Manitoba's earthquakes & tsunamis

Everyone has heard of earthquakes that happen in far-away places and, since December of 2004, of tsunamis as well. In my earth science classes I used to bring a poster that scared a lot of people. It had a faked tsunami on it, but it looked real. It was designed to scare Californians lying worriless on sandy beaches. So, why did I bother bringing it around?  I know this phenomenon is very unpredictable, can be deadly, but it doesn’t happen very often. So, for people who travel extensively today, one has to be aware. Unfortunately, tsunamis can happen anywhere along the shore of an ocean or lake and anytime. Well, we learned that lesson well at a high human cost.

Manitoba is in the middle of the continent, so it must be far away from events like that. Well it is not! And the ocean, Hudson Bay, is not far away. 

Until recently, Manitoba was the only province / territory in Canada not affected by these natural phenomena. However, things changed when water reservoirs of the so-called megadams were built in the lower Nelson River for hydroelectric development. We have all benefited by having an ample source of electricity. However, there are communities that may be in danger from the earthquakes and tsunamis, if they have not been affected already. By the way, at least one BU student comes from one of these communities on the Ontario coast of Hudson Bay.

Three major dams were built in the lower Nelson River. Their water reservoirs are huge lakes, almost joined together, occupying an area about 60 km long and up to 10 km wide.

The weight of the water in the newly formed lakes creates a stress on the ground. This in turn gives rise to re-adjustments in the crust, or earthquakes. This phenomenon is not new.

RIS is a relatively new phenomenon, but already there are more than 100 reported cases, six classic cases, and several international symposiums have been held to discuss the effects of reservoirs (Canadian Induced Seismicity Research Group). One study in 1977 (Parker et al) found 16 “accepted” and 35 “questionable” cases of RIS.  Most reservoirs are built along valleys and valleys usually form over weaknesses in the rocks below. The additional weight of water in a reservoir increases the stress below and the groundwater pore pressure and this results in a reduction of the rock strength leading to failure.

Some reservoirs produce earthquakes immediately upon filling (due to elastic changes), some after a delay (a result of pore fluid diffusion), and some after several years (USGS  Auburn Project Review Team, 1996).

These induced earthquakes can be triggered by very small stress changes even as small as the pressure of a car tire, some 1-3 bars. In a recent study by (Bevis, M., Geophysical Research letters, 2005) GPS monitoring of the level of water in the Amazon recorded an amazing vertical displacement of 5-7.5 cm over the course of a year. In other words, the changes in the weight of the river resulted in a change in elevation of the ground. This is a very elastic Earth ! The investigators concluded that the Earth behaves like a big bathroom scale.

The Paleozoic rocks in northern Manitoba are flat-lying and occupy a Basin overlying the Precambrian basement which is much older in age. The thickness of this Basin increases dramatically from the southern shore of Hudson Bay towards the north (GSC paper 68-53). Thus, the boundary between Paleozoic and Precambrian dips steeply under the floor of Hudson Bay. This boundary along the shores of Hudson Bay and possible cross structures appear to be where the earthquakes are taking place. There are 3 separate areas of earthquake epicentres and they coincide offshore from two permanent communities in Ontario (Fort Severn and Winisk) and one seasonal tourist settlement at York Factory in Manitoba.

In recent years flooding have affected the permanent communities with one fatality. The cause of the flooding appears to have been excessive water flowing into Hudson Bay (K. Harper, class presentation).

Table 1 shows no earthquakes in this area prior to dam / reservoir construction. The first reservoir was followed immediately by a group of earthquakes not close by, but along the shores of Hudson Bay several hundred kilometres away. The more reservoirs, the more earthquakes with epicentres moving further away.

Mega-Dam	TIME PERIOD	PERIOD OF QUAKES	#    OF  QUAKES	MAGNITUDE Richter Scale	Distance of Epicentre  from Dams
Before construction	1900-1965		0
Kettle Rapids	mid-1960s	1965-1971	13	2.4 -  2.8	Up to 300            km
Long Spruce	Late 1970s	1988-1989	3	2.6  --  3.0	Up to 180          km
Limestone	Early 1990s	1993-2006	28	1.7 --  4.0	Up to 350         km

Table 2 displays a peculiar phenomenon that can be attributed to these immense crustal changes. A pronounced ‘see-saw’ effect, like a scale tipping onc way, then the other.

There are two groups of epicentres, one along the shores of Hudson Bay, the second further inland scattered over a huge area. Movements started in the northern group, then the focus switches to the south, then back to the north and son on. Finally, in the last 3 years both areas reacted to the previous re-adjustments.

In Manitoba, Shamattawa is the nearest community to an epicentre, some 150 km away from a group of 3 earthquakes.

DATES	DURATION    IN YEARS	NORTH	SOUTH	AVERAGE MAGNITUDE
1965-1971	7	13		2.6
1983-1987	5		7	3.1
1988-1989	2	3		2.8
1991-1995	5		21	2.5
1996-1999	4	14		2.7
2000-2003	4		62	2.27
2004-2006	2	15	29	1.9
TOTALS	30	45	119

The following 5 figures show accumulated earthquakes in 10 -year increments after 1965. The earthquakes are recorded by Seismology Canada and are available in their website.

If there is a known reason for an earthquake such as a mine blast, it is recorded. There is no explanation for any of the earthquakes mentioned in this paper.

EARTHQUAKES TO OCT. 1965

EARTHQUAKES TO OCT. 2005

Tsunamis

One or more tsunamis are generated naturally when the epicentre of a displacement in under a body of water. The earthquakes under Hudson Bay have taken place just off shore from the existing communities and the seasonal settlement.
There are no records of tsunamis in this area, but their effects can be masked by episodes of flooding or high tides. Under a frozen Bay their effects would be difficult to predict, but they are capable of causing weaknesses in the ice cover.
No one can safely assume that the relatively weak earthquakes so far (with magnitudes up to 4) suggest that there is no danger in the future

CONCLUSION

The seismicity over this area gives an amazing amount of information on the underlying structure in the rocks and should be studied in detail.

The communities affected already or in the general area should be informed of the possible dangers that they could face from the earthquakes and tsunamis. It is not advisable to get caught unaware of the problem.