Provocative analysis of sea-floor cores suggests that quakes on the Cascadia fault off California can trigger tremors on the San Andreas. Two of North America’s most fearsome earthquake zones could be linked. A controversial study argues that at least eight times in the past 3,000 years, quakes made a one–two punch off the west coast of the United States. A quake hit the Cascadia fault off the coast of northern California, triggering a second quake on the San Andreas fault just to the south. In some cases, the delay between the quakes may have been decades long. The study suggests...