When a devastating magnitude 8.0 earthquake occurred roughly 200 km south of the Samoan Islands at 17:48:11 UTC on Tuesday September 29, 2009, it generated a trans-oceanic tsunami that spread at jet-like speeds throughout the Pacific Ocean.
At 04:57:33 UTC on 30 September, roughly 11 hours after the earthquake, five of our newly deployed Bottom Pressure Recorders (BPRs) and one pressure sensor installed at the ODP 1027 (Cascadia Basin) CORK began recording the leading waves from this event (click map below to see BPR locations). Capable of resolving sea level displacements as small as 1/10th of a millimeter, the BPRs installed within 15 km of our ODP 1027 (Cascadia Basin) location as part of the NEPTUNE “tsunami meter” provided Canadian researchers with amazingly clean recordings of the tsunami signal as the waves passed over the 2700 m deep Cascadia Basin off the west coast of Vancouver Island. Similar, but somewhat noisier tsunami signals were subsequently observed up to 50 minutes later by BPRs at continental margin sites ODP 889 (Clayoquot Slope), Barkley Canyon, and Folger Passage.
The high resolution NEPTUNE tsunami meter recorded a series of four well defined primary waves (click image below) with decreasing times between successive wave crests of 12 min 50 s, 10 min 54 s, and 9 min 54 s, indicative of wave dispersion effects. Trough-to-crest heights of the primary waves were typically around 4 cm but reached a maximum height of 5 cm during passage of the third wave in the wave train. Wave heights diminished to around 2-3 cm after the leading group had propagated through the array. The tsunami continued to be recorded for the next few days as waves from the event rebounded around in the Pacific basin.
Continuing shoreward, the tsunami waves amplified to around 10-20cm as they crossed the continental shelf and progressed into embayments along the British Columbia coast. Observed wave heights at coastal tide gauges were around 15cm at Bamfield and Tofino, 18cm at Port Alberni, 8cm at Victoria, and 18cm at Henslung on Langara Island in the northern Queen Charlottes. Wave periods in the coastal regions were spread over a range of 5 to 25 minutes owing, in part, to local resonance effects.
Deep ocean data records like these are invaluable for refining models that predict tsunami hazards in coastal areas and for providing early warning assessments. The only negative aspect of our highly successful Samoan tsunami measurements was the greatly reduced capacity of our high resolution tsunami “antenna” in Cascadia Basin due to the missing third (northeastward tending) arm of the array. Weather and technical glitches prevented its deployment this summer (see related blog post). Because the tsunami originated to the southwest, data from the northeast BPR would have greatly increased our ability to accurately define the directionality and transformations of the tsunami waves as they propagated toward the coast. Deployment of the third arm in 2010 will be critical to the long-term success of the Ocean Networks Canada long-wave BPR array.
The above story provided by the NEPTUNE Canada tsunami team: R.E. Thomson (1), M. Heesemann (2), S.M. Mihaly (1), E.E. Davis (2), J. Cherniawsky (1), A.B. Rabinovich (1), and I.V. Fine (1).
- Fisheries and Oceans Canada, Institute of Ocean Sciences, Sidney, B.C.
- Natural Resources Canada, Pacific Geosciences Centre, Sidney, B.C.