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Tsunami on the Delaware River? Study of Historical Quake and Early East Coast Seismicity

Tsunami on the Delaware River? Study of Historical Quake and Early East Coast Seismicity

PASADENA, Calif. Imagine the Delaware River abruptly rising toward Philadelphia in a tsunami-like wave of water. Scientists now propose that this might not be a hypothetical scenario. A newly published paper concludes that a modest (one-foot) tsunami-like event on the East Coast was generated in the past by a large offshore earthquake. This result may have potential ramifications for emergency management professionals, government officials, businesses and the general public.

Early in the morning of Jan. 8, 1817, earthquake shaking was felt along the Atlantic seaboard as far north as Baltimore, Md., and at least as far south as Charleston, S.C. Later that morning, a keen observer documented an abrupt rise in the tide on the Delaware River near Philadelphia, commenting on the earthquake felt earlier to the south, and remarking that the tidal swell was most likely “the reverberation or concussion of the earth operating on the watery element.”

Scientists have previously interpreted this earthquake to have a magnitude around 6 and a location somewhere in the Carolinas or slightly offshore. In a new study, USGS research geophysicist Susan Hough and colleagues reconsider the accounts of shaking and, for the first time, consider in detail the Delaware River account. They show that the combined observations point to a larger magnitude and a location farther offshore than previously believed. In particular, they show that a magnitude-7.4 earthquake located 400-500 miles off South Carolina or Georgia could have generated a tsunami wave large enough to account for the tidal swell on the Delaware. Using new computer-assisted research techniques, they uncover first-hand accounts from newspapers and ships’ logs that give a wider perspective on the 1817 event. Notably, the predicted timing of such a tsunami wave from this location matches the documented timing in the eyewitness account.

The USGS monitors earthquakes offshore, and in recent years has undertaken research to better understand shaking and tsunami hazard from offshore earthquakes and landslides. Scientific understanding of faults and geological processes in this part of the Atlantic is limited. Still, it has long been understood that large, infrequent offshore earthquakes may pose a tsunami hazard to the Atlantic coast. In 1978, a magnitude-6 earthquake occurred roughly 240 miles southwest of Bermuda, even farther offshore than the inferred location of the 1817 earthquake. In 1929, the magnitude-7.2 Grand Banks, Newfoundland, earthquake triggered a submarine landslide that generated a large tsunami. Waves 10-13 feet high struck the Newfoundland coast, killing 29 people and leaving 10,000 temporarily homeless. 

The inferred 1817 tsunami was significantly smaller than the Newfoundland disaster. However, the new interpretation by Hough and colleagues highlights the potential earthquake and tsunami hazard along the Atlantic seaboard from the still poorly understood offshore earthquake faults. The new study highlights that there is still work to be done to characterize this hazard in the southeastern United States. 

The study, “Reverberations on the Watery Element: A Significant, Tsunamigenic Historical Earthquake Offshore the Carolina Coast,” by Susan E. Hough, Jeffrey Munsey, and Steven N. Ward, is published in the September/October issue of Seismological Research Letters

USGS Newsroom

More information

Parameter Value Description
Magnitude mb The magnitude for the event.
Longitude ° East Decimal degrees longitude. Negative values for western longitudes.
Latitude ° North Decimal degrees latitude. Negative values for southern latitudes.
Depth km Depth of the event in kilometers.
Place Textual description of named geographic region near to the event. This may be a city name, or a Flinn-Engdahl Region name.
Time 1970-01-01 00:00:00 Time when the event occurred. UTC/GMT
Updated 1970-01-01 00:00:00 Time when the event was most recently updated. UTC/GMT
Timezone offset Timezone offset from UTC in minutes at the event epicenter.
Felt The total number of felt reports
CDI The maximum reported intensity for the event.
MMI The maximum estimated instrumental intensity for the event.
Alert Level The alert level from the PAGER earthquake impact scale. Green, Yellow, Orange or Red.
Review Status Indicates whether the event has been reviewed by a human.
Tsunami This flag is set to "1" for large events in oceanic regions and "0" otherwise. The existence or value of this flag does not indicate if a tsunami actually did or will exist.
SIG A number describing how significant the event is. Larger numbers indicate a more significant event.
Network The ID of a data contributor. Identifies the network considered to be the preferred source of information for this event.
Sources A comma-separated list of network contributors.
Number of Stations Used The total number of Number of seismic stations which reported P- and S-arrival times for this earthquake.
Horizontal Distance Horizontal distance from the epicenter to the nearest station (in degrees).
Root Mean Square sec The root-mean-square (RMS) travel time residual, in sec, using all weights.
Azimuthal Gap The largest azimuthal gap between azimuthally adjacent stations (in degrees).
Magnitude Type The method or algorithm used to calculate the preferred magnitude for the event.
Event Type Type of seismic event.
Event ID Id of event.
Event Code An identifying code assigned by, and unique from, the corresponding source for the event.
Event IDS A comma-separated list of event ids that are associated to an event.

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