Reverberations on the watery element: A significant tsunamigenic historical earthquake offshore the Carolina coast

We investigate an early nineteenth-century earthquake that has
been previously cataloged but not previously investigated in
detail or recognized as a significant event. The earthquake
struck at approximately 4:30 a.m. LT on 8 January 1817 and
was widely felt throughout the southeastern and mid-Atlantic
United States. Around 11:00 a.m. the same day, an eyewitness
described a 12-inch tide that rose abruptly and agitated boats
on the Delaware River near Philadelphia. We show that the
timing of this tide is consistent with the predicted travel time
for a tsunami generated by an offshore earthquake 6–7 hours
earlier. By combining constraints provided by the shaking intensity
distribution and the tsunami observation, we conclude
that the 1817 earthquake had a magnitude of low- to mid-M 7
and a location 800–1000 km offshore of South Carolina. Our
results suggest that poorly understood offshore source zones
might represent a previously unrecognized hazard to the
southern and mid-Atlantic coast. Both observational and modeling
results indicate that potential tsunami hazard within
Delaware Bay merits consideration: the simple geometry of
the bay appears to catch and focus tsunami waves. Our preferred
location for the 1817 earthquake is along a diffuse
northeast-trending zone defined by instrumentally recorded
and historical earthquakes. The seismotectonic framework for
this region remains enigmatic.