THE POSSIBILITY OF NORTHEASTWARD UNILATERAL RUPTURE FOR THE JANUARY 23, 1812 NEW MADRID EARTHQUAKE

CRAMER, C.H., SCHWEIG, E.S., U.S. Geological Survey, 3890 Central Ave.,Memphis, TN, 38152-3050, and TUTTLE, M.P., M. Tuttle and Assoc., 128 Tibbitts Lane, Georgetown, ME 04548, cramer@ceri.memphis.edu.

Recently, Mueller et al. (2004) and Hough et al. (2005) suggested that the January 23, 1812 New Madrid earthquake occurred in southern Illinois and not in the northern New Madrid seismic zone as traditionally thought. To support their hypothesis, they cite the distribution of intensity data, modeled Coulomb stress changes, liquefaction in White County Illinois, triggered earthquake activity in northern Kentucky, and the possibility that some liquefaction-related deposits previously attributed to one of the three published 1811-1812 mainshocks were actually caused by a M7.0 aftershock. However, an alternative explanation is that unilateral rupture within the New Madrid fault zone led to extraordinary strong motions to the northeast. Hough et al., (2005) argue, "The distribution of intensities provides prima facie evidence that the event was centered well to the north and east of the other mainshocks." Yet this statement does not consider rupture directivity, which can provide a similar intensity pattern. Ground motion models that include both directivity (Somerville et al., 1997) and the effects of surface geology show that relatively strong intensities could have extended well beyond the rupture to the northeast. For 1.0 s motions out to 300 km, the Somerville et al. model provides a 25-40% increase in ground motion to the northeast. Stronger increases are suggested by the 1988 Saguenay earthquake observations of a two-fold increase along a 35-degree azimuth range. Furthermore, there is evidence from some sites in the central New Madrid region showing a fourth, less severe, 1811-1812 liquefaction event (M7.0 aftershock?) (Tuttle, 1999; Tuttle et al., 2002). This evidence requires four strong shaking events in the New Madrid seismic zone. Unilateral rupture to the northeast in the New Madrid fault zone for the January earthquake is compatible with observations of triggered earthquakes in northern Kentucky and liquefaction in the northern New Madrid region and White County Illinois. The unilateral rupture hypothesis cannot be unequivocally established because of the absence of 1811-1812 intensity data southwest of the New Madrid seismic zone with which to show an asymmetric pattern of intensities.