Early Sunday morning here (just after midnight on Monday morning in New Zealand), a very large Magnitude 7.8 earthquake struck South Island. The extent of the faulting is somewhat unclear because it was a complex event, but the active fault appears to extend from the epicenter, 100 km north of Christchurch, to the Cook Strait just southwest of Wellington. This earthquake appears to be part of the general subduction of the Pacific plate under the Australian plate. Although the earthquake did generate a small 3-6 foot (1-2 meter) tsunami, the tsunami effects were minimal. Remarkably, only two deaths have been reported in the town of Kaikoura as a result of this earthquake, though many more are displaced or injured. Keep in mind that M 7.8 is the estimated size of the 1906 San Francisco earthquake, which killed about 3000 people. The relatively minor impact in the face of such a large earthquake raises some interesting points about earthquake physics.
Another recent earthquake of comparable size is the Nepal earthquake of April 2015. This was also a M 7.8 thrust-type earthquake, and it killed nearly 9000 and displaced millions. In a long-past blog post I talked about the different effects of earthquakes in Chile vs Haiti, with a look at the pronounced effect that a country’s resilience has on the outcomes of earthquakes. We don’t need to belabor that point again. Suffice to say that yes, New Zealand’s building stock is generally much more seismically resilient than Nepal’s. But if we set aside the human effects of the two earthquakes and look just at the ground motions, we find that the Nepal earthquake generated much more intense shaking than the New Zealand event.
Looking at the two shakemaps side-by-side, there’s a lot of dark orange-red in the Nepal shakemap on the right, representing MMI VIII-IX, while there’s no real red in the New Zealand shakemap on the left, and barely any deep orange areas. By the numbers, the maximum shaking level on the New Zealand map is about 60% g acceleration and 60 cm/s velocity, but there are several areas on the Nepal map where the observed shaking exceeded 100%, and even at times 120-130% g, and 100-110 cm/s! So yes, Nepal is much more vulnerable and the exposed population in Kathmandu alone (1.4 million) exceeds Wellington and Christchurch combined (about 800,000), but the earthquake itself was much more intense in Nepal than in New Zealand, despite the exact same moment magnitude.
In part, the difference in intensity is due to the depth of the earthquake: Nepal was 8 km deep while New Zealand was 23 km deep. Shallower faulting causes more intense shaking, all else being equal. However, there is another difference worth discussing: the earthquake rupture histories are very different. As mentioned in the discussion of directivity and its effect on ground motion, an earthquake occurs neither as a point in space nor in time. It is a slip event that takes place over a finite surface area, and it takes some time for the slip to occur as the fault “unzips” along the rupture path. That unzipping can occur smoothly or it can occur in fits and starts. In the Nepal earthquake, the slip model for the event (lower figure) shows relatively uniform contours of rupture time and a slip distribution that is smooth. The slip appears to have been mostly over by 50 seconds after the start of the earthquake. By comparison, the slip model for New Zealand (upper figure) shows a lot of unevenness both in the slip distribution and in the rupture speed. Note that the real slip doesn’t start until 50 seconds after the start of the earthquake, and nearly 100 km from the epicenter. So the rupture history for the New Zealand event is much more complicated. The moment release of this earthquake appears to have taken much longer than for the Nepal event. What’s more, the complex rupture might have absorbed more energy into starting and stopping the slip on different parts of the fault. These two effects combine to reduce the radiated seismic energy (which is felt as shaking) for this earthquake.
Note that the slip model for New Zealand is very preliminary and the story might change over the coming weeks as the data is analyzed more carefully. In the meantime though, we should be thankful that New Zealand appears to have largely dodged a bullet today.