What makes earthquakes so destructive

Large tsunamis which travel to the ocean floor to the surface are dangerous to human health, property, and infrastructure. Long lasting effects of tsunami destruction can be felt beyond the coastline.

Earthquake damage facts show fires caused by earthquakes are the second most common hazard. Gas is set free as gas lines are broken and a spark will start a firestorm. Every region of the Golden State holds earthquake risk. Most of us live within 30 miles of an active fault. How dangerous are earthquakes? Earthquakes can shake houses off their foundations, turn soil to liquid, and cause landslides. Liquefaction can also occur and turn the soil to liquid.

Ground shaking often leads to other hazards and types of damage, such as a house shifting off its foundation. The vibrations from an earthquake can lead to ground displacement and surface rupture. The surface rupture can cause other hazards, as well as damage to roads and buildings.

In this example, the surface rupture has caused large cracks and the collapse of a paved road. This could lead to injuries, loss of life, or impede people from getting home or to work. Earthquakes often trigger landslides, which can cause catastrophic damage to homes and towns.

This image of a landslide in El Salvador in shows how destructive landslides can be to people and their homes. The shaking from a major earthquake can shake and shift almost everything inside your home. According to a UCLA study , the majority of the injuries from the damaging Northridge earthquake were from heavy furniture and household objects falling on people. To prepare for next earthquake, evaluate the safety of your home.

Your home safety review ranks high on your earthquake preparation checklist, after preparing your earthquake safety kit and gathering essential supplies. Keep your family safe and prevent the injury of your loved ones by being prepared. One way is with a weight suspended from a spring. To measure vertical motion, seismologists note changes in the distance between the bottom of the weight and the base of the frame before the ground shakes and while the ground is shaking. A second way to measure seismic motion is with a weight on a pendulum that swings sideways.

Using this method, horizontal motion can be measured by noting changes in the distance between the side of the weight and the sides of the frame before and while the ground is shaking. In practice, seismologists often use a weight on a spring to measure vertical motions plus an east-west swinging pendulum and a north- south swinging pendulum to measure horizontal motions.

They need all three systems because the ground can move in any direction during an earthquake. Seismometers must be very sensitive because the seismic motions from distant earthquakes are often very small. A medium-sized earthquake in Alaska, for example, will produce a ground motion in New York of less than one millionth of an inch when the P-waves arrive.

If sensitive seismometers are placed in a large city or near an ocean beach, traffic or ocean waves produce vibrations that will interfere with the detection of distant earthquakes. In areas that are seismically active, strong-motion seismometers are often used. These instruments can measure very intense seismic motions that would cause sensitive seismometers to go off the scale. Thousands of seismometers are now operating at stations all around the world. Each is quietly waiting and "listening" for the seismic waves of a distant or a nearby earthquake.

When an earthquake occurs somewhere on Earth, many of these seismometers will record the motions of the sound waves, shear waves, and surface waves that are produced. By measuring the size of these waves and the times at which they reach each seismic station, seismologists can determine where the earthquake occurred and how large it was. Seismologists can locate the source of the waves—the place where the earthquake occurred—in several ways.

One way is to interpret all the seismic motions recorded at each station and to backtrack along the path the motions traveled to find the common point from which all the seismic waves originated. Another way to locate an earthquake is to study more than one type of seismic wave, taking into account the fact that different waves travel at different speeds. Both P-waves and S-waves, for example, start out at the same time like runners in a race.

P-waves travel faster, however. The farther they travel, the greater the time difference between when they arrive at a seismometer and when the S-waves arrive. By measuring this time difference, seismologists can tell how far away the earthquake is from each station.

They can then locate the source of the earthquake by studying the different times at which the waves reached different seismic stations. The place on the Earth's surface where an earthquake occurs is called the epicenter. This is usually the place where the shaking is strongest. The place where the seismic waves actually originate is called the hypocenter.

It lies below the epicenter. The hypocenter is the place where rock actually breaks along a buried fault. It is quite rare for an earthquake to occur on a fault that breaks through to the surface of the Earth. However, such surface faulting does sometimes occur—for example, in parts of California. Most earthquakes occur in the Earth's crust only a few miles below the surface. But about 10 percent of all earthquakes are deep, occurring more than 60 miles about kilometers below the surface.

Some hypocenters are as deep as miles about kilometers. Seismologists use three different ways to describe earthquake shaking and earthquake sizes. Intensity refers to the strength of shaking motions and the damage they can do. The intensity of shaking varies from place to place for the same earthquake. Seismologists use an intensity scale to describe the strength of seismic motions at a particular location.

To describe how big the earthquake itself is, seismologists refer to the magnitude. They use a magnitude scale, which compares the sizes of different earthquakes. Or they use the seismic moment of the earthquake. Seismic moment describes an important combination of physical conditions at the earthquake source. More than a hundred years ago, when people began studying earthquakes scientifically, they needed a practical way of describing the strength of an earthquake's shaking motions.

