On the morning of 28 September 2004, years of planning and anticipation came to fruition as the Parkfield segment of the San Andreas fault ruptured in a magnitude 6.0 event. It was big enough to provide lots of data, yet not so big that Parkfield was widely damaged. The records of this quake are a landmark in the history of seismology.
The upper left edge of the aftershock zone is the SAFOD site. These aftershocks outline the area of the fault that ruptured in the mainshock. Scientists will want to explain why the quake started so close to one end of the rupture zone. The mainshock was magnitude 6.0 and the first large aftershock, a few minutes later, was assigned magnitude 5.0 (but downgraded a few days later). Yellow squares are background seismicity from the preceding week.