Another largish Midwest aftershock (just now)

USGS ^ | April 22, 2008 | USGS

[kc8ukw]

Magnitude 4.5 Date-Time

* Monday, April 21, 2008 at 05:38:30 UTC * Monday, April 21, 2008 at 12:38:30 AM at epicenter

Location 38.473°N, 87.823°W Depth 10 km (6.2 miles) set by location program Region ILLINOIS Distances

* 8 km (5 miles) NW (323°) from Mount Carmel, IL * 12 km (7 miles) WSW (238°) from Allendale, IL * 12 km (8 miles) NE (37°) from Bellmont, IL * 35 km (22 miles) SW (230°) from Vincennes, IN * 60 km (37 miles) NNW (337°) from Evansville, IN * 211 km (131 miles) E (94°) from St. Louis, MO

Location Uncertainty horizontal +/- 6.1 km (3.8 miles); depth fixed by location program Parameters NST= 44, Nph= 44, Dmin=37.6 km, Rmss=1.27 sec, Gp= 47°, M-type="Nuttli" surface wave magnitude (MLg), Version=7 Source

* U.S. Geological Survey, National Earthquake Information Center: World Data Center for Seismology, Denver

Event ID us2008rcar

[kc8ukw]

Just in case you're again wondering if your house is settling... a lot.

[kingu]

This is what happens when you vote for Huc... *cough* I mean, I hope everyone’s alright, that is a strong size when you’re not in earthquake country.

[MediaMole]

People keep saying they’ve felt these, but I never notice. (500+ miles away)

I just hope this isn’t a prelude to something really damaging.

[BurbankKarl]

Oh Yeah, the Yellowstone Caldera is about to let go.

[OeOeO]

I felt and thought I heard the one the other day, and I'm outside Chicago.It's so unusual you think it's your imagination.

We are actually pretty near earthquake country, St. Louis is almost on top of it. The New Madrid fault sparked 2 huge quakes in the 1800's when the area was relatively unpopulated. They altered the path of the Mississippi river. People here are going to get the shock of their lives one day. Nothing as far as I know is built to Earthquake code.

[Grammy]

Mr G felt the table shaking, and I heard a very low rumble when it happened. I immediately started looking on the internet to confirm it, and knew it would be here.

Not nearly as intense as the one Friday. That one woke us up from a dead sleep, and the bed shook pretty well for a while. This one we would have slept through.

[stlnative]

this lastest one will be downgraded. we had a 4.6 after the 5.2 and this one (4.5) was not as strong as the 4.6.
I’d guess it as a 4.0 to 4.2 tops

it only lasted about 20 seconds compared to the 45 seconds for the first two.

but believe me I am worried also that this may a build up of something larger in the days ahead.

[kc8ukw]

I immediately started looking on the internet

That's exactly what I did after the first one! For 15 minutes I wondered why there was no FR post... and never thought to turn the radio on. What has happened to me?

[stlnative]

latest (not lastest)

[stlnative]

bookmark this... it shows up here first and it was up within a few minutes of the quake.

http://earthquake.usgs.gov/eqcenter/recenteqsus/Maps/US2/37.39.-89.-87_eqs.php

[shineon]

I felt this one two. Just a little shaking, shifting, but it woke me up.

[Grammy]

I found the first one on Freerepublic an hour before they reported it on Fox! After it got me so wide awake there was no way to go back to sleep without finding out what happened.

[shineon]

whoops ... this one too.

[MEpajamaMONSTER]

I hate when that happens!

[stlnative]

I think most of us are on the edge due to it being so active right now. It is amazing how far it can be felt.

I know it broke several home water lines (from the meter to the house) in Marion IL after the 5.2

[Global2010]

Unusual.

Earthquake Ping.

[txhurl]

Does.... this make the shakings y’all had.. foreshocks?

I’m in TX, we get every other disaster besides earthquakes, so I don’t understand.

[stlnative]

well they could be foreshocks, but let’s hope not.

Go to the link I posted and you will see how active it has been. It had been quiet for quite some time before the 5.2 we had on 4/18.

The 5.2 made it feel like a large person fell into the side of the bed and then the shaking was lighter for about another 40 seconds. You can hear the house rattle at the same time you are feeling it.

It woke me up instantly and kicked the covers off and headed for bedroom door way. It was still going by time I was in the doorway.

[shineon]

What Are Aftershocks, Foreshocks and Earthquake Clusters?

The calculations in this system are based on known behaviors of aftershocks. Scientists have shown that the rules governing aftershock behavior also apply to "aftershocks" that are larger than their main shock - i.e., the possibility that the first event was a foreshock. These rules include:

Aftershock Facts: In a cluster, the earthquake with the largest magnitude is called the main shock; anything before it is a foreshock and anything after it is an aftershock. A main shock will be redefined as a foreshock if a subsequent event has a larger magnitude. The rate of main shocks after foreshocks follows the same patterns as aftershocks after main shocks. Aftershock sequences follow predictable patterns as a group, although the individual earthquakes are random and unpredictable. This pattern tells us that aftershocks decay with increasing time, increasing distance, and increasing magnitude. It is this average pattern that this system uses to make real-time predictions about the probability of ground shaking.

Distance: Aftershocks usually occur geographically near the main shock. The stress on the main shock's fault changes drastically during the main shock and that fault produces most of the aftershocks. Sometimes the change in stress caused by the main shock is great enough to trigger aftershocks on other, nearby faults, and for a very large main shock sometimes even farther away. As a rule of thumb, we call earthquakes aftershocks if they are at a distance from the main shock's fault no greater than the length of that fault. The automatic system keeps track of where aftershocks have occurred, and when enough aftershocks have been recorded to pinpoint the more and less active locations, the system adjusts the probabilities on the map to reflect those local variations.

(Click for image for a larger version)

Time: An earthquake large enough to cause damage will probably be followed by several felt aftershocks within the first hour. The rate of aftershocks decreases quickly - the decrease is proportional to the inverse of time since the main shock. This means the second day has about 1/2 the number of aftershocks of the first day and the tenth has about 1/10 the number of the first day. These patterns describe only the overall behavior of aftershocks; the actual times, numbers and locations of the aftershocks are random. We call an earthquake an aftershock as long as the rate at which earthquakes occur in that region is greater than the rate before the main shock. How long this lasts depends on the size of the main shock (bigger earthquakes have more aftershocks) and how active the region was before the main shock (if the region was seismically quiet before the main shock, the aftershocks continue above the previous rate for a longer time). Thus, an aftershock can occur weeks or decades after a main shock.

Magnitude: Bigger earthquakes have more and larger aftershocks. The bigger the main shock the bigger the largest aftershock will be, on average. The difference in magnitude between the main shock and largest aftershock ranges from 0.1 to 3 or more, but averages 1.2 (a M5.5 aftershock to a M6.7 main shock for example). There are more small aftershocks than large ones. Aftershocks of all magnitudes decrease at the same rate, but because the large aftershocks are already less frequent, the decay can be noticed more quickly. Large aftershocks can occur months or even years after the main shock.

[stlnative]

Aftershock Facts: In a cluster, the earthquake with the largest magnitude is called the main shock; anything before it is a foreshock and anything after it is an aftershock. A main shock will be redefined as a foreshock if a subsequent event has a larger magnitude

Let's hope the 5.2 stays as the main shock and not foreshock!