On 8 April 2011 as of 16:28 UTC (12:28 p.m. DST) 261 earthquakes magnitude 2.5 or higher had hit somewhere in the world over the past seven days or 168 hours, which means planet Earth gets one of these mag. 2.5 or higher seisms every 38 minutes as an average. These earthquakes include the magnitude 7.1 aftershock off the coast of Japan’s Honshu island, the 6.5 magnitude in the state of Veracruz, Mexico and a 5.0 and 5.8 shocks in the western Caribbean near the Swan Islands north of Honduras. This seismic activity is reflected on the world map below:
In reality there is a lot more shaking on Earth that shown on this map. If we were to count all earthquakes magnitude 1.0 or higher we would be talking of at least one earthquake per minute or perhaps even more, and if we took the time to count all measurable natural seismic movements of earth the annual total would exceed 1.0 million, such is the nature of the planet we inhabit. While this appears to be quite a large number, averaging 1 shock every 30 seconds or so, what happens is that the vast majority of these temblors are only measured by instruments as anything below a 2.5 magnitude is not felt by most humans, also quite a bit of this shaking is centered in uninhabited regions or in the oceans.
What is important is to concentrate of where those earthquakes magnitude 2.5 or higher actually hit and with what frequency, paying close attention to earthquakes magnitude 5.0 or higher as these are the ones that account for most of the loss of life, injuries and physical damage that we see in the aftermath of major impacts such as the 9.0 that hit off the coast of Japan’s Honshu island on 11 March 2011, or the 8.9 that hit off the coast of Chile in 2011 and the earlier devastating earthquake in Haiti also in 2010.
The United States Geologic Survey (USGS) keeps track of earthquakes magnitude 2.5 or higher on a worldwide basis through a number of data sets. One of these data sets keeps a running tab of all mag. 2.5 or higher earthquakes taking place worldwide over the last 168 hours (7 days), and it is constantly updated.
By reviewing this running-168 hour data set on 8 April 2011 at 12:28 p.m. DST we discovered the following:
* A total of 34 earthquakes magnitude 5.0 or higher have hit somewhere on Earth in the past 168 hours;
* Of these 34 shocks, 5 were magnitude 6.0 or higher and one of them was a magnitude 7.1 in Japan, which triggered a new tsunami warning in that region;
* A total of 82 earthquakes hit the United States and an additional 52 hit Puerto Rico and the Virgin Islands;
* Alaska was has been hit by 49 magnitude 2.5 or stronger earthquakes over the last 7 days, while California suffered 11 hits, Hawaii 3 and Arkansas 12. All of the shocks in Arkansas have taken place over the last 24 hours within 50+ kilometers of Little Rock.
This 168 hours of earthquake activity are “par for the course” on planet Earth and should come as no surprise to any one. What should be surprising to all is the fact that we still see catastrophic effects, in terms of loss of life and structural and physical damages as well as indirect and consequential effects after each major impact. Also surprising is the disparity of damage and loss of life between different countries. It is clear that humankind still has a lot of work to do in preparing for and in mitigating the impact of major earthquakes.
It is also clear that radical paradigm changes are needed with respect to the way we establish design criteria, and the methods of construction, risk assessment and mitigation planning we employ in earthquake vulnerable regions. Along these lines, our focus on earthquakes is driven by recent catastrophic events including the major one in Japan and the realization that there is a lot whole of shaking taking place every day on planet Earth, but it is clear that the same concerns and needs exists when it comes to other natural hazards such as tropical cyclones, floods, drought and major weather storms, as well as the slow, but inexorably and incrementally acting consequences of global climate change!
On 11 March 2011 at 12:46 a.m. EST (05:46:23 UTC) a major earthquake, preliminarily measured at a magnitude of 8.9 occurred near the Japan Trench in a subduction zone, where the Pacific plate thrust underneath the North America Plate where Japan is located. The earthquake hit at the rather shallow depth of 24,4 kilometers [15.3 miles] near the coastline of Sensai, generating a tsunami that began impacting more than 2,100 kilometers of the eastern coast of Japan’s main island of Honshu, just a few minutes after the earthquake itself. Given the high magnitude of the earthquake the resulting tsunami was expected to impact Alaska and the west coast of Canada and the USA as well as Hawaii. Initial reports indicated 32 death from the earthquake and following tsunami, but this number is expected to climb as reports come in from a long stretch of densely populated coastal region to the north of Tokyo.
The area where this earthquake hit had seen a series of quite strong, magnitude 6.0 to 7.2, foreshocks starting on 9 March, a day where one 7.2 earthquake and three 6.0 shocks took place near the epicenter of today’s major quake.This map shows the northern portion on Honshu island in Japan, to the north of Tokyo, and the Japan Trench identified by a purple line. Each color dot represents a magnitude 7.0 or greater earthquake in this region since 1900. The orange star pinpoints the 8.9 magnitude 11 March 2011 earthquake.
The northern portion of Japan’s Honshu island has quite a history of seismicity with several magnitude 7.0 or greater earthquake having taken place since 1900. So the frequency of recurrence for major earthquakes is high as is the vulnerability of the region, in especial the eastern coastal region of Honshu.
The considerable risk of earthquake damage faced by northern Honshu is best illustrated by looking a seismic map of the region showing all earthquakes magnitude 2.5 or greater, which have occurred since 1990, that is just over 20 years!
The region where the 11 March 2011 earthquake hit is one of the most active sections of what is known as the Pacific “rim of fire”, which follows the boundary of the Pacific tectonic plate and a few other smaller plates, such as the Cocos, Nazca and the Philippines, as they interact with other plates such as the North America, South America, Australian and the Eurasian.
As these plates move past one another, or one under the other [subduction], friction makes the rock on each side of the boundary basically stick together until the force generated by accumulated energy is such that it overcomes friction and a rupture occurs. These boundaries between plates where these ruptures occur are know as seismic faults, or to put it simply: breaks in the crust of the Earth. In addition to generating most of the earthquakes on Earth, plate boundaries and seismic fault are also associated with the building of mountain ranges, volcanoes and islands.
It is clear we will have to monitor the aftermath of this events, the earthquake and the tsunami, closely to assess how much damage and loss of life and injury takes place. We could expect the death toll to climb in Japan as preliminary damage assessments are completed by civil defense and emergency management teams, and physical or structural damage to coastal locations around the Pacific ocean to mount as the tremendous amount of energy released by the break in the seismic fault, which generated the tsunami, expands in subsequent concentric rings (as you get when you drop a pebble in a pond) and generates multiple impact waves over a period of minutes or hours on coatal regions around the Pacific.
As with other major impacts by a natural hazard, today’s event presents us with invaluable opportunities to learn. From how much prediction was available, to the roles of response and recovery efforts, and especially how effective the numerous mitigation measures Japan has implement over the years through building codes and standards and methods of construction. we will have unique opportunities to evaluate what worked or not, and why. No amount of simulations or experimental work can replicate what nature has done and the value of directly observing the impact and consequences…..this is the value of the empirical aproach to risk management.