Wild Land

A pair of magnitude 7 earthquakes shook Hawke’s Bay during February 1931. The first was a magnitude 7.8 quake that struck on Tuesday 3rd February near the shoreline 8 to 24 kilometres north of Napier.

Writing in The New Zealand Journal of Science and Technology, July 1933, F.R. Callaghan of the DSIR described the earthquake as follows: “The consensus of opinion is that there were two shocks of very different characteristics, occurring within a brief interval of some thirty seconds of one another. The first shock developed rapidly in intensity, had a distinctly uplifting motion associated with violent and confused swaying. Then followed a pause of about half a minute, till the second shock occurred, with a motion resembling a sharp bump downwards.” Callaghan quoted the radio operator of the m.v. “Taranaki” as stating “… the tremors continued for two and a half minutes from the time of the first noticeable shock.”

The earthquake was felt throughout New Zealand with the exception of Northland and Otago. In Hawke’s Bay, the main earthquake was followed by a brief period of quiet, but before long tremors commenced and continued for some days. Official records recorded 151 aftershocks that day, with 55 on the 4th, 50 on the 5th, 29 on the 6th and 24 on the 7th. The number of aftershocks then eased to less than 20 per day.

However, the rapid decline of the aftershocks was not to last. At about 1.30 p.m. on Friday 13th February another huge earthquake shook the Hawke’s Bay area. This event was officially set at magnitude 7.3 and centred about 50 km east of the earthquake of the 3rd.

In his book “Quake Hawke’s Bay 1931” Matthew Wright reports that this second event was New Zealand’s fourth strongest recorded earthquake. He writes, “Power failed three seconds before it was felt in Napier. People from Napier to Dannevirke ran for their lives as damaged buildings cracked and fell.”

He adds, “Some inland parts of Hawke’s Bay felt this aftershock more strongly than the 3 February quake. In Taupo, goods were thrown from shop shelves, but ‘there was no damage of any moment’. People rushed into the streets in Dannevirke and Masterton. In Wellington all but one of the clocks stopped in the Dominion Observatory, and ceiling lights in the Evening Post offices swayed ‘more vigorously’ than they had the week before.”

The earthquake of February 13th 1931 is widely regarded as an aftershock of the larger event ten days earlier. But Messrs Adams, Barnett and Hayes commenting on the rapid decline in the frequency of aftershocks in the Journal of Science & Technology stated, “The fresh outbreak on the 13th February, due to the severe shock on that date, may almost be regarded as a separate disturbance, although it probably arose from conditions produced by the original shock on the 3rd.” Earthquake counts shot up to 81 on the 13th, before dropping to 23 on the 14th, 18 on the 15th, 19 on the 16th and then slowly dropping away.

In all, 597 earthquakes were recorded at Hastings during February 1931.

The next event of note was to occur further north on February 17th.

Geonet, the USGS (NEIC) and IGNS reported 35 earthquakes in the New Zealand area between the Kermadec Islands in the north, and the Auckland Islands to the south during January 2006. The magnitude distributions were as follows:
M5 to 5.9 (5), M4 to 4.9 (21) M3 to 3.9 (9).
An additional 3 events in the magnitude 2 range were deemed worthy of mention.

Seismological activity was noticeably quieter in January, compared with December. There were no distinct earthquake swarms, but the interesting sequence of deep earthquakes near the Bay of Plenty continued, as did the cluster of deep earthquakes further north in the Kermadec Islands.

The sequence of deep earthquakes offshore to the north of Rotorua and Gisborne continued with two distinct phases. The first phase involved a series of 7 earthquakes migrating southward along a line from 190 km north-east of Rotorua to 70 km south-west of the city between the 2nd and the 17th. As the epicentres progressed southward, the depths increased from 120 km to 280 km on the 15th, and then began rising to 157 km. One anomalous event defied this pattern at 12 km depth on the 5th.

