Hawke’s Bay Cleans Up

With the coming of daylight, residents and businesses in Hawke’s Bay on the east coast of New Zealand’s North Island have started cleaning up after last night’s magnitude 5.9 earthquake. Reports suggest that infrastructure has escaped the damage which seems to be limited to contents of houses and shops, broken windows and cracked plaster.

Only four minor aftershocks have been recorded overnight.

Many Hawke’s Bay residents were woken by a shallow magnitude 5.9 earthquake located 10 km south-west of Hastings last night. The 30 km-deep quake struck at 11:25 p.m. on Monday the 25th of August 2008, with some people reporting shaking that continued for nearly a minute.

Four aftershocks have been recorded by seismic instruments since the main event. Smaller quakes at 11:30 p.m. and 11:34 p.m. (not yet analysed in the GeoNet database) were followed by a magnitude 3.5 quake at a depth of 40 km at 11:35 p.m. A magnitude 3.4 quake then struck at 2:16 a.m. on the 26th slightly further south at a depth of 45 km.

A rule of thumb is that many earthquakes are followed by an aftershock an order of magnitude lower than the main event. This means that an aftershock of about magnitude 4.9 is possible if the sequence follows this rule, but it is by no means certain. The aftershock sequence has rapidly abated and the area has been relatively quiet in the 10 hours since the last reported aftershock.

Two other earthquakes have rattled the area this month. A magnitude 3.0 quake at a depth of 25 km woke some sleepers on the morning of Sunday the 24th. This quake was located 20 km north-east of Porangahau, 60 km south of Hastings. A magnitude 3.4 quake 50 km deep struck 20 km south-east of Hastings on Wednesday 13th.

A cluster of 5 earthquakes with magnitudes between 3.2 and 4.3 struck within 20 km of Hastings during July. All of the quakes were at depths of 30 to 40 km.

June was quiet, but two quakes with magnitudes of 3.3 and 4.5 struck to the east of Napier during May. During April a quake of magnitude 3.8 struck east of Napier and two quakes with magnitudes 3.0 and 3.7 struck 30 and 50 km south of Hastings.

Five quakes with magnitudes between 3.5 and 4.1 struck near Porangahau, Hastings and Napier during March. Only one quake was reported in the area during February, but four events with magnitudes between 3.4 and 3.9 were reported during January.

By midday today (Tuesday) more than 950 reports had been filed with GeoNet by people who felt the main quake. Damaging (MM7) reports were filed from Greenmeadows, Hastings, Marewa, Mayfair and Taradale, while 85 responses reported slight damage classified as 6 on the MM scale. A further 320 responses report minor damage equivalent to MM5 shaking. An explanation of the Modified Mercalli Intensity Scale, which is used to classify earthquake shaking and damage, can be found here.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST.]

Leave a Reply