Earthquake 4.0

CALIFORNIA BAY AREA - This evening east-northeast of Alamo, CA, an earthquake struck. It hit at exactly 9:00:15PM on Friday September 5, 2008.
It was a magnitude 4.03 (max MMI was a V) quake, and most people that registered with the USGS website said it felt like a IV. The epicenter was approximately (11 km) 7 miles from Concord. The hypocenter (not to be confused with the epicenter which is the term for where the quake erupted) was 10.1 miles below ground. The fault location was just off the Calaveras Falut, and on the Mt Diablo Thrust Fault. Most people think that Mt Diablo (the Diablo range) is volcanic in origin, but it’s not. Mt Diablo was formed by a type of fault known as a thrust fault, it’s a shallow fault that pushes upward(in this case).

If you sat still after the quake at 9:05 there was another quake, an aftershock further up on the fault line that was only a 1.9 in magnitude. That quake was 10 miles form Concord California.
Over 15 seismographs caught this quake, you only need three to pinpoint it’s exact location.

I could feel the very first indicator quake that was a mere bump, and then a second later I felt the real quake. That is how all earthquakes work once you leave the epicenter. There is a P wave and an S wave (primary waves that have other wave-types associated). The P wave hit first and then the main body of the quake the S wave comes in. The S wave is the more destructive wave, it bounces, while the P waves is a form of compression. To the right you can click on and enlarge the spreadsheet I got from USGS website that shows the waves as recorded from the seismographs!

MMI is the Modified Mercalli system, it’s a different type of earthquake measurement scale based on the movement of the earth and what happens to it because of the earthquake.  It's the more subjective way to judge an earthquake, which is why it is not used in modern times for accuracy anymore. It's most widely used to figure out how large (or small) quakes were before we have seismographs.

Here is the website where you can check out the earthquake:
www.usgs.gov