May 13 2011 - April 2011 GeoNet roundup

New website facilities

• Read about the most significant, documented, historical tsunami events to have affected New Zealand.

Additions and upgrades to the geophysical monitoring networks

• A continuous GPS site was completed near the southern end of Lake Taupo, at Rihia Road (TGRI).
• The last of the 36 PositioNZ continuous GPS sites was upgraded from a NetRS to a NetR9 receiver. This was the Chatham Island North site (CHTI), and was done in conjunction with maintenance of the tsunami gauge and the replacement of the temporary communications hut with a permanent building.

Earthquakes summary

• The most widely-felt earthquake of the month occurred on Saturday, April 16, magnitude 5.3, 20 km south-east of Christchurch at a depth of 11 km. We have received 2,319 felt earthquake reports to date - further structural damage and liquefaction occurred.
• The preliminary locations of March's earthquakes located by our analyst team are shown below.

Volcano surveillance update and field trips

Summary

• No eruptive volcanic activity occurred at any of New Zealand's active volcanoes during April.
• In response to the heating cycle of Ruapehu's Crater Lake (reaching 41 °C on March 1), high gas fluxes, changes in the lake chemistry and an increase in locatable shallow earthquakes, the Aviation Colour code was changed from Green to Yellow on April 5, indicating elevated unrest. The Volcanic Alert Level was not changed and remained at Level 1.
• Routine gas flights were completed at Ngauruhoe and Ruapehu.
• A 3 monthly Lake Taupo levelling survey was completed, which continued to record low rates of deformation.
• Visits were made to Whale Island to collect water and gas samples from the geothermal features.

Wednesday April 13 - Gas flight over Ruapehu and Ngauruhoe

Conditions were good but high clouds appeared at the end of the flight. The volcanoes were clear of cloud, and the wind was steady at around 6 knots. CO₂ emission at Ruapehu was lower at 606 T/Day. SO₂ emission was low and barely detectable by Cospec.

No gas was detected at Ngauruhoe.

Wednesday April 13 - Ruapehu Crater Lake visit

The installed tiltmeter near the outlet of Ruapehu's Crater Lake.

Ruapehu Crater Lake was visited to take water samples and to install a tiltmeter on the lake shore. The maximum temperature measured was 31.4 °C, the lake was lightly steaming and turquoise in colour and the central vent was upwelling. There was no flow through the outlet.

The tiltmeter was installed on a lava outcrop on the west side of the outlet. The tiltmeter samples at 1 Hz (1 sample per second) and is recorded by a datalogger. One reading every 15 minutes is sent over the Argos satellite system that also collects the lake temperature and level data. We don't expect to see the instrument again until next summer as it will soon be buried by snow!

Reported landslides from media sources

Tuesday April 5

• On-going problems at SH 35, Maraenui Hill, caused a brief road closure of the detour around an earlier large slip.

Tuesday April 26

• A landslide on Rocky Cutting Road at Kaiate Falls restricted traffic.
• Wairaka Road, Whakatane, was blocked by landslips.
• Belk Rd, Omanawa, 10 km south-west of Tauranga, was blocked by landslips.
• There was a slip covering the road at Muriwai Terrace/Wairaka Road, Whakatane.
• Flooding and slips affected many roads near Whitianga, Huntly, Ngaruawahia, Waihi and Matamata. Rocks were reported on the road near Waihi.
• Slips affected the Tahorakuri Forest area near Taupo.
• A slip on the Wellington-Wairarapa railway line near Maymorn station temporarily shut down train services to and from Masterton in the afternoon, and trains were replaced by buses.

Thursday April 28

• SH2 between Napier and Wairoa was closed by multiple slips, and there were many slips throughout the Hawke's Bay district.
• A large slip off Bluff Hill, Napier, at 12:45 am blocked Breakwater Road and eighteen houses in Seapoint and Hornsby Roads were evacuated as a precaution.

Saturday April 30

• A large slip damaged the water supply pipeline for Wairoa; the water supply was restored on Monday May 2.

Felt earthquakes of
March 2011
(372 located).

Shallow earthquakes of
March 2011
(2,465 located).

Deep earthquakes of
March 2011
(401 located).