The 1999 Chi-Chi earthquake: what happened and what it changed

One night in September

The earthquake began at 17:47 Universal Time on 20 September 1999, which was 1:47 a.m. on 21 September in Taiwan. A report by the Earthquake Disaster Mitigation Research Center of Japan’s RIKEN institute places the epicentre at 23.85° N, 120.81° E, just north of Chi-Chi in Nantou County, at a depth of about 7 kilometres. That is shallow, which is one reason the shaking at the surface was so violent.

Magnitudes differ by agency. The Central Weather Bureau, as the Central Weather Administration was then called, reported magnitude 7.3 on its local scale; the US Geological Survey gave a moment magnitude of 7.6. The two numbers measure different things and are not in conflict. Seismologists have described it as the largest earthquake to have occurred on land in Taiwan in the twentieth century.

A 100-kilometre rupture

The earthquake ruptured the Chelungpu fault, which runs north–south along the western foothills of central Taiwan. It is a thrust fault, dipping at a shallow angle of about 30° to the east: the eastern side was pushed up and over the western side. The rupture extended along roughly 100 kilometres.

The shaking near the fault was extreme. Ground accelerations in the southern part of the fault exceeded the acceleration of gravity, 1 g, at some sites, and the ground moved at speeds of up to 300 centimetres per second, with displacements of up to 8 metres. The main shock also triggered two further earthquakes of about magnitude 6, each near a known fault.

This is the kind of fault the collision of the Philippine Sea and Eurasian plates produces in central Taiwan, as explained in Why does Taiwan have so many earthquakes?

The human toll, and Taipei

The RIKEN report, citing the preliminary report of Taiwan’s National Center for Research on Earthquake Engineering from October 1999, gives 2,333 deaths, 10,002 injuries and 8,773 damaged buildings. These are preliminary figures from the weeks after the disaster, not a final count.

The damage was not limited to the epicentral region. The strong shaking reached cities as far as 150 kilometres away and destroyed several high-rise buildings in the Taipei Basin. That distant damage is one reason why seismologists pay close attention to how the soft sediments under Taipei respond to large earthquakes elsewhere on the island; the basin is described on City & Region.

Large as it was, Chi-Chi was not the deadliest earthquake in Taiwan’s recorded history. A US Geological Survey table of historic surface-faulting earthquakes lists more than 3,000 deaths for the earthquake of 21 April 1935 and 1,258 for that of 17 March 1906.

A seismological treasure

Chi-Chi struck at a moment that made it unique for science. In 1996 the Central Weather Bureau had completed an extensive strong-motion instrumentation programme, with more than 600 modern digital instruments. When the earthquake came, they recorded it.

The result, described in the Bulletin of the Seismological Society of America, was more than 30,000 digital strong-motion records from the main shock and thousands of aftershocks: the largest set of strong-motion data recorded from a major earthquake since strong-motion seismology began in the 1930s. More than 60 recording sites lay within 20 kilometres of the fault rupture, which the authors describe as a five-fold increase in the number of such near-field records available for the entire world.

Records taken close to a large fault are rare and especially valuable, because they show how buildings and the ground behave in the strongest shaking. They are used by seismologists studying how faults rupture and by engineers who write building codes. The CWA notes that the records of its strong-motion stations are among the most important inputs for the seismic design codes that engineers use.

What changed afterwards

The CWA’s Seismological Center traces a direct line from the disaster to the next stage of its work. After the 1999 Chi-Chi earthquake, the National Science Council set out to strengthen earthquake research and disaster prevention. In 2000 it launched an interagency programme on earthquakes and active faults, under which the Central Weather Bureau built a modern seismic monitoring system: broadband seismometers together with Academia Sinica and National Central University, GPS stations, and groundwater observation together with the Water Resources Agency.

Early warning moved forward too. In 2001 the centre developed a “virtual subnetwork” technique that gave metropolitan areas away from the epicentre the ability to receive early warnings, and from 2004 a new phase of work aimed to cut alert times to under 30 seconds. The early warning service was launched in 2013, and since 2016 alerts have also reached phones through the Public Warning System. How that system works today, and what it can and cannot do, is explained on Watching the ground move.

Academia Sinica’s Institute of Earth Sciences, which took part in the new networks, is described on Taiwan’s national academy.