On Magnitudes and Intensities
Ever see a report on TV about an earthquake, and have a hard time gauging how big it really was?
I'll try to convey what I've come to understand about earthquake ratings.
Over the past month, Japan, my chosen home, has lived through over a thousand earthquakes.
Some rattled us, some we barely felt.
With dozens of them, we got emergency alerts automatically to our cellphones (for those of us using smartphones, only those who have specific apps installed).
It's a loud, shrieking siren, and especially if you're in a crowded area, they're often more frightening than the actual quake - imagine dozens of phones around you suddenly shrieking, and everyone rushing to check what's coming.
By the way, this warning system did not work with the big quake.
That aside, these announcements, when they come in time, contain info that will tell you how much of a rattle to expect.
The number usually associated with earthquake reports is the Magnitude according to the Richter scale. This number for the 3/11 quake is 9 - the 4th largest ever recorded.
What does this number tell us? Think of a TV report illustrating a quake on a map. It's usually symbolized by a circle.
The Magnitude lets you know how big that circle will be.
9 - huge circle engulfing half of japan. 3 - small cirle barely extending beyond one or two prefectures.
The second number, which is actually more practical for understanding how scary a particular quake will be if you're in the epicenter, is what we call "Shindo" - literally "degree of shaking." This is divided into 7 steps on the Japan Meteorological Agency seismic intensity scale.
A 1? chance is 90% of people won't have noticed it. If you did, you were probably indoors and sitting, or lying down, or watching something hanging from a thread, like a lamp (and noticed it swung).
A 7? Probably feels like the end of the world. The description in the link above says "(People are thrown by the shaking and impossible to move at will." Holy shit.
What we felt on 3/11 was a 5+. Stuff on higher shelves fell (our bookshelf was pretty much cleared out), some buildings and pavement had cracks, some reclaimed areas had problems with liquefaction.
Walls were creaking. People were yelling "holy shit, this is huge!" Over the phone, I heard my colleague Miyahara, even farther away from the epicenter than me, yelling at his colleagues in our shipping facility to prepare to flee the building. When I looked out the window after it calmed down, there were literally hundreds of people from nearby office buildings on the street.
It was the first time an earthquake really, truly scared me. (I was close once before when an Intensity 4 quake surprised me while in bed)
After 3/11, I watched the emergency alerts very carefully.
Here's how I categorized them for myself:
Magnitude: If it's in Fukushima/Miyagi, and it's below M5, I probably won't feel it. If it's over 6, I will probably start holding down my monitor or the books or in the shelf, because we'll probably get a decent rattle.
If my phone says it's an M7, I'll think "oh shit, here we go again." There will be casualties and injured near the epicenter. (An M7 is what devastated Haiti, and what we got on 4/11) Trains, and maybe electricity for a short while, will be knocked out.
Intensity: If it's a 4 in Fukushima, I'll watch something hanging and see if it's swinging. If it does, I'll lean back satisfied. Very few people will receive damage from this, even in the epicenter.
If it's a 5-6 in Fukushima, I'll get worried. But not for my health. Trains will probably stop for a few minutes to an hour, maybe some of my coworkers living outside Tokyo will have trouble going home. In the epicenter, there will be at least injured, and significant buildings that are already damaged.
Problem is that Magnitude and Intensity aren't always proportional.
Sometimes you'll get an M5 I4, meaning we'll feel it in Tokyo.
On other days, there's I5+ M3, which means in the epicenter it's freaking scary, but here we have no clue. (Like this: http://emergency.yahoo.co.jp/weather/jp/earthquake/2011-04-19-04-14.html It's a five, but look at how tiny the spread is)
Before the big quake, there were actually several M5+ quakes near the area, with an Intensity of only 2 or 1 (Because they were deep down).
Yahoo is pretty helpful with their earthquake info. This page usually has a detailed analysis of a quake as soon as it's done shaking (And a fast flash before it does).
We just had another earthquake that was reasonably strong. Here's the analysis: http://typhoon.yahoo.co.jp/weather/jp/earthquake/2011-04-19-23-10.html
The next time you hear about an aftershock in Japan, why don't you check out the Yahoo page and see which areas really had how much Intensity, instead of just looking at the overall magnitude circle? It's actually pretty fascinating. (The source for this info is http://tenki.jp/earthquake/)
To finish, here's the data from the quake on 3/11:
I'll try to convey what I've come to understand about earthquake ratings.
