Tsunamis are instrumentally recorded by sea level recorders
such as coastal tide gauges, near-shore wave and
GPS buoys, and deep-ocean bottom pressure gauges.
Coastal tide gauges have various types such
as mechanical type with a float, and pressure, acoustic or radar sensors.
After the 2004 Indian Ocean tsunami, more coastal tide gauges have been installed in the Indian
Ocean region. Currently, sea level data at several hundreds
of stations are available in real-time. Near-shore gauges include wave
gauges using ultrasonic waves and GPS buoys. They measure
offshore sea levels at water depths of 50 to 200 m, and
can detect tsunamis before their coastal arrivals giving
some lead time for issuing tsunami warnings. Tsunami
waveforms are much simpler in deep oceans, where they
are free from the effects of coastal reflection or refraction
due to bathymetry. A kinematic GPS analysis of a ship in
open ocean detected the 2010 Chile tsunami. Deep ocean
measurements of tsunamis have been made by
using bottom pressure gauges for early detection and
warnings of tsunamis. The Deep-ocean Assessment and
Reporting of Tsunamis (DART), developed by NOAA
(National Oceanic and Atmospheric Administration) of
the USA, records water levels using bottom pressure
gauges, and sends signals to a surface buoy via acoustic
telemetry in the ocean, then via satellites to a land station
in real time. After the 2004 Indian Ocean tsunami,
the total number of DART stations in the Pacific as well as
Indian Ocean increased from 6 in 2004 to about 60 in
2013. An alternative way to retrieve data from deep
ocean bottom pressure data is to use submarine cables.
Around Japan, more than 10 bottom pressure gauges were
installed at the time of the 2011 Tohoku earthquake, and
more cabled networks, DONET along the Nankai trough and S-net along the Japan Trench are being
deployed.