For someone who has lived through a massive earthquake (as I did in Taiwan in 1989), the disturbing thing, even for someone who has lived in earthquake zones for years, is the aftershocks. They build up to a doubt about whether the earth can ever be counted on to lie still. Now after more than a decade of living somewhere where earthquakes are unknown, I largely am back to counting on the earth beneath my feet to lie still. (The danger here, and it is a considerable danger, is slipping and falling on ice. That paralyzed my dad for the last six years of his life.) The earthquake news from Japan brings back a lot of memories from the Ring of Fire. Many people there will be wondering over the next few weeks if the term "solid ground" has any meaning at all.
I lived in Phoenix when there was an Earthquake somewhere down south in Mexico and I felt it. The earth was moving back and forth, the blinds on my window moved back and forth and not ever having felt an Earthquake it was the weirdest feeling in the world.
I can't imagine a strong earthquake or what it would feel like, but Phoenix which is definitely not known for its earthquakes moving underneath my feet made me more wary about what I am walking on than ever before.
I'll raise you - a friend of a friend's grandad sent his family for a summer vacation and was sitting in bathroom reading a book, enjoying the quiet. The bathroom door was left open and he could see out into the apartment. With a corner of his eye he caught a chandelier in the living room that started swinging and then he felt the floor moving underneath his feet. Since this was in Moscow, the city sitting on the plateau, his first thought was that he was having a stroke. And the second - that it'd be a shame to collapse with his pants down, real shame. It did not occur to him it could be a quake, which it was. The only time in modern history a noticeable quake was felt in Moscow. So yeah... quakes can really sneak up on you ;)
Not necessarily a story about earthquakes sneaking up on you, but a funny one. I work in Shenzhen right now in China. One day in the office, 11th floor, everyone suddenly feels the floor shake for a few seconds. We're all thinking earthquake. So what do you do when there's an earthquake? Well, I remember being taught in grade school that if you're outside, you make for an open field or area where things can't fall on you. If you're inside, you crawl under a desk, table, anything that can protect you from falling stuff.
What does everyone do? Stand up, look at each other, and dash to the window to look outside. I felt kinda stupid being the only one just sitting there, so I went to look out the windows as well. Even more funny? We had an rehearsal later that afternoon for the entire building, to practice what to do in the case of an emergency disaster.
Last year, sometime around April if I remember correctly... let me check my Facebook backup. (Yay for searching a big huge HTML document)
April 4, 2010 is when it happened, sometime late in the morning, afternoon. I just remember just waking up and trying to decide if I should go take a shower or not, and thus lying in bed.
Now that my sense on quakes seems unreliable, probably due to series of aftershocks, I just have installed SeisMac (a seismometer app for Mac) to tell if it is really an aftershock.
It's disturbing even at a subconscious level. I left two windows open, creating a cross breeze, and a brief gust at 2am days after the quake caused my window blinds to rattle. I awoke from a deep, deep sleep with all the hair standing up on my body and the proverbial "colon turned to water" feeling of stark terror.
I experienced the 7.2 earthquake on Easter of 2010 in Southern California (San Diego area) and the thing I remember the most is that I felt at least one aftershock per day for at least the next 30-45 days. Nothing serious, they were always in the 4.5-5 scale region.
Have a look at this. http://www.geonet.org.nz/earthquake/quakes/recent_quakes.htm...
Christchurch, New Zealand. Main quake, 7.1 on the scale (and with hindsight, not all that bad), was almost six months before a very shallow 6.3 aftershock which was very bad and had lots of deaths. So far there have been something like 8000 aftershocks. People from the area often seem shell-shocked.
An aftershock is, essentially, an earthquake caused either by the continuing event that caused the first quake or by the resettling of the earth in the area, and especially the latter can take a long time.
It will not greatly affect the orbit of any satellite, because the Earth's center of gravity can't be shifted by any action internal to the Earth itself by conservation of momentum, excepting somehow raising some part of Earth above the relevant orbits which no earthquake is going to do. Otherwise to change the trajectory of Earth's center of gravity requires an external force of some sort. Some small effect from a different mass distribution will be caused and it will have to be taken into effect, but when you care about these effects as far as I know you have to measure them empirically anyhow, because if you care at that level of detail it turns out there's a lot of noise as the Earth warms and cools and has various flows within it and as water moves hither and yon and so on. So while it will have some effect it is merely one of very many such things.
Technically speaking, it won't require any recalibration either, because when the satellites tell you that you are 2.4 meters away from where you were yesterday when standing in the "same location", they're right. (Assuming this number is reliable.) GPS doesn't really tell you where you are "on the surface", they tell you where you are within the sphere they encompass and from there we map that back to the surface location with other knowledge our systems have. If Madagascar took it upon itself to go dock with Australia tomorrow, the GPS satellite system itself would not need to be updated. All the things that use GPS and have some reason to expect Madagascar to be in a certain location would need to be updated.
