Northern Sumatra Earthquake and the
Subsequent Tsunami on 26 December 2004


The International Monitoring System (IMS) of the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), which is being built to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty, has recorded the earthquake west of northern Sumatra, Indonesia, on 78 of its waveform monitoring stations within seconds to minutes of the event on 26 December, 2004. Of the 78 stations, 71 were using the seismic, six the hydroacoustic and one the infrasound technologies.

Following current practice, the first automatic event list containing the Sumatra earthquake was released by the International Data Center (IDC) in Vienna to States Signatories about two hours after the event, that is at about 03:00 UTC on 26 December, 2004. The second and third automatic event lists followed at 6 hours and 12 hours after the event.

Raw data from the monitoring stations are communicated to National Data Centers in near real time for analysis, if  States Signatories have subscribed to this service. The States Signatories in the afflicted region that have set up their National Data Centers and are receiving data from the International Data Center of the Preparatory Commission for the CTBTO include Australia, Indonesia, Thailand, Kenya, Malawi, Oman and South Africa.

The IDC automatically located about 1,500 aftershocks during the following two days. These aftershocks were included in the automatic event lists issued to States Signatories according to the normal schedule, together with other events located worldwide.

The IMS does not include equipment to detect tsunamis directly, and the IDC processing does not include a capability to determine the mechanism of an earthquake automatically. The issuance of a reliable tsunami alarm requires the rapid determination of the earthquake mechanism, the incorporation of data from pressure detectors designed to detect tsunami waves directly, and appropriate arrangements for rapid dissemination of alarms to responsible entities who can take actions at national level.

Monitoring data collected by the IMS stations is provided to States Signatories which decide how they wish to use the data.

During the period prior to Entry into Force of the Treaty, the CTBTO is mandated to provide data and processed information to States Signatories in order to support the testing and evaluation of the emerging monitoring network and processing system. Accordingly, the schedule and availability of these data and processed information to States Signatories are tailored to the requirements of testing and evaluation of the Treaty verification system, in anticipation of moving to an operational phase when the Treaty enters into force.

Signal detection at the IDC is performed automatically, without human intervention, within about ten minutes of receiving data. Further processing is done automatically to locate events (including earthquakes). The first automatic event list is issued to states signatories about two hours after data are recorded. Second and third automatic event lists are issued six hours and twelve hours after data are recorded. These subsequent event lists are of higher quality and take advantage of data from additional IMS seismic stations whose data are provided only in response to automatic requests.

Of the 321 monitoring stations envisaged by the Comprehensive Nuclear-Test-Ban Treaty about 130 are providing data at present to the International Data Center in Vienna. These comprise seismic, hydroacoustic, infrasound and radionuclide stations.

As of 5 January, 2005 the Treaty has been signed by 174 States, and ratified by 120 States, including 33 of the 44 States whose ratification is required before the Treaty enters into force.

For further information on the CTBTO, please see or contact:
Annika Thunborg, Chief, Public Information  
T    +43 1 26030-6375  
M    +43 699 1459 6375