7.5 magnitude earthquake in Japan has ripple effect across globe & in NC

The massive earthquake that struck central Japan on Monday at 4:10 p.m. local time immediately registered on nearby seismometers. It was also recorded in the minutes that followed around the world, including here in North Carolina.

Earthquakes occur with a sudden release of strain energy at the hypocenter below the Earth's surface and are first felt at the epicenter directly above. Monday's more than 6 miles beneath the Sea of Japan near the town of Suzu on the Noto Peninsula in Ishikawa prefecture.

That energy travels outward in all directions, in waves like ripples in a pond, both along the Earth's surface and through its core.

Primary (P) waves travel fastest and arrive first, compressing the rock horizontally. Shear (S) waves arrive next delivering destructive up and down motion. Seismometers record both the horizontal and vertical motion but the charts you generally see are of that most destructive vertical component.

Looking across a sampling of seismometers provided by the Incorporated Research Institutions for Seismology (IRIS), the quake registered on a seismograph in northern Alaska 9 minutes after the quake struck Japan. The first seismic wave reached Casper, Wyoming, after 12 minutes, then Pittsboro 14 minutes later at 2:24 a.m. EST.

But, like waves moving outward from a pebble tossed in a pond, seismic waves are strongest nearest the source. The speed of motion recorded in Japan reduced by a factor of several thousand times as it reached North Carolina.

Motion continues after the first shock, along the surface as well as through the earth's core. Though aftershocks are always anticipated after a large earthquake like this, the waves in the chart below are likely still from that initial shock as the earth rings like a bell with energy reflected off the crust and through the core. The seismometer in Pittsboro, like others around the globe, continued to record this motion for several hours after the initial shock.

The Python source code used to retrieve waveforms from the four seismic stations to create the charts and maps in this story is available in this GitHub repository.