FAQ Answer

FAQ from IRIS Reddit AskScience AMA

For the full IRIS AMA please visit Reddit AskScience AMA page

 

 

 

GENERAL QUESTIONS

Q: What's the biggest earthquake that's ever happened?

A: The biggest earthquake that we've ever recorded was the M 9.5 Chilean earthquake in 1960. That doesn’t mean that it’s the largest earthquake that’s ever happened. The magnitude of an earthquake is dependent on the area of the fault that ruptures, so faults that are both long and deep (and stay brittle enough to break even deep in the earth) are the ones that have the biggest earthquakes. These are almost always along subduction zones, or places where one tectonic plate dives down beneath another one. The earthquake in Chile happened on a very long and very deep fault, so a M 9.5 earthquake is one of the largest we would expect to see.


Q: How does anyone know anything about Earth’s core, when no one has ever been there?

A: Great question! In fact, we get that question so often that we made an animation explaining how we know about the Earth’s interior - https://www.youtube.com/watch?v=UD7GHzIRI-s. In brief, we look at seismic records from around the world from a single earthquake. As the recording seismic stations get farther from the epicenter of the earthquake, the arrival time between the P-waves and S-waves will increase. Also, at a certain distance from the epicenter of the earthquake, the interior of the Earth causes some seismic waves to not appear on the seismogram for those stations. Between 104 degrees and 140 degrees from the epicenter of an earthquake P-waves are not detected; S-waves are not detected more than 104 degrees from the epicenter of an earthquake. This led to the discovery of the liquid nature of the Earth’s outer core. This lack of seismic waves is known as the Shadow Zone. The video will explain in more detail.


Q: Are earthquakes increasing? Is there anything we can do to stop them?

A: To answer your question about the frequency of earthquakes, you should definitely watch the IRIS webinar given by Dr. Thorne Lay (UCSC). From his abstract alone, you can find out that during the decade 2004-2014, 18 M8 and larger earthquakes occurred around the world, which in some cases caused horrendous destruction and loss of life. The annual rate of such events was 2.5 times greater than had been experienced over the previous century of seismological observations, however, variation in earthquake size and frequency is expected. There are a couple of additional reasons why earthquakes and their effects seem to be on the rise - 1) We have more seismic instruments and those instruments are more sensitive so we detect more earthquakes, and 2) We have global media and so we hear about earthquakes in remote or distant areas more than we used to. These factors, coupled with the fact that we are seeing an uptick in large earthquakes, means that earthquakes are in the news more often.

These very large earthquakes occur due to natural, plate tectonic processes though, and human activity occurs far away from these plate boundaries, and at much shallower depths than these earthquakes occur. Thus, human activities cannot influence these large, typically deep, earthquakes.


Q: A while ago, there was a soccer match at the local stadium (it has a capacity of 25,000). I've heard from a few friends that the fans went so crazy when the match ended that the seismograph registered an earthquake (they said it registered at around 0.3). Is that actually possible?

A: That actually happens fairly often. The energy from celebrating Seattle Seahawks fans has been captured by seismometers (https://www.pnsn.org/seahawks) as have more recent sports celebrations (http://www.dailymail.co.uk/sport/football/article-3481839/Leicester-City-fans-celebrating-minute-winner-against-Norwich-created-0-3-magnitude-EARTHQUAKE.html). It's important to remember that these are not tectonic earthquakes, or earthquakes caused by movement along a fault. These are just recordings of energy that has traveled through the ground and has been picked up by nearby seismometers. When they say it was magnitude 0.3 “earthquake” what they mean is that the energy the fans put into the ground by jumping and stomping is equivalent to the energy that would be released by a magnitude 0.3 earthquake. The fans didn’t create an actual earthquake.


Q: I have heard that geological events like earthquakes and volcano eruptions can get triggered by tidal motions from the Moon and the Sun. Is there any evidence to support this? And what is the mechanism that makes this happen? Do the tidal forces need to be applied in the right direction or is it enough to just apply stress to the formations?

