A strong foreshock preceded the mainshock by about 20 to 25 seconds. The strong shaking of the main shock lasted about 42 seconds. The shaking intensity as described on the Modified Mercalli intensity scale reached VIII in San Francisco and up to IX in areas to the north like Santa Rosa, where destruction was devastating. There were decades of minor earthquakes – more than at any other time in the historical record for northern California – before the 1906 quake. They have been widely interpreted subsequently as precursory activity to the 1906 earthquake.