Test Glossary Import

The height of the sea at a given time measured relative to some datum, such as mean sea level.

A system consisting of a device such as a tide gauge for measuring the height of sea level, a data collection platform (DCP) for acquiring, digitizing, and archiving the sea level information digitally, and often a transmission system for delivering the data from the field station to a central data collection centre. The specific requirements of data sampling and data transmission are dependent on the application. The GLOSS programme maintains a core network of sea level stations. For local tsunami monitoring, one-second sampled data streams available in real time are required. For distant tsunamis, warning centres may be able to provide adequate warnings using data acquired in near-real time (one-minute sampled data transmitted every 15 minutes or better). Sea level stations are also used for monitoring long-term sea level change and climate change studies, where an important requirement is for the very accurate location of the station as acquired through surveying techniques.

Rarotonga sea level station, Avarua Harbor, Cook Islands.

The fiberglass electronics package (a), antenna (b), solar panel (c) were installed on a pier. Conduit (d) containing cables connecting the sensor, located at a depth of five feet below low-tide water level, to the data collection platform containing the electronics above, was externally attached to the tube containing the sensor (e). Courtesy of University of Hawaii Sea Level Center.

GLOSS sea level stations employ a number of instruments to measure sea level, including down-looking radars to measure sea level. Port Louis, Mauritius. Photo courtesy of University of Hawaii Sea Level Center.

Satellite altimeters monitor Sea Surface Height (SSH), and can record a snapshot of the propagating tsunami if the satellite orbit is located above the tsunami. During the 2004 Indian Ocean tsunami and 2011 Tohoku tsunami, several satellites captured the tsunami as it propagated across the Indian and Pacific Ocean, respectively.

Radar altimeters aboard the Jason-1 satellite recorded the 26 December 2004 Indian Ocean tsunami in a snapshot taken two hours after the earthquake. Bottom profile superimposing MOST model calculations on the satellite data shows a maximum wave amplitude of about 60 cm. Image courtesy of NOAA.

A seiche may be initiated by a standing wave oscillating in a partially or fully enclosed body of water. It may be initiated by long period seismic waves (an earthquake), wind and water waves, or a tsunami.

Tsunamis are sometimes referred to as seismic sea waves because they are most often generated by earthquakes.

A descriptive tsunami intensity scale, which was later modified into the modified Sieberg sea-wave intensity scale (Ambraseys, 1962).

The average height of the one-third highest waves of a given wave group. Note that the composition of the highest waves depends upon the extent to which the lower waves are considered. In wave record analysis, the average height of the highest one-third of a selected number of waves, this number being determined by dividing the time of record by the significant period. Also called characteristic wave height.

When referring to tsunami waves, it is the spreading of the wave energy over a wider geographical area as the waves propagate away from the source region. The reason for this geographical spreading and reduction of wave energy with distance travelled, is the sphericity of the earth. The tsunami energy will begin converging again at a distance of 90 degrees from the source. Tsunami waves propagating across a large ocean undergo other changes in configuration primarily due to refraction, but geographical spreading is also very important depending upon the orientation, dimensions, and geometry of the tsunami source.

The permanent movement of land down (subsidence) or up (uplift) due to geologic processes, such as during an earthquake.

The 26 December 2004 earthquake resulted in 1.2 m of land subsidence in the Car Nicobar, Nicobar Islands, India, leaving houses that were once above sea level now permanently submerged. Photo courtesy of ICMAM, Chennai, DOD, India.