 2001 at 18:54:32.78UTC
striking the Olympia, Tacoma, Seattle metropolitan area. The seismic moment
tensor of the earthquake was determined using long-period (50 to 200s) complete
three-component waveforms recorded by the COR (IRIS), NEW (USNSN), MOD (BDSN),
YBH (BDSN) broadband stations. The moment tensor inversion yielded a pure
double-couple solution with a steep dipping (strike=357, dip=69, rake=-90) and
a shallowly dipping (strike=178, dip=21, rake=-89) nodal planes. The normal
type mechanism indicates in-slab tension induced by bending. The best centroid
depth was determined to be 44 km, several. The figure below illustrates the
moment tensor result and compares the waveform fits, which are found to be
quite good.
The two possible nodal planes from the moment tensor inversion were tested in a series of finite source inversions. These inversions make use of the same stations, with the addition of stations, HAWA, OCWA and LLLB to improve azimuthal coverage. The results of these inversions indicate that shallow dipping plane fits the seismic data slightly better than the steeper plane. Figure below compares the variance reduction vs. rupture velocity for the two nodal planes.
Interestingly both nodal planes recover slip to the north of the hypocenter,
however the shallow plane allows for a component of slip to the northeast. As
the figure below shows this slip aligns in the direction to Seattle, and may be a partial explanation for the relatively large ground motions observed there.