A: There is a correlation between the solid Earth tide (the body force of the Sun and Moon on the Earth) and the timing of microearthquakes at the 9º50' East Pacific Rise hydrothermal system. (Stroup et al., 2007 and Stroup et al., 2009). Other scientists have also investigated the tidal effects on the timing of earthquakes on a global scale, and before/after large megathrust earthquakes.

A great paper came out on this subject in Nature (Stein et al., 1999) that reports stresses, even as small as 1 bar (atmospheric pressure at sea level), could trigger earthquakes. Tides impose only a fraction of this stress level, BUT they also impose a dynamic change in stress - basically, a change in the stress over a short period of time (like the semi-diurnal tide in a 12 hr span of time) - which can cause stress fluctuations and trigger earthquakes. Thus, not only does the level of stress matter, but the stressing rate makes a difference, which is why tides can trigger earthquakes.

To answer your last question, tidal forces do need to be applied in the right direction for a given fault orientation. It's also really important to know that tides are basically the 'straw that breaks the camel's back' so to speak, as far as tides that trigger earthquakes (or allow earthquakes to occur sooner than if tectonic stresses were applied alone). Tides are not the only stresses that are causing these earthquakes, but are an additional stress on top of the tectonic (and in some cases thermal) stresses in a particular region.


Q: I live in Seattle, and am aware of the ring of fire, but don't know a lot about it, anything I should know?

A: The Ring of Fire is what we call the very seismically and volcanically active area around the Pacific Ocean. As a resident of Seattle this is relevant to you because Seattle has both volcanoes and earthquakes! Here are some great videos and animations about earthquakes and volcanoes (and related hazards) in the Pacific Northwest. Additionally, here some information on how to prepare for an earthquake.


Q: I only have a basic understanding of earthquakes so correct me if I'm wrong; if rocks are essentially overcharged batteries that burst releasing energy shouldn't there be a way to extract it or reduce their intensity?

A: Rocks themselves don't really have any "inherent" energy in the way that you've described, but they can store energy as they are deformed. The surface of the earth is covered by a thin skin of rock called the crust, and this crust is broken up into pieces which are slowly moving around and interacting with each other. The places where these pieces (or plates) come together are the places where we have the most earthquakes. For instance, California has a lot of earthquakes because that's where the Pacific and North American plates are slide past each other. These plates are moving past each other in different directions and at different speeds and this puts stress on the rocks along the areas where the plates met. Eventually the built up stress in the rocks overcomes the strength of the rocks and the rocks break. When the rock breaks, it releases energy in the form of waves that we feel as shaking. This is what we call an earthquake. Because the energy that builds up is basically potential energy stored inside the rocks themselves there is no good way for us to reduce or extract it.


Q: Are earthquakes along mid-ocean ridges different from terrestrial settings?

A: Earthquakes are earthquakes, regardless of whether they occur on land or under the ocean. In the ocean, earthquakes can generally be divided into those that occur at mid-ocean ridges or at subduction zones. Earthquakes at mid-ocean ridges tend to only reach a magnitude of around M7, while large M9 megathrust earthquakes (like the 2004 Indonesia earthquake or 2011 Japan earthquake) can occur along subduction zones. Also, because of the volcanic nature of mid-ocean ridges, earthquakes tend to occur in swarms - or a lot of little earthquakes that occur all at once. This behavior is also observed at other volcanic systems (terrestrial or oceanic) around the world.


Q: If there is significant melting of the ice on Greenland and Antarctica will this cause a change in behavior of the tectonic plates? That is: how will the change in distortion of the crust due to the rearrangement of mass influence the movement of the plates?