> All the things that use GPS and have some reason to expect Madagascar to be in a certain location would need to be updated.
In this case, various street turn-by-turn GPS based systems (for locations in JP) would now be off by 2.4m (assuming the article is accurate). Likewise the ground coordinates for runways would be shifted slightly (if I'm following this correctly).
> It will not greatly affect the orbit of any satellite, because the Earth's center of gravity can't be shifted ...
Even if the center of gravity is not moved, there is an impact on trajectories. We are used to use a simplification that consider only the gravity center (using this theorem http://en.wikipedia.org/wiki/Shell_theorem) but this theorem doesn't apply if we want to be really precise.
Is it the case that you might explain such a thing with words that look something like "Some small effect from a different mass distribution will be caused and it will have to be taken into effect, but when you care about these effects as far as I know you have to measure them empirically anyhow, because if you care at that level of detail it turns out there's a lot of noise as the Earth warms and cools and has various flows within it and as water moves hither and yon and so on. So while it will have some effect it is merely one of very many such things."? If only I had thought to add such words to my original post. Alas.
Earthquakes definitely alter the Earth's gravity field, because they change the Earth's mass distribution. These changes have been routinely noticed by people who track satellites.
In the early 2000s there was a mission called GRACE to precisely measure the gravity field and its temporal variation. It's important to know it exactly for all sorts of purposes.
The second item there shows a change in the acceleration due to gravity that was caused by the Sumatra earthquake of December 2004. It is measured in nano m/s^2.
The movie above that item shows the changes in gravity due (mostly) to sloshing water on a global scale.
I'm not educated on satellites at all myself but wondered the same thing. Surely satellites have beacons or towers around the earth to calibrate itself?
Here's a surprising fact. If you allow after-the-fact reprocessing of the GPS and significantly increased modeling technology, you can get millimeter accuracy from civilian GPS.
For more on this global network of re-analyzed GPS, see:
These plots are lat/lon/height (in centimeter units) versus time. In the height plot, you can see a several-cm drift which has annual/seasonal features. This is due to subsidence due to pumping of ground water in the summer. You can also see some green lines which I believe are large earthquakes.
Besides airborne and (coming soon) satellite radar interferometry, reprocessed GPS is one of the main ways we have to observe seismic displacement fields.
My GIS certificate is ancient at this point and I have never managed to work in the field I trained for, but my recollection is that they are accurate to within only about 10 meters, whether personal or professional, unless you spend quite a lot of time gathering data from multiple satellites (on the order of several hours, depending upon the level of accuracy you are seeking). EDIT: In other words, "I just turned mine on" shouldn't get any reading on any system that is more accurate than about 10 meters. It takes time to get accurate readings with GPS.
Someone with a more recent credential/professional experience want to confirm or deny that recollection -- or even update it with more recent facts? (For reference to my question of "more recent", IIRC, I got my certificate in 2002.)
According to http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System#C... normal GPS systems will be accurate to within 2.5 m 95% of the time. Better accuracy can be achieved using differential GPS systems and things like WAAS, but they rely on ground-based systems and thus might not be a good way to measure how much the ground shifted.
I've felt a distant earthquake once, in Michigan (not exactly an earthquake zone) - it was deeply unnerving, particularly in an office on the fifth floor of an old factory building. I can't imagine being trapped amidst suddenly collapsing buildings, or knowing that it could potentially happen any time. I hope the worst is over and people have reconnecting with their families by now.
Also, I don't want to trivialize what the Japanese are going through, but why does Japan move in metric while the Earth moves in imperial units? Sheesh.
How long ago was that? I felt one from Michigan myself a few years back, that originated down in southern Indiana. That was around 3 or 4AM, though, so unless you were working late or living in your office...
The American midwest may not be earthquake central, but it has its faults. New Madrid, in particular:
The power of the northridge earthquake was indescribable. To think that this quake in Japan was more powerful blows me away. During the northridge quake, it literally felt like the entire world was coming to an end.
Last time it hit, the bells range in Massachusetts around 1890. If one were to hit today expected loss of life is in the 1000's. They would expect a total loss of infrastructure for stuff that has been built over 100 years ago. They have no earthquake building codes so imagine something that would destroy buildings just like the one did in Japan. Happily only several hundred were lost compared to what is expect of the Madrid fault.
I live in Florida, and I'll take the risk of hurricanes and tornadoes any day over earthquakes. Just the sudden and unavoidable nature of earthquakes makes them seem orders of magnitude worse to me.
Let the whole state go under. I'll be long gone by then. (All kidding aside, I haven't really heard anything about that, and it isn't a pleasant thought. Any particular articles on the subject that you've seen?)
Again, I don't know how likely this is, but Florida is one of the most vulnerable states in the US. The prize probably goes to Louisiana with North Carolina in the top 3.
If the rotation speed of earth changed (and this is definitely possible), then by a really, really tiny amount (something like a day getting a thousandth of a second longer or shorter)
