THE 1906 EARTHQUAKE – LESSONS LEARNED, LESSONS FORGOTTEN, AND LOOKING FORWARD

The 1906 Mw7.8 earthquake on the N. San Andreas Fault marked the birth of modern earthquake science. For the first time, the effects and impacts of a major seismic event were systematically investigated and documented. The resulting publication, the so-called “Lawson report” (named for the principal investigator), contained many “firsts”:

• the entire 300-km-long surface rupture was mapped, surface offsets documented, and co-seismic surface displacements inferred from geodetic measurements

• analysis of local seismic data yielded an epicenter ~40 km NW of the current best location offshore from San Francisco - impressive considering how little was known of local velocity structure and that P and S waves had only been identified by seismologists <10 yrs before

• comprehensive mapping of intensity showed the strongest shaking occurred in areas of “made land” (fill) and soft sediment including China Basin and present day Marina district—two San Francisco neighborhoods heavily damaged again in 1989

• surveys of damage to structures showed destruction was closely related to building design and construction--a painful lesson oft repeated around the world

• interpretation of the pre-and co-seismic deformation patterns led Henry Reid to propose the elastic rebound hypothesis--that earthquakes represent sudden release of elastic energy along a fault resulting from a cycle of slow strain accumulation produced by relative displacements of neighboring portions of the crust. It is still accepted today with minor modifications, even though the basis for large-scale horizontal displacements wasn't established until the plate tectonic revolution five decades later.

As earthquake science evolves, reanalysis of the 1906 earthquake data continues to yield new insights about that event and the behavior of large strike-slip faults in general. A ~60 yr period of seismic quiescence in N. California after 1906 remains the best example of a regional “stress shadow” resulting from reduction of stress on adjacent subparallel faults by slip in a major earthquake. Looking to the future, a dense array of continuous GPS recorders in N. California, part of EarthScope's Plate Boundary Observatory, can search for fault interactions and determine if an acceleration of strain rate precedes the next big earthquake as it may have prior to 1906.