A warning sign for one of these objects is a phenomenon called a drawback, although early detection of them is difficult because of shoaling.
The intensity of these objects can be measured on the twelve-point Papadopoulos-Imamura scale. One of these objects led to the somewhat recent rediscovery of relics at Mahabalipuram [muh-HAH-buh-lee-poo-ruhm]. The assessment and reporting of these objects, which can be as small as a couple of centimeters, is made with the DART system. A warning sign for one of these objects is a phenomenon called a drawback, although early detection of them is difficult because of shoaling. These objects can have a height of 100 feet, and their name is Japanese for "harbor wave." For 10 points, name these very large waves produced by landslides, earthquakes or volcanic activity. 
The intensity of these objects can be measured on the twelve-point Papadopoulos-Imamura scale.
tsunamis [be nice and prompt on "tidal wave"] 