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Live Web Chat Transcript: Earthquakes - How, Where and Why?
03.04.10
 
haze over Santiago, Chile, after earthquakeHaze lingered over the metropolitan area of Santiago, Chile, following a magnitude 8.8 earthquake on February 27, 2010. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA's Terra satellite captured this view. Image credit: NASA

JPL's Margaret GlasscoeNASA/JPL scientist Margaret Glasscoe will answer questions during a live chat. Image credit: NASA/JPL
This is a feature from the NASA/JPL Education Office.

A magnitude 8.8 quake in Chile over the weekend is the latest in a spate of very large and deadly earthquakes. What causes these temblors? Is there any way to forecast them? Is the recent earthquake activity unusual? Earthquakes provide visible examples of plate tectonics in action.

Join NASA/JPL scientist Margaret Glasscoe live on the Web as she answers these questions and more. Glasscoe is geophysicist who studies the long-term effects of earthquakes and fault interactions using computer models. She specializes in studying earthquakes in California.

Transcript

Margaret: Hi everyone. I'm really glad to be here to take your questions. We also have lots of emailed ones so here's the first answer: The figure axis of the Earth is different from the rotation axis. The rotation axis is the axis about which the Earth spins, but the figure axis is the axis about which the Earth's mass is balanced. An earthquake can shift the figure axis by shifting the mass of the Earth around. This is how it can make the Earth rotate more slowly or more quickly. If you imagine a figure skater who is spinning on the ice, if she were to spread her arms out away from her body, she would spin more slowly. If she brought she arms in closely, she would spin more quickly. In this way, the skater is shifting her mass and changing her angular momentum. This is similar to what is happening to the Earth when a large earthquake shifts its mass around and changes its figure axis. If it changes it enough, it can make the Earth rotate more slowly (lengthening the day) or more quickly (shortening the day).

(Moderator) Jason: To ask a question, please type out your question below and hit the "Ask" button on the right. This is a moderated chat. We're answering questions as soon as possible. Many thanks for your patience.

kamp: Do earthquakes in one part of the world lead to earthquakes in another part of the world.,

Margaret: There is evidence showing that in some cases one earthquake can trigger another event in the same region. The M 6.4 Argentina event was considered a trigger event from the M 8.8 Chile earthquake. Another example was the Big Bear earthquake that occurred only 3 hours after the Landers earthquake in eastern California in 1992. This triggering effect can happen as a result of an earthquake relieving stress on one fault and then loading stress on another fault nearby, causing that fault to also fail in an earthquake. Faults interact in a very complex manner so we don't know globally the effect, but we do know there is a regional effect.

RogerFoster: Are animals able to sense an earthquake shortly before it happens? If so, how?

Margaret: There's a lot of anecdotal evidence for this but not any scientific data as far as I know.

sndeveau: Are all of the recent earthquakes an indicator that something unusual is going on within our planet?

Margaret: Earthquakes happen all the time. Many small earthquakes happen every day and people don't notice them because they are so small. We tend to notice the large earthquakes because they are so dramatci and can do a lot of damage.

(Moderator) Jason: We really appreciate all the great questions we are getting. We're working on getting you good answers right now. Please bear with us.

Nicademus: I know that the statemnt about Earthquakes isn't "If but When" So my question is what is the best way to be prepared for an Earthquake?

Margaret: A very helpful site for earthquake preparation tips is The Great California Shake Out http://www.shakeout.org/ . You should definitely have an earthquake preparedness plan ready for your home, work, and if you have one, a car. Keep in mind that if you live in California, it's not a matter of if, it's a matter of when. So the better prepared you are for an earthquake, the better off you will be. Here are a few specific things you should do to prepare: You should have at least 3 days of water, some cash on hand, food and any prescription medications. It's also a good idea to have a pair of shoes handy next to your bed because there could be broken glass after an earthquake. Also, it's good to some first-aid supplies and consider the people around you who may have special needs. For instance, babies and small children, people on prescription medicine, elderly parents, etc. But again, check a site like The Great California Shake Out (http://www.shakeout.org/ ) for more thorough information.

Margaret: Here's another we received in the email leading up to our chat. A fourth grade class in Rhode Island asked: Why are the plates starting to move so much now and isn't it a good thing we (in Rhode Island) sit in the middle of a plate and not on the edge of one? The plates are not moving more now as opposed to before, they move at a constant rate (or nearly constant rate, they can change over time, but it is gradual, rather than sudden). It is in general safer to live in the middle of a plate, yes, because earthquakes tend to focus along the plate boundaries. However, they can still occur in the middle of plates, like in Hawaii, where there is a hot spot, and even in the Eastern United States, as a result of the crust relaxing back from the retreat of the ice sheets from long ago.

