Why is an earthquake dangerous?
earthquake, any sudden shaking of the ground caused by the passage of seismic waves through Earth’s rocks. Seismic waves are produced when some form of energy stored in Earth’s crust is suddenly released, usually when masses of rock straining against one another suddenly fracture and “slip.” Earthquakes occur most often along geologic faults, narrow zones where rock masses move in relation to one another. The major fault lines of the world are located at the fringes of the huge tectonic plates that make up Earth’s crust.
Little was understood about earthquakes until the emergence of seismology at the beginning of the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur.
About 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce substantial damage if their centres are near areas of habitation. Very great earthquakes occur on average about once per year. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property.
What causes earthquakes?
Earthquakes are triggered by a variety of processes including volcanic eruptions, landslides and even meteor strikes. But the most common cause of earthquakes lies deep below our feet in the form of plate tectonics.
Sandwiched between the atmosphere above and the asthenosphere below (the upper layer of the earth’s mantle) lies the outermost layer of Earth — the lithosphere. This layer consists of numerous pieces, or plates, that jostle around on top of the asthenosphere like an energetic jigsaw puzzle. Temperatures in the asthenosphere range from 2,370 degrees Fahrenheit to 3,090 degrees Fahrenheit (1,300 degrees Celsius to 1,700 degrees Celsius) and the depth ranges from 62 miles to 155 miles (100 km to 250 km) below Earth’s surface. The high temperatures result in the asthenosphere layer having enough elasticity to “flow” — despite being solid. This ductile layer can flow slowly under heat convection and help move magma and rocks through Earth, contributing to the movement of tectonic plates.
When two plates attempt to move past each other, friction prevents them from gliding on by with relative ease, causing stress to build up at the point of contact. Though their movement is hindered, the plates never stop moving, so ultimately something has to give.
Eventually, the rock slips, releasing vast amounts of energy in waves that travel through Earth’s interior to the surface and generate the shaking we perceive during an earthquake. The point on Earth’s surface that lies directly above the focus — or hypocenter — of the earthquake is known as the epicenter.
Earthquakes can arise anywhere between Earth’s surface and around 700 kilometers deep according to a statement from USGS . They’re prevalent along the edges of plate boundaries and according to the British Geological Survey , over 80% occur around the edge of the Pacific Ocean, in an area known as the “Ring of Fire.” Some earthquakes, however, can appear far from boundaries, right in the middle of the plate. These are known as intraplate quakes and although little is known about them, some scientists believe that they result from preexisting faults that formed within Earth’s crust long ago.
how are earthquakes recorded & detected?
When the Earth trembles, earthquakes spread energy in the form of seismic waves. A seismograph is the primary earthquake measuring instrument. The seismograph produces a digital graphic recording of the ground motion caused by the seismic waves. The digital recording is called a seismogram.
A network of worldwide seismographs detects and measures the strength and duration of the earthquake’s waves. The seismograph produces a digital graphic plotting of the ground motion of the event.how is earthquake magnitude measured?
An earthquake has one magnitude unit. The magnitude does not depend on the location where measurement is made. Since 1970, the Moment Magnitude Scale has been used because it supports earthquake detection all over the Earth.
Earthquake Measurements
To get a better idea of the strength of the shaking and damage, the Moment Magnitude Scale was developed to capture all the different seismic waves from an earthquake to worldwide seismic networks.
Earthquake intensity scales describe the severity of an earthquake’s effects on the Earth’s surface, humans, and buildings at different locations in the area of the epicenter. There can be multiple intensity measurements. The Modified Mercalli Scale measures the amount of shaking at a particular location.
Earthquake Magnitude Scale
An important piece of information to keep in mind is that the magnitude scale is logarithmic. In other words, it is “comparing amplitudes of waves on a seismogram, not the strength, or energy, of the quakes,” according to USGS. This helps us understand that while the size (amplitude) differences between small and big quakes are big enough, it is the strength (energy) differences that are meaningful. Try out USGS’s “How Much Bigger…?” calculator to learn more about how to measure the magnitude of an earthquake.
