The German engineers drew on a 60-year history of tsunami early warning research dating to the National Oceanic and Atmospheric Administration's Pacific Tsunami Warning Center,. The new system incorporates many recognizable elements from the legacy NOAA and Japanese systems, but also features new research and design elements and integrated, newly designed software and hardware.
The Japanese early warning system is considered by researchers to be the most advanced in the world in its scale and distribution. The basic tools are seismometers, tidal gauges, pressure sensors and waterborne buoys—instruments on which the Indonesian system is based. It also links a new earthquake alert circuit that went online in 2007—connecting more than 1,000 land-based and ocean-bottom seismometers under the aegis of the Japan Meteorological Agency—with Pacific-wide tsunami alert coordinating centers in Japan, Alaska and Hawaii. The GFZ-designed system, in comparison, is a bit more compact, more connected to GPS monitoring, and integrated with newly developed software. This SeisComP 3.0 software analyzes seismological and sensor data, and is linked to a local grid of tsunami wave data models. It is all designed to make the system faster and more responsive to the near-field tsunamis that threaten Indonesia.
In a near-field event the most crucial element is the seismic network, Lauterjung says. Each node is equipped with a seismometer recording ground motion over a wide range of frequencies and along three axes. The stations are linked to SeisComP 3.0, which uses the inbound data to calculate earthquake location and magnitude. Measurements from the whole system are matched with a database holding thousands of modeled tsunami simulations calculated across the Sunda Arc: some 3,500 now, with another 7,000 in the works.
Not every earthquake triggers a tsunami—it takes a vertical shock to transfer energy to a water column. The GFZ design accounts for this distinction by including GPS units in the seismic stations. These concrete-mounted units transmit data continuously and align with geodetic stationary, or base, points, allowing observers to calculate nearby deformation in Earth's crust.
Offshore, 4.5-meter steel buoys are attached to pressure plates on the ocean floor set with instruments: an acoustic modem for transmission, another seismometer for the seafloor, and gauges which detect sudden changes in water pressure above. The pressure plates can measure changes in sea level within an accuracy of several millimeters. Also, a GPS-referenced data stream from the buoy, overlaid with an algorithm and mathematical filtering, calculates sea-surface heights by measuring the buoy's movement compared with a set of land-based reference stations.
Last but not least are the tidal gauges measuring sea-level changes off the beach. The stations look like orange boxes sitting on dock pilings, and comprise two component sets: One is a set of three different kind of tide gauge sensors, including a radar hanging like a lure at the end of a rod. The other contains data processors and—again—an integrated GPS antenna, which monitors the stability and displacement of the station during earthquakes. Onboard algorithms for the sensors go into motion if there is a sudden tidal outrush and indicate both the timing of a potential tsunami arrival and its relative strength.
The satellite-linked network now stretches like a veil over the coastal face of Sumatra and its smaller islands: 10 GFZ tidal stations dot the coastline, along with 21 seismometers, 10 buoy systems and 20 GPS stations. The Indonesians themselves ordered another 105 seismic stations in addition to the 15 donated by Japan, 10 from China and several from France. The upshot, Lauterjung says, is that Indonesia now has one of the densest seismic networks in the world.