A: In a local sense, any changes of the Greenland and Antarctica ice sheets, including melting, changes the normal loading on the fault system. Thus, changes in the stress state on a fault due to mass changes can either inhibit fault slip or cause a fault to slip. In a global sense, this does not change the plate tectonic behavior. Plates are affected by forces nearby, as well as far away. Basically, the plates cover our earth like huge pieces of a jigsaw puzzle. Plates have switched direction before, like the Pacific plate around 50-42 Ma, but this rearrangement was due to global scale changes in plate tectonic activity (like the Pacific-Farallon and Pacific-Antarctic relationships) - or changes in the global scale arrangement of the puzzle pieces, creating a new puzzle.

To summarize, mass changes due to melting may cause local earthquake behavior, but will not have far reaching consequences to overall plate tectonic activity.


Q: I just recently saw a video claiming we have found an underground reservoir larger than the oceans combined in between the upper and lower mantle. It was said this reservoir was found by scientists who were studying seismic activity. Can you confirm on this and provide any more information on this particular subject matter?

A: Sure. There's been quite a bit of confusion surrounding this particular study. There are not actually oceans of water down in the mantle like there are oceans on the surface. What the researchers found was that there are minerals deep in the earth that are capable of containing water in their crystal structures even at great depth and high temperature. The amount of water that these minerals contain is thought be roughly equivalent to the amount of water in the ocean - in other words, significant. Here is a piece that explains it pretty well. The actual peer reviewed paper is here. This new finding gives us insight into the water cycle of the earth, as well as information about the mineral composition of the mantle. This work was done using seismic information collected from the EarthScope USArray.


Q: What can one say about [the] recent Nepal earthquake [Mw7.8 Ghorka event]?

A: We actually had quite a bit to say about the Nepal earthquake, so we made a video - https://www.youtube.com/watch?v=5VjaSFEf4BU&feature=youtu.be

 

INDUCED SEISMICITY

Q: Is there any correlation between man's activity and earthquakes?

A: Yes! People have been causing small earthquakes through mining activities for a long time, but recent advances in extraction techniques for oil and gas production have greatly increased the size and frequency of man made earthquakes. http://earthquake.usgs.gov/research/induced/


Q: I've seen conflicting studies on the theory that fracking can cause earthquakes. What are your thoughts on this? If you do believe that fracking can cause earthquakes, do you believe that using something similar to preemptively cause smaller, more frequent earthquakes along major fault lines might be used to keep larger more devastating earthquakes from happening near large population centers?

A: A paper that address the myths and misconceptions of hydraulic fracturing is Rubinstein and Babaie Mahani, SRL, 2015. The process of hydraulic fracturing itself can cause earthquakes, but I the biggest difference is between felt and non-felt earthquakes. For instance, southern California has around 30 earthquakes per day on average, but hardly any of them are felt. The same is true about hydraulic fracturing.

The purpose of hydraulic fracturing (hydrofracking) is to increase the production of oil and gas wells by increasing the number of pathways for fluids to flow between the rock formation and the well. The process of hydraulic fracturing achieves this by injecting fluids (usually water) into the ground at high pressure, such that it will fracture the rock or create a small earthquake, which typically isn't felt. Thus, fracking increases the fracture density and allows fluid to flow, and therefore be accessed by the oil or gas well. In the US, the largest earthquake observed that was correlated with hydraulic fracturing is a M3 event in Ohio see Skoumal et al., 2015. In Alberta, Canada, earthquakes related to hydraulic fracturing have been larger, around M4.

However, earthquakes related to wastewater injection, the deep injection of fluids after general oil and gas production processes can produce much larger (felt) earthquakes as compared to hydraulic fracturing. The largest earthquake ever observed to be correlated with induced seismicity is the 2011 M5.6 Prague, Oklahoma earthquake. I would highly suggest reading the Rubinstein and Babaie Mahani, SRL, 2015 paper, and watching Dr. Justin Rubinstein's IRIS webinar on the subject.

