These definitions and frequently asked questions were distributed to reporters last year by John Bwarie, a local strategy and communications officer for a project of the U.S. Geological Survey, and an advisor to seismologist Lucy Jones.
To his list of tips I would add, for TV anchors and reporters, don't say out loud on the air the speculations, suppositions and exaggerations that the people around you speak in the minutes after an earthquake. Your viewers and your Twitter feed will be more excited and more unreliable than usual. You should not be, please. Your goal should be to help the rest of us by staying unexcited and relentlessly sifting the bad info from the good.
For everybody: don't call Lucy Jones a doctor. Unless you refer to all scientists, lawyers and English professors with doctorate degrees as doctor. (She has a Ph. D. in geophysics from MIT.)
For more info: LA Observed earthquake news, science and resources
Part 1: Defining Earthquake Terms
To understand earthquakes, here is a short primer on some of the terms scientists use and what they mean:
Earthquake: An earthquake is the sudden slip of one block of the earth’s crust past another that produces shaking as one of its effects. Just like the slip of one finger past another when snapping your fingers produces a sound wave, the slip along a fault produces waves that are perceived as earthquake shaking.
Magnitude is a number that represents the total energy released during an earthquake. The smallest earthquake ever recorded is about magnitude -2 (yes, like temperature magnitudes can be negative), and the largest historical event was magnitude 9.5. Although there is no theoretical limit to magnitude, it is unlikely that an earthquake much larger than 9.5 will occur. Each unit of magnitude represents a 32 times increase in the energy released by the fault. So a magnitude 7 earthquake has 32 times more energy than a magnitude 6 earthquake, and more than thousand times (32 x 32) more energy than a magnitude 5.0 earthquake, and a million times more energy than a magnitude 3.0 earthquake. There are no “points on the scale”. When seismologists say “point” it is to express the decimal point - “magnitude 6 point 5” means magnitude 6.5.
Intensity is a number (written as a Roman numeral) describing the severity of an earthquake in terms of its effects on the earth's surface and on humans and their structures. Several scales exist, but the ones most commonly used in the United States is the Modified Mercalli Intensity scale sometimes written “MMI”. Unlike the magnitude, which has one value for each earthquake, the intensity depends on your distance from the earthquake and decreases with distance from the event.
The fault is the surface across which two blocks of crust slip in an earthquake. This planar surface may intersect the earth’s surface as an identifiable fault trace. Faults vary in size from centimeters to thousands of kilometers long. A fault zone may be a complicated set of fractures up to hundreds of kilometers wide. The magnitude of an earthquake is proportional to the area of the fault that slips and how much it slips. A magnitude 3.0 happens over a fault surface of 1-10 square meters. A magnitude 5.0 requires slip on a fault a few kilometers across, while a magnitude 8.0 needs a fault several hundreds of kilometers long. Big earthquakes occur only on big faults, but a little earthquake could occur on a big fault if only part of it slips. Small quakes may also happen on a little “secondary” fault near a big fault or on a tiny fault.
The slip is the amount of movement that occurs between the two sides of the fault surface during an earthquake. The amount of slip can range from a few centimeters for a magnitude 4.0 up to 10 meters or more for a magnitude 8.0. For smaller quakes this slip may all occur miles deep in the earth and not reach the surface.
The epicenter is the point on the earth’s surface above the hypocenter, which is the point at depth on the fault where the earthquake begins. When an earthquake occurs the slip doesn’t happen all at once. The earthquake begins at a point and ruptures across the fault. The rupture moves at about 3 kilometers per second, so a bigger earthquake lasts for a longer time.
An earthquake cluster, or earthquake sequence, is a group of earthquakes that are close in time and space. Every earthquake changes the stress in the surrounding rock and increases the probability that another earthquake will occur nearby. This probability dies off quickly with both time and distance, so mostly they are near the fault surface that has been moving. A big earthquake is on a big fault and therefore produces more aftershocks.
A mainshock is the largest earthquake in a sequence. A foreshock is any earthquake that happens near and before the mainshock. An aftershock is any earthquake that happens near and after the mainshock. Foreshocks, mainshocks, and aftershocks are all earthquakes and these terms simply describe the relationship between events in a sequence. For example, as a sequence progresses a quake dubbed a mainshock may have its status changed to foreshock if it is followed by an even bigger quake. Sometimes the largest aftershock or largest foreshock is so close in size to the mainshock (exactly the same magnitude or only 0.1 or 0.2 units apart) that the two events are called a doublet. However, generally the largest aftershock is about one magnitude unit smaller than the mainshock.
Triggered earthquakes are earthquakes that occur right after a big earthquake but are too far away from the mainshock fault to be called aftershocks. The first time we observed this clearly were earthquakes triggered by the magnitude 7.3 1992 Landers earthquake, which included a magnitude 5.7 earthquake in Nevada (over 200 miles away).
