Hello,

In compiling my yearly earthquake charts I noticed a curious pattern of delayed aftershocks that occur after large quakes off the coast of Northern California.

In September of 1994 there was a 6.6 quake. The first aftershock occurred 20 minutes later, and was followed by series of consistent 3.0+ aftershocks. It was about an hour and a half before there was another 20-minute gap between aftershocks.

In February of 1995 there was a 6.8 quake off the coast of Northern California. There was about an eleven-minute gap before the first 3.0+ aftershock was recorded. Again it was about an hour and half before another eleven-minute gap occurred.

On June 15 of 2005 there was a 7.2 quake off the coast of Northern California. The gap this time was one hour. It was about four hours before another one-hour gap was observed.

A couple of days later a 6.6 quake occurred in the same region. A 17-minute gap was observed before a vigorous aftershock sequence was observed.

I know that some aftershocks that occur immediately after the main shock can get “lost” in the continuing vibrations as seen in the seismographs. Can this be the contributed to that fact especially considering these quakes are occurring at distance from the nearest seismographs? Or is there something else going on?

The data I use for my research comes exclusively from the ANSS catalog. You can find my charts at the linked web site.

-Brad

• West Coast Quakes