The second phase of the sequence commenced activity on the 22nd at 190 km north of Rotorua but lacked the steady southward trend of the first phase. Epicentres in this series of 5 earthquakes ranged between 185 km north-east and 50 km south-west of Rotorua and depths ranged between 33 km and 252 km.

Earthquakes in both phases of the sequence ranged in magnitude between 5.5 and 4.0. The final event in the sequence was a magnitude 5.5 quake at 170 km depth located 30 km north-west of Taupo on the 28th. This earthquake was widely felt because of its magnitude and depth, and Geonet advise that it was felt from diverse locations in Hawkes Bay, Manawatu, Kapiti Coast and Wellington.

Activity in the cluster of earthquakes in the Kermadec Islands eased from its January peak, with 5 events between magnitude 4.5 and 5.2 occurring between the 17th and 30th. A subset of 3 events occurred between 150 km and 180 km north-east of L’Esperance Rock with magnitudes between 4.9 and 5.2 and depths between 10 km and 43 km.

A cluster of 3 earthquakes in southern Hawkes Bay occurred on the 7th, 23rd and 28th of January. Magnitudes ranged between 3.6 and 4.4 and depths between 20 km and 50 km. The magnitude 4.4 event on the 23rd which was located 10 km north of Waipukurau was one of the month’s 3 damage-causing earthquakes, with minor damage being noted by Geonet.

The other notable sequence of earthquakes was located within 5 km of Upper Hutt. When looked at in isolation, the pair of earthquakes seem to be a classic foreshock and mainshock pair. The first event was magnitude 3.3, 30 km deep on the 1st. The second was magnitude 4.3, 35 km deep on the 13th. However, these seem to be members of a long-term swarm that has been happening near Upper Hutt at various intervals since April 2004.

Only minor damage resulted from the three damaging earthquakes during January according to Geonet. They were:
January 7th, mag. 4.1, 30 km deep, 10 km west of Porirua. Items off shelves.
January 23rd, mag. 4.4, 20 km deep, 10 km north of Waipukurau. Unspecified minor damage.
January 29th, mag. 3.9, 5 km deep, 20 km east of Arthurs Pass. Unspecified minor damage.

Vulcanologists report that activity at the nation’s volcanoes is little changed from December. Their status can be summarised as follows:
Mt Ruapehu (Alert Level 1).
White Island (Alert Level 1).
Mt Tongariro (Alert Level 0).

Mt Ruapehu’s crater lake cooled to 28° C during the first half of the month, with the lake level rising slightly due to snow melt. Seismic activity remained at a low level.

White Island’s crater lake dropped slightly during the month, and seismic activity remained low. Surface features such as small collapse holes, mud pots and dry areas developed during January. Sulphur dioxide emissions increased slightly.

The numerous small earthquakes beneath Mt Tongariro continued at the increased level observed in recent months.

Some Tawa residents felt the small earthquake that occurred 10 km north-west of Porirua this morning. The magnitude 3.3 quake occurred at a depth of 40 km at 1:01 a.m. Today’s earthquake is the latest in a series of small events that have been occuring near Porirua since December.

The first event in the series was a magnitude 4.6 quake at 30 km depth, 10 km north-east of Porirua on December 13th at 9:09 p.m. This event was felt from Wanganui to Wellington, and in Wairarapa and Marlborough. Geonet reported that items were thrown from shelves around the Wellington region.

On January 6th, a magnitude 4.1 earthquake at 30 km depth centred 10 km west of Porirua was felt from Levin to Wellington and in Marlborough at 5:24 a.m. Geonet reported that, once again, items were reported to have tumbled from shelves in the Wellington region.

On February 1st, a magnitude 3.7 earthquake at 40 km depth centred 10 km west of Porirua was felt at Linden (near Tawa) and Titahi Bay at 10:01 a.m. Geonet did not report any damage from this event.

Earthquakes have also occurred nearby at Upper Hutt, Waikanae, further west near Farewell Spit, as well as near Seddon and the Marlborough Sounds during January and February.