Download Data Package 5 - Tsunamis June 2013

Data Package 5 – Tsunamis
Background:
Literally “harbour-wave” in Japanese, the term tsunami has come to refer to any oceanic waves
that are caused by the displacement of large volumes of water. Tsunamis differ from normal
wind-forced sea-surface waves in that they have much longer wavelengths. Much like
earthquakes, tsunamis are primarily created by sudden vertical movements along a fault in the
Earth’s crust. Fault movement in the sea floor displaces a large volume of water above mean sea
level (Figure 1). The energy produced from this displacement is transformed into a tsunami
wave. Tsunamis are most common in the Pacific Ocean, which is surrounded by a ring of
subduction zones capable of producing earthquakes of large magnitude. Other events such as
landslides (both submarine and from subareal debris), underwater volcanoes, calving icebergs
and meteorite impacts can also generate a tsunami.
Figure 1. Tsunami caused by vertical fault movement in the Earth’s crust. Image credit:
www.surfersvillage.com (Accessed November, 2012)
Tsunami wavelengths can exceed 200 km and can move through the open ocean at speeds over
700 km/hr (194.4 m/s). As the disturbance propagates away from the source it forms a short
wave train due to the effect of dispersion. The height of a tsunami can vary from 0.05 meters in
the open ocean to above 30 meters in shallow coastal waters. Because of their very long wave
length, tsunamis behave as shallow water waves (water depth is less than 1/20 of the
wavelength), even in the deep ocean. As the tsunami approaches the shoreline, the wave slows
resulting in shorter wavelength and an increase in amplitude due to the conservation of energy
(Figures 2 & 3). Depending on the nature of the seafloor motion causing the tsunami, the trough
or the crest makes contact with the shore, either causing the sea to recede from the coastline or
immediate inundation. Embayment’s such as harbours can resonate with the tsunami wave and
either amplify or diminish the wave.
Figure 2. Basic components of a tsunami wave. Wavelength measures the distance between each
successive wave (crest to crest or trough to trough); crest and trough represent the maximum and
minimum heights of a wave, respectively; amplitude is the vertical distance between the wave
baseline and either the trough or the crest.
Figure 3. Tsunami approaching the shoreline. As the tsunami approaches the coast the amplitude
increases, producing a steeper leading wave. Image credit: www.beachapedia.org/Tsunami
(Accessed: November, 2012)
The speed at which tsunamis propagate through the ocean can be calculated by the formula for
shallow water wave propagation, v=√gh, where g is equal to 9.81 m/s2 (acceleration due to the
Earth’s gravity) and h represents the depth (m) of the ocean. From this formula, it is evident that
the speed of the wave is a function of the depth of the water. When provided with the speed of a
tsunami as well as the distance from its origin, one can approximate the amount of time it would
Δd
take for a tsunami to reach a specific location with the formula v̅= Δt , where d is equal to the
distance travelled (m), v is equal to velocity (m/s), and t (sec) is equal to time (Thurman &
Burton, 2001).
Tsunamis can be detected by seafloor bottom pressure recorders by using the hydrostatic
1p
equation (h= ρ g , where ρ is the density of seawater, p, the pressure at the seafloor and g, the
acceleration of gravity) to determine the height of the overlying water column. When the tsunami
waves pass over the seafloor pressure sensors, the water column height increases and a greater
pressure is exerted on the sensor below. The pressure variations can be used to establish the
waveform of the tsunami as it propagates by the location of the pressure recorder.
Study Location:
Ocean Networks Canada is made up of VENUS, the coastal array and NEPTUNE Canada the
offshore array. NEPTUNE Canada is located in the northeastern Pacific Ocean off the west coast
of Vancouver Island and consists 812km of cable to five instrumented study locations which
collect a variety of oceanographic data. The NEPTUNE Canada network extends from a depth of
20 meters at Folger Passage on the continental shelf, to a depth of 2660 meters at Cascadia Basin
(renamed from ODP 1027) on the abyssal plain. This network collects video, hydrophone,
oceanographic data and many other kinds of data which allow scientists to study ocean
phenomena on a continuous timescale.
Figure 4. The Ocean Networks Canada observatory located off the coasts of Vancouver Island
and British Columbia mainland. Image credit: Ocean Networks Canada
On March 11, 2011, tsunamis generated from the Japan earthquake were detected by NEPTUNE
Canada’s deep ocean pressure sensors. The earthquake was a Mw9.0 mega-thrust interface
subduction earthquake and occurred 130 km off the northeast coast of Japan in the Pacific Ocean
at the Japan Trench (Fraser, 2013) (Figure 5). The tsunami spread across the Pacific Ocean and
was detected by seafloor pressure sensors at sites Cascadian Basin, Clayoquot Slope (renamed
from ODP 889), and Folger Passage. When the tsunami waves passed over the pressure sensors,
the water column height increased and a greater pressure was exerted on the sensors below.