An earthquake swarm is an earthquake cluster that has several earthquakes close to the largest size (rather than a mainshock or a doublet). Unlike typical mainshock/aftershock sequences where the number of quakes dies off rapidly with time swarms may persist for longer periods of time. Swarms are characteristic of certain locations in California, especially volcanic and geothermal areas such as the Imperial Valley and Mammoth Lakes.
Part 2: Frequently Asked Questions
1. When do fault locations matter?
All earthquakes occur on faults but often the faults are too small to be recognized at the surface - or even to extend to the surface at all. But to have a big earthquake, there has to be a big fault. So when an earthquake occurs near a big fault, it could trigger (a bigger earthquake on that nearby big fault. The first earthquake need not be on the big fault to trigger another earthquake.
2. How are earthquakes assigned to faults?
The only way to be certain an earthquake occurred on a particular fault is to see actual surface slip on that fault, usually as cracks at the surface. Surface slip is almost never seen in an earthquake smaller than magnitude 5.0 and sometimes not for even larger earthquakes. If no surface slip is observed a focal mechanism can still allow scientists to estimate the orientation and direction of slip on the fault. If that is parallel to a mapped fault and the location is very near that fault, it might be on the fault - or it might be on a secondary fault around the main fault. Without surface slip, it may take quite a bit of research to make the assignment.
**The USGS usually doesn’t try to assign a fault for earthquakes below magnitude 5.0.
3. How do you determine the depth of an earthquake?
When an earthquake happens, the seismic waves (ground shaking) travel from the earthquake and arrive at seismic stations distributed across southern California. By measuring the time these waves reach each station, we triangulate the location of the earthquake including the depth. Because all our stations are on the surface, we cannot determine the depth as accurately as the horizontal location. To determine the depth accurately, we need to have at least one station as close to the horizontal location as the earthquake is deep. So for the shallowest earthquakes, it can be very difficult to know exactly how deep they are.
4. What’s the difference between an earthquake and an aftershock?
Nothing. An aftershock is an earthquake.
5. Can aftershocks trigger another earthquake?
Absolutely. An aftershock is an earthquake and every earthquake makes another one more likely.
6. Are we overdue for a big earthquake?
Earthquakes are not regular enough to talk about "overdue". On the central section of the San Andreas fault, there are intervals as short as 40 years and as long as 400 years between individual events at the same spot.
7. When smaller earthquakes happen, do they release pressure so big ones are less likely?
No. Seismologists have observed that for every magnitude 6.0 earthquake there are 10 of magnitude 5.0, 100 of magnitude 4.0, 1,000 of magnitude 3.0, and so forth as the events get smaller and smaller. This sounds like a lot of small earthquakes, but there are never enough small ones to eliminate the occasional large event. It would take 32 magnitude 5.0 's, 1,000 magnitude 4.0 's, 32,000 magnitude 3.0 's to release the same energy as one magnitude 6.0 event. So, even though there are more small events than large ones, there are never enough to release all the stress in the earth’s crust and eliminate the need for the occasional large earthquake.
8. What kind of earthquake can be triggered by a magnitude 3.5 event?
Anything. But most likely the triggered earthquake will be smaller. About five percent of the time the triggered earthquake is bigger than the first earthquake, but even then it is probably only a little bigger. Only one in a thousand magnitude 3.5 earthquakes trigger something as large as magnitude 5.
9a. What is the normal rate for earthquakes in the LA area over the past 80 years?
Since 1932, Los Angeles County had three earthquakes larger than magnitude 6.0 and about 30 larger than magnitude 5.0. (There have been many more earthquakes in adjacent counties which are also felt and have caused damage in LA.)
9b. What is the normal rate for earthquakes in Southern California over the past 80 years?
Since 1932, Southern California has had 3 earthquakes with a magnitude over 7.0 and 16 earthquakes that were between magnitude 6.0 and 7.0.
10. How can you tell if one earthquake is related to another earthquake?
We don’t have a definitive way to determine that. We assume it is related when they are very close in time and space and happening at a rate higher than background.
11. How do you determine how “long” an earthquake lasted?
For seismologists the duration of an earthquake the time it takes the rupture to travel from the hypocenter down the fault until the slip stops. Therefore the duration depends on the length of the fault rupture, which increases with magnitude. This is the biggest factor that determines how long someone feels the shaking, but the length of shaking experienced at a specific location is also affected by how far that location is from the fault, the local soils and what the person considers strong shaking. For the largest California events strong shaking can last more than a minute.
12. What is the probability that an earthquake is a foreshock to a larger earthquake?
Worldwide the probability that an earthquake will be followed within 3 days by a large earthquake nearby is just over 6%. In California, that probability is also about 6%. This means that there is about a 94% chance that any earthquake will NOT be followed by a bigger quake. In California, about half of the biggest earthquakes were preceded by foreshocks; the other half were not. There is no way to tell in advance that an earthquake is a foreshock until a larger event follows it. So, foreshocks can only be recognized in retrospect.