TSUNAMI HAZARDS TO THE U.S. ATLANTIC COAST (Invited Presentation)

Tsunamis pose a low-probability, high-impact hazard to the U.S. Atlantic seaboard. The coast is believed subject to infrequent tsunamis from nearby submarine landslides and from distant tectonic and volcanic sources. The coast is vulnerable to tsunamis because of residential and industrial development in low-lying areas. Attempts to clarify the hazard have focused on nearby submarine landslides and, farther afield, on the Puerto Rico Trench (PRT), the Azores-Gibraltar (A-G) plate boundary (source of the 1755 Lisbon tsunami), and the Canary Islands. Over 100 landslides have been mapped along the Atlantic continental slope and rise from Cape Fear northward. Most of these slides removed thin (20-200 m) Pleistocene-age layers and transported them 10s-100s of km downslope. The few slide dates available are in the range 50,000-7,000 ybp; hence, tsunami generation by these slides cannot be readily tested by studying Holocene tidal-marsh deposits. The landslide frequency is low because of the low frequency of triggering earthquakes along the passive margin and the cessation of sediment input into the margin at the end of the last glacial maximum. Centuries or millennia probably elapse between earthquake-triggered landslides on the formerly glaciated New England margin, because the triggering earthquake likely needs to exceed M 5 and to be located within 100 km of the continental slope. Earthquake frequency farther south is unknown, but salt diapirs off Cape Hatteras may contribute to slope instability there. Earthquake frequency is also low along the PRT and A-G plate boundary because the relative plate motions are small. The potential for a M 9 earthquake from the PRT appears to be low based on combination of evidence from bathymetry, seismic reflection, GPS measurements, post-Columbus earthquakes, seismic tomography and modeling. Landforms and deposits east-northeast of Puerto Rico provide evidence that the eastern PRT produced a tsunami in AD 1200-1450 from either interplate thrusts or outer-rise normal faults [1], but the impact of this tsunami on the U.S. Atlantic coast is unknown. Tsunami modeling suggests that bathymetric features in the eastern Atlantic protected the U.S. East coast (but not Newfoundland, Florida, and the Caribbean) from the 1755 Lisbon tsunami. [1] Atwater et al., AGU Fall meeting, 2012.