Earthquakes and Tsunamis – PTWC/NOAA (2016)

The Pacific Tsunami Warning Center (PTWC) of NOAA has produced a set of educational videos to inform public about earthquakes and tsunamis. The playlist includes seismological analysis of many destructive earthquakes and dynamics of subsequent tsunamis from tectonically active regions of the world.

The first animation in the playlist graphically compares the relative sizes of some notable earthquakes by their moment magnitudes. Each circle’s area represents its relative energy release. Color of the circles indicate their tsunami potential. Their labels list their moment magnitude, location, and the year.

Richter magnitude scale was devised in 1930s for measuring earthquakes. Since the 1970s scientists measure earthquakes with the moment magnitude scale. The moment magnitude scale (abbreviated as MMS; denoted as MW or M) is used by seismologists to measure the size of earthquakes in terms of the energy released. The moment magnitude scale is more useful especially for tsunami forecasting. Moment magnitude numbers scale such that the energy release increases by a factor of about 32 for each next whole magnitude number. For example, magnitude 6 releases about 32 times as much energy as magnitude 5, magnitude 7 about 32 times as much as magnitude 6 (and so on).

The second animation in the playlist shows every recorded earthquake chronologically from January 1, 2001, through December 31, 2015. Each second of the animation corresponds to 30 days. The earthquakes initially appear as flashes then hold as colored circles before shrinking gradually in order not to block earthquakes happening afterwards. The size of each circle represents the earthquake magnitude. Their color represents their depth within the earth’s crust.

In the end, the animation first shows all quakes in the 15-year period simultaneously. Next, it shows earthquakes greater than magnitude 6.5 only. This magnitude is the smallest earthquake size known to generate a tsunami. Lastly, animation shows the earthquakes with magnitudes of magnitude 8.0 or larger. These “massive” earthquakes are most likely to pose a tsunami threat when they occur under the ocean or near a coastline and when they are shallow within the earth (less than 100 km or 60 mi. deep).

The period between 2001 and 2015 includes some remarkable events. Several large earthquakes caused devastating tsunamis, including 9.1 magnitude in Sumatra (26 December 2004), 8.1 magnitude in Samoa (29 September 2009), 8.8 magnitude in Chile (27 February 2010), and 9.0 magnitude off of Japan (11 March 2011). Like most earthquakes these events occurred at plate boundaries, and truly large events like these tend to occur at subduction zones where tectonic plates collide. Other, much smaller earthquakes also occur away from plate boundaries such as those related to volcanic activity in Hawaii or those related to fracking wastewater injection wells in Oklahoma.