Ground-shaking News

By Martin Scherwath

Exactly one week after the Alaska earthquake it's happening right outside our front door: An earthquake, initially estimated magnitude 6.7, later downgraded to 6.4, occurred just offshore of Vancouver Island, 20 km from Nootka Island. This location is very close to the Nootka Fault Zone where we have the greatest offshore seismicity, right between the Juan de Fuca and the Explorer tectonic plates. So, September 9, 2011 will certainly go into the local seismology history book as a big one, and we expect many immediate aftershocks around this location in the near future.

NEPTUNE Canada's main seismometers at sites ODP889 and ODP1027 show this earthquake again very clearly, but also bottom pressure recorders (BPR) show strong signals. Below are the two seismograms and one BPR record, showing ten minutes of data:

NEPTUNE Canada data specialist Dilumie scanned more stations with a pressure response to the earthquake and found that our Piezometer (at our ODP 1027 location), newly installed in July exhibits a great response, changing with depth. In this plot, we can see three sensors measuring differential pressure at the seafloor and depths of 1.6 and 3.2 metres.

And all this happens while NEPTUNE Canada is mobilizing for our September 2011 maintenance & installation cruise.

Busy times!

Alaskan Earthquake: 2 Sept 2011

Earthquake!

On Friday, September 2, 2011, 10:55 UTC (03:56 AM PDT), a relatively large magnitude 6.8 subduction thrust earthquake occurred in Alaska's Fox Islands (part of the Aleutian Island chain). (USGS information on this earthquake.) Initially, the magnitude was estimated as 7.1 which immediately triggered a tsunami evaluation for the NE Pacific with a wave height up to 50cm locally. As more seismic data became available, the earthquake magnitude estimate was downgraded and the tsunami warning was cancelled. Nevertheless, This large earthquake left clear signals on the NEPTUNE Canada broadband seismometers at nodes 1027 and 889:

Our seismic data show clearly how the initial P-wave energy arrived approximately 6 minutes after the earthquake, and the P-wave energy subsided after about 2 minutes. Approximately 12 minutes after the earthquake, S-waves appeared, and another two minutes later, the very large amplitude, low frequency surface waves were recorded.

At the time of this analysis, only 1 Hz sampled data were available, but we hope to obtain higher-resolution data for analysis later. The Canadian and US Navies sporadically embargo our high-resolution seismological data for security purposes, but we can often obtain the data after an embargo is lifted.

Tsunami! (not)

As for a tsunami, the initial forecast was 2-4cm for the coast of Vancouver Island arriving at approximately 15:30 UTC (08:30 PDT) but was later downgraded. Data inspection of our bottom pressure recorders showed that no clear signals from the tsunami stand above the background signals (storm waves and tides), so the actual tsunami height must have been quite small, perhaps only a couple of millimetres.