Massive earthquake struck off the coast of New Zealand

A M 7.4 earthquake struck off the coast of New Zealand raising concerns about a potential tsunami.

The seismic shock occurred on June 18, 2020, at 12:49 am local time. Its epicenter was located about 650 kilometers northeast of Bay of Plenty and south of the Kermadec Islands in New Zealand at a depth of around 30 kilometers. According to the United States Geological Survey (USGS), the tremblor occurred on a strike-slip fault near a trench where the Pacific plate is moving under the Australian plate at a rate of 5,5 centimeters per year.

This area, which is known as the Tonga-Kermadec subduction zone, is an earthquake-prone region that has experienced more than 20 large earthquakes (>M 6.5) in 50 years, however, similar massive shocks have recently occurred only once (in May 2006).

Authorities immediately issued an alert about a potential tsunami that could be caused by the massive shock, however, the announcement was later revoked. Nevertheless, New Zealand's Emergency Management Agency stated that abnormal currents may strike some coastal areas, therefore, people should avoid local beaches. 

According to New Zealand's GeoNet, a project operating a state-of-the-art earthquake monitoring network in the country, the impact of the earthquake was minor and the tremblor was characterized as "weak". This fact is quite strange (given the magnitude of the shock) but it is related to the large distance between the earthquake and the inhabited regions.

"Why was the earthquake called ‘weak’ when so many people felt it? This is due to its distance from New Zealand 700km north-east of Gisborne. Our shaking intensity scale gives the impact of an earthquake on people living on the earth's surface, instead of magnitude which is the energy released during an earthquake, and so it can be more useful as an indicator of the earthquake's significance to the community," GeoNet, reports.

The seismic shock was felt by around 9,000 people that reported weak-to-moderate shaking. There are still no reports about damage caused in infrastructure.

A major factor that prevented a tsunami event, was the focal mechanism of the earthquake. Strike-slip faults generate a lateral displacement that does not highly affect the seafloor and thus, it does not destabilize the equilibrium of the water. On the contrary, earthquakes of that magnitude on normal or reverse faults may cause devastating tsunamis.

NZ Earthquake Commission suggests that, in the worst-case tsunami scenario, waves as high as 12 meters could hit the coastal areas of New Zealand. In that case, prediction models determined that the fatalities and injuries would reach 60,000 while the potential economic loss was calculated at $45 billion. A crucial factor that would lead to those casualties is the fact that the tsunami would hit the vulnerable areas within an hour.