Relative Sizes of Some Recent Seismic Events

A few things about this plot (all numbers are approximate, but they are correct enough for this level of analysis):

kT = kilotons

2013 DA14 refers to how big an event the near-Earth asteroid that missed us on Friday, Feb. 15, 2013 would have been if it crashed.

Japan quake is the 2011 mega-quake, and Haiti quake is the 2010 quake that devastated Haiti.

Haiti quake (magnitude 7) is very small (in terms of magnitude) compared to Japan quake (magnitude 9).

Everything else is very small compared to Japan quake.

In very rough numbers/guesstimate: The meteorite that killed the dinosaurs was equivalent to something like magnitude 10 to 12. If we use the number 11, that’s at least 100 times bigger than Japan quake. So, if we plotted the dinosaur meteorite, everything else here would probably be too small to see.

Hurricane Sandy Recorded by Seismographs: Interdependency and Interrelationships Within the Earth System

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College 

Seismology provides an interesting window into the interdependency and interrelationships within the Earth System.

The seismograms of Hurricane Sandy shown below were recorded by the Boston College Educational Seismology Project (BC-ESP) on our BC campus seismograph. This is a good example of seismology as a window into the interdependency and interrelationships within the Earth System. Hurricane winds and waves generate seismic waves that are recorded by seismographs. And by coincidence, this is not only a fascinating recording of an historic hurricane, but it happens to also include one of the most well-recorded earthquakes I have ever seen on our educational seismographs. Plus, we just happened to record aftershocks of that well-recorded quake on the same seismogram as the main shock.

And, if you look very carefully near the beginning of the October 30 seismogram, you can see a magnitude 6.2 aftershock “hiding” beneath the hurricane waves.

Boston Area Reports of the October 16, 2012 Earthquake in Maine

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

The magnitude 4.0 earthquake that occurred near Hollis Center, Maine on October 16, 2012 was widely felt across New England. Below are reports from people in the Boston area describing their experiences of the quake. Click here to read the reports.

To put these reports in a larger context, Figure 1 shows the distribution of felt reports from the U.S. Geological Survey’s Did You Feel It? website.

Figure 1: U.S. Geological Survey “Did You Feel It?” reports.

To compare these personal descriptions of what people experienced with what is recorded on seismographs, consider the seismogram shown in Figure 2, which was recorded at Boston College. When I asked people how long the shaking lasted, the responses ranged from 3 to 20 seconds, with an average of 9 sec. Also, a few people reported a “rumbling” sound, which (combined with their estimates of the duration) suggests that they were feeling/hearing the S waves and the lower-frequency surface waves.

Figure 2: Seismogram of October 16, 2012
earthquake recorded at Boston College.

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Boston Area Reports of the October 16, 2012 Earthquake in Maine

“The frame of the house, the doors and the windows shook and vibrated for a several seconds.”

“I was on the top floor of my friend’s dorm with a group of friends when the tremors started. We first heard a table rattle against the wall and then the shaking moved up through the couches and distributed evenly across the whole room. Right away someone asked if it was an earthquake, but we laughed it off and assumed it was just people running down the hall on the floor below us. I’d say the shaking lasted a total of 5 or 6 seconds”

“I was in my bedroom (3rd floor, wood house in Brighton) during the earthquake. For about 10 or 15 sec, the house shook from side to side. I thought that the downstairs tenant was moving some very heavy furniture up the stairs.”

“It seemed I heard it more than felt it – actually looked out the window to see if it was some distant gas explosion. It sounded like a rumble with some vibration, like a caravan of large army trucks going by. Lasted about 5 sec…”

“… I did not feel the earthquake myself, however my family … felt it. … While she was walking across the hall in the house she felt everything (all the furniture in the house) move, including herself…”

“I think I felt the earthquake. It felt like a large truck/train was going by for about 10 seconds. I was sitting down at my desk in my third floor apartment, and my apartment seemed to vibrate not shake or anything more intense.”

