Earthquakes in Lombok – Expert Reaction

A magnitude 6.9 earthquake hit the Indonesian tourist island of Lombok on Sunday evening at 7.46pm (local time), reportedly killing at least 82 people.

It comes a week after a 6.4 magnitude quake hit the same island, killing a reported 17 people.

We gathered the following comments from experts. Please feel free to use these comments in your reporting.

Dr John Ristau, seismologist, GNS Science comments:

“The magnitude 6.9 Loloan, Indonesia earthquake was a reverse faulting earthquake at a depth of 31km.

Image credit: California Geological Survey

“In this area the Sunda plate to the north subducts beneath the Indonesian Arc, and the mechanism of the earthquake is consistent with reverse-faulting on the Indonesian Arc thrust.

“The Pacific Island arc from Samoa to Indonesia is one of the most seismically active regions on Earth, and earthquakes of this size are not unusual. In the region within about 500km of the epicentre there have been at least 15 earthquakes of magnitude 6.5 and above in the last 10 years.

“Earthquakes of this size will produce significant aftershock activity. However, plate interface earthquakes tend to have less vigorous aftershock sequences than shallow crustal earthquakes of comparable size.”

No conflicts of interest.

Professor Brendon Bradley, Professor in Earthquake Engineering, University of Canterbury, comments:

“The fact there was a magnitude 6.4 a week before this one means there can be some relationship between the two. We would call the 6.4 a foreshock, and the second one (the 6.9) a mainshock, but they are each earthquakes in their own right.

“A 6.9 is quite a lot bigger than a 6.4. A 6.9 event has ~5 times more energy release than 6.4. The magnitude tells you how much energy is released, but it doesn’t tell you how strong the shaking will be. The intensity always comes down to the question of where you are – the closer you are the stronger, on average, the shaking will be.

“After an earthquake, there’s a gradual tapering off of activity as the plates try to rebalance themselves. You have the initial earthquake that stirs things up and as days go on the earthquake rate reduces, until the next earthquake happens. So you get this tapering off of activity, but combined with this long term build-up at the same time from the movement of the tectonic plates.

“We can predict aftershock sequences relatively well statistically but we can never talk precisely about them. These earthquakes are totally normal – this is a really active area of the world. These quakes are right on the boundary of Australian and Indonesian plates. This is the same part of the world that experienced a magnitude 9 quake in 2004.

“Earthquakes of this size are happening around the world quite often – but they are not evenly portrayed by media (unfortunately, it’s typically the reality that only when people die that we see it on the mainstream news). Globally we see hundreds of magnitude 6 quakes each year and tens of magnitude 7 quakes.

“The distance to New Zealand is far too large for these quakes to have any direct influence on us.”

No conflicts of interest declared.

The below comments were collected by our friends at the Australian SMC. Feel free to use these comments in your reporting:

Dr Simon McClusky, Senior Fellow in the Research School of Earth Sciences, The Australian National University (ANU), comments:

“Our results draw a new kinematic framework for active deformation in the eastern Sunda-Banda Arc, highlighting the need to reconsider the level of seismic hazard there.

“Several of the active faults identified here directly threaten socioeconomic assets vital to Indonesia.

“The Kendeng thrust passes through the southern outskirts of Surbaya, Indonesia’s second largest city with a population of over 2.5 million, and traverses a 300 km length of East Java, with a population density of over 800 people per square kilometer.

“The Semau Fault skirts the city of Kupang, the main commercial centre of Nusa Tenggara with a population of around 500,000. Finally, earthquakes along the back-arc thrust beneath the sea floor extending 1700 km from eastern Java to Timor could generate regional tsunamis threatening the coastlines of the Flores Sea.

“Further studies, including earthquake, geodetic, and paleoseismic, should be undertaken to better understand these threats.”

No conflicts of interest declared.

Professor Phil Cummins, researcher of Seismology & Mathematical Geophysics in the Research School of Earth Sciences, The Australian National University (ANU), comments:

“The magnitude 7 earthquake that struck Lombok Island, Indonesia, last night occurred just below the Island’s northern coast. Its location is every close to the magnitude 6.4 earthquake that occurred one week ago, and its faulting mechanism appears to be very similar.

“These were both shallow earthquakes, and both caused considerable damage. But last nights earthquake was 4 times larger and released much more energy. It caused strong shaking throughout Lombok and Bali and was even felt in eastern Java.”

No conflicts of interest declared.