They began by describing the pattern of damage to buildings and making maps to show different levels of damage at different places.

Seismologists call this approach the study of earthquake intensities. Today they usually work with twelve different damage zones, using what is called the modified Mercalli Intensity Scale.

Each zone is assigned a Roman numeral. Each of the descriptions corresponds to a particular intensity of shaking. After an earthquake, seismologists make intensity maps based on people's experiences and the types of damage that have occurred.

By referring to old newspaper accounts and personal diaries, it is even possible to make intensity maps for past earthquakes. Intensity and damage from an earthquake can be abnormally high in certain places because of the type of soil or surface. Areas with soft sedimentary layers of material are more susceptible to severe damage from shaking than surrounding areas of harder rock.

Extensive damage is likely to occur in landfill areas. In these areas sandy material has been dumped into a lake or a bay to create a surface upon which buildings are constructed. Buildings in these areas should be specially strengthened. The intensity scale describes the strength of seismic motions in different places.

It does not tell whether the earthquake that caused the motions was large or small. Shaking at intensity III, for example, could occur near the epicenter of a small earthquake or at a great distance from a large earthquake. Earthquake Magnitude. In the 's, the American seismologist Charles Richter studied thousands of seismograms of earthquakes that had occurred in southern California.

He realized that it would be useful to have a numerical scale for comparing the size of earthquakes that went beyond describing them as just large or small.

Richter knew that he would have to take two things into account in devising such a scale. One was the distance from the epicenter. The other was the great difference in size of ground motion between small and large earthquakes. The result of Richter's work was a method of assigning magnitude that we know today as the Richter scale. It is based upon a measurement of the size of the largest wave recorded on a certain type of seismometer that was commonly used in Richter's day.

He took account of the distance from the epicenter by finding how to make a correction to the actual measurement. In this way he knew how big the seismic waves would be at a distance of exactly kilometers about 60 miles from the epicenter. He also found a way to take account of the great difference in size of ground motion for different earthquakes.

On his scale an increase of one unit for example, from magnitude 7 to magnitude 8 means an increase in earthquake shaking by a factor of ten.

Thus a magnitude of 8 is times greater shaking than a magnitude of 6 and a million times greater than a magnitude of 2. He took 0 zero on his scale as the smallest earthquake he cared to work with at that time— million times smaller than an earthquake of magnitude 8. Very small earthquakes are rated up to about 2. Moderate earthquakes rated up to magnitude 5 can cause minor damage. Earthquakes of magnitude 6 and higher are major earthquakes.

They can cause widespread damage and loss of life. Today there are many different magnitude scales in addition to the Richter scale. All are based on ways of measuring the sizes of different seismic waves on different seismometers. The largest earthquakes on these scales range up to about magnitude 8 or 9.

Using sensitive instruments, seismologists have detected earthquakes as small as magnitude —3 or —4. An earthquake with magnitude of —3 is 1, times smaller than magnitude 0, the smallest number on the original Richter scale. Seismologists often prefer to describe the size of an earthquake in terms of the physical conditions at the earthquake source itself rather than in terms of the shaking it produces. To do this, they use seismic moment rather than magnitude.

The island of Hispaniola, which includes the countries of Haiti and the Dominican Republic, sits atop the Caribbean tectonic plate, which is surrounded by a sea of other plates. Between the jostling of the North American, Cocos, South American, and Nazca plates, the Caribbean plate is constantly shoved and squashed by tectonic movements. The key juncture that sparks shaking on the surface in Haiti lies just to the north of the island nation, where the Caribbean plate creeps eastward roughly three-quarters of an inch each year relative to the North American plate.

Yet the boundary between the plates is not one straight fracture. As the plates grind against each other, the forces produce a series of fractures that crisscross the region. Both the event and this latest quake—as well as multiple older quakes—occurred within one set of these breaks, which are known collectively as the Enriquillo-Plantain Garden fault zone. Scientists believe that the event is likely connected to today's temblor. Analyses of the region after the event suggested that the shifting of the surface increased stresses both eastward toward Port-au-Prince and westward toward the epicentre of today's magnitude 7.

Hough adds that a similar increase in stress on faults in this area was seen during the s, when a spate of earthquakes struck in , , and Stress also tends to accumulate most at bends or curves in the faults, Saint Fleur says, and today's event seemed to strike at one such bend.

The epicentre is near the site of the quake, which, at an estimated magnitude 7. Yet even with this information, it's still not possible to predict quakes, Hough notes. But "there's no way to know which domino might go next. One such quake, clocking in at a magnitude 8. But the Aleutian Islands are sparsely populated, so that quake caused little damage.