Tsunamis are really complex things to model. Our highly skilled tsunami scientists use two distinctly different types of data to analyse tsunami threats to New Zealand after a large earthquake in the Pacific Ocean and to predict if and when tsunami waves are likely to arrive.

These two types of data are available at different times during the response to an earthquake.

The first data source available to rapidly assess the tsunami potential of an earthquake is seismic data - the recordings of earthquake ground shaking. These tell us preliminary information about the earthquake source and are typically the only data available for the first hour or so following a major earthquake.

After last night's Mexico Earthquake, based on early seismic data, both the Pacific Tsunami Warning Center (PTWC) and the US Geological Survey’s National Earthquake Information Center (NEIC) determined the earthquake was M8.0, which is too low to generate significant impacts in New Zealand.

The NEIC’s first, and current, earthquake location was deep within the earth, on a fault that is incapable of generating a tsunami. We of course now know that a tsunami was generated but based on the first estimates of magnitude a significant tsunami was considered to be unlikely. The PTWC subsequently upgraded the magnitude to M8.2, which in that location is large enough to potentially be of concern to marine and beach areas in New Zealand.

The second type of data available is a direct measure of the tsunami once it has been generated. This can be either near-shore tide-gauge measurements or open-ocean DART gauges that directly measure water height. These are the best data with which to analyse the tsunami hazard, but we receive this data only when the tsunami has reached the places where the gauges are located. Three DART stations captured last night’s tsunami within the first ~3 hours after the earthquake, almost immediately after the waves travelled past them. Once these data were available, models of predicted wave heights in New Zealand were rapidly generated and calibrated against these data.

These model predictions proved to be very close to the waves we measured arriving on New Zealand shores today, and our predicted timing was spot on.

As last night's Mexico quake was a distant event with 12+ hours before potential tsunami impact in New Zealand, we were able to use this more accurate but later-arriving dataset to generate tsunami forecasts. But this explains why alert levels were updated and changed over time.

Only messages issued by the Ministry of Civil Defence & Emergency Management represent the official warning status for New Zealand.