The outer shell of the earth is composed of a number of almost rigid "plates" that slowly move against each other. It is this geological feature that provides the conditions for major earthquakes to occur. Although they can occur anywhere on the planet with little warning, the most extreme earthquakes occur near the plate boundaries. Stresses can build up at these boundaries, caused by the general movement of the plates against each other over time, which is "bottled up" at the plate boundaries. It may then be released suddenly, in the form of an earthquake. Several boundaries are under deep water, but the effects spread for many miles, and so can be felt on land in these cases too. Tsunamis may be triggered bringing damage to coastal communities, in a wide area. Powerful earthquakes may also initiate volcanoes to erupt, in the vicinity or further afield, although this is not always the case, as it depends on the state of the magma chamber at the time.
A measurement of earthquake magnitude is the Richter scale. On a logarithmic scale this measures the size and energy released from an earthquake. On this scale, there are usually dozens of "earthquakes" occurring daily, with a magnitude of below 2.5 . These are usually not felt by humans. It takes a much stronger earthquake for damage to occur. For example, a magnitude 6.0 earthquake is ten times larger than a magnitude 5.0, but it has 32 times the amount of energy released, so is more likely to cause damage.
An earthquake registering between 6.0 and 6.9 could be considered fairly major. Above 7.0, the earthquake is considered more serious, with a larger area of damage anticipated. The two most important variables affecting earthquake damage are the intensity of ground shaking caused by the quake and the quality of the structures in the region. The level of shaking, in turn, is controlled by the proximity of the earthquake source and the types of rocks that seismic waves pass through en route. Thus, any loss of life is dependent on location (close to settlements etc.) as well as whether or not buildings can withstand the earth tremors. The larger the magnitude, and the weaker the building structures, the more likely fatalities will occur. The largest earthquake in recent history measured about 9.5 (Chile, 1960), however this did not result in the largest fatalities. People can’t stop earthquakes from happening. People can however significantly mitigate their effects by identifying hazards, building safer structures, and learning about earthquake safety.
It is not beyond possibility that a massive earthquake of magnitude 10.0 ( 10 )or 10.1 on the Richter scale could be produced on the planet, in extreme circumstances. This level of quake would cause damage to the earth's crust on a truly global scale, with the fault line likely to circle a large part of the planet, causing massive destruction on many continents and probably unbelievably high death tolls - although, by contrast, the energy produced by such an earthquake as this is only a tiny fraction of the energy the earth receives every day from the sun in the form of light and heat! Try the "Earthquake Simulator" to see what it might take to produce a "Mega-Quake" here
Below is a graph showing how the number of all magnitude earthquakes has grown over recent years. Although we are primarily concerned with long term trends for larger magnitude earthquakes in this report, it is worth noting the trend as it applies to all earthquakes - whether large or small, over a 30 years period or so from the mid-1970s. However, this trend needs to be understood in relation to the increase in seismographs. Certainly, in the last 25 years, more lower intensity earthquakes have been noticed because of a general increase in the number of seismograph stations across the world and improved global communications. This increase has helped seismological centres to locate many small earthquakes which were undetected in earlier decades. Therefore, an upward trend is not unexpected in the graph, although the rise in the number of large earthquakes will be of more significance to our assessment of the trend. By limiting the range of earthquakes being counted in this report to magnitude 7 or above, it means these earthquakes can easily be identified by a limited number of seismographs, and we have ensured that any increase in frequency for these larger earthquakes cannot be down to the increase in overall detection rates in this 25-year period, as all these larger earthquakes are able to be easily detected with fewer seismograph stations.
Graph above provided by DL Research. - http://www.dlindquist.com
The fluctuations in earthquake numbers for individual years show how unpredictable earthquakes are, and viewing on a year by year basis might make trend analysis harder to pinpoint.
For the purposes of statistical recording below, only those earthquakes registering ABOVE 6.9 (ie a level of 7.0 or above) are now counted below. Major earthquakes, rather than the more regular and frequent lower intensity earthquakes, are judged to have a greater impact on geographic reach and damage, and are more likely to be accurately recorded over a longer period of history.
It is interesting to note the following statistical records for earthquakes since 1863 across the planet. I have chosen a relatively long period each time, to allow true trends to be seen, and smooth out the effects of short term counts. (see below).
When it comes to detection rates for major earthquakes over 100 years ago, it is worth noting that our ability to record all earthquakes of 7.0 or above occurring across the planet prior to 1901 is uncertain, and so there is likely to be some incomplete data in the 19th century, because of this. It was only in 1897 that a global network of seismographs was deployed. The numbers shown for the period from 1863 to 1900 may therefore be lower than actual, although it is unlikely to be more than three or four times that shown. (records for major 'quakes were fairly good from the 1880s onwards). It is acknowledged that care and good judgement must be exercised in using global seismic data, as there may still be incomplete data from our sources even after 1901. This report has been based on data available from USGS (NEIC) and other online sources, such as Wikipedia, in an attempt to cover as much of the available data as possible .
DATES FROM & TO PERIOD NO. EARTHQUAKES (Mag. > 6.99)
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1863 to 1900 incl 38 yrs 12
1901 to 1938 incl 38 yrs 53 Reference list 1901 to 1938
1939 to 1976 incl 38 yrs 71 Reference list 1939 to 1976
1977 to 2014 incl * 38 yrs 164 (to Mar. 2011) predict >190 in total. Reference list 1977 to date
* Although periods are shown up to 2014, this report was initially written in 2006. Therefore the final period (from 1977) will be updated as required until the end of 2014. In the meantime a predicted total is shown.
The earthquake (OFF EAST COAST OF HONSHU, JAPAN) - incidently one of the largest
The earthquake (OFF EAST COAST OF HONSHU, JAPAN) - incidently one of the largest
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