PDSP6: Do you study earthquakes on site or from a distance?

Margaret: I use computer models to study the long term effects of earthquakes and then compare these models to satellite observations being collected today. We try to understand the whole earthquake cycle in order to get a better picture of what is going on before and in between earthquakes, as well as during an after. The computer models allow us to take a look at parts of the earthquake cycle that we can't otherwise observe, since these processes take a very long time to occur. Whereas it takes a few hours to run on a computer, it would take hundreds or even thousands of years to take place in the real world. The use of computers allows us to take a glimpse into the parts of the earthquake cycle we wouldn't otherwise be able to see. Typically we observe earthquakes using seismic networks, which record the seismic waves as they propogate through the Earth. And we can also use satellite observations like GPS and radar to see how the Earth has moved as a result of the quake. Scientists can also go out in the field and directly observe how the Earth has moved as a result of the quake. So scientists don't usually experience an earthquake unless they happen by chance to be where it is.

game70: Why do earthquakes occur even in the middle of plate boundaries such as Eastern Canada/USA? Can they ever be as powerful as ones that occur along plate boundaries?

Margaret: Earthquakes can occur anywhere, though they occur most frequently at the boundary of plates. Earthquakes that occur in the easternl U..S./Canada are associated with the relaxation of the Earth's crust due to the recession of the ice sheets after the last glacial retreat. They tend to be smaller in magnitude because they are not along active plate boundaries.

(Moderator) Jason: We really appreciate all the great questions we are getting. We're working on answering questions as soon as possible. Many thanks for your patience.

RogerFoster: What was the most powerful earthquake ever recorded in the US?

Margaret: The 1964 Magnitude 9.2 earthquake in Alaska.

Calvin_Christian_Middle_School: Can an earthquake trigger a volcanic eruption?

Margaret: If a fault is in a region that is also volcanically active, there is a chance that an earthquake can trigger volcanic activity. In fact, volcanic eruptions are often associated with earthquake activity.

RogerFoster: What is the liklihood that scientists will be able to forecast earthquakes in the future?

Margaret: There is no real way to predict earthquakes. Those who work in the field are working on forecasting earthquakes, like meteorologists forecast the weather, since predicting the exact time and location of earthquakes is likely to be impossible. The field of weather forecasting saw a huge revolution in the 1970s when weather satellites were launched and we began observing storms and other weather phenomenon. Those in the field of meteorology were better able to model and forecast weather because of these improved satellite observations. With the improved models, short term forecasts of weather dramatically improved. To date, there are very few continuously recording satellites observing the deformation of the Earth. (Deformation is when the solid Earth changes, as it does in an earthquake.) These observations are necessary to understand the earthquake process and better our forecast models. NASA is currently working on a satellite mission (DESDynI : http://www.desdyni.org) that will launch an imaging radar and a lidar (very similar to radar, but uses lasers rather than radio waves to detect the changes in position from the satellite) to continuously measure the Earth and observe the small scale motion of the Earth's surface. This will help us better understand the earthquake cycle and improve our models, which will enable us to some day hopefully forecast earthquakes like we do the weather.

Margaret: Here's more that we received in email. This bunch came from sixth graders in Port Chester, New York:

1. Why can't earthquakes, especially powerful ones, be more accurately predicted? We are working to better forecast earthquakes -- since prediction of earthquakes will most likely not be possible. At the moment though, we do not have adequate data or models to fully understand the earthquake process, but we are working on improving these in order to allow us to forecast earthquakes much like weather is forecast.

2. Why don't more cities have building codes? Cities in earthquake-prone regions in the U.S. are required to adhere to building codes. However, in other countries, building codes are regulated by whatever government is in charge of that particular country. If that country chooses not to enforce those codes, then buildings will not withstand the force of an earthquake.

3. Are there different kinds of earthquakes? Earthquakes can be related to faults, volcanoes and explosions . Some can even be caused by the relaxation of the Earth's crust from the retreat of the ice sheets.

4. What is the difference between a P wave and an S wave? The P-wave is the primary wave. It is a compressional, or longitudinal, wave and can travel through any medium. The S-wave is the secondary wave and is a shear, or transverse, wave. It travels horizontally through a medium, and can only travel through solids. It travels roughly 60% the speed of the P-wave. The P-wave arrives at a seismic station first, the S-wave second, because of its slower speed. This page may also be of help: http://en.wikipedia.org/wiki/Seismic_wave

5. Can an earthquake be measured all the way around the Earth? Depending on the size of an earthquake, it can be measured at seismic stations all over the Earth. Since seismic waves can travel both through and around the Earth, they can be recorded at stations at great distances from where the earthquake originated.

fsavage: How does the depth of the quake relate to its intensity?

