How often do earthquakes occur in California
No state in the USA is more densely populated than California. Los Angeles alone already has a population of just under four million. And this despite the fact that the region is on extremely shaky feet: California is an earthquake region. At some point the earth will tremble violently here again. Experts warn that it could soon be the case. Is the big quake - "The Big One" - imminent?
For many years it was remarkably quiet in California. Too calm, some fear. It is true that there are constantly smaller tremors and many of them are so weak that they cannot be felt. But the last great quake occurred 100 years ago: In April 1906 it destroyed San Francisco. The reason for the increased risk of earthquakes is that a large crack runs lengthways through California: Two plates of the earth's crust meet at the San Andreas Graben. If their rock masses move jerkily, there is a huge crash on the surface of the earth. And the tension on the edges of the panels is growing every day.
Geologists have calculated that a devastating earthquake greater than 6.7 magnitude is very likely in the next 20 to 30 years. According to experts, the question is not whether the disaster will come, but when. The Californians seem to take the tension in the rock calmly. Although the skyscrapers are earthquake-proof, many older buildings are not. It is uncertain whether the nuclear power plants are safe enough. And every now and then radio stations and newspapers encourage the population to have the essentials in the house in the worst case: food, drink and cash.
The great San Francisco quake
It was 100 years ago, but the people of California still have terrible memories of the day: In the early morning hours of April 18, 1906, the earth in and around San Francisco began to shake violently. The main earthquake lasted only about 40 seconds, but the consequences were fatal. Whole quarters of the up-and-coming city collapsed. Fires broke out in many parts of the city, adding to the extent of the disaster. How many people perished in the earthquake is not certain. The official death toll is 3,000. Thousands upon thousands have been left homeless. There were also numerous victims and destruction in the San Francisco area. The property damage was in the tens of millions. The quake hit California completely unexpectedly. Nobody expected such a catastrophe at the time.
Where plates scrape past each other
The residents of San Francisco and Los Angeles live on a powder keg: at any moment an earthquake can shake the California coast. The region has already experienced many quakes, one of which was particularly devastating. On April 18, 1906, the earth trembled so badly that entire neighborhoods of San Francisco collapsed, killing around 3,000 people. But why is the danger of earthquakes so great on the west coast of the USA in particular?
Two plates of the earth's crust move past each other along the California coast: the North American and Pacific plates. Both are drifting northwest, but the Pacific plate is a little faster. It is therefore slowly "overtaking" the North American record. So it happens that Los Angeles and San Francisco get closer and closer, by about 6 centimeters every year. If they move at the same pace, Los Angeles will be on the Pacific plate north of San Francisco, which is on the North American plate, in around 12 million years.
Where the plates meet, there is a clearly visible long crack through the land. This San Andreas trench is over 1100 kilometers long. Here, the different speeds of the earth plates cause extremely strong stresses in the rock. Because the two plates don't just slide past each other, they hook into each other. At some point the tension between the rock masses is so great that the faster Pacific Plate moves forward with a jolt. Such jerky movements of the plate are expressed in more or less strong earthquakes. Because of this, California will continue to be shaken by tremors. Some researchers even claim that a tremendous quake would be imminent in a few years. But no one can predict exactly when that will be.
When the earth shakes
The earth trembles, cracks gape in the ground, trees sway and houses collapse - earthquakes are forces of nature with destructive power. When the earth shakes, entire districts can collapse. The earth shakes particularly often in certain areas, namely where the plates of the earth's crust adjoin one another. This is the case, for example, in Japan, on the west coast of the USA or in the Mediterranean region.
The cause of earthquakes is the movement of the plates. These float on the viscous material of the earth's mantle, whose currents propel them like a motor. Where two slabs adjoin each other, their rock masses can get stuck and come to a standstill. The problem is: the current in the interior of the earth drives them on. This creates enormous tension between the two plates. If the tension becomes too great at some point, one of the plates jerks forward. The tension discharges: the earth shakes.
Earthquakes often happen where two plates slide past each other at different speeds, such as on the coast of California. This also does not go smoothly where plates collide. For example, the African drifts towards the Eurasian plate and dives under it. Because this plate boundary runs in the Mediterranean area, the earth shakes again and again in Italy or Turkey. There are also tremors where the earth's crust is being pulled apart, for example in the Upper Rhine Rift. These were not as strong in the past centuries, but there have already been violent tremors here too: In 1356, a strong earthquake caused great damage in the city of Basel.
The movement of the plates is not "to blame" for an earthquake every time. Collapses can also shake the area. This happens when natural or man-made cavities collapse. Such quakes do not reach as far and are not as strong as quakes caused by the movement of the earth's plates.
