We reviewed a map of the last 30 days of earthquake activity and explored the 24 Aug, South Napa Earthquake sequence (located in California). The magnitude 6.0 mainshock was the largest earthquake in the Bay Area since the magnitude 6.9 1989 Loma Prieta earthquake.

Then students completed an in-class activity using observed numbers of earthquakes with specified magnitude ranges since 1973. These “event” counts were used to estimate the typical number of earthquakes that occur each year (and month and week) in various magnitude ranges. Finally, by plotting the logarithm of the number of events in each magnitude range versus magnitude, students discovered one of the most important relationships in earthquake science – the Gutenberg-Richter relation. The number of earthquakes greater than a given magnitude decreases logarithmically – there are roughly 10 times as many magnitude six earthquakes as there are magnitude seven earthquakes.