When the magnitude 8.9 earthquake off the coast of Japan struck on March 11, 2011, it took nearly eleven hours for the tsunami to race 5,000 miles across the ocean to Santa Barbara. But it took the shock wave only 12 minutes to get here. The graph below shows the effect of the earthquake on the Santa Barbara Courthouse tower clock. The earthquake struck at 05:46 UTC, and it disturbed our pendulum 12 minutes later at 05:58 UTC.

The dark red line in the graph is the pendulum rate. It shows the first shock reaching us just before 2200 hours, followed by another shock about 40 minutes later. The second event appears to be caused by an 7.1 aftershock. Seismic waves travel through the earth's crust at speeds up to 18,000 miles per hour.

 

The following graph shows the effect of the same earthquake on a schoolhouse clock in my living room. The dark blue line is the pendulum rate. It shows disturbances at the same times as the tower clock.

 

The following graph was provided by Research Geophysicist Justin Rubinstein at the Earthquake Science Center of the USGS. It shows the seismic waves from the Japanese earthquake arriving in Santa Barbara. The left edge of the graph is 9:45 PM on the night of the earthquake. The numbers across the bottom are tens of seconds. It shows the first event arriving about 780 seconds after 9:45, or 9:48 PM. This corresponds to the time just before 2200 shown in the MicroSet graphs. The 7.1 aftershock comes in about 3150 seconds after 9:45, or about 10:40 PM. This corresponds to the second disturbance in the Courthouse clock graph.