What you’re referring to is broadly called the seismographic network. We have sensors kinda…everywhere. All over the place, globally.
It’s also worth noting that given the precision of modern instruments you don’t need to measure *at the quake*. We have instruments sensitive enough to pick up the effects of an earthquake hundreds, if not thousands of miles away.
And we generally don’t determine the magnitude of an earthquake based on the reading of *one* seismograph. We base it on readings of multiple ones. And based on when each individual seismograph registers the quake, how strong it was at that point and time and based on a whole lot of math and triangulation, we can determine where and when it was.
We don’t *need* seismograph in Los Angeles to know when an earth quake hit Los Angeles, or when, or how strong it is. If we have one in Hawaii and one in Idaho and one in Rio, and we know when each one went off, we can compare the strength and time of its effect in each of those three areas and calculate backwards. If we know how fast the seismic wave is moving, and we know it’s strength at 3 separate locations, we can triangulate backwards.
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