The exact point from which an earthquake emanates is the focus of the earthquake, also known as the hypocenter. From here, the earthquake waves spread in all directions - comparable to the waves after a stone has plopped into the water. The greater the distance from the focus of the earthquake, the weaker the earthquake waves that cause the earth to sway.
The epicenter on the surface of the earth lies directly above the focus or hypocenter. The destruction of an earthquake is usually greatest around this epicenter. How strong an earthquake is can be measured with special devices. Usually the strength is given with values on the Richter scale, which is open at the top. The strongest earthquake recorded so far was that of Valdivia on May 22, 1960, also known as the Great Chile Earthquake. It reached a strength of 9.5 on the Richter scale.
Continents on the move
For a long time it was thought that the land masses of the earth would stand rigidly in place. It later turned out that the opposite is the case. The continents of our planet are moving! Like huge ice floes, they drift in different directions, albeit not very quickly. Their speed corresponds roughly to the growth of a fingernail. But why is it that the continents are constantly on the move?
The earth's crust that envelops our planet is brittle and cracked. It resembles a cracked egg shell and is made up of seven large and many smaller plates. Some of them make up the continents, others make up the ocean floor. These plates of the earth's crust drift around on a hot, viscous rock slurry and are driven by movements in the earth's interior, more precisely: by currents in the earth's mantle. Experts also say: you are drifting. All these processes related to the movement of the earth's plates are called plate tectonics, and the movement itself is also called plate drift.
The earth is particularly active where the individual plates adjoin one another. At some of these plate boundaries, hot rock penetrates upwards from the earth's mantle and cools down. Here new earth crust forms: the two plates grow and are thereby pushed apart. On the other hand, where two plates collide, the lighter one of them - the continental crust - is crumpled up and unfolded to form mountains. The heavier of the two - the oceanic crust - is slowly disappearing into the depths. Due to the heat in the earth's interior, their rock is melted again. As the edge of the plate sinks into the depth, it pulls the rest of the plate behind it and thus additionally drives the plate movement.
Volcanic eruptions, earthquakes, long mountain ranges and deep ocean trenches accumulate along such plate edges. Most of the unrest on the earth's surface brings with it the largest of its plates: it is the Pacific plate, which is moving northwest at a rate of about 10 centimeters per year. Most of the world's active volcanoes can be found at their edges, and violent earthquakes shake the region. Because of the frequent volcanic eruptions and earthquakes, this plate boundary is also called the “Pacific Ring of Fire”.
The outermost shell of the earth
Like an egg from an eggshell, the earth is also surrounded by a hard shell. This outermost layer surrounds the earth's mantle and is called the earth's crust. If you compare the earth to a peach, the earth's crust is - in relative terms - as thick as its skin. Under continents it reaches an average of 40 kilometers deep, under the oceans it is only about seven kilometers.
Below is the outer part of the earth's mantle, which extends to a depth of around 100 kilometers. It is also solid, but consists of heavier rock. The earth's crust and this outermost part of the mantle together are also called the “lithosphere”. This solid layer of rock has broken into slabs of different sizes, which slowly drift around on the hot, viscous mantle of the earth.
Where the rock melt penetrates upwards from the hot earth's mantle, the earth's crust can break up. Then lava flows out, which becomes the new crust of the earth. This mainly happens where the plates of the lithosphere adjoin one another, such as on the mid-ocean ridges.
In Iceland, for example, these plate boundaries are easy to recognize: cracks and furrows run through the earth's crust, where the Eurasian and North American plates drift away from each other. There is also a plate boundary in the Mediterranean region. Because the African plate is pressing against the Eurasian plate here, there are many volcanoes in Italy and there are always earthquakes.
The crust is covered by the bottom. The soil of the land masses is formed from weathered rock and remains of animals and plants. The sea floor, on the other hand, develops from deposits such as clay and sunken remains of marine organisms. On the coasts, the sea floor also consists of deposited rubble that was removed from the mainland and washed into the sea.
During a meeting of the Geological Society in Frankfurt, the meteorologist and polar researcher Alfred Wegener put forward a daring theory: In his opinion, the continents move on earth. Geology colleagues are skeptical or even negative.
If Alfred Wegener had claimed that the earth was flat, he would hardly have caused astonishment among his listeners. According to Wegener, all the continents of our earth are said to have been united into a single land mass long ago. He calls this supercontinent Pangea, which moved on the Earth's mantle and split into two parts 200 million years ago. These two continents are said to have further divided and shifted. There are clear indications of the breaking and moving of the continents: They fit together like pieces of a puzzle. It is also noticeable that the same animal species occur on different continents.
