Everybody knows that description of the feelings they often experience during intense lovemaking. How many of us have said that we 'felt the earth move', as we trembled in excitement. That statement is far more prophetic than you might ever have imagined, because somewhere on the planet, every thirty seconds of every day, an earthquake happens somewhere. Of the 1,000,000 quakes that happen every year, only 100 cause serious damage.
These movements in the Earth's crust are caused by the continuous shifting of tectonic plates, huge land masses that are in constant motion, migrating across the face of the planet. The Pacific Plate is driving slowly under the North American Plate, right where the San Andreas fault line lies. In the Far East, much of the ground movement is caused by the so-called 'Ring of Fire', a huge circular area of intense volcanic activity.
The San Francisco earthquake of 1906 is the most famous in Western history, though by no means the most destructive or lethal ever recorded.
Seismic events, as earthquakes are scientifically called, can be so small as to pass completely unnoticed or catastrophic in their intensity. The destructive power of massive earthquakes makes we humans realise just how small and puny we are, utterly helpless in the face of nature's most violent fits of temper. When they happen on a large scale they are awesome in their effects, and this is far more frequent than you can imagine.
Earthquake severity is measured on the Richter Scale, invented by Charles F. Richter in 1934. This tells the amplitude of the largest seismic wave recorded for the earthquake, and is based on a logarithmic scale (base 10). For each whole number you go up on the scale, the severity of the quake goes up ten times. So a magnitude 5 earthquake would result in ten times the level of ground shaking as a magnitude 4 earthquake – actually resulting in 32 times as much energy being released.
If you were to compare this with explosives, a magnitude 1 quake equates to blowing up 6 ounces of TNT, whereas a magnitude 8 equates to detonating 6,000,000 tons. Any tremor of 2.5 or below is hardly noticeable, but as you can plainly see, the bigger the measurement the more destructive the event. There have been many such cataclysmic occurrences in our history, both ancient and modern.
80% of the world's earthquakes happen along the rim of the zone called the Pacific Ring of Fire. Sometimes there are many small earthquakes before the big one. These small ones are foreshocks. After the mainshock, there may again be many small quakes called aftershocks.
Aftershocks can follow an earthquake on and off for days or weeks. An earthquake can last for a few short seconds or several long minutes. Most last a minute or less. Many earthquakes happen beneath the sea and big waves can form afterwards. The shaking of the ground is not what kills most victims of earthquakes, but mainly falling buildings, fires, landslides, avalanches and tsunamis. The largest ever recorded earthquake was a 9.5 quake in Chile in 1960.
Most of the worst earthquakes in history, in terms of death toll, have occurred in China. In addition to lying along the earthquake-prone Ring of Fire, China also has always had a high population density. On January 23rd 1556, in Tangshan – part of the Shaanxi province – a magnitude 7.5 Earthquake caused the deaths of over 800,000 Chinese people, the second highest ever death toll from such an event. The same area underwent a similar experience on July 28th 1976, when official figures state that 242,000 died.
On November 1, 1755, at 9:20 in the morning, the Lisbon earthquake took place. It was possibly the most destructive and deadly in history, killing well over 900,000 people. It struck in mid-morning during a high religious holiday, All Souls Day. Shortly afterward, three large tsunamis swept over the city's harbour and killed many thousands of refugees. A week later, after untameable fires and unremitting aftershocks, essentially the whole city of Lisbon was in ashes, its people scattered, and perhaps half of its population dead.
On 1 September 1923, on the Japanese mainland, a quake hit the Kanto plain and destroyed Tokyo, Yokohama and the surroundings. About 140,000 people fell victim to this earthquake and the fires caused by it. In 1931, the New Zealand cities of Napier and Hastings were devastated. At least 256 people died in the magnitude 7.8 earthquake – 161 in Napier, 93 in Hastings, and 2 in Wairoa. Many thousands more required medical treatment.
At Aleppo in Syria, on August 9, 1138 an earthquake of unknown magnitude killed 230,000. On May 22, 1927 Xining in China underwent a magnitude: 7.9 event which saw 200,000 more die. Most recently, on January 12th 2010, an earthquake measuring 7.3 on the Richter Scale hit the island of Haiti, causing 222,517 fatalities.
A lot of myth and folklore surrounds earthquakes. Ancient Jews and Christians believed God sent earthquakes to punish the wicked. The sixtieth psalm in the Old Testament reads: “Thou hast made the earth to tremble; thou hast broken it...” Ancient Greeks believed the god Atlas had lost a war against Zeus, and was condemned to bear the Earth on his shoulders. To ease this burden, he sometimes shifted it from one shoulder to the other, and the Earth shook.
People in China thought that the Great Dragon, who lived deep inside the Earth, shook the ground when annoyed. Japanese earthquakes were caused by sudden movements of the Giant Catfish which carried the world on its back, or so they said. We have no idea how to predict when an earthquake is due, but some animals seem able to, with hundreds of reports of cats, dogs, cockroaches and rats becoming disoriented and disturbed prior to earthquakes.
In 1975, people in Northeastern China spoke of mice and rabbits leaving their burrows and snakes coming out of hibernation before a huge earthquake occurred. Horses were reported as being panicky on the night prior to the 1906 event, and on the day before the 1964 Good Friday Earthquake, Alaska's Kodiak bears came out of hibernation weeks before schedule.
However advanced humanity may be in a technological sense, we are nonetheless helpless to either predict or prevent nature from venting her awesomely destructive power whenever circumstances dictate it. Shifting plates may never before have seemed so potentially deadly, but they surely can be, dependent only upon what size they are.