These pressure peaks were used to track and identify the Japanese tsunami.
Figure 5. Propagation and arrival times of the Japanese tsunami throughout the Pacific Ocean.
Image source: NOAA Center for Tsunami Research, Pacific Marine Environmental Laboratory
(Accessed March 19, 2012).
Table 1: Bottom Pressure Recorders’ (BPR) locations and depths
Instrument Location
Sensor Used
Sensor Depth (m)
Barkley
Canyon
BC Upper Slope
BPR
392.0
Approximate Sensor Distance
From the University of
Victoria1 (rounded to the
nearest km)
211
Endeavour
RCM North
RCM South
Folger Deep
BPR
BPR
BPR
2155.29
2230.0
100.0
431
433
150
North East
12.5km
North East 25km
North West
12.5km
South 12.5km
South East 25km
West 25km
Bullseye
BPR
2644.7
327
BPR
BPR
2630.0
2623.5
321
346
BPR
BPR
BPR
BPR
2668.0
2600.0
2681.0
1258.0
344
335
360
261
Folger Passage
Cascadia Basin
Clayoquot
Slope
(1NOAA/National Weather Service. April 2013. “Latitude/Longitude Distance Calculator.” Accessed
May 23, 2013. http://www.nhc.noaa.gov/gccalc.shtml)
Data Analysis:
The graphs showing pressure of the water column between 1:00pm-5:00pm on March 11, 2011
as determined by sensors at different sites are provided below in this data package. Each site,
depth and the instrument that recorded this data is provided in the figure title below the graphs.
Questions:
 Look up what time the earthquake in Japan occurred and compare to the time in the
graphs below. How many hours after the earthquake did the sensors measure the tsunami
approaching British Columbia’s shore?
 Overall there is a general decline in pressure over time. What causes this decline?
 What are the benefits of having instruments, such as the bottom pressure recorder (BPR),
in succession? Speak to the importance of this in emergency planning on Canada’s West
Coast.
Available Data:
Figure 6. Japan tsunami detected by NEPTUNE Canada’s seafloor bottom pressure sensors at
sites: A.) ODP 1027 (ODP 1026); Instrument: CORK; Depth: 2660 m B.) ODP 889 (Bullseye);
Instrument: BPR; Depth: 1250 m C.) Folger Passage (Folger Pinnacle); Instrument: ADCP 2
MHz; Depth: 23 m on March 11, 2011 (UTC).
Accessing the Data (access to this data requires a login, to obtain one please
visit https://dmas.uvic.ca/Registration) :
1. Go to the data portal login http://dmas.uvic.ca/PlottingUtility
2. Make sure the location is set to NEPTUNE Canada (this should be the default but if it isn’t
follow this pathway: Click on the Tools tab -> navigate down to Network Preference ->
Change the Network Preference by clicking Switch to NEPTUNE Canada)
3. On the left side of the Plotting Utility page you can choose the location you wish to choose,
the instrument used to collect the data and the variable you wish to plot. (e.g. Cascadia Basin
→ ODP1026 → CORK → Uncompensated Seafloor Pressure)
4. On the top of the Plotting Utility page you can enter the time and date that you want to plot the
data from. (e.g. For the Japan Tsunami enter: Date From: March 11, 2011 13:00:00 Date To:
March 11, 2011 17:00:00)
5. Click the “Plot” button to see the graph which will show up in the middle of the screen. You can
also create multiple plots for that time by not changing the time but clicking a different sensor on
the left hand side. (e.g. Clayoquot Slope → Bullseye→ BPR→ Seafloor Pressure)
6. To download the individual plots click the “Options” drop down button then the “Image of
Plot” button. Simply save these images by right clicking the mouse and clicking “Save As”.
References:




Thurman, H. V., & Burton, E. A. (2001). Oceanography and earth science. (9th ed., pp.
278-307). Upper Saddle River, New Jersey: Prentice-Hall, Inc.
NEPTUNE Canada, 2012. NEPTUNE Canada: An Invitation to Science. Victoria, BC:
University of Victoria.
Fraser, S. 2013. Tsunami damage to coastal defenses and buildings in the March 11th
2011 M w 9.0 Great East Japan earthquake and tsunami. Bulletin of earthquake
engineering. 11(1).page 205.
R Core Team (2012). R: A language and environment for statistical computing. R
Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL
http://www.R-project.org/.