To answer your second question, when a small earthquake occurs, sometimes it can trigger a larger earthquake, creating a domino effect. I observed this during the 2011 Prague, Oklahoma earthquake, where the first M4.8 earthquake was induced by nearby wastewater injection (not hydraulic fracturing), and it triggered subsequent failure along the fault system, including the much larger M5.6 earthquake see Sumy et al., 2014. Because this is the case, it's best not to try to create a lot of small ones because you do not know how large the earthquake can get (how much of the fault will rupture), nor do you know what the resulting hazard along the fault will be.


Q: Is it plausible that these human-induced quakes could increase the risk of a larger quake in a nearby fault system?

A: Yes, a human-induced earthquake could increase the risk of a larger quake on a nearby fault system, but perhaps not as far away as New Madrid. The 2011 M4.8 Prague, Oklahoma earthquake, which is correlated with nearby wastewater injection, likely triggered the nearby M5.6 Oklahoma earthquake (see Sumy et al., 2014). That's an example of an earthquake occurring and effecting the static (permanent) stress changes on the fault triggering another earthquake. Another case is dynamic, where passing seismic waves over a fault can trigger an earthquake. Note that dynamic stress triggering does not result in permanent stress changes along a fault system, and can usually only trigger earthquakes that are ready to go. van der Elst et al., 2013 found that passing seismic waves from the M8.8 Maule, Chile earthquake could have triggered earthquakes in Oklahoma. For dynamic triggering to occur, especially at these far distances, the earthquakes have to be large, like a M8.8.


Q: Has there been any additional study into the 200+ mile diameter area surrounding Enid, Oklahoma to determine if there is some new fault or new geologic formation happening in the area? Is this too distant to be related or connected to the New Madrid (Missouri area) fault?

A: The areas around Prague, Jones, and Fairview, Oklahoma are intensely studied, and continue to be intensely studied by seismologists. Oklahoma is a highly faulted state, and you can see all of the known faults here. Larger earthquakes occur on pre-existing fault structures. Hydraulic fracturing in its very nature creates fractures in an existing rock formation, but cannot create a new geologic formation. These earthquakes are not connected to the New Madrid region in MO. [Footnote–The IRIS Wavefields Project was conducted in summer 2016 in northern Oklahoma. This experiment made use of cutting-edge three-component nodal-type sensors. These 5Hz sensors are about the size of a paint can, have onboard GPS timing, and can run independently for up to 30 days. IRIS deployed these sensors as a piggyback to an existing nodal deployment. The deployment took advantage of the 1000+ single channel nodes and 45 broadband sensors from Keranen (data available after moratorium) with instruments provided by IRIS and IRIS community members (~300+ 3-C nodes, 40+ broadbands, and 10 infrasound sensors) deployed in the same area. https://www.iris.edu/hq/event/wavefields_community_demonstration_experiment]


Q: I thought Oklahoma's earthquakes were unrelated to fracking. Much hydraulic fracturing is going on in other parts of the country, yet none are showing anywhere near this level of activity. Why are other states not having similar results? Where are some good sources of follow-up information?

A: Most of Oklahoma's earthquakes are unrelated to fracking, but are widely thought to be related to wastewater injection. There are thousands of wastewater injection wells in Oklahoma, which have been operational for decades in some cases. Oklahoma both has a much longer history of wastewater injection operations, and is heavily faulted - which is very different from other states. To relate this to your earlier question, neither hydraulic fracturing or wastewater injection creates new geologic formations. Check out earthquakes.ok.gov for more information.


Q: There is a lot controversy about oil drilling and mining operations can trigger massive earthquakes. Being a civil engineer, I don't give much credit to this controversy. Do you?

A: This question depends on what you mean by 'massive' earthquakes. The largest earthquake correlated with oil and natural gas activities (to date) is the 2011 M5.6 Prague, Oklahoma earthquake. Seismologists would call this a 'moderate' earthquake. However, we don't fully understand the potential for small earthquakes to trigger larger earthquakes. The 2011 M4.8 Oklahoma earthquake occurred in close proximity to nearby wastewater injection operations, and this earthquake triggered the M5.6 earthquake (see Sumy et al., 2014). If small earthquakes can trigger a larger earthquake along a fault system, then potentially, this could cause a larger earthquake. Again, depends on your definition of 'massive'.

