I didn't see any videos that showed shaking as bad as I experienced near the epicenter of the 1989 M7.0 Loma Prieta (World Series) quake. But the Japan quake measured at about M7.6. The videos must have been taken quite a distance from the epicenter.
The amplitude (and speed) of earthquake shaking decreases over distance from the epicenter. Often the frequency content of the shaking is lower a distance away. Right on top of the event it can be like a whip, with movement that is faster.
Before the 1989 quake I used to chat on the phone with the late geologist, Jim Berkland. I learned quite a bit from him. Part of his theory was that the gravitational forces from the Sun and Moon can tend to trigger a quake event.
Quakes are like a "relaxation oscillator" the energy slowly builds and builds until it finally "breaks to camel's back". When the rock breaks this built up energy is released rapidly.
The theory is that the external gravitational forces applied to the tectonic plates can cause the event to trigger when these forces combine with the built up stress in the plates.
These gravitational forces also affect the tides. Watching to tides can often tell you when quakes are more likely. Berkland called this the "Earthquake Window"
I become more concerned when we are in a major earthquake window. I'm not sure if an event in Japan necessarily means we in the Pacific Northwest are in a major window.
The last M9.0 event on the Cascadia Subduction Zone was in late January 1700. The local tribes did not record the time of the event. But in January 1700, the Japanese recorded a tsunami event at a time when there was no associated earthquake in Japan.
In 1989 the early P-wave was "frisky" or bouncy". I immediately grabbed a door jamb, and it's a good thing I did. The S-wave main shocks were nasty. I thought the house was going to be ripped right off the foundation. And in fact, some nearby brand new homes got torn right off their foundations.
After the trauma of a large temblor, the aftershocks can drive you batty.
Wiki link on Jim Berkland.
Posted by Izzy Weird on January 2, 2024, 3:16 pm, in reply to "The Japan quake."