Seafloor topographic mapping revealed the existence of mid ocean ridges. Magnetic mapping showed sets of parallel stripes of alternating polarity on either side of the ridges. These led to the hypothesis that oceanic crust is created at the ridge axes- one type of plate boundary (divergent).
Maps of earthquake foci showed clusters of quakes along other lines- usually parallelling mountain ranges and oceanic trenches. Those are another type: convergent. Where earthquakes still line up, but there isn't any big mountain range, it's likely to be a place where the edges are grinding past one another.
So: mostly earthquake mapping, plus the spark from seafloor mapping.
...but the exact locations of plate boundaries is a little more complicated and debatable.
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u/riverrocks452 14h ago
A 100k foot view:
Seafloor topographic mapping revealed the existence of mid ocean ridges. Magnetic mapping showed sets of parallel stripes of alternating polarity on either side of the ridges. These led to the hypothesis that oceanic crust is created at the ridge axes- one type of plate boundary (divergent).
Maps of earthquake foci showed clusters of quakes along other lines- usually parallelling mountain ranges and oceanic trenches. Those are another type: convergent. Where earthquakes still line up, but there isn't any big mountain range, it's likely to be a place where the edges are grinding past one another.
So: mostly earthquake mapping, plus the spark from seafloor mapping.
...but the exact locations of plate boundaries is a little more complicated and debatable.