“On October 16th at 7:13 I was reclining in my coach … and then I heard/felt what sounded and felt like a Grand Piano or really big weight “hit” the flat roof above my apartment, “I yelled” “Knock it off!” as I thought it was some workers moving equipment on the roof which sometimes happens, then I heard what I thought was a collision of something really big with either the side of the building or in the hallway, what I felt then was from the side, and then there were 2 or 3 other loud “collisions” which then I knew I was experiencing my first earthquake! The total time I would say was approximately 10-12 seconds.”

“I was sitting in a corner study lounge on the 5th floor of my dorm when I started to feel the shaking. I noticed it in the walls around me. My first thought was that it was an earthquake, but was so surprised that I didn’t actually believe it until I checked USGS. But I felt about 4 to 5 seconds of shaking, accompanied by a rumbling sound (which could have come from the building or the earthquake itself). “

“My family was sitting at the dinner table in our third-floor apartment when the quake hit… probably about 5 seconds of swaying — definitely enough time to say “that’s an earthquake!” Based on our experiences in Christchurch … guessed it at a 4.5 magnitude. That was a pretty good guess.”

“We were sitting in our apartment in the third floor when it happened. Me and my wife were on the sofa talking and my daughter was playing on the floor when the whole apartment started shaking. My daughter got scared and jumped on the sofa and sat next to me. She remained frozen with fear as we watched the floor, curtains and the walls shake. In the kitchen china rattled and our LCD TV started oscillating back and forth on its base on the table. It lasted for about 15 seconds.”

“During the earthquake I felt my futon slightly moving back and forth, it was the sort of disturbance you feel when you live in an old house and someone is having a dance party in the floor above you. It must have taken me about 6 seconds to realize that this was happening and then I felt a rather strong shake that made my tv move slightly. The stronger shake must have last about 3 seconds, since I remember quickly standing up, looking at the window and noticing that everything was calm again.”

“I definitely felt the earthquake, like a giant truck rumbling too close to our house – the kitchen floor shook with quick vibrations. … I also heard the glass bottles rattling on the open shelves.”

“My wife and I were walking our dogs in Jamaica Plain when I heard an odd low rumble coming from the direction of downtown Boston. At first I just assumed it was a jet or bus, but as the sound grew louder and closer I could hear that it was more broad and not a point source. Within a few seconds the sound rolled right though our neighborhood and I could hear what was clearly the sound of creaking buildings and a low earthly rumble. The wave of sound continued past to the south. I commented on how weird the whole thing was, and only later found out that what we experienced was the quake propagating from north to south through the greater Boston metro.”

“Cellular Seismology”: Does Past Seismicity Delineate Zones Where Future Large Earthquakes are Likely to Occur?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Although earthquake prediction is not (currently?) possible, people of course still want to plan as well as they can for the impact of future large earthquakes. An obvious component of such planning, if it were possible, would be knowing where future large earthquakes are likely to occur.

Plate tectonics provides a very successful model for describing where the vast majority of earthquakes occur, i.e., at or near plate boundaries. The development of the theory of plate tectonics in the 1950s and 60s depended to a large extent on the fact that most earthquakes repeatedly occur within the same zones, zones that we now know are plate boundaries. But one of the fundamental questions that still remains to be answered is: What causes some (occasionally quite large) earthquakes to occur outside of those zones, in locations that are not near plate boundaries?

Figure 1: Global seismicity for two 19-year time periods, 1973-1991 and 1992-2010 (magnitude 5 and greater). For any time period long enough to represent a good statistical sample, the vast majority of earthquakes map out plate boundaries, demonstrating that on a global scale most earthquakes keep occurring in the same places.

These non-plate boundary (NPB) earthquakes are either strictly “intraplate” earthquakes (i.e., those that occur deep in the interiors of plates) or earthquakes that occur in diffuse zones near, but not on, plate boundaries. Although we know that, on a global scale, most earthquakes repeatedly occur in well-defined zones, we don’t know if that is the case for NPB earthquakes. Do NPB earthquakes repeatedly occur in specific zones, or do the zones they occur in “migrate” over time from one place to another such that they might eventually occur essentially anywhere in plate interiors? If NPB earthquakes occur in specific zones that remain stationary over time, then given a long enough record of past seismicity, we should be able to discern where large NPB earthquakes will occur in the future. But if NPB seismicity migrates over time, then past seismicity will not be a useful indicator of where future large NPB earthquakes will occur.

