Earthquakes- Frequently Asked Questions and Common Misconceptions
Below is a list of common questions you may have about earthquakes. Having heard so much about earthquakes in the news over the past few years, with little background in earth sciences there may be significant gaps in your understanding about earthquakes. This page intends to fill those gaps so you can put what you hear from the press about earthquakes into context, and of course so you can show off your sharp earthquake knowledge and understanding to your friends
The Basics
Q: What is an earthquake?
A: An earthquake is term used to describe sudden slip along a fault, and the resulting energy released in seismic waves which we feel as "tremors/ground shaking" caused by sudden stress changes in the earth's crust along a fault, volcanic or magmatic activity, or the slip along the fault itself
Q: Okay, well what is a fault?
A: I guess it's my "fault" for not telling you that beforehand.. but a fault is a fracture in the earth's crust where blocks of crust on either side of the fracture have moved relative to one another. Faults are "active" either when blocks are in motion, or when stress is accumulating on the fault from usually tectonic forces attempting to push the one block relative to the other block. Faults are dormant when stress is no longer accumulating in the rocks surrounding the fault; the tectonic forces are no longer present; however, evidence of slip and a fracture plane are still present in the rock.
Q: How does an earthquake happen?
A: When an active fault is in this stage of stress accumulation (before an earthquake) the fault is said to be "locked" as forces are attempting to push the blocks past one another, but the friction between each block is too great for the blocks to actually move. Because the energy from the pushing forces needs to go somewhere, the stress from the pushing forces is stored in the rock surrounding the fault as strain. Earthquakes are the result of the release of this stress stored as strain, very suddenly, when the accumulating stress overcomes the amount of frictional force holding the fault locked. The blocks proceed to slide relative to one another because the frictional force has been surmounted and slip occurs on the fault.
(See the terminology section at the bottom of the page for definitions of stress and strain)
Confused??? -A tangible example would be two overweight men walking down a narrow corridor in opposite directions (one to the north end of the hall one to the south end of the hall); they attempt to walk past one another and get totally stuck against one another and between both walls. Two new people enter the corridor from each end and notice the large men stuck against one another in the center of the hall. Each of the two new people begin to push on one of the large men to free them; one person pushing their large man towards the north end of the hall, the other person pushing their large man towards the south end of the hall. At first the large men do not budge, but as they push harder and harder they finally reach a point where both of the men slide past one another and are freed in a seemingly quick burst of energy, whew! now they can go about their business. This is basically what happens in the events leading up to an earthquake. In this example the corridor represents the fault, and the earthquake is represented by the surge of energy as the two large gentlemen are freed and can go about their business. In real life however, earthquakes repeat along a fault, so it would really be like a continuous cycle of large people getting stuck in the hall and then freed over and over again. Get it!? It's an interesting concept.
A: An earthquake is term used to describe sudden slip along a fault, and the resulting energy released in seismic waves which we feel as "tremors/ground shaking" caused by sudden stress changes in the earth's crust along a fault, volcanic or magmatic activity, or the slip along the fault itself
Q: Okay, well what is a fault?
A: I guess it's my "fault" for not telling you that beforehand.. but a fault is a fracture in the earth's crust where blocks of crust on either side of the fracture have moved relative to one another. Faults are "active" either when blocks are in motion, or when stress is accumulating on the fault from usually tectonic forces attempting to push the one block relative to the other block. Faults are dormant when stress is no longer accumulating in the rocks surrounding the fault; the tectonic forces are no longer present; however, evidence of slip and a fracture plane are still present in the rock.
Q: How does an earthquake happen?
A: When an active fault is in this stage of stress accumulation (before an earthquake) the fault is said to be "locked" as forces are attempting to push the blocks past one another, but the friction between each block is too great for the blocks to actually move. Because the energy from the pushing forces needs to go somewhere, the stress from the pushing forces is stored in the rock surrounding the fault as strain. Earthquakes are the result of the release of this stress stored as strain, very suddenly, when the accumulating stress overcomes the amount of frictional force holding the fault locked. The blocks proceed to slide relative to one another because the frictional force has been surmounted and slip occurs on the fault.
