"Bart Bailey" wrote in message
...
In posted on Thu,
5 Jun 2008 05:15:35 -0700, David Eduardo wrote: Begin


An aftershock is a separate earthquake or temblor. It's a separate
incident.

Then why are Northridge or Loma Prieta each
referred to as an earthquake (singular) instead of clusters?

The Northridge quake occurred at 4:31 AM on January 14, 1994. Everything
else was an aftershock.

Nether Northridge nor Loma Prieta were clusters. A cluster is a long
sequence of many similarly sized events that runs at about the same level
for many days and even weeks, such as the one last year centered around the
Southeastern end of the Salton Sea. Northridge was a major quake followed be
decreasing and smaller aftershocks where the average daily activity is on a
decline. Clusters often remain at comparable intensity and frequency levels
for many, many days.