Complex sentence: A relationship exists between the depth of the mainshock and the occurrence of aftershocks, and several Pacific Northwest earthquakes illustrate this link, like the February M5.5 1981 Elk Lake event in southwest Washington that was followed by more than 1,000 in the first two years.
Split into simpler sentences: A relationship exists between the depth of the mainshock and the occurrence of aftershocks. Several Pacific Northwest earthquakes illustrate this link, like the February M5.5 1981 Elk Lake event in southwest Washington that was followed by more than 1,000 in the first two years.