The image you see below is copied from a pdf document which can be found at this website.
If you click on it, you will see a hi res version which provides more detail. I suggest that you downlod the hi-res version and study it carefully. Can you can find anything anomolous in it?
This image was used as the basis for Popular Mechanics debunking of one of the key '9/11 Myths' - namely that the seismic data supports the hypothesis that a large underground explosion occured several seconds before each tower collapsed.
A sharp spike of short duration is characteristic of explosive blasts.