Tsunami StudyWhat is a Tsunami?
In the open ocean tsunami waves are generally only a few tens of centimetres high, but as they approach shore they may grow to heights of 10 m or more (Fig. 2, 3). Because of this, they may cause fatalities and destroy property around the entire margin of the ocean basin.
Potential Sources of Tsunamis at the Fraser River Delta
Tsunamis that might reach the Richmond foreshore have three possible sources: (1) great earthquakes beneath the seafloor at subduction zones bordering the Pacific Ocean; (2) smaller, although still large earthquakes at shallow crustal depths beneath the Strait of Georgia or Juan de Fuca Strait; and (3) a large collapse of the front of the Fraser River delta into deep waters of the Strait of Georgia west of Richmond.
Subduction Zone Earthquakes
The tectonic plates that underlie the Pacific Ocean are being subducted beneath continental margins around the perimeter of the ocean basin. At each subduction zone the interface between the converging oceanic and continental plates is locked for long periods of time, and the accumulating strain compresses and deforms the continental margin. The accumulated strain is released every few decades or centuries in great (magnitude (M)=8) plate-boundary earthquakes. During these earthquakes the sea floor above the locked zone abruptly rises, generating a tsunami, and the coastal zone suddenly subsides, with flooding of low-lying areas. After the earthquake the plate interface relocks, and the cycle begins again. The great (M=9.3) earthquake that occurred off the coast of Sumatra in 2004 is an example of this rupture process.
If these model predictions are valid, we conclude that the sea dikes of the southern margin of the Fraser delta may be overtopped by tsunami waves about two hours after a great earthquake if the tsunami arrival coincides with a high tide and a strong onshore wind. At low tide the sea dikes would have more than two metres of freeboard and thus would restrain a tsunami wave. If the sea dikes had been weakened, or had collapsed, as a result of earthquake shaking, they might be locally breached by the tsunami even at low tide.
Other Subduction Zones Around the Pacific Rim
Of the subduction zones that surround the North Pacific, only the Alaska-Aleutian margin represents a significant tsunami threat to the west coast of Canada. Great earthquakes have ruptured this subduction zone six times in the last 4000 years. On the last occasion (27 March 1964; M = 9.2) tsunami waves up to six metres high devastated several communities on the outer coast of Vancouver Island. The islands at the north and south ends of the Strait of Georgia, however, effectively blocked the tsunami from reaching inland waters. At the east end of Juan de Fuca Strait, the largest wave from the 1964 Alaska tsunami was one to two metres high, whereas in the southern Strait of Georgia it diminished to less than 0.5 metre. We conclude therefore that tsunamis triggered by distant plate-boundary earthquakes do not constitute a significant source of hazard at the Fraser River delta.
Local Earthquakes at Shallow Crustal Depths
Earthquakes on faults within the North America plate represent an additional tsunami hazard to coastal communities of the Pacific Northwest. Faults in central and northern Puget Sound are known to have ruptured in large (M 7) earthquakes in the last few millennia. Some low-lying areas around Puget Sound were flooded by the tsunamis generated by these earthquakes. The narrow, winding passages of this inland sea, however, cause rapid loss of tsunami wave energy, and we consider it highly unlikely that tsunamis generated by submarine earthquakes in Puget Sound or Juan de Fuca Strait have ever inundated the lowlands of the Fraser River delta.
What is less certain, however, is the tsunami potential posed by submarine faults beneath the Strait of Georgia? If any of these faults are active, and if the seafloor is displaced during a future earthquake, then the ensuing tsunami would likely present a hazard to coastal areas in the Strait that are orientated parallel to the fault zone. The southern foreshore of the Fraser River delta is therefore likely at greater risk than the western foreshore from seismic tsunamis generated by east-west trending fault zones beneath the southern Strait of Georgia.
Collapse of the Front of the Fraser River Delta
A strong local earthquake could conceivable cause a landslide at the front of the Fraser River delta, where it slopes into deep waters of the Strait of Georgia. The landslide, if large and sudden enough, might generate a tsunami. The unconsolidated sediments forming the delta front may also fail without a seismic trigger, and yet still produce a tsunami.
Landslide-induced tsunamis are particularly dangerous because the waves may locally be very large and the warning time very short. For example, in November 1994 a submarine slide in Taiya Inlet created a wave that reached a height of nine to eleven metres at the shoreline in Skagway, Alaska, causing one fatality and over $20 million of damage.
It has long been recognized that the western front of the Fraser delta is at risk from submarine landslides. The Fraser River discharges about 17 million tons of sediment into the Strait of Georgia each year, and much of this sediment accumulates on the steep frontal slope of the delta. Small slides are common in this unconsolidated material; however, these slides are shallow-seated and move down the delta front over a period of hours, consequently they do not produce tsunami waves.
Researchers have investigated the ability of large submarine slope failure at the southwestern delta front. They conclude that a large slide could generate tsunami waves up to 18 m high on the east shores of Galiano and Mayne Island, but that the tidal flats of the delta foreshore would deflect and reduce wave energy, meaning that waves at the shoreline in Tsawwassen would likely not exceed two metres.
Documenting Past Tsunamis from the Geological Record
Has the Richmond area ever experienced a catastrophic tsunami? No tsunami has affected this area in the historic period (the last 100 years), but what about further back in time. Scientists can answer this question by searching for telltale tsunami deposits in the sediments of the Fraser River delta.
- widespread in areas near the shore,
- distinctly different in character from the surrounding material and have a sharp base,
- thin landward (because the wave loses energy and drops the sediment it carries),
- fine landward (e.g. pebbly sand becomes sandy mud), and
- contain the remains of marine organisms not found in the surrounding material.