Sumatra Earthquake: Tremendous Progress in the Warning Chain

On 11 April 2012 at 8.38 AM UTZ a strong earthquake with a magnitude of 8.6 Mw occurred about 500 km west of the Indonesian province of Aceh in northern Sumatra, which resulted in a tsunami warning in Indonesia and the Indian Ocean. It was followed by a very strong aftershock with a magnitude of 8.0 Mw about 2 hours later.

In addition to the two major earthquakes (Mw = 8.6 and 8.0), the GEOFON Seismological Service of the GFZ has evaluated more than 60 aftershocks. They are aligned along two zones running in essence from east to west on the western side of the Sunda trench. The two main earthquakes are unusual events. Firstly, they occurred in the so-called "outer rise" area outside the actual subduction zone, and secondly they do not exhibit the more typical normal fault mechanism for this zone, but a horizontal strike slip fault. This type does not generate a vertical displacement of the ocean floor and therefore no distinct tsunami. The  earthquake epicenter is located in the transitional zone between the Indian and the Australian part of the Indo-Australian plate and has a diffuse seismicity, in which predominantly normal faults occur as well as sporadically strikeslip faults. However, events of this magnitude were not previously observed. Moreover, it seems that these two earthquakes are the world's largest recorded strike slip quakes that caused no significant damage.

The tsunami early warning system in Indonesia, which was built with German assistance under the auspices of the GFZ since 2005 and was handed over to Indonesia in March 2011, has worked as intended. The first warning was already issued about 4 minutes after the first big quake and distributed throughout the various communication channels to the affected provinces and districts. Simultaneously, the public was informed via TV and radio. An identical message was issued shortly afterwards from the BMKG in its role as a "Regional Tsunami Service Provider" to all national warning centers in the countries bordering the Indian Ocean. At this early stage the earthquake mechanism was not yet known and with a quake of this strength a major tsunami had to be expected, not only for Indonesia but also in e.g. Malaysia, Thailand, India, Sri Lanka and the west coast of Australia. Consequently, in India, Sri Lanka and Thailand as well as in Indonesia evacuation measures were initiated promptly. It was evident that local authorities and the public knew exactly how to react and what to do. This shows that the years of education and training work in the area of the "Last Mile" have had a very positive effect.

In the end, due to its mechanism the strong earthquake did not generate a large tsunami. Gauges in northern Sumatra registered a small tsunami 1.5 hours after the quake that reached a maximum height of 1 meter. Gauging stations in Sri Lanka, the Maldives or the Cocos Islands indicated tsunami heights of about about 20 cm.

Conclusion: The entire warning chain from the measurement and determination of the earthquake via the handling of the situation in the warning center and the information of the affected regions to the reaction of the population has worked and demonstrates the tremendous progress that the region around the Indian Ocean and especially in Indonesia has experienced since the catastrophic tsunami in 2004.

A team of GFZ scientists from sections 2.1 and 2.4 have developed a special event page for the Sumatra earthquake geofon.gfz-potsdam.de/eqinfo/special/gfz2012hdex.php