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Was late afternoon on march 27. 1964. When the ground in south central alaska began began to shake as it never had before in north america and never has since more than four minutes centuries of accumulated compression were suddenly released a magnitude nine point two earthquake.
Caused part of the alaskan coast to lurch forward more than 20 meters the resulting landslides and mudslides devastated towns and cities in some areas unstable land began to behave like water causing buildings and roads and pipelines to sink into the ground and dozens of tsunamis. The largest of which topped out at 67 meters high caused catastrophic damage along the bays and inlets of the coast. More than 125 people died in a good friday.
Earthquake which was felt over an area of about 13 million square kilometers. And it released more energy that the combined power of every north american quake. Since its been 50 years since north americas largest recorded quake.
And its legacy remains enormous because it was this natural disaster that ushered in the modern age of earthquake science. Much of what we know today about earthquakes can be traced to work conducted by a handful of geologists in the months and years that followed and they all started with the same question. How did this happen music to understand what caused the great alaskan earthquake and really the majority of all earthquakes.
We have to start with plate tectonics. Take a look at a map of earthquake activity. Today.
And youll find that most of the seismic events. All over the world take place along the edge of oceanic in continental plates. There are seven of them all told that together make up the earths lithosphere.
The outermost shell of the planet. Where you find the crust in the upper mantle these plates are constantly moving and they interact with each other in lots of different ways. It can move apart from each other creating a space known as a divergent plate boundary.
When this happens magma.
The molten rock beneath the surface usually seeps up to fill the crack and form new crust along the boundary plates can also push up against each other creating whats called a convergent plate boundary tectonic plates move slowly just a few centimeters a year but over millions of years these collisions can create whole new mountain ranges as one plate is pushed up and the other sinks below the surface finally tectonic plates can slide slowly alongside each other creating. What we call a transformed boundary. You will generally find any mountain ranges or lava along these boundaries.
But you will find the makings for an active earthquake zone. All of these interactions seem pretty obvious to us today. But not so much just a half a century ago the theory of plate tectonics had actually been proposed to full 50 years before the 64 quake by german meteorologist.
Alfred wegener. The idea that the earths surface floated around in pieces. Was ridiculed at the time and vecnas career was all but ruined but in the decades that followed new generations of geologists began discovering things like huge rifts on the ocean floor.
Which turned out to be the boundaries of vecnas plates. Even so by the early 1960s. Some scientists saw new crust forming in the middle of the oceans.
Those divergent plate boundaries as evidence that the planet was actually growing larger each year that was the only way they can think of to explain where the other end of the plate had gone that alaska quake. Changed all that at first early in the quakes aftermath. Some geologists tried to explain the disaster by theorizing that massive plates had just rotated past one another counterclockwise.
But a handful of usgs scientists led by george plath kerr. Knew that something else had happened something wed never studied before hrothgar and his team went to alaska to study the land and what they found was weird and some areas forests had been submerged under saltwater. But in others rocks with barnacles on them were standing on dry land.
The land hadnt moved from side to side it had moved up. We know it today as a megathrust earthquake. When one plate slowly slides beneath another in a process known as subduction when the forces build up between these overlapping plates and one of them slips.
The result is enough energy to move an entire coastline once plath kerr and his team figured out what was going on they realized that the earth wasnt somehow growing in circumference.
Like some geologists thought instead the plates were recycling themselves in these subduction zones. As one plate slipped under another it was forced down into the magma where it returned to a molten state. It turned out that the two plates involved in the alaska quake.
The pacific in the north american plates were the worlds largest making up about 1 3 of the worlds surface and of. Course theyre still colliding at a rate of about 58. Centimeters per year with the lighter north american plate being pushed upward.
Today we know that all of the earthquakes with a recorded magnitude of 90. Or higher. And there have been six of them since 1900 have been the result of one of these megathrust quakes.
But thankfully most earthquakes are not mega thrusts. The majority occur along those transform boundaries where plates slowly slide past each other its this movement that will eventually cause. Los angeles.
Which is situated on the pacific plate to slide right past san francisco. Which sits on the north american plate and will at least make commuting. Easier course.
This part of the pacific plate moves about 50 millimetres to the northwest each year. So thatll take a while wow. Earthquakes usually occur where tectonic plates meet they can also happen far from their edges.
Because plate boundaries are just one type of fault or fracture in the crust faults can form just about anywhere on a plate. As the result of a plate jostling around and bumping into other ones or sometimes helped along by things like local volcanic activity. No matter what their origin the constant build up and release of energy in these faults are what caused earthquakes far from plates boundaries.
So called normal faults for example form.
Where the crust is being pulled apart and the ground above the fault zone. Drops one of the largest normal faults in the us. Is the 240 kilometer along wasatch fault.
Which extends through idaho and utah. As the north american plate moves to the southwest it stretches in this air crest pulls. It apart forming the normal faults reverse faults meanwhile occur when the opposite occurs and pieces of crusts are being compressed together.
So the ground above the fault zone is being pushed up these kind of faults are the earths great mountain builders. The sierra nevada. The rocky mountains.
Even the himalaya are all the result of reverse fault dynamics. And its also the type of fault that caused that megathrust in alaska then we have strike slip faults where pieces of the earth rub against each other sideways. California san andreas.
Fault is probably the most well known of this kind in north america. While most of the earthquake activity in southwestern europe. Is the work of turkeys anatolian faults like many other aspects of earth science.
Were still trying to understand all of the forces that caused earthquakes and while the alaskan quake ushered in a new era of knowledge in terms of how the earth is pieced together scientists are still trying to get a handle on questions like how different temperatures within the earth. Affect seismic activity. And then theres the big question of whether and how we can predict them itd be nice.
If we could just monitor tectonic plates and send out warnings when a quake is imminent. But those plates are usually about a hundred kilometers thick and the process that occurs before an earthquake is subtle and pretty much impossible to observe. But maybe the most lasting legacy of the 1964 earthquake is all of the scientific institutions.
That now exist to study and monitor the activity of the earth.
The us geological survey. Is earthquake. Hazards program.
The tsunami. Warning center. In the advanced national seismic system.
Which. Monitors seismic activity. Around the clock.
Were all created. In the wake of the alaska quake to get a grasp on all the science. Thats going on right beneath her feet.
And theres no shortage of data for them to study. There are more than half a million detectable earthquakes around the world each year a hundred thousand of which can be felt by humans. A hundred of which caused damage more we learn the better scientists will become at predicting when and where earthquakes may strike.
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