Margaret: The size of an earthquake is dependent on the area of the fault that slips (so the length and depth) as well as tne amount of slip that occurs. So if the fault is deeper it's likely to produce a larger earthquake.

Margaret: Here's more from our email questions. A middle school science teacher sent these questions:

Why don't trenches fill with pieces of rock when the continents move? The oceanic trenches do actually have sediment in them but they are still deep enough that they are not completely filled. See this visual: http://abyss.uoregon.edu/~js/images/plate_boundaries.gif

Why did the earthquake in Chile shorten the day? As I explained previously in the chat, the earthquake in Chile caused the mass of the Earth to shift, which caused the figure axis (the axis about which the mass of the Earth is balanced) to change. This change in the mass of the Earth caused a changed in the rotation rate of the Earth, making it speed up slightly, thus shortening the day.

Why are their earthquakes in Illinois? Earthquakes can occur pretty much anywhere, though they occur most frequently at the boundary of plates. Illinois is located near a large mid-continental fault in the United States called the New Madrid fault, which had a large event in the late 1800s . Earthquakes that occur in the middle part of the continental US are usually associated with the relaxation of the Earth’s crust due to the recession of the ice sheets after the last glacial retreat.

Calvin_Christian_Middle_School: What is the strongest recorded earthquake?

Margaret: The largest recorded earthquake was the 1960 M 9.5 Chile event.

kamp: Which earthquake caused more damage - the one in Haiti or the one in Chile? We are wondering because the magnitude in Chile was greater but were their buildings built better so they could sustain a larger earthquake?

Margaret: The earthquake in Haiti caused more damage because the buildings in Haiti were not built with the same regulations that the buildings in Chile were. In Chile, they have contended with a number of large earthquakes over the past several decades, so they were prepared for the eventuality of another large earthquake; the last large earthquake in Haiti was in the 1700s, and they usually contend with hurricanes. Also, the Chile earthquake was a number of miles off-shore and deeper, whilst the Haiti earthquake's epicenter was very near the city of Port-au-Prince.

Margaret: Here's more from the email from an Earth science classes in Torrance, Calif:

1. How different are the building codes in Chile as compared to California building codes? Chile has some of the strictest building codes outside of the U.S. and Japan. All three of these countries are in earthquake-prone areas and so have strict building codes for earthquakes.

2. Were there any warning signs that a large earthquake was going to strike in Chile? The earthquake process is still as yet not well understood. As far as I know, there were no warning signs preceding the Chile earthquake.

3. Will we ever be able to predict when an earthquake is going to happen? As I explained above, we will probably never be able to predict earthquakes but our hope is that with better satellite observations and improved models, we will be able to better forecast them, like the weather.

4. How much stress is built up on the southern section of the San Andreas Fault and when will we have an earthquake? It is actually very difficult to measure stress on a fault directly. Based on the history of the fault and observations of the accumulation of the strain along the San Andreas, an earthquake along the southern segment is due soon, but we are not sure exactly when. The best thing that anyone can do is to be prepared in case that there is an earthquake, because we know that there will eventually be one, even if we don't know exactly when it is going to happen.

5. Could this earthquake add stress to the San Andreas Fault? Faults in the Earth interact in a very complex manner, but usually faults within a closer geographic region are more likely to have an effect on the stress field of any given fault. But we don't have a perfect understanding of how stress is transferred, so it could be that there is very long-range stress interaction occurring on a global scale.

6. How much dirt and particulates were was released into the air from the earthquake? Depending on the strength of the earthquake, and how much shaking occurs, a large amount of dirt and particulates can be released in the air. Additionally, if there are landslides that are triggered as a result of an earthquake, as there were following the 1994 Northridge earthquake, then dust and particulates can also be released into the air.

7. Are there moon quakes on the moon and how big are they? The moon is indeed seismically active and does have moonquakes. These are caused mostly by tidal forces of the Earth on the moon. They tend to be far smaller than those on the Earth (about Magnitude 5) but can last for tens of minutes to hours because the waves do not dampen like they do on Earth because the rocks on the moon are old and cold.

8. How deep was the Chile earthquake? The Chile earthquake was 35 kilometers (22 miles) deep.

9. Does the P and S waves travel faster with a big earthquake than a smaller earthquake? P and S waves travel at the same speed no matter what the size of an earthquake. A P-wave travels at ~7 km/s and an S-wave at ~4 km/s, depending on the material.

