Earthquakes near Upper Hutt

For the second time in a week, many residents of the Wellington region were woken early this morning by a local earthquake rattling doors and windows. Whilst last Saturday’s magnitude 4.1 earthquake was centred near Porirua, this morning’s was further east and shallower. The magnitude 4.3 earthquake, which struck at 4:12 a.m., was 25 km deep and had an epicentre within 5km of Upper Hutt.

Geonet report that the earthquake was widely felt in the Wellington region, but no damage reports have been mentioned by local news media.

A magnitude 3.3 earthquake early on January 1st was also located within 5 km of Upper Hutt, but slightly deeper at 30 km. This event may have been a foreshock for today’s earthquake or a member of a swarm, it is too early to tell.

Last year, the Upper Hutt area hosted a series of 5 earthquakes which commenced on January 21st with a magnitude 5.5 event at 7:56 a.m. It was followed 6 minutes later by a magnitude 3.7 and 25 minutes later by a magnitude 3.6 quake. The following evening a magnitude 4.2 quake occurred followed by a magnitude 3.6 quake on the following afternoon. This turned out to be a standard earthquake and aftershock sequence, with minor damage being reported throughout the Wellington region.

The series of quakes near Upper Hutt occurred during a lengthy swarm of 7 offshore quakes near Martinborough which ran between January 18th and February 1st 2005. The largest event, a magnitude 5.4, occurred at the end of the sequence. This swarm of quakes also caused minor damage, and interrupted elevator and electrical services in Lower Hutt.

A year earlier, in 2004, Upper Hutt experienced two swarms of earthquakes in short succession between April 3rd and May 10th. The first swarm of 8 earthquakes occurred within 1 hour and 17 minutes just after midnight on April 3rd. The larger events detailed by Geonet were a magnitude 3.9 at 3 minutes past midnight, 6 minutes later a mag. 4.6, 1 minute later a mag 4.6, 36 minutes later a mag. 3.4, and 34 minutes later a mag 3.8 – all within 5 km of Upper Hutt. Seismologists described the series as a foreshock/aftershock sequence. Activity eased, with a mag 3.5 occurring late on the evening of the 4th.

On April 17th a magnitude 3.7 quake shook Upper Hutt and all was again quiet until April 26th when a magnitude 3.8 earthquake occurred. Another burst of activity occurred within 5 km of Upper Hutt on May 9th with a magnitude 4.1 at 7:31 a.m. followed by another mag. 4.1 39 minutes later. A magnitude 3.4 earthquake was felt 14 minutes after that. The final earthquake in the sequence was magnitude 3.3 on May 10th 2004.

On May 26th 2004, the Institute for Geological and Nuclear Sciences issued a fascinating media release about the Upper Hutt earthquake sequences, and an unusual land surface movement detected by GPS (Global Positioning System) in the hills behind Paekakariki. Analysis of GPS data showed that the steady westward movement of the Kapiti Coast had slowed from 25mm per year to 15 mm per year between May 2003 and May 2004. It was suggested that the interface between the Australian and Pacific tectonic plates had slipped about half a metre underneath the Kapiti Coast, increasing the tectonic stress under Upper Hutt, and thereby triggering the earthquake swarms.

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