Horizontal displacements such as those of this earthquake rarely trigger large tsunamis. This usually requires a vertical movement of the seabed. When a block of rock beneath the water surface rises or falls rapidly due to an earthquake, the water column above it is raised or lowered and a tsunami is triggered. The exact cause of the tsunami in the Bay of Palu is not yet known. Possible causes are the pronounced topography of the coastal area and the seabed, whereby horizontal movements could also shift large masses of water, and / or underwater landslides caused by the quake.

After the earthquake, the tsunami waves took about half an hour to reach Palu. This appears long in comparsion to the relatively short distance to the epicentre. The reason for this is the depth of the water; the greater the depth, the faster the waves spread. At a shallow water depth of 200 m, as it is likely to prevail between the epicentre and the city of Palu, the speed of propagation is about 160 km/h. This is much slower than the speed of the seismic waves, and also much slower as if the sea depth was at 4 km, typical in the deep ocean. In this deep case, the tsunami waves would have arrived at Palu in under 10 minutes. In addition, the narrow geometry of the bay north of Palu is very likely to have significantly amplified the height of the tsunami wave.

As always, the sequence of strong quakes has redistributed the tectonic stresses in the region. In the coming days, further, sometimes violent aftershocks at the Palu-Koro fault as well as at neighbouring fault lines are to be expected. Indonesia lies on the Pacific Ring of Fire, where 90 percent of all earthquakes worldwide occur.

Die Region um Saxon wurde bereits im Februar dieses Jahres von einem Erdbeben der Magnitude von 2.6 erschüttert. Im Wallis und vor allem in einem Streifen, der parallel der Haupt-Talrichtung von Leukerbad über Derborence Richtung Lac d'Emosson verläuft, ist es in den letzten Jahren immer wieder zu kleineren, spürbaren Erdbeben gekommen. Solche Erschütterungen sind für diese Region nichts Ungewöhnliches, ist das Wallis doch der Kanton mit der höchsten Erdbebenaktivität in der Schweiz.

Indonesia sits atop the Pacific ‘Ring of Fire’, a seismically highly active area that also includes countries such as Japan, the Philippines or New Zealand. Ninety percent of earthquakes worldwide occur along the Ring of Fire; therefore, Indonesia is subject to a high earthquake hazard. Despite the regional differences within the country, earthquakes as strong as the Lombok earthquake can occur anywhere at any given time. Such strong shocks can be expected to occur less frequently in areas that are less at risk, such as Lombok, than in high risk areas such as Sumatra; however, the possibility can never be fully ruled out.

People who would like to travel to Indonesia during the next days or months are recommended to refer to their travel agency or local accommodation for more information on the current situation. Travellers currently in Indonesia should follow the recommendations of the local authorities as well as the most recent developments on the news. You can find general tips on what to do before, during and after an earthquake abroad here.

More information:

USGS Information about 6.9 earthquake

Blog entry by Temblor

As earthquakes occur without warning, it helps to know in advance what you should do if one does happen. The standard recommendation in Switzerland and elsewhere is: seek cover. That might be under a sturdy table or by holding a pillow over your head in bed. Many earthquake injuries are caused by falling objects such as shelf contents or ceiling or wall materials. If you run around in a panic during the quake, you are more likely to be hit by such objects. What's more, even keeping your balance can be difficult during a powerful earthquake, so you'll probably want to cling on to something that offers protection (the Director of the Seismological Service found this out for himself, as you can see in the next episode of Einstein to be broadcast on SRF).

However, as so often, there are exceptions to this rule. If you're near the sea, running inland is a better option due to the possibility of a subsequent tsunami. In Alaska, they have the following rule of thumb: if the quake is strong enough to almost knock you over, run up the nearest hill. Likewise, if you are indoors but very close to an exit, head outside. If you're already outside, stay there and keep away from buildings. Detailed recommendations on what to do can be found here. Essentially, the right action to take during an earthquake very much depends on the circumstances (the type of earthquake, the type of building, where you are, and so on).

This makes it difficult for seismologists to give general recommendations without causing undue anxiety. The most important protection against earthquakes is and will always be appropriate construction. However, judging how earthquake-resistant a building is just by looking at it is hard even for experts, so this is probably not something that you can take into account when choosing accommodation. That said, it's often the case that the newer a building is, the more likely it is to comply with the latest building standards and therefore to be earthquake-resistant. 

Once the tremors are over, heavily damaged buildings should be evacuated. While making your way outside, beware of falling objects such as bricks and tiles or disconnected power lines. Aftershocks, some of them strong, are likely to occur after a major quake, and these may cause further damage. In genaral, always follow the instructions of the local authorities.

Should you wish to take further precautions, this page lists recommendations for things you can do ahead of shorter or longer trips. All the information can also be downloaded as a fact sheet.

Last but not least, don't forget that, while earthquakes are real, the risk of being harmed by one is extremely small, even in seismically active areas. For example, if you're travelling to California, the risk is 1 in 1,000,0000 per year. Driving a car there is much more dangerous, with around a 1 in 11,428 chance of being involved in a serious accident. So the message is: make a few simple preparations and then enjoy your well-deserved holidays!