CAPTURING AN EARTHQUAKE AND OTHER ODD PURSUITS IN CALIFORNIA

In 1984, a moderate earthquake on the Parkfield Segment of the San Andreas Fault was predicted to occur sometime before 1993. The earthquake eventually occurred on September 28, 2004, more than 10 years after it was expected. While parts of the prediction did not succeed, the strong motion recordings from many stations are now providing unprecedented insights and changing the way we visualize earthquakes. 56 strong motion records were obtained closer than 20 km from the epicenter - more than in all past California earthquakes combined. The nearest recording station was practically a bull's eye, less than 1 km from the epicenter. For the first time, abrupt variations in ground motion near the rupture have been captured in detail. Counter-intuitively low ground motions within a few kilometers of the epicenter are situated near other stations that experienced motions several times higher than required for the design of new buildings, albeit with short duration. The implications for engineering practice are numerous. The concept that average ground motion tends to attenuate or dissipate at sites that are further from the rupture still seems to work well. However, with this new data showing wide variations in ground motion near the fault rupture, the notion of simple attenuation rules based solely on the distance from the fault appears rather simplistic, if not misleading. The engineering profession's challenge is to assimilate emerging scientific knowledge when building new structures and evaluating existing structures. The bar to that challenge has just been raised several notches by the results of the Parkfield Earthquake of 2004.

• INTRODUCTION

• CENTRAL CALIFORNIA

• HISTORY DOES REPEAT

• BEFORE THE PREDICTION

• THE EARTHQUAKE PREDICTION

• LAYING THE TRAP

• THE ONSLAUGHT OF INSTRUMENTS

• GETTING THE PUBLIC READY

• SEPTEMBER 28, 2004 EARTHQUAKE

• PRECURSORS?

• RESULTS FROM OTHER INSTRUMENTS

• IMPLICATIONS FOR PERFORMANCE-BASED EARTHQUAKE ENGINEERING

• HAZARD ESTIMATION

• CONCLUSIONS WORLDWIDE SUPPORT FOR INSTRUMENTATION

• REFERENCES