(See the terminology section at the bottom of the page for definitions of stress and strain)
Confused??? -A tangible example would be two overweight men walking down a narrow corridor in opposite directions (one to the north end of the hall one to the south end of the hall); they attempt to walk past one another and get totally stuck against one another and between both walls. Two new people enter the corridor from each end and notice the large men stuck against one another in the center of the hall. Each of the two new people begin to push on one of the large men to free them; one person pushing their large man towards the north end of the hall, the other person pushing their large man towards the south end of the hall. At first the large men do not budge, but as they push harder and harder they finally reach a point where both of the men slide past one another and are freed in a seemingly quick burst of energy, whew! now they can go about their business. This is basically what happens in the events leading up to an earthquake. In this example the corridor represents the fault, and the earthquake is represented by the surge of energy as the two large gentlemen are freed and can go about their business. In real life however, earthquakes repeat along a fault, so it would really be like a continuous cycle of large people getting stuck in the hall and then freed over and over again. Get it!? It's an interesting concept.
More Questions?
Q: Where do earthquakes happen?
A: As you may have guessed, earthquakes happen along faults. Generally, active faults are located along tectonic plate margins, where entire plates (plates are generally the size of continents, for scale) are trying to moving past one another. Along these plate boundaries many different blocks of crust are accumulating stress and could potentially reach a point where they will slip and cause an earthquake.
Q: Can we predict when an earthquake is going to happen?
A: No. Along a subduction zone, transform boundary, or any active fault, strain accumulation is extremely difficult to analyze. Knowing when an earthquake will occur would require geologists to know the amount of strain accumulated and strain threshold of each square meter of rock along every fault in the fault system. Currently we do not possess the technology or resources to do that.
Terminology:
Q: What is the epicenter of an earthquake? And what is the difference between that and the focus of the earthquake??
A: Earthquakes almost always occur at some depth at least 2-3km below the earth's surface; these terms are useful in distinguishing exactly where the earthquake first ruptured and where that first rupture is relative to us.
When an earthquake nucleates you might hear the press talk about the epicenter of the earthquake; the epicenter of an earthquake is the location on the surface of the earth directly above where the earthquake first nucleated. The focus of an earthquake is the exact location of the earthquake's nucleation, some depth below the earth's surface.
Q: What is stress? (in a geologcial sense)
A: Stress is force applied to an area. So for example, if I push on a table with my hand, the stress is the force which i push with times the area of my hand (which is in contact with the table).
Q: What is strain?
A: Strain is deformation, or changes in length which come from an applied stress. Ex. squishing a ball between your hands or stretching a rubber band
Q: What is a seismogram, seismometer, and seismologist?
A: A seismometer is a geophysical instrument which measures vibrations propagating through the ground; a seismometer then generates a seismogram which is a plot of ground vibration over time; and finally, a seismogram is then interpreted by a seismologist, which is a particular scientist who studies earthquakes.
A: Earthquakes almost always occur at some depth at least 2-3km below the earth's surface; these terms are useful in distinguishing exactly where the earthquake first ruptured and where that first rupture is relative to us.
When an earthquake nucleates you might hear the press talk about the epicenter of the earthquake; the epicenter of an earthquake is the location on the surface of the earth directly above where the earthquake first nucleated. The focus of an earthquake is the exact location of the earthquake's nucleation, some depth below the earth's surface.
Q: What is stress? (in a geologcial sense)
A: Stress is force applied to an area. So for example, if I push on a table with my hand, the stress is the force which i push with times the area of my hand (which is in contact with the table).
Q: What is strain?
A: Strain is deformation, or changes in length which come from an applied stress. Ex. squishing a ball between your hands or stretching a rubber band
Q: What is a seismogram, seismometer, and seismologist?
A: A seismometer is a geophysical instrument which measures vibrations propagating through the ground; a seismometer then generates a seismogram which is a plot of ground vibration over time; and finally, a seismogram is then interpreted by a seismologist, which is a particular scientist who studies earthquakes.