USGS lowered the magnitude of this latest quake to 5.9 and they are still studying to determine if this was an aftershock or a quake on a different fault triggered by the earlier one. From the Eureka Times Standard (FYI Petrolia is the little town closest to the epicenter): The discussion here about what makes a quake cause damage is accurate - many variables are involved. Engineering structures for earthquakes is still an evolving process. A few years back in one of the Southern California earthquakes a million gallon water tank, fully engineered to withstand major earthquakes, collapsed because of an unanticipated action - the quake threw the full tank straight up off the ground about a foot and the force of that landing was too much for the bottom to withstand. No one had designed for an earthquake that could do that. The tank could have easily withstood a "typical" 7.9 rolling quake, but a relatively close lesser quake with significant vertical sheer forces hadn't been anticipated. All the retrofit efforts notwithstanding, when THE BIG ONE occurs most structures will have some damage and significant numbers of injuries and deaths will occur. Access and prioritizing rescue and relief efforts will present challenges. No matter what, government will get criticized for being unable to have crews everywhere at once.