As a civil engineer, you may be interested to know that even moderate earthquakes (like the ones in Oklahoma) can cause damage in the close, proximal regions to the earthquake. The M5.6 Oklahoma earthquake was felt in 17 states, caused chimneys to fall, and brick houses and foundations to crack.


Q: On the topic of induced earthquakes, what are your thoughts on fracking? Are countries blowing things out of proportion by banning such things, or do you believe that further research might be required?

A: Regarding the induced seismicity question, I think the subject of hydraulic fracturing, and especially its influence on earthquake activity, still warrants further investigation. Typically, hydrofracking does cause small earthquakes, but wastewater injection (the deep injection of large amounts of fluid) has the potential to trigger larger earthquakes. In Oklahoma, the main cause for concern is the wastewater injection activities, not the hydraulic fracturing. It's very important that government regulatory bodies, scientists, and concerned citizens work together to come up with the best solutions to their states problems.


Q: Does waste water injection happen without fracking? Or is it part of fracking?

A: Wastewater injection for the purposes of disposal (what's causing the earthquakes) DOES happen without fracking. In fact, much of the wastewater injection in Oklahoma is related to produced water. The following is from Rubinstein and Babaie Mahani, SRL, 2015: '...spent hydraulic fracturing fluid represents 10% or less of the fluids disposed of in salt-water disposal wells in Oklahoma (Murray, 2013). The vast majority of the fluid that is disposed of in disposal wells in Oklahoma is produced water. Produced water is the salty brine from ancient oceans that was entrapped in the rocks when the sediments were deposited. This water is trapped in the same pore space as oil and gas, and as oil and gas is extracted, the produced water is extracted with it. Produced water often must be disposed in injection wells because it is frequently laden with dissolved salts, minerals, and occasionally other materials that make it unsuitable for other uses.'


Q: Gail M. Atkinson et al. recently reported that induced seismicity from hydraulic fracturation [sic] activities, while still relatively infrequent, could reach magnitudes somewhat greater than what was previously believed (which used to be below a magnitude of about 3.0, usually closer to 2.0), roughly on par with that of events triggered by disposal wells (say up to the 4.0 to 4.8-ish magnitude range).

Question 1: For areas where both activities are taking place, how would one be able to distinguish induced seismic events due to hydraulic fracturation from those induced from wastewater disposal?

Question 2: Also what kind of follow up would an induced seismic event (whether from fracking or reinjection) of a magnitude between 4 and 5 warrant from both Industry and Government regulators?

A: Dr. Atkinson did an IRIS Webinar on this subject. Check it out!

A1) It's commonly thought that wastewater injection and hydraulic fracturing occur side by side, but more often than not, that's not the case. For instance, the state of Pennsylvania allows hydraulic fracturing, but not wastewater injection - thus, the two operations occur nowhere near each other. In general, we determine whether an earthquake is correlated with hydraulic fracturing or wastewater injection if: 1) there's a deviation from the background level of earthquake activity, 2) the earthquake occurs in close proximity and at the same depth as these types of activities, and 3) during the same time frame (but not always) as hydrofracking or injection activities are taking place. There's a great paper that goes through this by Davis and Frohlich, 1993.

A2) A M4-5 earthquake would require regulators to examine closely whether they need to shut off current wastewater injection activities or shut down hydraulic fracturing operations. The 'traffic light' system is a good protocol that some states and countries are trying to use. A great paper on the risk mitigation strategies of induced seismicity was published by Bommer et al., 2015.


Q: So the earthquakes we've been having more and more in Kansas could be from wastewater?

A: Yes, a lot of those earthquakes in Kansas are related to wastewater injection. A great suggested paper to read on the subject is by Weingarten et al. [2015].


Q: Could you imagine a day where geologists use fracking techniques to release pressure on large faults to reduce risks of more powerful, natural earthquakes?