If we are to ever understand the cause of NPB earthquakes, we will have to resolve this question of whether or not the locations where these NPB earthquakes are occurring are persistent. And we will have to determine the characteristics of Earth processes in those zones that are seismically active, as compared to zones that aren’t. My attempt to answer these types of questions has led me to invent the method known as “Cellular Seismology” (CS). Details of my CS research have been published in scientific/technical papers (e.g., Kafka, 2002; Kafka, 2007; and Kafka and Ebel, 2011). What follows here is my explanation of the essence of CS in (what I hope to be) simple, non-technical language.

CS is an intentionally simple method of systematically investigating the relationship between locations of past and future earthquakes in a given region. The name “Celluar Seismology” was chosen because it is analogous to a cellular phone system, with past earthquakes acting analogously to a cell phone tower. The cell tower is associated with a circular zone, extending some radius away from the tower, within which cell phones can receive a signal from the tower. Analogously, we envision that some circular zone surrounding the epicenter of a past earthquake is a zone that presumably has the necessary geophysical characteristics to generate future earthquakes.

CS involves analyzing what seismologists refer to as “earthquake catalogs”, i.e., databases of times, locations and magnitudes of earthquakes in a given region. To implement CS we construct circles of a given radius around each epicenter in an earthquake catalog (which we call the “Pre-CAT”), and investigate the percentage of later-occurring earthquakes (in what we call the “Post-CAT) that were located within that radius of at least one previous earthquake. These Post-CAT earthquakes that occurred near a Pre-CAT earthquake are referred to as “hits.”

We then systematically analyze the observed percentages of hits in an attempt to discern the extent to which patterns emerge in the relationship between locations of past and future earthquakes. We have found what seems to be a stable pattern of at least 2/3 to 3/4 of future earthquakes occurring near past earthquakes in most regions, and we are now investigating how the patterns compare and contrast for different regions.

Figure 2: Hypothetical region, showing how Cellular Seismology works. When a red (Post-CAT) earthquake occurs within a green zone, i.e., a region surrounding a Pre-CAT earthquake, that Post-CAT earthquake is referred to as a “hit.”

Figure 3: Cellular Seismology results for Northeastern United States. This analysis is shown for the case of earthquakes with magnitude of 3 and greater. The circle radius around the Pre-CAT earthquakes is chosen so as to fill 33% of the map area within the blue polygon. For 33% map area, there are 84% hits for this case.

Figure 4: Cellular Seismology results for California. This analysis is shown for the case of earthquakes with magnitude of 4 and greater. The circle radius around the Pre-CAT earthquakes is chosen so as to fill 33% of the map area within the blue polygon. For 33% map area, there are 93% hits for this case.

This CS research has been occurring within the context of a resurgence of interest among some seismologists in earthquake forecasting. It is well accepted among most seismologists that “earthquake prediction” (which refers to predicting the specific time, place and magnitude of an earthquake) is not currently possible, but some level of “earthquake forecasting” (which refers to more long-term estimates of the probability of earthquakes occurring within some region) is considered to be a reasonably attainable goal.

We applied CS to earthquakes in California where there have been recently published earthquake forecasts based on past seismicity. The results are somewhat counter-intuitive. We are finding (so far?) that there does not appear to be anything in the record of past seismicity that is any more predictive of where future earthquakes are likely to occur other than the simple notion that future earthquakes tend to occur near past earthquakes.

The underlying philosophy behind CS is to prefer the simplest (most parsimonious) approach to analyzing any phenomenon of interest. We tried using more complicated approaches to analyzing the relationship between past seismicity and later-occurring earthquakes, and found that more complicated methods showed insufficient gain in predictability to warrant any more complicated approach to this problem than the simple CS method. Thus, we argue that before invoking a complicated solution to predicting locations of future earthquakes, that complicated approach should be checked to see if it performs any better than CS, which we think is a reasonable, least astonishing, hypothesis.

Click here to see data visualization movies of Cellular Seismology results.


