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Extreme weather Natural phenomena

Tsunami measurements

This blog-post dives into the measurements of the 2011 11’th March Tsunami from the ‘Great Sendai Earthquake’. The Great Sendai Earthquake, caused a loss of lives in the order of 20.000 and insurmountable amounts of economic damages.

Tsunami measurements from NDBC buoys

Lets focus on measuring the Tsunami itself. We do this to try and understand how the tsunami spread across the globe. To investigate this we will have a look into some wave buoy measurements from the National Data Buoy Center in short, NDBC.

The NDBC wave buoy measurement equipment, are located at different distances from the earthquake epicenter across the Pacific Ocean. Figure 1, showcases all buoy measurement locations of the NDBC Tsunami program within the pacific ocean. These buoys are specifically designed for Tsunami detection purposes.

Screen dumb of NDBC-buoy locations across the pacific ocean with stations where no-data is available, recent data and historical data.
Figure 1: NDBC NOAA wave buoy locations with historic, recent and decommissioned stations.

The water depth and distances vary with thousands of kilometers between the buoy observation stations. However we will see that measuring a tsunami is still possible even across the vastness of the pacific ocean.

Related read: What caused this catastrophic event? How do we mitigate future catastrophes?

Measurements from the (National Data Buoy Center) NDBC buoys – (North Oceanic and Atmospheric Administration) – NOAA

Measurements are extremely valuable for understanding the natural world in general and in particular the oceans dynamics.

Although the vastness and shear scale of the oceans makes such a task almost implausible we still try and measure wave heights, wind speeds and current speeds by placing buoys out at sea.

Buoy data and placements

The NDBC-buoys are located offshore and for this exercise we will specifically look into buoys 43451, 21418 and 21413 which was in place during the March 11’th 2011 Tsunami. The driving mechanism behind the Tsunami was the “Great Sendai Earthquake”. We can see the Earthquake epicenter and aftershock locations in Figure 2.

Related read: Top ten misconceptions about earthquakes, Top ten misconceptions about tsunamis

In order to showcase the differences in measured surface elevations between the NDBC-buoys. I have created Figures 3-8 where measurements are shown and GIS (Geographical Information System)-figures are created of the buoy locations in question. Here we can see that the measurements of the Tsunami are occurring at different times across the pacific ocean.

Earthquake epicenters. GIS- (Geographical Information System) image of the magnitude 8.9 earthquake nearby Sendai. Measurements of aftershocks and epicenter of earthquake is shown with red circles.
Figure 2: Epicenter of the magnitude 8.9 Earthquake “Great Sendai Earthquake” prior to the Tsunami hitting shores of Japan among other locations.
Tsunami measurement at NDBC-bouy 43451. Data show the initial disturbance of the wave profile.
Figure 3: NDBC Buoy 43451 de-trended measured surface elevations across the Tsunami event where the high-frequent disturbance is clearly available.
Tsunami measurement at NDBC-bouy 21418. Data show the initial disturbance of the wave profile due to the measured Tsunami.
Figure 4: NDBC Buoy 21418 de-trended measured surface elevations across the Tsunami event where the high-frequent disturbance is clearly available.
Tsunami measurement at NDBC-bouy 21413. Data show the initial disturbance of the wave profile due to the measured Tsunami.
Figure 5: NDBC buoy 21413 de-trended measured surface elevations across the Tsunami event where the high-frequent disturbance is clearly available.
Tsunami measurement at all relevant NDBC-bouys showcasing the differences in time between measured Tsunami signals. Data show the initial disturbance of the wave profile due to the measured Tsunami at different time-stamps enabling the calculation of a wave-train celerity.
Figure 6: All NDBC buoy measurements of the Tsunami from the “Great Sendai Earthquake” on 11’th of march 2011. Each have been individually de-trended.
GIS-Figure of the location of buoy measuring center NDBC buoy 21418 and NDBC buoy 21413. Terrain and bathymetric data are all showcased with blue and terrain colored maps following the GEBCO bathymetries.
Figure 7: NDBC-buoys across the western part of the pacific ocean with locations of buoys 21418, 21413 and the epicenter of the Great Sendai Earthquake. Terrain heights and water depths are displayed following GEBCO and google-maps imprinting
GIS-Figure of the location of buoy measuring center NDBC buoy 43412. Terrain and bathymetric data are all showcased with blue and terrain colored maps following the GEBCO bathymetries.
Figure 8: NDBC-buoy across the eastern part of the pacific ocean with locations of buoys 21418, 21413 and the epicenter of the Great Sendai Earthquake. Terrain heights and water depths are displayed following GEBCO and google-maps imprinting

Tsunami wave celerity

The Figures above show that there are differences in the Tsunami surface elevations. The Tsunami wave disturbance develops and travel across the globe changing underway.

