The earthquake near New Salem, Illinois–on an extension of the New Madrid fault–is a reminder that, for all of the publicity that earthquakes in California get, the fault system that caused this quake is the most dangerous in the continental United States.
The reason is twofold.
First, California has an extensive system of faults. While the number of faults increase the sheer likelihood of quakes, the faults also provide natural "firewalls" against seismic wave propagation from a quake, which is why quakes that occur in one place are felt in others. In the case of the New Madrid fault, there are few faults surrounding it, so there’s little to stop the wave propagation. This is why, during the major New Madrid quakes in 1811 and 1812, they were felt so far away.
The second reason is that the states surrounding the New Madrid fault have not been as proactive as California in preparing their structures for a seismic calamity. It’s true that, in recent years, they have taken steps to retrofit the major bridges and other structures for such an event, but upgrading the building codes to weed out unreinforced masonry structures–the worst type of structure in an earthquake–has been sorely lagging. A parallel problem with timber structures exists down the Mississippi River, which are in turn unprepared to withstand calamities such as Hurricane Katrina.
One time I was reading an article in a technical journal about the New Madrid fault, and the researcher being interviewed frankly confessed that we really don’t know why there are earthquakes in the New Madrid region. It’s a reminder that, for all of the confidence that we exude about our knowlege, there’s still a lot to learn about God’s creation, and a little humility–in addition to proper preparation–never hurt anyone.