Friday, July 20, 2012

Tsunamis, Rogue Waves, and Tidal Waves

Geology started out as the science of uniformitarianism: What we could see in the rock record is what we could expect to see in the future. The initial assumption of Lyell and other early geology pioneers was that the “Great Flood” of the Bible was not to be taken seriously, and that every geologic phenomenon in the past was like what we observe today: calm and steady and slow – like weathering.  However, in the past several generations, geologists have come to recognize that there have been short-lived, phenomenally catastrophic events that have changed the face of the landscape. One of these is the tsunami, a word of Japanese origin where it was first described scientifically. The word was chosen about a generation ago to distinguish one kind of wave event (a tsunami) from a tidal wave or a hurricane storm surge. A tidal wave is a twice-daily feature associated with Lunar and Solar cycles. In Southeast Alaska and the Bay of Fundy in eastern Canada, these can reach 15 meters in height – especially if focused into an east-west-oriented narrow bay or fjord such as at Fundy. A “tidal bore” is a wave that moves in with a rising tide, and in shallow estuaries like Turnagain Arm in Southeast Alaska, these can be walls of water several meters high – sufficient to overturn or “pitch-pole” a medium-sized boat.

What causes tsunamis? Can one happen in the US?

-Jared W

There are four different kinds of events that have caused tsunamis in the past:
1.       Asteroid impacts. There are huge tsunami deposits on Haiti stemming from an asteroid impact 65 million years ago.  This was the dinosaur-era-ending Chicxulub asteroid, which impacted on what is now a small village of that name on the northern tip of the modern Yucatan peninsula of Mexico. Fragments of this explosion apparently also went sub-orbital and landed as far away as Montana and the mid-Pacific ocean. Estimates of a mega-tsunami wave in the Caribbean up to 3 kilometers in height have been suggested – enough to completely inundate a large island such as Madagascar.

2.       Landslides. The face of a mountain fell off into Lituya Bay in southern Alaska in 1958. It created a wave at least 500 meters high, judging from surrounding mountains stripped of trees to at least that elevation. Surviving witnesses describe their vessel being floated over a large raft of logs, and the modern coastline remains largely denuded.

3.       Volcanoes. When the volcano Krakatau exploded in 1883, 45-meter-high waves reached as far as 10 kilometers inland on Sumatra, and swept people, animals, and debris back into the Sunda Strait. More than 36,000 people died in this event, and contemporary descriptions report that a person would walk across the Sunda Strait on bodies and logs without getting their feet wet. The tsunami from the catastrophic eruption of Thera volcano (modern Santorini in the Aegean) 3,500 years ago apparently ended the Minoan civilization on nearby Crete. The language of modern science is substantially Greek-based (with Latin) as a result of that single event.

4.       Earthquakes. In January 1700 AD, a subduction earthquake in the Cascadia region of northwestern North America sent a tsunami across the Pacific Ocean that devastated villages on the Sendai coast of Japan. The earthquake sunk a forest in Puget Sound below sea level. The wave that reached Japan was called the “Orphan Tsunami”, since it was not associated with any locally-felt earthquake or typhoon – it arrived without warning under a clear blue sky. In 1946 a subduction earthquake off the Chilean coast of South America caused what one geologist friend referred to as “unplanned urban renewal” many hours later in Hilo, Hawaii. I have personally seen signs marking the wave run-up on telephone poles 5 meters above my head in the modern downtown area. The Great Sumatra Earthquake of December 2004 killed over 250,000 people from Indonesia to India. The wave reached Sri Lanka many hours after it was initially triggered, but there was no infrastructure in place at the time to warn the millions of affected people in its path. The Great Tohoku Earthquake of 2010 triggered a tsunami that devastated northeastern Japan, directly led to a melt-down at the Fukushima Dai-Ichi nuclear plant - and destroyed docks and ships many hours later on the Oregon coast.

It is important to understand that relatively few earthquakes cause tsunamis. The basic requirement is that the causative fault must have a normal or reverse component to it - part of the seafloor must drop or lift suddenly. Modern tsunami warning systems are based on a two-tiered approach: an initial earthquake beneath an ocean floor or ocean margin is detected. If the fault system is well known (for instance is understood to be a subduction fault), then an initial warning is issued. Deep ocean buoy systems are then monitored – these waves may travel at more than 500 kilometers per hour, so they take a relatively long time to cross an ocean. If a wave-front is noted passing through this system, then warning sirens light up on the threatened coast.

Technically, hurricanes (Atlantic Ocean) and typhoons (Pacific Ocean) do not cause tsunamis, but they DO generate low-pressure-driven storm-surges that could top 10 meters above normal sea level in the worst cases. These are not sharp-edged waves like a tsunami, but instead are long-wavelength, very broad surges of seawater tracking the eye of the hurricane or typhoon as it hits land. Hurricane Katrina in 2005 did most of its damage with a huge storm-surge that overwhelmed the levees and barriers designed to protect New Orleans, a city that over time since its founding has sunk below sea level.

There is another class of large water waves called “Rogue” or “freak” waves. There is a long history of “disappeared” ships in the history of humankind, and anecdotal stories of waves exceeding 30 meters in amplitude that somehow left survivors. Recently, sea-height-measuring radar satellites have allowed this sort of feature to be quantified. The physics concept of constructive interference of waves comes into play, but there may also be other factors involved, including diffractive focusing and non-linear effects. For instance, the southwest-flowing Agulhas current in the western Indian Ocean has long been known to interfere with westerlies to create a zone of dangerous rogue waves of unusual frequency and intensity.

By the way: that Biblical story of the Great Flood? As scientists we must be careful and not just dismiss something out of hand - like this one was. Geologic evidence now suggests that the Black Sea was a continental basin that flooded catastrophically around 5,600 B.C.E. 


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