We can calculate the average wave celerity using the time difference of the measured wave disturbance between individual buoys. The arrival time of the wave at the eastern NDBC buoy 43412 is approximately 30 hours after the first measured signal. See Figures 7 and 8 for locations.

The approximate distance between NDBC buoy 21418 and 43412 is 15.000 km’s. Using this approximate distance together with the time difference of 30 hours and we get that the celerity of the Tsunami wave is approximately 500 km/hr!

In conclusion, the Tsunami moves with approximately 500 km/hr across the deep ocean of the Pacific. That is faster than a racing formula 1 car!

lets now try and examine the wave-shape of the Tsunami itself.

Tsunamis – Single or multiple waves?

Lets investigate Figures 3-6 more in-depth. In all figures, we can see that the initial wave disturbance comprises of multiple individual waves with different frequencies. Each of these waves travel independently and exchange energy with one-another. For instance, Figure 6 show individual wave buoy signals plotted together. In order to process the wave buoy data to reach the above signals we do the following steps:

  • Firstly, we download the individual raw wave data.
  • Secondly, we preprocess the data series removing unwanted data.
  • Thirdly, we detrend the data removing mean water depths from the equation.
  • Finally, we plot the results against each other with date-stamps.
  • In conclusion we can now investigate the Tsunami wave amplitude individually.

We can see that the large amplitude in buoys 21418 and 21413 are smoothed out in buoy 43412. This confirms the hypothesis that the wave dispersion relation transforms energy from bound high frequency harmonics towards carrier low frequency harmonics resulting in the smoothing out the of wave signal across distance.

Tsunami wave trains

In reality, Tsunamis consist of a plethora of waves with different amplitudes, frequencies and directions, all interacting resulting in what we call a Tsunami. When we think of an individual Tsunami wave, in reality we think of a wave-train of multiple waves with different frequencies, amplitudes and directions whose sum and difference make up the Tsunami.

In linear wave theory, we assume that these individual waves are all travelling independent of one-another, resulting in a wave-train of motion where the net wave train moves at speeds different from the individual frequencies.

This agrees with the above measured wave celerity as the individual tsunami wave-train moves with 500 km/hr while the incoming individual Tsunami wave frequency only travels of few tens of kilometers an hour.

Tsunami amplitude

Lastly lets look into the tsunami wave amplitude. We have now concluded that the Tsunami wave consists of a train of individual waves whose frequencies and directions all interact. We have also concluded that the wave-train travels across the ocean with approximately 500 km/hr whenever the water depth is large O(4000 m’s). The last interesting piece of the puzzle is the most feared one, the Tsunami amplitude.

We can see that the amplitudes out in the deep sea are O(1/10 m’s) above figures and small compared to the amplitudes experienced near the shore O(10 m’s). Now one may wonder why are there so large differences in the observed wave amplitude between the deep and shallow ocean?

Why are Tsunamis wave heights increasing at shorelines?

The answer lies in the dynamical nature of wave shoaling. As waves near the shore, the wave-energy transforms from the sub-harmonic space into the super-harmonics. Shoaling causes the wave to slow down and transform increasing the wave amplitudes of the incoming wave until breaking occurs. Check out this blogpost explaining shoaling in detail.

The wave period of the Tsunami wave endures throughout the breaking causing the water to keep rushing in over shores, low breakwaters damaging infrastructure and potentially people.

Conclusion

In this blogpost we have examined the measurements of the 11’th March 2011 Tsunami caused by the Great Sendai Earthquake.

We have looked into the placements of NDBC buoys along with measured amplitudes of the Tsunami.

We have estimated the wave celerity at deep water O(4000 m’s) to be 500 km/hr, faster than a racing formula 1 car.

We have explained in short details why wave heights at nearshore is substantially different from the deep water measurements.