Over the past month, Japan, my chosen home, has lived through over a thousand earthquakes.
Some rattled us, some we barely felt.
With dozens of them, we got emergency alerts automatically to our cellphones (for those of us using smartphones, only those who have specific apps installed).
It's a loud, shrieking siren, and especially if you're in a crowded area, they're often more frightening than the actual quake - imagine dozens of phones around you suddenly shrieking, and everyone rushing to check what's coming.
By the way, this warning system did not work with the big quake.
That aside, these announcements, when they come in time, contain info that will tell you how much of a rattle to expect.
The number usually associated with earthquake reports is the Magnitude according to the Richter scale. This number for the 3/11 quake is 9 - the 4th largest ever recorded.
What does this number tell us? Think of a TV report illustrating a quake on a map. It's usually symbolized by a circle.
The Magnitude lets you know how big that circle will be.
9 - huge circle engulfing half of japan. 3 - small cirle barely extending beyond one or two prefectures.
The second number, which is actually more practical for understanding how scary a particular quake will be if you're in the epicenter, is what we call "Shindo" - literally "degree of shaking." This is divided into 7 steps on the Japan Meteorological Agency seismic intensity scale.
A 1? chance is 90% of people won't have noticed it. If you did, you were probably indoors and sitting, or lying down, or watching something hanging from a thread, like a lamp (and noticed it swung).
A 7? Probably feels like the end of the world. The description in the link above says "(People are thrown by the shaking and impossible to move at will." Holy shit.
What we felt on 3/11 was a 5+. Stuff on higher shelves fell (our bookshelf was pretty much cleared out), some buildings and pavement had cracks, some reclaimed areas had problems with liquefaction.
Walls were creaking. People were yelling "holy shit, this is huge!" Over the phone, I heard my colleague Miyahara, even farther away from the epicenter than me, yelling at his colleagues in our shipping facility to prepare to flee the building. When I looked out the window after it calmed down, there were literally hundreds of people from nearby office buildings on the street.
It was the first time an earthquake really, truly scared me. (I was close once before when an Intensity 4 quake surprised me while in bed)
After 3/11, I watched the emergency alerts very carefully.
Here's how I categorized them for myself:
Magnitude: If it's in Fukushima/Miyagi, and it's below M5, I probably won't feel it. If it's over 6, I will probably start holding down my monitor or the books or in the shelf, because we'll probably get a decent rattle.
If my phone says it's an M7, I'll think "oh shit, here we go again." There will be casualties and injured near the epicenter. (An M7 is what devastated Haiti, and what we got on 4/11) Trains, and maybe electricity for a short while, will be knocked out.
Intensity: If it's a 4 in Fukushima, I'll watch something hanging and see if it's swinging. If it does, I'll lean back satisfied. Very few people will receive damage from this, even in the epicenter.
If it's a 5-6 in Fukushima, I'll get worried. But not for my health. Trains will probably stop for a few minutes to an hour, maybe some of my coworkers living outside Tokyo will have trouble going home. In the epicenter, there will be at least injured, and significant buildings that are already damaged.
Problem is that Magnitude and Intensity aren't always proportional.
Sometimes you'll get an M5 I4, meaning we'll feel it in Tokyo.
On other days, there's I5+ M3, which means in the epicenter it's freaking scary, but here we have no clue. (Like this: http://emergency.yahoo.co.jp/weather/jp/earthquake/2011-04-19-04-14.html It's a five, but look at how tiny the spread is)
Before the big quake, there were actually several M5+ quakes near the area, with an Intensity of only 2 or 1 (Because they were deep down).
Yahoo is pretty helpful with their earthquake info. This page usually has a detailed analysis of a quake as soon as it's done shaking (And a fast flash before it does).
We just had another earthquake that was reasonably strong. Here's the analysis: http://typhoon.yahoo.co.jp/weather/jp/earthquake/2011-04-19-23-10.html
The next time you hear about an aftershock in Japan, why don't you check out the Yahoo page and see which areas really had how much Intensity, instead of just looking at the overall magnitude circle? It's actually pretty fascinating. (The source for this info is http://tenki.jp/earthquake/)
To finish, here's the data from the quake on 3/11: