Scientists assessing what happened during the magnitude 7.8 Kaikoura earthquake and what will happen next are working around the clock analysing data from a range of sources explains Victoria University of Wellington geophysicist Associate Professor JOHN TOWNEND.

The challenge for geoscientists in the aftermath of a large earthquake is determining where aftershocks are likely to occur, how often, and what their effects will be. To do this, scientists need to know what has happened, what is happening now, and what past experience of aftershock sequences tells us is likely to happen in coming days, weeks, or months.

Working out what is happening is a team effort making use of many different types of observation and scientific expertise.

GNS SCIENCE A GNS Science technician services a solar-powered GPS monitoring station at Gunn Ridge in Fiordland National Park. The instrument is one of about 600 in the GeoNet nationwide network of instruments.

Every second, data from more than 450 seismometers and 200 global positioning system (GPS) instruments spanning the country pour into GeoNet's computers. Something like 70,000 pieces of data are analysed in near real-time every second by an automatic system that detects the seismic waves produced by earthquakes and computes each earthquake's location and size.

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The duty seismologist will review any automatic results indicating a large earthquake has occurred, and if necessary will manually analyse data from additional seismometers in order to refine the automatic solution.

Seismologists will also scrutinise data transmitted by tsunami/tide gauges and liaise with the Ministry of Civil Defence and Emergency Management, which will issue tsunami warnings if necessary. Separately, the levels of ground-shaking recorded at sites throughout New Zealand are also recorded, forming the basis for engineering assessments of buildings and other infrastructure in the event of strong shaking.

Following a large earthquake, GeoNet's instrumental data are augmented with other types of observations. Field parties map the location and amount of fault slip that has occurred, and the locations and scales of landslides. Already, 80,000–100,000 landslides are estimated to have occurred following the Kaikoura earthquake, and some of these pose significant hazards in their own right having formed temporary dams across rivers.

SVEN BOS Slips being cleared in the area between Oaro and Goose Bay.

Determining what has happened involves a combination of painstaking fieldwork in often arduous conditions, and extensive computer analysis of aerial and satellite imagery. In conjunction with the data from GeoNet's other sensors, these measurements reveal where the ground has moved and which parts of the country have been strongly shaken.

As soon as possible, geodesists begin analysing data from permanent GPS recorders and extra instruments deployed by field teams around the faults that have ruptured. GPS recordings made during an earthquake show how the ground has moved, in three dimensions, and are used in conjunction with seismic data, field observations and satellite imagery to delineate the faults that have slipped.

Before and after images of the Marlborough region acquired along different orbits by European and Japanese satellites have already yielded astonishing pictures of ground deformation and show that the Kaikoura earthquake involved slip on multiple separate faults.

ROSS GIBLIN/FAIRFAX NZ Massive slips cover SH1 and the rail line north of Kaikoura after the 7.8 earthquake. The highway is somewhere under the slip. `

The radar observations and global seismological measurements epitomise the international scientific response elicited by a large earthquake.

In addition to the contributions made from afar, scientists from the United States, United Kingdom, Japan and elsewhere are arriving in New Zealand to contribute to field effort and analysis. Geotechnical experts from the US are assisting with mapping and interpreting landslides and other damage, while US and Japanese seismologists work with New Zealand counterparts to deploy extra seismometers across Marlborough and the southern North Island to record aftershock activity.

Evolving models of where and how much slip has occurred during a large earthquake are used from day one to investigate how other faults have been loaded or unloaded. Just as one person getting off a crowded bus will cause other passengers to shuffle about and find more comfortable positions, so slip on a fault causes stress to be distributed in the surrounding area.

This stress redistribution process increases the stress on some faults, bringing them closer to the point of failure, and reduces stress on others. An earthquake the size of the Kaikoura event causes stress changes over a large region—in this case, much of the northern South Island and Cook Strait—and an immediate priority for scientists is working out how other large faults have been affected.

Meanwhile, it's also important to understand the hazard posed by ongoing aftershocks. Seismologists are often quoted as saying that the number of earthquakes of a particular size in a region decreases by a factor of ten for every one magnitude unit increase in size.

In other words, for every ten magnitude 4 earthquakes, scientists expect to record approximately one magnitude 5. This rule of thumb, known as the Gutenberg-Richter relationship, holds well for earthquakes in most parts of the world, and enables an estimate to be made quite quickly of how many aftershocks of a particular size are anticipated following a large earthquake.

Knowing what earthquakes to expect is only part of the challenge, however. What is of more importance to engineers and civil defence authorities, and to the public at large, is an assessment of the amount of shaking different parts of the country may experience. To calculate this, seismologists combine the aftershock probabilities with models describing the decay in shaking with distance from an earthquake (attenuation) and the effects of different soil types and geographic features (amplification).

GeoNet uses a combination of probabilistic earthquake forecasts and quantitative assessment of specific scenarios to communicate the evolving hazard posed by aftershocks and other possibly large earthquakes. It has to be remembered that these assessments are made objectively on the basis of a huge range of instrumental observations and past experience of New Zealand earthquakes.

However, complexities of the earthquake source, the ground deformation, and the ensuing patterns of aftershocks make it impossible to know exactly what will eventuate.