INSTANT EXPERT – EARTHQUAKES
Earthquakes originate along the plate boundaries of Earth’s crust. The most intense tremors can move and shake entire regions to such an extent that they interrupt Earth’s rotation, and can reduce the length of a day on Earth by a few milliseconds.
There are three types of earthquake, but no accurate way to predict them.
The 1960 earthquake in Chile lasted for 10 minutes and rated at least 9.5 on the Richter scale – an underground tremor so intense that it altered Earth’s mass distribution, accelerated the planet’s rotation around its own axis, and reduced the length of the day by 1.26 microseconds.
“The entire world appeared as if God had seized one end of it like a rope and slung it as hard as he could,” an eyewitness later said.
In the wake of the earthquake came tsunamis of up to 10 metres height.The waves hit the coasts of Chile first, but spread out to later cause havoc as far away as Hawaii, the Philippines and Japan.
That 1960 earthquake killed 1700 people, injured 3000 more, and left upwards of two million without a home. And although it was the strongest yet recorded, it was not by any means the first or the last time the nation has been shaken by an earthquake. Off the west coast of South America, three of Earth’s crust’s tectonic plates border each other. All earthquakes originate at such boundaries. The tremors occur when built-up tension in Earth’s crust is released by the plates either moving away from each other, colliding, or touching each other.
The most powerful earthquakes typically originate when plates collide and an ocean plate that makes up the bottom of the ocean is forced under a much thicker continental plate that forms Earth’s surface. Such a plate boundary is known as a subduction zone, and that’s the kind of boundary which exists close to Chile.
Earthquakes are measured by means of seismographs. A simple seismograph consists of a mass suspended from a spring. When the earth trembles, the frame of the seismograph moves in proportion to the mass. The strength of the earthquake is calculated by measuring the maximum impact on the seismograph and comparing it with the distance to the earthquake. By recording precisely when the tremor reached different seismographs, scientists can calculate the location of the earthquake’s epicentre. The magnitude of an earthquake was long defined on the Richter scale developed in 1935 by seismologist Charles Francis Richter, but this has more recently been judged as only locally relevant (to California) and unable to provide accurate estimates for large magnitude earthquakes. Today the moment magnitude scale, abbreviated MW, is widely preferred, and is applicable globally.
Since 1900, Earth has experienced slightly more than 10,000 powerful earthquakes, with a few of the worst in this millennium. The
EYEWITNESS EARTHQUAKE IN CHILE 1960
The entire world appeared as if God had seized one end of it like a rope and slung it as hard as he could.
most fatal ever struck Indonesia in 2004, causing the ’Boxing Day’ tsunami through the Indian Ocean that ended up killing around 230,000 people. In 2010 and 2011, Chile and Japan were affected by some of the most powerful quakes of modern times, taking thousands of lives and causing destruction worth billions of dollars.
The global frequency of earthquakes is fairly constant – although it can vary a lot from one year to the next. According to the National Earthquake Information Center (NEIC), about 20,000 earthquakes take place annually, corresponding to 50 earthquakes a day. Statistically, a major earthquake of 8+ Richter will typically take place 0-0.2 times a year. Earthquakes of 7+ strike 10-20 times annually, whereas earthquakes of 6+ Richter are recorded about 150 times a year.
At the present point in time, it is still impossible to predict exactly where and when a powerful earthquake will originate. So in cities and other areas which are located within a region known to suffer regular and powerful earthquakes, the best defence is preparation, with measures that can minimise the damage when the inevitable occurs.
In Japan, and particularly in the capital of Tokyo, the authorities have long faced up to the consequences of being located in one of the world’s worst earthquake zones. Nine out of ten large buildings in the city of Tokyo are earthquake-resistant. High-rises have pendulums on their roofs that are set in motion in case of an earthquake. These can neutralise a tremor so that the building will rock back and forth but not collapse.
Scientists agree that Tokyo will face an earthquake disaster eventually. Their estimates put a 70% risk on Tokyo being struck by an earthquake of 7.0+ on the Richter scale some time before 2050. As it says in the earthquake instruction manual that has been handed out to millions of people in Tokyo: “This is not a ’what if’ scenario. It will definitely happen in the very near future.”