A. Hydraulic fracturing tends to take place in much shallower formations than wastewater injection, which is one (of many) reasons why larger earthquakes can occur from wastewater injection processes. One of the reasons is that earthquakes that start deep tend to rupture to shallower portions of the fault, as it's the 'path of least resistance'. Basically, there is less overburden. Would it impact the amount of seismic activity? Hard to say, as it depends on the fault structure, the stress on shallower portions of the fault, and myriad other factors. Plus, it would take an incredible number of small earthquakes to equal the amount of energy released by just 1 larger earthquake. For example it would take 32 magnitude 5 or 1,000 magnitude 4 or 32,000 magnitude 3 earthquakes to equal the energy released by just one magnitude 6 earthquake! To answer your question about the interaction between cool water and hot rock, the thermal differences like this are observed in both geothermal and oceanic hydrothermal settings, and can cause earthquakes due to thermal contraction. These earthquakes tend to be very small though, and tend to be in swarms (lot of little earthquakes that occur all at once). I like to call swarms 'popcorn earthquakes', but that's just me!

 

EARTHQUAKE PREDICTION

Q: Can earthquakes be predicted yet?

A: Unfortunately we can't predict earthquakes. BUT, we can use the information we do know about different faults and their earthquake history to "forecast" the potential for shaking from an earthquake in a particular area over a certain time interval. This may sound like splitting hairs but it's actually an important distinction (Earthquake Forecasting and Earthquake Prediction: Different Approaches for Obtaining the Best Model). Forecasting earthquakes means that we can tell you the likelihood of an earthquake of a certain size in a particular area over a particular time interval–similar to a weather prediction. So even though we can't say to expect an earthquake of a certain size on a certain day we can tell you what you should expect in a particular area over, say, the next 30 years. For example, in June 2016 the USGS released an earthquake forecast for the San Francisco Bay area (https://pubs.er.usgs.gov/publication/fs20163020) stating that there is 72% possibility of a M6.7+ earthquake occurring in the Bay Area before 2043. This (theoretically) gives city planners and officials the information they need to make informed decisions about retrofitting and strengthening infrastructure. People that live in earthquake country should be prepared for the inevitable earthquake regardless of when it happens. You can find more information about earthquake preparedness here - http://www.earthquakecountry.org.


Q: I've read that California is in for a BIG quake and I'm fairly concerned. Friends of mine have even moved out of state in anticipation of it. Your thoughts?

A: Undoubtedly California will have a large, destructive earthquake at some point in the future. The Pacific Plate and North American Plate are continuing to move, which is causing stress to build along the boundary of those 2 plates. The San Andreas Fault is technically the plate boundary, but there are a whole myriad of other faults that help to accommodate this motion. Eventually, the built up stress along one of these faults will exceed the strength of the rocks and the fault will break, causing earthquake shaking. The loss of life and property that will occur as a result of that shaking is dependent upon many factors including, but not limited to, where the earthquake occurred (in a populated or remote region), the quality of the infrastructure (were buildings and roads retrofitted?) and the readiness of the population. Additionally, there is much that you can do to prepare for an earthquake. Make sure your home or apartment is retrofitted, secure large objects to the wall, have an emergency plan and appropriate supplies. You can learn more about how to be prepared for an earthquake here.


Q: How much time is left before the "Big one" happens on the pacific plate?

A: That depends on whether you’re talking about the “big one” on the San Andreas Fault in California or a “big one” in the Pacific Northwest. The short answer in both cases is that we don’t know for sure. We know from historic records and paleoseismology (the study of ancient earthquakes) about how big earthquakes in these areas can be and about how often they happen. But there are a lot of variables.

