Some disturbance of the sea on the New Zealand coast resulted from yesterday’s great quake near Iquique, Chile.
Thursday, 3rd April 2014
As expected, the tsunami generated by yesterday’s magnitude 8.2 earthquake centred off the coast of Tarapaca, Chile resulted in minor disturbance of the sea in some New Zealand coastal areas.
The tsunami reached the Chatham Islands at about 3 o’clock this morning where it caused half-metre changes in sea level, fourteen hours after the earthquake occurred. The disturbance began at Gisborne and Castlepoint less than 2 hours later, with sea level changes at Gisborne of about 20 cm.
[click for larger image] This snapshot of GeoNet’s New Zealand tsunami gauges taken at 4:30 this afternoon shows sea level changing from normal about twelve hours before the snapshot was taken. These changes are not twenty centimetre waves that swimmers would experience while frolicking in the shallows, they are abrupt changes in sea level – a much more powerful thing to experience.
[click for larger image] This snapshot of GeoNet’s tsunami gauge network taken a few hours later (just before 9 p.m), shows that small changes are occurring at more sites in New Zealand, but the changes at Gisborne, though minor, are more pronounced.
When a massive magnitude 9.5 earthquake struck off the Chilean coast in 1960, both Gisborne and Lyttelton experienced significant tsunami waves, and the disturbance at Gisborne continued for days.
Warning systems nowadays take this historical information into account, and estimate likely risk based on past experience.
Following yesterday’s great quake, the Pacific Tsunami Warning Centre issued an animation showing how it expected the tsunami waves to propagate across the Pacific Ocean. The arrival times, calculated from past experience, were remarkably accurate. The animation can be found here.
If readers still think that tsunami waves are simply like big waves at the beach, there is a simple but effective animation on the PTWC page that shows how a tsunami is generated by an earthquake, and how the wave behaves when it reaches land. This animation can be viewed here.