10. What specifically caused the size of the earthquake to be so big in Chile? The earthquake in Chile was so large because it was a subduction zone earthquake. This means that the fault was not only wide in extent (length) but was also very deep. The total area of the fault that slipped determines in part how large an earthquake is. Because the fault that broke was so deep, the earthquake that resulted was much larger.

11. How long before an earthquake can you accurately predict it? Again, we probably will never be able to predict earthquakes. At this point, the best forecasts are within the 5-10 year range, but we are hoping to improve these as better data and models become available.

jgoulet: How large is the largest tectonic plate?

Margaret: I did some quick research and what I find is the Pacific plate is the largest at over 103 million square kilometers.

Madero307: We are a sixth grade class in Chicago. What caused the Chile earthquake? Was it convergent, divergent, or translation plate boundery?

Margaret: It was a convergent plate boundary, which means the Nazca plate is subducting underneath the South American plate.

andreamayo: This questions is from 8th grade students at Holy Saviour - St. John Fisehr School in Linwood, PA How is it possible that an earthquake has a negative magnitude?

Margaret: Since it's a logarithmic scale it is possible to have negative values for magnitudes of earthquakes.

Madero307: Are there any ways to prevent earthquakes or the damage they cause?

Margaret: There is no way to prevent earthquakes but there are ways to be better prepared for them. I gave some tips earlier. Again, here is a site to check for specifics: http://www.shakeout.org/

(Moderator) Jason: We really appreciate all the good questions we are getting. We're answering questions as soon as possible. Many thanks for your patience.

ccpayne: Is it possible for California to have an earthquake and break away from the United States?

Margaret: No. The San Andreas Fault is a strike-slip fault and forms a transform plate boundary. This means that the Pacific and North American plates are moving past each other in a northwest - southeast direction. California is not going to break off and fall into the ocean but Los Angeles and San Francisco will be neighbors in a few million years.

Margaret: We are getting lots of questions about how the Chile earthquake affected the tilt of the Earth's axis and the length of the day. We answered it already but here's more: A science teacher in Florida asked: My students have asked me about the information out that states that the Chile 8.8 earthquake has cause a permanent tilt in the Earth's axis. If this is correct, could you explain how this might impact us and whether or not this will impact our future weather conditions? As my colleague Richard Gross, who calculated the change in the Earth's rotation from the recent Chile earthquake explains, there are actually 3 axes involved. There is the north-south axis, which is the reference axis for the geographic coordinate system (that is, the reference for the latitude-longitude grid). This axis does not move. Then there is the axis about which the Earth's mass is balanced. This is the figure axis and it is not the same as the reference axis, being offset from it by about 10 meters. This is the axis that moved by about 3 inches. Finally there is the rotation axis, which is the axis about which the Earth rotates. The rotation axis is different from either the reference axis or the figure axis. Since the rotation and figure axes are not the same, the Earth wobbles as it rotates. The change from these earthquakes is all rather small, so it is not going to affect to weather conditions .

MCES6: Do earthquakes happen on other planets?

Margaret: There is evidence of seismicity on Mars and perhaps on Venus. Several moons of Jupiter and Titan, a moon of Saturn, show signs of tectonic activity so they would likely have seismicity associated with this activity. Our own moon also is seismically active.

Sierra: Thank you for doing this talk on Earthquakes. My science club is studying geology and how the Earth works. It all fits together perfectly!!!!!!

Margaret: Great to hear. We appreciate your interest.

Brimleybays: How much damage do earthquakes do? Does one magnitude of earthquake create more damage than another?

Margaret: The amount of damage an earthquake does depends on its size and the intensity of its shaking. But the intensity of shaking is also dependent on the type of rock. Soft soil or sediment shakes a lot harder than hard rock so a smaller earthquake can cause a lot more damage if you're on softer ground. Note that the magnitude of an earthquake is a logarithmic scale so one step in size means that an earthquake would be 32 times more powerful. (So an 8.0 is 32 times more powerful than a 7.0)

Sierra: What kind of classes in college did you have to take to know all about earthquakes and plate tectonics?

Margaret: You can take a introductory geology course to get you started and if you want to pursue further studies, you can take geopyhsics and structural geology.

mr.dog_14: can earthquakes occur any time and day?

Margaret: Yes, earthquakes can occur any time of day.

andreamayo: Alexis in 8th grade would like to know how an earthquake causes a tsunami.