We know that different parts of the San Andreas Fault behave in different ways – some sections have lots of small earthquakes and creep along, whereas other sections are “locked” or stuck. Those are the segments that produce damaging earthquakes. You may have heard that Southern California is "overdue" for an earthquake on the San Andreas. What that means is that the time since the last earthquake on that part of the fault is longer than the average time we have observed for earthquakes on that section. There was a great discussion about this after Tom Jordan of the Southern California Earthquake Center (SCEC) made a comment about how the San Andreas is "locked, loaded and ready to go". The best discussion, IMHO, is this one.

The Pacific Northwest also has significant earthquake hazard (and volcano hazard) because the Juan de Fuca plate is diving down (or subducting) beneath the North American plate. (https://youtu.be/_belQwGNolY) The largest earthquakes in the world happen along these types of plate boundaries. However, there are lots of things that government officials, policy makers and the public can do to mitigate risk due to earthquakes. http://www.earthquakecountry.org/booklets/index.html

Here are some videos and animations from IRIS that talk about the geology, earthquake and tsunami risk in the Pacific Northwest. https://www.youtube.com/playlist?list=PLngDHXr1w29SVVyip5E9bYnuCcEeL-CGw

 

GENERAL SEISMOLOGY

Q: I was wondering what measuring the earthquake depth tells you about what's going on under the surface (causes, results, etc.). Do you measure the depths by triangulating from different stations?

A: We do use methods similar to triangulation to determine the location of earthquakes, including the depth at which they occurred. The USGS has a great explanation here, and IRIS has several activities on locating earthquakes at https://www.iris.edu/hq/inclass/search#type=3&4/concept=5

The depth at which an earthquake occurs is actually very important, particularly when you think about damage to human structures. This is somewhat oversimplified but is a good way to think about it- If you have an earthquake that occurs at a depth of 500 km then that earthquake is 500 km from the nearest populated area. If you have an earthquake that occurs at a depth of 5 km then you are only 5 km from the nearest populated area. Of course, deep earthquakes usually occur on subduction zone boundaries, which cause some of the largest earthquakes. Earthquakes on strike slip faults, for example, tend to be much more shallow. You can see this in the IRIS Interactive Earthquake Browser. The dots are earthquakes and the purple dots are shallow earthquakes, whereas the red and orange dots are deeper earthquakes. Just by looking at the distribution of dot colors on the map you can get insight into what type of plate boundaries are where. Be sure to use the 3D button on the lower right side to rotate the image and look inside the earth. So, the depth of an earthquake can tell you something about the type of fault on which it occurred.

More telling, however, is the focal mechanism. We sometimes call these beach balls. Focal mechanisms give us information about the way that the fault moved. Here is a great animation about focal mechanisms - https://youtu.be/MomVOkyDdLo


Q: Is it possible to model the waves of an earthquake mathematically and accurately?

A: Yes, seismograms can be simulated using the estimated elastic properties of earth materials, the structure of the Earth obtained from decades of scientific research, and differential equations. These are now routinely produced for moderate to large earthquakes by a group in Princeton (http://global.shakemovie.princeton.edu/), and also hosted by IRIS (http://ds.iris.edu/ds/products/shakemoviesynthetics/). Using these techniques, we can simulate what the waves from a specific earthquake would look like even if it hasn't happened yet. The more accurate our models of the earth are, the more precise the simulations become.


Q: What's going to happen to the transportable array when EarthScope is done? Will these instruments just enter the general pool of seismometers managed by IRIS? A follow up question, what's the lifespan of a seismometer that's part of the instrument pool?

A: The EarthScope Transportable Array (TA) is a temporary deployment of seismometers from west coast to east coast that started in 2004 and completed in the lower, contiguous 48 states in 2014 and is currently being installed in Alaska. At each location, a seismometer is in place for 2 years. The TA will complete its journey in Alaska in 2018. Our hope is that some of these stations will be adopted in Alaska. As for the others, we're currently deciding how they will be dispersed among other programs, such as PASSCAL, the Global Seismographic Network, and perhaps even for use in the Ocean Bottom Seismograph Instrument Pool. One of the metrics we examined was the age of the existing instrument pool, broken down into the various programs. The average age of our instruments is around 10 years old, though some of instruments are already around 25 years old (like those use in the GSN).

