Tsunamis, Earthquakes and Cyclones Oh My!

This week we learned about Tsunamis, Earthquakes and Cyclones- how they occur, how their behaviors are modeled, and how the probability of their occurrences are measured. The knowledge we have about each hazard allows us some ability to plan for disaster resilience and risk management. Unfortunately there are limitations to our knowledge, ability to plan, and public understanding of these hazards.



Tsunami’s, also called tidal waves, or seismic sea waves are defined by a high wave (sometimes tens of meters high) caused by the displacement of a large body of water. They occur unpredictably and often with little time to respond. They can be generated by earthquakes, landslides, volcanic eruptions, and meteorite impacts. These sources of tsunamis can be difficult to predict. Earthquakes and meteorites follow the power-law distribution, with smaller events occurring more frequently than larger ones. It is very difficult to determine a frequency distribution for submarine landslides as they can be triggered by earthquakes, sea level rise, excess pore pressure, weak layers, underwater explosions such as nuclear detonations, glacier calvings, and tectonic over steepening of the local slope. The impact of tsunamis is mainly focused on coastal areas, but they can affect entire ocean basins. Tsunamis can affect places along the same coastline much differently, with higher waves in one place than another. This can come from the “Fingers of Death” or mid ocean ridges and mountain ranges, or sometimes underwater landslides, channeling water towards the shore. In the Dec 2004  tsunami waves were channeled through these “Fingers of Death” towards Nova Scotia and Peru. This concentrated the force of water and hit those areas harder. After a tsunami happens geologists and other scientists measure the inundation- distance traveled inland, and the run-up-vertical distance above sea level that the waves reached. There are multiple monitoring centers that watch for tsunamis and their causes using bouys, tide gauges, seismograph stations, and pressure recorders, in order to try to warn people about tsunamis coming their way. In the case of earthquakes or underwater land slides causing a tsunami, the size and features of the event can be used to try to predict the size and path of a possible tsunami. Seismic waves travel faster than tsunami waves, so in the case of earthquakes causing tsunamis there can be an early warning of the hazard. Volcanic eruptions also cause seismic tremors and these can be used to make tsunami predictions. Meteorites can be difficult to predict, and if large enough, can have such a devastating impact that potential tsunamis might be the least of your problems.



“Fingers of death” https://www.livescience.com/9314-tsunami-waves-channeled-globe-2004-disaster.html



Earth quakes are caused by tectonic plates or earth’s crust rubbing or pushing against each other, often along fault lines, where two tectonic plates meet. 97% of earthquakes occur along plate boundaries with only 3% of earthquakes occurring in the interior of plates. Using pressure and force to disturb the earth causes seismic tremors. These tremors are measured on the logarithmic Richter scale which measures seismic oscillations. The Hokie’s have been able to generate seismic readings in scientific labs on campus on the Richter scale from jumping to Enter Sandman in Lane Stadium. Just like excited humans can cause seismic waves, so can man-made fracking or pumping waste water deep into the ground. (see https://theconversation.com/injecting-wastewater-underground-can-cause-earthquakes-up-to-10-kilometers-away-102008 ) Seismic hazard maps are drawn illustrating tectonic plate boundaries and places where earthquakes are likely to occur. These maps can be used to update building codes for earthquake preparedness in earthquake prone places. Seismic hazard analysis is done to determine the probability that a hazard will occur in a given amount of time in a location, this can give predictions as to how likely an area is to experience a certain magnitude earthquake. PGA or Peak Ground Acceleration is measured to determine the maximum ground acceleration that occurred during an earthquake, this is equal the largest amplitude recorded on an accelerogram.  Maps can be made of PGAs to better understand the hazard. Whereas the Richter scale measures the total energy of an earthquake, the PGA measures how much the earth shakes at a geographic point and tells you more about possible damage to buildings in the area. Seismic hazard maps are created to show the likely PGA values and probability of exceedence for each area. Earthquake warning systems rely on accelerometers seismometers alarms and communication systems to warn the public should a triggering event start. Earthquake prediction is not yet capable of decisive event warnings.



Tropical Cyclone’s and their accompanying Storm Surges are perhaps the most commonly occurring of the three hazards we discussed. Tropical cyclones include hurricanes and typhoons, and by definition form in the tropics. They are compact and generally have moderately strong to severe winds and the potential to create large waves and storm surges. Extratropical Cyclones are usually more spread out than tropical Cyclones with winds typically being weaker than those of tropical cyclones, but the duration and extent of high winds, big waves, and large storm surges can be longer for extratropical cyclones. Both types of cyclones cause high winds, storm surges, large waves and precipitation. Cyclones have been responsible for many deaths throughout human history, and lots of destruction of property, particularly from the storm surges they cause. Tropical cyclones such as Hurricanes are divided into categories based on the Saffir-Simpson Hurricane Wind Scale, which does not account for the factor of storm surge. A hurricane’s category is often used to communicate with the public how severe the hazard is, when really the higher ranking hurricanes have stronger and faster winds, but there are more hazards that come with a hurricane than their winds. The fact that Hurricane Katrina was classified Category 3, and produced 28ft storm surge, whereas the predecessor Hurricane Camille of Category 5 had hit Louisiana with less storm surge confused the public and encouraged people to stay in harm’s way.

Storm surges occur when pressure drops and high wind blows on the ocean surface to produce vertical circulation to cause high waves and rising seas, often causing significant flooding, property damage, and in some cases human fatalities.

The probability of a storm surge is measured by the Annual Exceedance Probability AEP, defined by the probability an event occurs in a given year and that its magnitude exceeds a prescribed value.  A flood is described by the return period, or 1 over the AEP. This gives way to the common name of a 100-yr flood. Having the calculations handy of AEP’s  gives us the chance to see what areas are within the 100-yr flood zone. This is helpful for determining where to build infrastructure and seeing on a map what areas could be more likely to flood during a storm surge. Scientists and FEMA use maps of different years of flood zones to understand at risk areas and populations and place storm surge mitigation efforts. Unfortunately the name of a 100-yr flood is very misleading to the general public. It sounds as though an area within the 100-yr flood zone is only likely to flood once every 100 years, but actually a 100-yr flood zone means that there is a one in four chance of a flood occurring within a 30 year time frame in that area. This gives home-buyers and construction companies/ investors a false sense of security and can encourage people to buy property or live in harms way. While there is legislation about building within the 100-yr flood plain, in some US cities there are grandfathered exceptions, or a lack of public zoning laws which can expand the issue of people living in frequently flooded areas.



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