article: Nov 4 2005 - Marlborough Earthquake Swarm Near Seddon

Several sharp earthquakes occurred on Tuesday November 1, 10 km east of Seddon in the Marlborough region of the northern South Island.

Three occurred within 40 minutes of each other in the late morning, with the largest having a magnitude of 4.6. A report of goods falling off shelves was received from Blenheim, and a report from a farm near Ward of cupboard doors being thrown open.

A further tremor occurred in the early evening, and was the largest earthquake so far at magnitude 4.8. Two reports from Seddon were received, noting that a photo frame had fallen off a wall, the glass in a wall unit had smashed, and many items had toppled from shelves. People and animals were alarmed. Several more reports of items falling off shelves and objects sliding slightly were received from the Blenheim area.

The accompanying map shows the levels of shaking as measured by GeoNet seismographs throughout the region for the evening earthquake. The colours are coded to approximate the shaking effects described by the Modified Mercalli Intensity scale. Our seismograph station near Cape Campbell recorded the highest shaking. (General information about this site may be found by searching the Seismic Station section of the DELTA database and entering CMWZ for the station code.)

Since Tuesday, a total of eight earthquakes in the tight cluster east of Seddon have been felt. At 2:16 am this morning another magnitude 4.8 earthquake occurred, but was located off the tip of Cape Campbell and at the greater depth of 15 km. This earthquake is likely to be in response to the activity near Seddon, but is not part of that localised swarm.

Similar earthquakes have occurred in this region on June 30 2003 (magnitude 4.8) and March 17 2004 (magnitude 4.5). However, both these earthquakes were located deeper than the current swarm, at about 15 to 20 km below the surface. The swarm earthquakes have been placed at 5 km depth, which for a magnitude 4.8 earthquake is potentially damaging near the epicentre. Whilst we cannot say exactly when these earthquakes will stop, past records show that similar sized and smaller earthquakes may continue to occur in the region for several more days or a few weeks.