Answer
To make this InSAR image, scientists took two pictures of the volcano: one before it stated bulging, and one after. The combined image shows how much the volcano bulged (moved upwards) during that time period.
There are three sets of colored rings between the arrows (we start at purple, so once we get to red that counts as on full set of rings). 3 sets times 3 centimeters per set equals 9 centimeters (3.4 inches) of uplift.