References:

• Kafka, A.L (2002). Statistical analysis of the hypothesis that seismicity delineates areas where future large earthquakes are likely to occur in the central and eastern United States, Seismological Research Letters, 73(6), 990-1001.
• Kafka, A.L. (2007). Does seismicity delineate zones where future large earthquakes will occur in intraplate environments?, In Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, Geological Society of America Volume, Geological Society of America Special Paper 425, Edited by S. Stein and S. Mazzotti, 35–48, doi: 10.1130/2007.2425(03).
• Kafka, A.L. and J.E. Ebel (2011). Proximity to past earthquakes as a least astonishing hypothesis for forecasting locations of future earthquakes, Bulletin of the Seismological Society of America, 101(4), August 2011, doi: 10.1007/s00024-011-0315-1.

“Cellular Seismology” Data Visualizations of How Patterns of Earthquake Locations Evolve Over Time

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

I have begun to explore the use of “data visualizations” to illustrate the results of my research on how patterns of earthquake locations evolve over time. You can go directly to the data visualizations by scrolling to the bottom of this post, but you may want to read the following explanation first.

These data visualizations are based on the concept of “Cellular Seismology” (CS), which I developed to test the hypothesis that past seismicity delineates zones where future earthquakes are likely to occur (Kafka, 2002, 2007). CS studies address one of the fundamental (and as yet unresolved) questions in seismology: To what extent do past earthquake locations delineate zones where future large earthquakes are likely to occur? Many claimed methods of forecasting earthquakes depend on the assumption that past seismicity indicates where future earthquakes are likely to occur. But this assumption, while true in general, is not yet verified at all the levels of detail necessary to directly relate past seismicity to specific future earthquakes and thus to predict exactly where future large earthquakes are likely to occur.

CS is an intentionally simple method of systematically investigating the relationship between locations of past and future earthquakes in a given region. The name “Cellular Seismology” was chosen because it is analogous to a cellular phone system, with past earthquakes acting analogously to a cell phone tower. The cell tower is associated with a circular zone, extending some radius away from the tower, within which cell phones can receive a signal from the tower. Analogously, we envision that some circular zone surrounding the epicenter of a past earthquake is a zone that presumably has the necessary geophysical characteristics to generate future earthquakes.

CS involves analyzing what seismologists refer to as “earthquake catalogs”, which are databases of times, locations and magnitudes of earthquakes in a given region. To implement CS we construct circles of a given radius around each epicenter in an earthquake catalog (which we call the “Pre-CAT”), and investigate the percentage of later-occurring earthquakes (in what we call the “Post-CAT”) that were located within that radius of at least one previous earthquake. These Post-CAT earthquakes that occurred near a Pre-CAT earthquake are referred to as “hits.”

We then systematically analyze the observed percentages of hits, in an attempt to discern the extent to which patterns emerge in the relationship between locations of past and future earthquakes. We have found what seems to be a stable pattern of at least 2/3 to 3/4 of future earthquakes occurring near past earthquakes in most regions, and we are now investigating how the patterns compare and contrast for different regions.

Technical details regarding how the patterns compare and contrast for different regions are described in Kafka (2002), Kafka (2007), and Kafka and Ebel (2011). The data visualizations presented here provide a visual representation of the evolution of locations of earthquakes in Post-CAT relative to CS maps of the locations of earthquakes in the Pre-CAT for the following four regions:

• Northeastern United States
• California
• Caribbean
• Whole Earth

Click on the above links to see the data visualizations of the CS results.

In each of these movies, the Pre-CAT consists of earthquakes occurring from some start date up to 1999, and the Post-CAT is shown for each year from 2000 to 2011. The CS map of the Pre-CAT is shown in grey, and the locations of the Post-CAT earthquakes are color-coded such that they correspond to a spectrum ranging from blue (2000) to red (2011). Also shown for each year is a bar with its height representing the percentage of hits.

As you view these movies, see if you are able to discern any patterns in how the locations of the Post-CAT earthquakes evolve over time relative to the locations of the Pre-CAT earthquakes.

