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Latest E. Coli reading, 07 February 2023: 389 (orange).

We’re all moving east round here

Map with arrows showing we are moving east.

We’re moving east — about 4 cm so far this year. That’s thanks to a slow-slip earthquake on the Hikurangi subduction plate boundary offshore the North Island’s east coast.

We tend to think of earthquakes as short sharp shocks, but:

slow slip events are like earthquakes in slow motion, unfolding over weeks to months and cannot be felt by humans.

The Hikurangi subduction zone – the boundary between the Australian and Pacific Plates – extends along the east coast and dives westward underneath the North Island. We frequently have slow-slip events on the Hikurangi subduction zone.

When slow-slip earthquakes occur we often see an increase in the number of small earthquakes in the same region. This is because the slow slip event produces changes in stress in the earth’s crust, causing some nearby faults to rupture in small earthquakes.

Previous Manawatu slow slip events typically last 1-2 years and can involve up to 30 centimetres of movement along the plate boundary, so we expect this one to continue well into 2023.

Watch the brief but interesting video at the page linked above to learn more about earthquakes and the Hikurangi Subduction Zone beneath our feet.

This item was updated on 22 January 2023 08:12:18