References

https://www.ndbc.noaa.gov/

https://www.britannica.com/event/Japan-earthquake-and-tsunami-of-2011

https://www.sciencelearn.org.nz/resources/596-tsunami-shoaling

Categories
Extreme weather

Tsunamis – top 10 misconceptions

This blogpost outlines top 10 common misconceptions about Tsunamis. Tsunamis are dangerous events that require effective investment strategies, in order to mitigate damages.

1. Tsunamis are a single huge wave travelling across the sea.

People often have the misconception that tsunamis are a singular wave with a huge crest. They think that Tsunamis come plunging in and destroying nearby infrastructure as it crashing in onshore.

This way of visualizing a tsunami is wrong in many different ways. For instance, the tsunami amplitude is typically overestimated. The problem is not the amplitude, but that the wave keep rushing in with water. This means that the amplitude provide the means, while the water provides the method of destruction.

As an example of this effect consider the video below of the 2004 tsunami hitting the shores of Sri Lanka.

Video 1: 2004 Tsunami of Hitting Sri Lanka

2. Tsunamis travel underwater before arising up near the coast.

Another common misconception is that Tsunamis travel underwater. People think of them as an intermittent wave with high amplitude that is unnoticeable due to the deep water.

That is wrong. Water is very nearly incompressible. This means that waves at the sea surface can be felt at the bottom off the ocean. Large amplitude waves are therefore not possible to hide deep under the water without influencing the surface.

Deep water misconception on tsunamis

We would therefore have the opportunity to measure the wave amplitude of the tsunami without needing deeper measurements.

3. Tsunamis only occurs due to earthquakes

This misconception is a simplification of the reality. Often times tsunamis are caused by earthquakes, However that is not the only explanatory event.

In reality, tsunamis can be caused by multiple effects such as; landslides above and below water, tilting icebergs, subduction holes, volcanic eruptions and meteor impacts. This is only to mention some of the most significant events.

4. Tsunamis are ‘deep’-water waves.

First of all let’s answer some common questions like; what is a deep water wave? What is a shallow water wave? And what’s the difference between a deep and shallow water wave?

In short: a deep water wave have almost circular particle orbits whose diameter Shriners with depth until a minuscule radii.

A shallow water wave is a wave whose particle orbits are approximately horizontal straight lines. This means that matter is transported back and forth during a wave period. The transport distance is approximately equal and non-varying with water depth.

The difference between a shallow and deep water wave is that the bottom is almost unaffected by deep-water waves while shallow water waves influence the bottom of the sea-floor. The main characteristic defining what type of wave is considered is the period.

The nature of a tsunami is that is a shallow water wave, meaning that it has a long period and high amplitude, thus the water keeps rushing in and although it is present at several thousand meters of depth, it is still considered a shallow water wave due to the long period of often multiple minutes.

5. Tsunamis are most dangerous due to a high wave crest

Similar to an earlier question the amplitude of the wave is dangerous in the sense that inundation is governed by the amplitude and terrain of the shoreline. However the combination of the high amplitude and long period is really what makes the tsunami dangerous.

The high amplitude enables the long period wave of water to keep rushing inwards destroying and inundating large areas of inland area.

This means that the amplitude in and of itself is not dangerous without the large volume of water which accompanies it.

6. Tsunamis are only dangerous due to the incoming wave

Tsunamis are dangerous because of the large volume of water which keeps rushing inwards, however as tsunamis consists of wave-trains there are typically multiple instances of flooding during a tsunami events where water draws back before rushing back in.

This way of water pulling debris and people back towards the sea is another aspect of the tsunami event that proves quite damaging and dangerous.

7. Tsunami impact areas cannot be predicted

In previous times, it was impossible beforehand to accurately estimate the impacts and inundation areas of potential tsunami waves. With the advent and innovation of modern computers and computational fluid dynamics this becomes feasible to create storm floods and inundation maps.

The inundation maps created based on high-fidelity hydrodynamic simulations allows engineers and scientists to test difference approaches towards mitigation of potential tsunami events. From these studies it is therefore feasible to predict the damages before the catastrophic events such as tsunami flooding.

GIF of an incoming tsunami

8. Tsunamis only hit in warm places

As tsunamis can be caused by landslides, volcanic eruptions, subductions and iceberg tilting they can occur on places where waters and temperatures are cold.