References:

• Kafka, A.L (2002). Statistical analysis of the hypothesis that seismicity delineates areas where future large earthquakes are likely to occur in the central and eastern United States, Seismological Research Letters, 73(6), 990-1001.
• Kafka, A.L. (2007). Does seismicity delineate zones where future large earthquakes will occur in intraplate environments?, In Continental Intraplate Earthquakes: Science, Hazard, and Policy Issues, Geological Society of America Volume, Geological Society of America Special Paper 425, Edited by S. Stein and S. Mazzotti, 35–48, doi: 10.1130/2007.2425(03).
• Kafka, A.L. and J.E. Ebel (2011). Proximity to past earthquakes as a least astonishing hypothesis for forecasting locations of future earthquakes, Bulletin of the Seismological Society of America, 101(4), August 2011, doi: 10.1007/s00024-011-0315-1.

Where was the only nuclear power plant in the U.S. that was ever automatically shut down because of an earthquake?

No, it wasn’t in California… It was in Virginia.

The North Anna Nuclear Power Plant in Virginia was automatically shutdown when it was shaken by the August 2011 magnitude 5.8 earthquake in Virginia.

And…

North Anna was also the only nuclear power plant in the U.S. where measured ground shaking ever exceeded the level it was designed to withstand.

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Last Year’s Virginia Earthquake May Have Been Felt by More People Than Any Other Earthquake in U.S. History!

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Most people living in the eastern U.S. probably think of places like California and Japan when they hear the word “earthquake.” But last year’s magnitude 5.8 earthquake in Virginia was a stark reminder that large and damaging earthquakes do occur in the eastern United States. And when eastern U.S. earthquakes occur, their effects can be quite dramatic compared to earthquakes of the same magnitude in the western United States. Here’s why:

Because of the difference in the structure of the Earth’s crust beneath the eastern U.S. versus the western U.S., seismic waves propagate more efficiently in the East than in the West. This difference is due to the fact that the crust beneath the East is older, warmer, and more solid than the crust underlying the West. The result of this difference is that when an earthquake of a given magnitude occurs in the eastern United States it is felt (and causes damage) over a much wider area than if that same size earthquake were to occur in the western United States.

This can be dramatically seen in the figure below, which shows areas where the Virginia earthquake was reported as being felt, as compared to areas where a magnitude 6 earthquake in California was reported as felt. The Virginia earthquake, although similar in size to the California quake (actually a bit smaller), was felt over an area about six times as large as that of the California earthquake.

This difference in the efficiency of seismic wave propagation, coupled with the fact that the region surrounding the Virginia quake is so densely populated, results in a surprising conclusion:

It is quite likely that the Virginia earthquake was felt by more people than any other earthquake in the history of the United States!

So even though earthquakes are obviously more frequent in California than they are in the East (due to California being on the plate boundary between the North American and Pacific plates), earthquake shaking and damage are nonetheless very real phenomena in the eastern United States.

If you felt this earthquake, please tell us your story by submitting a comment here.

References and Additional Reading:

For additional information about the Virginia earthquake, and more about the differences between eastern versus western U.S. earthquakes, see:

Learning from the 2011 Virginia Earthquake, Congressional briefing testimony of J.W. Horton, Congressional Hazards Caucus, March 29, 2012, www.hazardscaucus.org/briefings/horton-eqeast0312.pdf.

Mineral Virginia Earthquake Illustrates Seismicity of a Passive-Aggressive Margin, E. Wolin, S. Stein, F. Pazzaglia, A. Meltzer, A.L. Kafka, and C. Berti (2012), Geophysical Research Letters, 39(L02305), doi:10.1029/2011GL050310, www2.bc.edu/alan-kafka/ Virginia_082311/Virginia_GRL.pdf.

The Enigma of Why a Magnitude 5.8 Earthquake Occurred in Virginia, A.L. Kafka, akafka.wordpress.com/2011/12/27/the-enigma-of-why-a-magnitude-5-8-earthquake-occurred-in-virginia.

An Amazing 40-Year-Long Seismogram: A Whole New Way of Seeing Our Planet Quake

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

This seismogram, produced by the Albuquerque Seismological Laboratory (one of the all-time best seismic stations on Earth), shows a 40-year record of seismic recording!

(Higher-quality versions of this seismogram can be found here and here.)

This is a view of the Earth quaking that has never been seen before. It’s that mind-boggling!