So Africa and South America should have been one? To the professional world, Wegener's speech sounds as believable as a fairy tale from the Arabian Nights. To this day we are still convinced that the earth's crust is firmly connected to its subsurface. As far as we know, the continents are fixed and were once connected to each other by land bridges. Many geologists still disparagingly refer to Wegener's continental drift as the “geopoetry of a weather frog”. The main thing that remains unclear is the motor of movement: what drives the continents? But research can no longer ignore Alfred Wegener's theory. Can it also be proven?
Alfred Wegener - an airship?
The meteorologist Alfred Wegener became famous for a record he set in balloon flight: On April 5, 1906, he ascended with his brother Kurt and stayed in the air for over 52 hours. This exceeded the previous world record by 17 hours. But the balloon flight not only served for fame, but above all for science: The Wegener brothers wanted to explore the atmosphere and test methods of location determination. Alfred Wegener's interest is not only in the weather and aviation, but also in the eternal ice. In the year of his world record, he set out to explore Greenland. He returned from this Greenland expedition in 1908. Since then, the 32-year-old scientist has been a lecturer in meteorology, astronomy and physics at the University of Marburg.
A series of violent tremors on October 18, 1356 shattered the city of Basel. From afternoon to midnight the earth trembled again and again. Many houses and part of the Basel Minster collapsed. The quake and the fire that followed razed the city to the ground. It is not yet known exactly how many victims there were.
It is the worst natural disaster in Switzerland's history. Around four in the afternoon, the first quake caused numerous houses and the choir of the Basel Minster to collapse. Many of the residents of Basel were panicked and fled to the fields at the gates of the city. Escape was her salvation. Because in the late evening there were again strong vibrations that could still be felt at a distance of 50 kilometers. Even the strongest walls collapsed.
To make matters worse, the city began to burn: Fireplaces and candles in the collapsing buildings led to multiple fires. What the earthquake did not destroy was destroyed by the fire that raged for eight days after the quake. When the flames finally died down, the cathedral and many churches and monasteries were badly damaged. Not a hundred houses remained standing, most of the buildings are only in ruins. The number of deaths is estimated at 300 at the moment. Many survivors no longer have a home. They now live makeshiftly in tents at the gates of the city.
Help from the neighborhood
Many Basel residents have lost everything in the earthquake and fire. There is a lack of essentials, especially food. The rural population in the area is very helpful and helps those affected with food and drink. Cities that are further away, such as Strasbourg, Freiburg and Colmar, also offer their support. They send helpers with tools and horse carts to help clear alleys and rebuild houses. Donations have also been promised. As soon as possible, the long-suffering city of Basel with its cathedral should be back in the same position as it was before the great earthquake.
The seaquake that hit the seabed off the coast of Japan on Friday, March 11, 2011, has devastating consequences. The quake triggered a tsunami over 10 meters high, which raced towards the coast at several hundred kilometers an hour. The water flooded a 1,000-kilometer stretch of coastline and left an image of devastation. How many people died in the disaster is not yet known. The monster waves also hit the Fukushima nuclear power plant and destroyed the cooling system. Explosions at the reactor blocks raise fears of a core meltdown.
With a magnitude of 8.9, it was the worst earthquake in Japan's history. It began at 2:45 p.m. local time and its epicenter was 130 kilometers east of the city of Sendai in the northeast of the country. A few minutes later, a tsunami over ten meters high washed over the coast of the Japanese main island of Honshu. The massive tidal wave hurled ships inland, tore buildings with it and buried the coastline under a layer of mud. Tens of thousands of people were victims of the floods, countless are missing. The emergency shelters are completely overcrowded. 100,000 soldiers are now helping with the clean-up work.
At the same time, fear of a worst-case scenario is growing in Japan. The cooling of the fuel rods at the Fukushima nuclear power plant has failed. The meltdown may have already started. The government has declared a nuclear emergency. Thousands of residents have since been evacuated.
Tsunami warning for Europe?
Europe is also not safe from a tsunami. Because the Mediterranean is tectonically active, earthquakes are the result. After particularly strong seaquakes, the dreaded giant waves could pile up in the Mediterranean and even in the Atlantic. This happened about 2000 years ago when a tsunami followed an earthquake off Crete that devastated the coasts of the eastern Mediterranean.In 1755, a quake off Portugal and the subsequent tsunami destroyed the city of Lisbon. And there was also a giant wave during a quake off Sicily in 1908. Because people are now more aware of these dangers again, experts are now working on a tsunami early warning system for Europe.
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