The Richter Scale
From 1935 until 1970, the earthquake magnitude scale was the Richter scale, a mathematical formula invented by Caltech seismologist Charles Richter to compare quake sizes.
The Richter Scale was replaced because it worked largely for earthquakes in Southern California, and only those occurring within about 370 miles of seismometers. In addition, the scale was calculated for only one type of earthquake wave. It was replaced with the Moment Magnitude Scale, which records all the different seismic waves from an earthquake to seismographs across the world.
Richter’s equations are still used for forecasting future earthquakes and calculating earthquake hazards.
Moment Magnitude Scale
Today, earthquake magnitude measurement is based on the Moment Magnitude Scale (MMS). MMS measures the movement of rock along the fault. It accurately measures larger earthquakes, which can last for minutes, affect a much larger area, and cause more damage.
The Moment Magnitude can measure the local Richter magnitude (ML), body wave magnitude (Mb), surface wave magnitude (Ms).
Earthquake Magnitude Classes
Earthquakes are also classified in categories ranging from minor to great, depending on their magnitude. What’s the difference between a light and moderate quake?
These terms are magnitude classes. Classes also provide earthquake measurement. The classification starts with “minor” for magnitudes between 3.0 and 3.9, where earthquakes generally begin to be felt, and ends with “great” for magnitudes greater than 8.0, where significant damage is expected.
how is earthquake intensity measured?
A second way earthquakes are measured is by their intensity. Earthquake Intensity measurement is an on-the-ground description. The measurement explains the severity of earthquake shaking and its effects on people and their environment. Intensity measurements will differ depending on each location’s nearness to the epicenter. There can be multiple intensity measurements as opposed to one magnitude measurement.
The Modified Mercalli Scale
The Modified Mercalli (MM) Intensity Scale is used in the United States. Based on Giuseppe Mercalli’s Mercalli intensity scale of 1902, the modified 1931 scale is composed of increasing levels of intensity that range from observable quake impacts from light shaking to catastrophic destruction. Intensity is reported by Roman numerals.
An earthquake intensity scale consists of a series of key responses that includes people awakening, movement of furniture, damage to chimneys and total destruction.
Why is an earthquake dangerous?
Earthquakes can be very dangerous, if you are in the wrong place. They can make buildings fall down and set off landslides, as well as having many other deadly effects.
An earthquake that occurs at the bottom of the sea can push water upwards and create massive waves called tsunamis. These waves can reach speeds of up to 500 kilometres per hour and cause massive devastation to anything in their path. The strength of earthquakes is measured on a scale of magnitude. The higher the number on the scale, the more powerful the quake. The more powerful a quake is, the more damage it can cause.
Earthquakes have killed hundreds of thousands of people even though scientists are able to make buildings much safer and stronger than in the past. Unfortunately many quakes happen in parts of the world where people can’t afford to spend lots of money on safety measures.
how to prepare for a high magnitude quake
An earthquake is a sudden, rapid shaking of the earth caused by the shifting of rock beneath the earth’s surface. The size of an earthquake and the energy released by an earthquake will affect how much you feel it. Major earthquakes strike without warning, at any time of year, day or night.
Prepare before the next big one:
- Create an earthquake safety plan. Discuss with your family what to do, where to meet if separated, and how you will communicate when an earthquake strikes. Check work, childcare and school emergency plans.
- Practice DROP, COVER, and HOLD ON with all members of your household.
- Don’t rely on doorways for protection. During an earthquake, get under a sturdy table or desk. Hold on until shaking stops.
- Pick safe places in each room of your home.
- Create an emergency survival kit that provides you and your pets with three days of nonperishable food and water, medicines, emergency radio and first aid materials. Keep in a reachable place.
- Identify an out-of-the-area friend or relative that family members can check in by mobile texting.
- Find out if your home is in need of earthquake retrofitting and eligible for a grant.
- Identify and fix potential earthquake hazards in your home.
