Geologists at the University of Texas at Austin say those cities, like Port au Prince, Haiti, and many others, lie near a coast and near an active geologic feature called a strike-slip fault, where two tectonic plates slide past each other, a university release reported.
The tsunami risk in such areas was always considered low because when such faults rupture they do so without much vertical displacement of the sea floor, which is how tsunamis are generated. But researchers now say a tsunami can be generated by even a moderate earthquake on a strike-slip fault through submarine landslides, raising the overall tsunami risk in these places.
"The scary part about that is you do not need a large earthquake to trigger a large tsunami," Matt Hornbach, research associate at UT's Institute for Geophysics, said.
"Organizations that issue tsunami warnings usually look for large earthquakes on thrust faults," said Hornbach. "Now we see you don't necessarily need those things. A moderate earthquake on a strike-slip fault can still be cause for alarm."
Within minutes of the magnitude 7 Haiti earthquake a series of tsunami waves, some as high as 9 feet, hit parts of the shoreline, generated by weak sediment at the shore that collapsed and slid along the seafloor, displacing the overlying water, researchers say.
The UT scientists are conducting a research project in nearby Jamaica to assess the tsunami threat there.
"The geology of Kingston, Jamaica, is nearly identical to Port Au Prince, Haiti," Hornbach said. "It's primed and ready to go and they need to prepare for it. The good news is they have a leg up because they're aware of the problem."