Understanding the Natural Causes of Earthquakes

 

san francisco earthquake

Picture by sjb3949 – pixabay

Earthquakes are among the greatest of natural disasters and they can cause tremendous devastation over vast areas. Yet even people who have lived through earthquakes often don’t know what causes them. Part of this is because several phenomenon can cause earthquakes.



Defining Earthquakes

It needs to be explained that we are really talking about any large vibration in the earth when we talk about earthquakes. This means that many quakes are man-caused. Detonating explosives on or in the ground cause quakes, though usually not over a large area. In fact, scientists who study earthquakes often use explosives in their research.

This said, explosives wouldn’t be considered to be a natural cause. Other causes also won’t be discussed at length, because the effect is localized. For instance, if a slab of concrete was dropped nearby, you’d feel the tremble in the earth as the weight slammed onto the ground. However, someone standing a mile away probably wouldn’t feel a thing. Thus, we will focus on quakes that have longer ranging effects.

Rockslides and volcanos

Rockslides and mudslides can cause quakes that can be felt for miles, if the slide is large enough. The same is true of snow avalanches. In both cases, this is because of the total weight that shifts.

Volcanoes also cause quakes, but for a different reason. Prior to an eruption and as an eruption occurs, molten rock moves into a magma chamber under the volcano and then from the chamber to or toward the ground surface, where it often breaks through as lava, dust, ash and superheated gasses. The movement of magma and the eruption causes the ground to tremble and depending on the quantity of material that is moving, along with how explosive the volcano is, the quake can be often felt over a large area.

In some cases, the eruption can even be heard over a much larger area than it can be felt. This isn’t surprising if you consider that sound is merely vibration, usually of air, just as an earthquake is a vibration of the earth.

The explosive eruption of Krakatoa is an example. The eruption was so great that the volcano virtually blew itself apart. It caused an enormous amount of damage in the area, primarily because of tsunamis, but the explosion was actually heard over 3,000 miles away because of the sound waves the explosion produced.

Plate Tectonics

The main cause of the largest earthquakes, though, is mostly due to plate tectonics. A cross section of the earth would show it divided into the inner core, outer core, mantle and crust. Most of the volume is in the mantle, which is molten rock that has somewhat the consistency of taffee or thick syrup. The land masses and oceans lay on the crust.

The important thing to know about the crust is that it isn’t one solid piece. It is in sections, known as tectonic plates. Because the mantle is so hot and constantly in motion, the plates are moving in respect to each other. Two plates may move away from each other, toward each other or they might slide past each other. Sometimes the movement is slow and sometimes it isn’t. In many places, the plates are moving more than four inches per year in respect to the next plate.

The place where plates meet is called a fault or convergence zone and while the plates are in motion, the rock that is above the fault is solid rock. This means that the rock ‘sticks’ on each side of the fault, even though the plates are moving. When the stress in the rock along the fault line gets too great for the rocks to bear, the rocks fracture or break. This sudden release of energy is felt as an earthquake.

One of the best examples of this is the San Andreas fault, part of which reaches through part of California. From the air, the fault line can be clearly seen in places. This is also why there are so many earthquakes in an area reaching from Los Angeles to beyond San Francisco.

There can also be numerous earthquakes in one location, because the crust can be ten to twenty miles thick and it doesn’t fracture all at once, top to bottom. This is a good thing, since the energy that could be released if it all let loose at once would be far more devastating than what we normally see during a quake.

Plate Subduction

When plates collide, one of two things usually happens. Either the surface is forced upward, creating mountains like the Himalayas or one plate is forced beneath the other. This second action is called subduction. Both types of plate collisions cause earthquakes, however in subduction, the plate edge that is being forced beneath another plate is melted due to friction. Since the molten rock is lighter than solid rock, it tries to move upward. When and if it succeeds and breaks through the surface, it is a volcanic eruption. This is the cause of the ‘ring of fire’, a loop of active volcanoes that reaches from the South Pacific, up past Asia and Russia, then around the Aleutian Islands of Alaska and down the west coast of North and South America. The San Andreas fault previously mentioned is part of the ring of fire.

It should be noted, too, that the deepest known points under the oceans are all in subduction zones. Most of the Philippines lays just west of a large subduction zone. The same is true of Japan. In fact, the deepest point known under the ocean is called the Challenger Deep and it is located in the Marianas Trench, which is east of the Philippines, south of Japan, north of Papua New Guinea and just south of Guam.

challenger deep in the marianas

Picture by Wallace, CC SA 3.0, https://en.wikipedia.org/wiki/File:Marianatrenchmap.png



Earthquakes can occur far from plate boundaries, however most happen very close to where plates meet. The stresses between plates is easily the main natural cause of earthquakes. This also explains why some areas, like California, Puget Sound in Washington, the Philippines, Japan and the Aleutian Islands of Alaska are so prone to earthquakes.






