article: Oct 24 2007 - Previously unknown earthquakes at White Island detected

A new earthquake sensor recently installed at White Island volcano by the GeoNet project has shown previously unknown earthquakes that it is hoped will help to unravel what drives eruptions at the volcano.

An example of a typical earthquake from White Island and the VLP recorded by the new sensor. The wave from the VLP vibrates much more slowly than the typical earthquake.

“We hope that by understanding what causes these earthquakes we can gain insights into what causes eruptions at White Island”, said GNS Science volcano-seismologist Dr Art Jolly.

The earthquakes have a very slow vibration and are called Very Long Period (VLP) earthquakes by scientists. Many earthquake sensors cannot measure such slow vibrations and it has only been since a new sensor was installed at White Island in April that we have begun to record these earthquakes.

Scientists were initially cautious about reading too much into the new observations and put a temporary recorder on the island to see if it could also record the VLP events. “We hit the jackpot with another VLP that was clear on both recorders. That meant we could rule out any kind of problem with the new sensor and we are sure that the signal comes from the volcano”, said Dr Jolly. “The next thing is to plan how we collect enough data to try to model exactly what is producing these earthquakes”, he added.

VLP earthquakes have been recorded at several overseas volcanoes and with the installation of new sensors at some of New Zealand’s volcanoes they have begun to be recorded here too. “These VLPs have been recorded during volcanic eruptions, associated with hydrothermal (hot spring) activity, and with more typical earthquakes. These sensors at Ruapehu caught a VLP earthquake associated with the September 25 eruption”, said an excited Dr Jolly.

GeoNet project director Dr Ken Gledhill said, “One of the real benefits of the GeoNet project is that it can install these new, sensitive instruments. With them, what we learn about our volcanoes will feed directly into assessing the day-to-day hazards. This is exactly what the project is about”.