Q: Are there computer programs used to determine the source location of an earthquake? How do they work? Is there a particular mathematical algorithm used?

A: The USGS put out a great FAQ about how to locate earthquakes and here at IRIS we have a great activity to learn how to locate earthquakes. Sophisticated algorithms to locate earthquakes include HYPOINVERSE, VELEST, and hypoDD, among others - this is a very small sampling of earthquake location algorithms.

One of the reasons why different earthquake catalogs (or listings of earthquakes) may have different locations and depths is because there are many earthquake algorithms that can be used to locate earthquakes, and there is no particular algorithm that is the standard earthquake location tool.


Q: As someone who is not a seismologist but spends a lot of time looking at earthquake locations/distributions, why can locations and depths of earthquakes be so variable between catalogs? Different algorithms for locations? Different velocity structures?

A: Seismologists can use different algorithms for locations, as well as a different distribution of stations to determine the location, and different velocity structures. Also, different parameters in the same algorithms can result in changes to the location and depth, as can differences in where the P- and S-wave arrivals are picked.


Q: When comparing earthquake locations between local catalogs (e.g. a particular countries seismic network that they might not share widely or a temporary array deployed after a major event) and teleseismic catalogs, should deference always be given to the local data as being closer to the 'truth' in terms of locations?

A: For earthquake catalogs, local data is usually better because the arrivals are more clearly seen on a seismogram, as the waves have not had time to attenuate as they can at far distances. Local data has not been as perturbed by the earth's structure as seismograms at further distances. You also want 360 degree coverage too, so if local deployments do not accomplish that, then regional and teleseismic distances can help with that. Local data is a huge help, but not always possible.


Q: Do you know of any other novel ways that are in development for handling the noise from stations located over thick basins? I know that usually the way to lessen the spurious arrivals is through stacking of multiple events, but are there any others I haven't come across? I primarily use data from your organization and SPREE, so thank you for your work!

A: This has been a challenging issue in converted wave imaging for decades, in both sedimentary basins and more recently for stations that are being deployed atop thick ice sheets, on mountain tops (due to an inverse basin effect), and at the bottom of the ocean (from reverberations with the water column). Presumably you are working with P-wave receiver functions if basin reverberations are a problem. If so, then depending on whether you're just interested in 3D imaging or just wanting to extract certain properties (e.g. Moho depth, crustal Vp/Vs) there is a fair amount of existing work which you may be able to work from. Recent approaches include incoherency filtering of arrivals if you have good azimuthal sampling, modeling the response of your shallow low wave speed layer and removing it from the deconvolution, or extracting properties from the receiver function in sequence.

 

ADVICE FOR STUDENTS

Q: Do you have any general advice for students who are intending to study geology? I am starting college this fall and would love to hear some advice.

A: My advice would be to take lots of different types of classes in addition to your geology classes. Of course you need the standard requirements like math and physics and chemistry, but also make sure you are competent with ArcGIS (or other GIS systems), learn how to code, and take at least one art class. The utility of the first few class suggestions is probably clear, but maybe not the art class part. A geologist is a trained observer, and art classes will teach you how to observe details in a systematic way, as well as give you some skills that will enable you to transcribe and record your observations. Invaluable! Good luck!!


Q: What do you think would be the best way to get the younger generations interested in Earth Sciences? What sort of public outreach do you think is the most successful when it comes to drawing up public awareness for the sorts of things you do, or getting interest in the sciences in general?

A: Getting students/kids engaged at local museums is always a great way! Getting kids outdoors also helps to introduce them to earth sciences at a young age and can inspire their curiosity about the natural world. Science fairs are also a way to meet scientists, and they are usually FREE! At IRIS, we participate in events at the Smithsonian museums, at the USA Science and Engineering Festival, and at other events around the country. Most importantly, encourage kids to ask questions!