Margaret: Earthquakes can cause tsunamis in two ways. The first is if an underwater earthquake causes a part of the ocean floor to displace upwards or downwards, causing a large amount of water to be vertically displaced. The other way is for an earthquake to cause an underwater landslide, which also displaces a large amount of water and can lead to a tsunami. See also http://en.wikipedia.org/wiki/Tsunami

(Moderator) Jason: We really appreciate all the great questions we are getting. We're going to continue to answer questions here for another few minutes. We may not be able to get to all of them but we are trying! At the end of the chat we will post some useful links about earthquakes and more.

Soaring: Why do aftershocks happen?

Margaret: Aftershocks occur as stress is relieved in the crust surrounding the fault around the main earthquake.

squirrles: what was the size of the earthquake in San Francisco 1960

Margaret: I think you meant the 1906 Great San Francisco Earthquake, which ruptured the northernmost 477 km (296 miles) of the San Andreas fault was a magnitude 7.8.

Margaret: Here are more answers to some of the questions we got from via email. Heidi emailed this question after seeing a quake occurred in Oklahoma on Saturday: Is the Saturday quake in Oklahoma related to the quake in Japan a day earlier? Can the shock waves travel through the Earth and resurface at a location far from the epicenter? The earthquake in Oklahoma was probably not related to the one in Japan but seismic waves do travel through the Earth and can be recorded at various points on the surface, far from where they originated.

(Moderator) Jason: Thanks very much for all the great questions. We have time for just a few more before we end this chat. A lot of questions were also e-mailed up front and we're going to answer several of them right now.

Margaret: Here's an email from a teacher in Tallahassee, Florida: Our sixth grade students have been learning about convection currents in liquids and gases. Prior to the Earthquake in Chile they also learned how currents in magma affect plate movement. Could you expand on this to help them relate what they are learning to "real world science?" Convection in the mantle of the Earth is what drives the plates, and it is at the boundaries of these plates (and very rarely in the middle of these plates) where earthquakes occur. As hot material moves up in the mantle and spreads out along the bottom of the plates, it moves them along, then cools and sinks at subduction zones. See this figure for a visual: http://www.dstu.univ-montp2.fr/PERSO/bokelmann/convection.gif

Margaret: A sixth-grade science class in Long Beach, Calif., asked:

1) Do seismologists still use the process of triangulation to locate the epicenter of an earthquake? The location of an earthquake is determined by means of using the arrival times of the earthquake waves as they reach seismometers in a seismic network. First, a guess is made as to the depth, location and origin time of the event. Then, by means of a computer and some calculations, the arrival times of the waves recorded by the seismometers are used to keep refining that guess until the differences in the calculated and observed arrival times are small enough. Then, you have determined the location of the earthquake. (For a detailed explanation, see http://earthquake.usgs.gov/learn/faq/?categoryID=2&faqID=118/index.html)

2) Can an earthquake in one location trigger and earthquake in another location along the same plate boundary? There is evidence showing that in some cases one earthquake can trigger another event in the same region. The M 6.4 Argentina event was considered a trigger event from the M 8.8 Chile earthquake. Another example was the Big Bear earthquake that occurred only 3 hours after the Landers earthquake in eastern California in 1992. This triggering effect can happen as a result of an earthquake relieving stress on one fault and then loading stress on another fault nearby, causing that fault to also fail in an earthquake. Faults interact in a very complex manner so we don't know globally the effect, but we do know there is a regional effect.

3) How much of Southern California's topography is a result of continuous earthquakes versus ancient plate tectonic movement? Much of the topography in the Los Angeles Basin is the result of younger fault motion related to the San Andreas fault, which is the newer plate boundary. So for instance, the Hollywood Hills or the Santa Monica and San Gabriel Mountains are all younger, fault-related topography. The Peninsular Ranges further south are much older and many of the mountains in eastern California are related to older tectonic events, though there are younger faults cutting through eastern California as well.

4) And every class has asked...are there earthquakes on other planets? Marsquakes? Venusquakes? There is evidence of seismicity on Mars and perhaps on Venus. Several moons of Jupiter and Titan, a moon of Saturn, show signs of tectonic activity so they would likely have seismicity associated with this activity. Our own moon also is seismically active.

(Moderator) Jason: We've had a great time answering your questions about earthquakes. Here are some useful links about earthquakes, JPL and NASA. So long from JPL in Pasadena, California. United States Geological Survey: http://education.usgs.gov/; Caltech Earthquake page: http://www.seismolab.caltech.edu/earthquakes.html; Great California Shake Out: http://www.shakeout.org/; JPL Education: http://education.jpl.nasa.gov; NASA: http://www.nasa.gov
 
 


Susan Watanabe
NASA's Jet Propulsion Laboratory, Pasadena, Calif.