As an example of cold weather tsunamis consider the Alaskan tsunami event of Lituya Bay, see Figure 1, believed to be caused by an enormous landslide event resulting in what is considered the largest recorded Tsunami on the planet.

Figure 1: Lituya Bay Tsunami caused by a landslide. The white areas on the picture show the inundation of the Tsunami, scraping away vegetation as it travelled down the fiord.

The height of the tsunami in the valley was devastating to the local wildlife, biodiversity and tree life. The amount of damages from the tsunami event was recorded in the water level where trees where cut down due to the tsunami.

9. Tsunamis are fully understood phenomena

Although much documentation, theoretical considerations and measurements have been utilized in an effort to understand and describe tsunamis radical nature.

Presently, it is not a fully understood phenomena and to this day there is still undergoing state-of-the-art research especially considering the development of numerical weather prediction tools providing early warning to these events.

Some of the questions which need to be understood in more detail are: What are the main mechanisms behind the creation of tsunamis, what amplitudes are created and how are the periods created during earthquake and landslide events.

Complete understanding of phenomena’s

10. Tsunami catastrophes cannot be mitigated

There exist multiple measure to try and counteract the terribly powerful destructive forces which tsunamis bring forward.

Shoreline flooding walls provide the much needed cover of tsunami events and will provide the users

Bonus: You can’t do anything to protect yourself against Tsunamis

Although the technology to accurately predict and stop Tsunamis before they hit shores doesn’t exist, you can still do something in order to mitigate the impacts of an incoming Tsunami.

The number one rule when trying to survive a Tsunami attack is seek shelter high up and act fast! Tsunamis are dangerous and hit within minutes of the alerts. This means that every second is precious when trying to escape and thus easy access to high terrain is critical when seeking shelter from the incoming danger.

The signs that a Tsunami is underway is explained in this blogpost, in summary some of the signs are: A receding coastline, a recent nearby earthquake, volcanic eruption or landslide.

Categories
Extreme weather

The Fremantle Doctor – The summer sea-breeze cooling down the area around Perth

The Fremantle doctor is not what the name suggest, a person with a doctorate in medicine, rather it is a sea-breeze in western Australia, where cold fresh air from the Indian ocean typically flows through the west coast of Australia around 4 pm in the afternoon.

Location of Fremantle within Perth

The “Fremantle doctor” name stems from the suburb city harbor located in Fremantle within the city of Perth, see figure below. The saying arose because sailors used to wait for the Fremantle Doctor to take the sail-boats out to sea, utilizing the regular winds blowing over the city side as drift for the sails.

Meteorological Phenomena

The sea-breeze is a meteorological phenomena occurring when cooler air-masses approach the land from the sea providing a cooling sensation on the skin during warm summer days. The Fremantle doctor happens due to the temperature gradients between the cooler seas to the west and warmer Australian land mass to the east.

The above figures are created in order to showcase the temperature variation on a warm summer day. As an example the particular day of Friday 2’nd of December 2022 was chosen.

The figures show 2 m temperature and 10 m wind across the entirety of Australia. The forecasting models are based on meteorological models from ECMWF. The temporal difference between the two figures is 6 hours between 9 am (top) and 3 pm (bottom).

Now compare the above forecast estimates around the city of Perth. First, it can be seen that the 2m temperature and 10 m wind estimates changes during the day. Secondly, in the afternoon by bringing in cooler fresh air from below the southern coast deep into the land. Finally, this sea-breeze bring in relief from the scorching heat brought onto the land by the sun.

Effect of Climate change on the Fremantle Doctor

According to the IPCC report 6 on climate change assessment of Impacts, Adaptation and Vulnerability from 2022. The number of extreme heat temperature events will be occurring at an increasing frequency and will (with high confidence) cause significant changes within the Australasia domain.

Whether the Fremantle Doctor’s cooling effect will increase in a similar manner is hard to say. The need for relief from high temperatures however will remain a constant during the summer months of Australia’s warm climate. Thus, the Fremantle Doctor will continue to bring relief to habitants living in the greater area of Fremantle, Perth.

References

https://www.ecmwf.int/ – Forecasts for ECMWF and open data available.

https://www.ipcc.ch/report/ar6/wg2/ – IPCC Report 6, assessment of Impacts Adaptation and Vulnerability