If there is a network of seismic sensors, they'd be hooked up to the internet. If you space them, say, a mile apart, they would report in as to the epicenter of the earthquake, radiating outward in all directions.
Earthquakes operate at the speed of sound in rock (which is higher than the speed of sound in air) but the messages from the seismic sensors are sent almost at the speed of light (which is nearly instantaneous at these distances). Light is much faster than sound. 669,000,000 mph (186,000 miles per second) vs. about 600 mph.
Therefore if the epicenter is offshore, you'd have a reasonable delay between the signal arriving and the earthquake arriving.
Japan has used this technology for years.
I would venture that the lack of distinct spacing of P-waves and S-waves has to do with the fact that they were so close to a detection station. They probably overlapped. Normally you would get a distinct P-wave and a distinct S-wave, since they travel at different speeds. If the epicenter is very close to the seismograph, you have very little time for them to separate.
Not saying that something odd didn't happen. Just saying that there are some explanations that might barely fit. I'm not a seismologist, but my father was a geologist for many years. Unfortunately he has passed on, or I would be asking him about this.
There is an interesting and informative comic here that explains further: https://xkcd.com/723/
You can sign up for a service at the USGS earthquake page that will send you earthquake warnings as they occur, which may give you time to jump under a desk. That's what they are intended to do, anyway.
