I was reading (or trying to read) an article about seismic stress change along the Anatolian Fault in Turkey and how it could apply to the San Andreas, but I am confused about a very simple thing. How do they measure the increases in stress in bars? For instance, on page 17 of this article (link below) it discusses the 1992 Landers quake which was followed 3.5 hours later by the Big Bear quake (due to increased bars of stress transferred). Then, it states "the Landers earthquake increased the stress 40 km away on the San Andreas fault by an estimated 2-6 bars,...with (approximately) 150 mm slip needed to relieve the imposed stress."

How are these stress changes measured? And how did they know where to look for stress changes after these back-to-back events?

So much to learn -- so little time.

• Progressive failure on the North Anatolian Fault since 1939 by earthquake stress triggering