Also shown below for comparison is a more typical 24-hour seismogram, with each line representing an hour. In the 40-year seismogram, instead of each line representing an hour, each line represents a month. The vertical axis shows the year, the horizontal axis shows the day of the month, and the labels on the right side of the vertical axis identify significant, globally recorded earthquakes.

Three relatively small earthquakes can be seen in the 24-hour seismogram (recorded in Massachusetts at Weston Observatory on August 23, 2011):

Virginia, Magnitude 5.8
Colorado, Magnitude 5.3
Colorado, Magnitude 4.6

Three mega-quakes, the largest earthquakes recorded since 1972, can be seen on the 40-year seismogram:

Sumatra 2004, Magnitude 9.1
Japan 2011, Magnitude 9.0
Chile 2010, Magnitude 8.8

Also seen on the 40-year seismogram are the many smaller earthquakes that happen every month. The smallest earthquakes that can be seen on this seismogram are about magnitude 6 (i.e., about as big as the Virginia earthquake shown on the 24-hour seismogram).

Note: The title of this blog has been updated from “BC-ESP Discussion Forum” to “Musings in the Quake Zone” to reflect what it has evolved into.

Broken Garage Door Springs, Earthquake Prediction, and Earthquake Triggering

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

A few days ago, one of the huge springs on our garage door opener broke, with quite a big KABOOM!

It seems to me that this is a perfect example of the problem of earthquake prediction. Here’s what I see as the connection:

In the morning, the garage door repair guy (who happened to be there fixing something else) looked up at the spring and said, “That deformed spot in the spring is ‘metal fatigue’ – it might break in 5 minutes, or it might break in 10 or 20 years.” Then, two hours later, it broke! (Perhaps he disturbed the internal stress field while he was fixing something else?)

This seems to be a perfect analog of the Elastic Rebound Theory of earthquakes, and the phenomenon of Earthquake Triggering!

It also explains why we knew that an earthquake was lurking in Haiti, but we couldn’t tell when it would occur: The tectonic stresses were ready to be released in an earthquake, but nobody could tell if the earthquake would occur in minutes, in years, in decades, or in centuries…

What is Earthquake Magnitude?

Alan Kafka
Weston Observatory
Department of Earth and Environmental Sciences
Boston College

Earthquake magnitude is a concept that can be quite confusing. Although it is well-recognized as a measure of the size of an earthquake, the principles underlying earthquake magnitude are not as simple as one might expect. Here is a short, and hopefully not too technical, explanation of the essential characteristics of earthquake magnitude:

Magnitude is a measure of the size of an earthquake at the location where it occurred. This is not the same thing as the amplitudes of the seismic waves where they are recorded, because (as can be seen in the figures below) the amplitudes of those waves decay with distance as they travel from the epicenter to the station. To estimate the magnitude of the earthquake from seismograms recorded at stations around the world, it is necessary to adjust the amplitude you observed at your station for the amount that the waves decayed as they traveled from the earthquake to your station.

A magnitude scale is a scheme for making these amplitude adjustments to determine a number that represents the size of the earthquake. Since exactly how to make these adjustments is a science in itself, there are many formulas that have been developed by seismologists to make the necessary adjustments. The original magnitude formula was developed by Charles Richter (in 1935), which is why the magnitude of an earthquake is often loosely referred to as the size of the earthquake on the “Richter Scale.”

Since the development of the Richter Scale, there have been many other magnitude formulas developed by other seismologists. This can lead to quite a bit of confusion, but all of these formulas should give roughly the same result. To minimize this confusion, the U.S. Geological Survey (USGS) publishes an “official” magnitude for each earthquake they report. This official magnitude is the USGS’s estimate of the magnitude that is most appropriate to release to the public given all of the complications discussed above. These official magnitudes reported by the USGS are often the values that seismologists are referring to when they discuss a given earthquake publicly. But sometimes a seismologist’s reported magnitude of an earthquake might be that of a seismic observatory or research institution other than the USGS. This sometimes happens when an earthquake occurs in an area where seismologists operate seismographs close to the epicenter, and thus they feel they have a more accurate estimate of the magnitude of that particular earthquake than the magnitude reported by the USGS.