Here, from a recording station in St. Louis, is a seismic chart thingy of the April 18, 2008 Mr. Carmel, IL earthquake.
I get that the huge blue splotch is the main earthquake itself. What’s the green line leading up to it? Is that a foreshock? How about the wee little blue line to the left of the main earthquake, that seems to pick up in intensity right before the Big One? Is that a foreshock as well?
I gather that the big red splotch towards the bottom is the aftershock a few hours later? What’s the black splotch on the far right between 1315 & 1715?
You need to understand that the lines on the chart are actually one continuous reading starting at the upper left and going across and then down just like text. Each line represents a fifteen minute interval.
The green line “preceding” the blue splotch is actually the tail of the blue splotch, which has wrapped around the end of the “page” and continues on the next line.
Same thing with the black and red splotches. The black splotch is the start of the aftershock and it continues on the next line as red.
The small blue squiggle before the blue splotch is probably a very small aftershock that occured about one hour after the main event.
The seismic chart is a continuous line. In order to read it properly the color changes as you view it top to bottom. The big blue scrawl is the actual earthquake and as it gets smaller, this reflects the dwindling intensity. It carries over to the next line so what you are seeing is not a foreshock but a continuation of the main shock.
The black splotch appears to be another shock that continues on the next line as red.
It makes a lot more sense now. Thanks guys!