Ballpark Guess of Seismic Wave Intensity Loss at 90º
For example, compared to its intensity fifty miles from the epicenter vs. at Earth’s full circumference, a wave would theoretically be at most 1% as intense when maximally stretched to encircle Earth’s full circumference of 25,000 miles,* the green stage in these images.† But it also loses energy as it crashes and slams through Earth’s rocky interior, hence intensity shrinks to some fraction of 1% then.
* circumference at 50 miles is 314 miles,
barely more than 1% of 25,000 miles
† see second, much larger globe below
Detail: To accurately represent the collapse in intensity at halfway around Earth, the green wavefront at 90º distant would be close to if not invisible compared to the blue one at 22.5º.
Reminder: At halfway around Earth a seismic wave continues racing onto Earth’s opposite side, where its ellipse begins rapidly shrinking, hence its energy begins concentrating and growing stronger and more forceful as it races toward re-uniting near its antipode, opposite its origin (epicenter) on the other side of Earth.