  • Comments

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        Indeed. A lot of people in that area won’t even pay any attention if a quake isn’t at least 4.0. I was living in Southern Oregon when the town I was in was hit with a 7.2. Few people, even from LA news agencies, could believe that there was so little damage. The secret is that the town sits on solid volcanic bedrock, except for the fault, which doesn’t actually go through town.

    1. Profile photo of Francine Labelle
      Francine Labelle

      I have lived through a few very small earthquakes. The only thing that I felt was some vibration and rattlings of windows. Of course, people who lived more closer to the quake centre were more affected than me. Thank goodness I do not live in a very strong earthquake area. But I certainly feel sickened when I see photos of reported earthquakes around the world. Your article, by the way, was very well written and researched. Just the type of article I enjoy reading. Thank you…

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        Thank you. I’ve been thinking about the quake that hit central Italy a few days ago and leveled several towns. That is why I included the youtube video.

    2. Profile photo of Dawnwriter
      Dawnwriter

      We had a terrible earthquake in Pakistan in 2005 that flattened the the town of Balakot. It occured at 9am in the morning and many children were in school. The memories and images still haunt us to this day.

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        Some quakes leave lasting scars, both physically and emotionally. I have a friend who lived through the great Alaskan Earthquake of 1964 that nearly destroyed Anchorage, Alaska. I have another friend who was living in Crescent City, California at the same time the Alaska Earthquake hit. The two towns are separated by nearly 3000 miles or almost 5000 km. Anchorage was almost destroyed, but less than a day later the tsunami created by the Alaskan Earthquake hit Crescent City and virtually wiped it off the map. Somewhere around 10% of Crescent City was left standing.

        Sometimes it isn’t the shaking that destroys and in the case of Crescent City, though they have rebuilt, the obvious signs of that destruction are still easy to see, over 50 years later. The emotional scars run deeper. The friend who lived in Crescent City lost family members and virtually all of his friends and people he knew.

    3. Profile photo of
      Peter Streep

      Great post, yes I had to think about Italy too when reading the post. I remember one earthquake in Holland (something that’s uncommon over there) a very strange and surreal sensation.
      Recently there have been more small earth tremors as they are pumping a lot of gas out of the earth in the northern part of Holland for decades now. (Holland has the largest gas field of Europe, one of the foundations of the wealth of the country…)
      It’s not something I wish to experience again!

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        I’d rather not live through one again, either. The sensation of the quake is hard to explain to someone who hasn’t been through one. It is sort of like standing on big ocean wave. Most of the damage doesn’t come from the up and down movement, though, it comes from the side to side shaking.

        A lot of times, man is responsible for the quakes around oil and gas fields because water is often pumped in to the wells to force the oil and gas upward. I don’t know if that is the case in Holland, but if so, the water would be the issue. It acts as a lubricant so the rocks can slide more easily past one another. The same thing has been occurring near the Aswan High Dam in Egypt, though not because they are pumping water underground. Rather, the Nile has backed up and the water has been allowed to sink deeper, over a greater area.

    4. Profile photo of S.L. Luna
      S.L. Luna

      one clear manmade cause of earthuakes is CERNs activity, during their collisions, it releases tremendous energies and force under, shaking the earth. it was very timely and not coincidnetal with the Vanuatu earthuake in July of this year. also in Chile last year. the Japan uake this year . Natural cause– Nibiru.

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        People tend to think of the earth as solid, timeless and static. It isn’t any of these things and it is quite dynamic, yet even with the proof around us all the time, it is hard to see because we are so tiny in comparison to even this small planet.

    5. Profile photo of Gina  M. Menorca
      Gina M. Menorca

      Thank you. this article adds to my knowledge on an earthquake. At the present, We, that live near the west valley point which is a fault. there is a compulsory attendance to all seminars and awareness program. because they believe that this fault will be active this time. They call it the Big One.

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        Seismologists try to predict the ‘big one’ for every major fault. They are seldom very successful. All they can say for certainty is that quakes will happen. Incidentally, a 6.7 quake struck near Papua New Guinea less than a day ago, along the same fault system I mentioned above, in reference to the Marianas Trench.

      1. Profile photo of Rex Trulove
        Rex Trulove Post author

        Yes. It was a 7.1, off North Island. That is an extremely active seismic area, with a major subduction zone not far away.

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