Coseismic Strain and the Transition to Surface Afterslip Recorded by Creepmeters near the 2004 Parkfield Epicenter

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The Mw 6 Parkfield earthquake at 11:15 local time, 28 September 2004 has dealt a possibly fatal blow to the notion that significant surface slip precedes earthquakes. In 1966, anecdotal reports of surface fissures along the fault zone had been reported in the preceding week, and a water pipe fractured 11.6 hours before the mainshock. This offered hope that substantial surface slip may have occurred had displacement sensors been in place to measure it. In the days, hours, and minutes prior to the 2004 Parkfield mainshock, displacement sensors were in place, and nothing happened. According to data from eight creepmeters crossing the fault, the fault remained effectively locked at the detection level of each sensor (6-20 μm) (Langbein et al. , 2005, this issue). This absence of surface slip was accompanied by an absence of significant strain on the borehole dilatometer and shear-strainmeter array in the region, and by the absence of any unusual displacements on the GPS array embracing the 2004 epicenter. Although preseismic strain signals seen on some borehole strainmeters suggest that epicentral strain was not completely indifferent to the pending earthquake, the signals are close to instrumental noise levels and pose new challenges for their reliable future detection. In contrast to the absence of preseismic fault displacement, the creepmeters all recorded an abrupt coseismic dextral offset at the time of the mainshock. The step corresponds to dextral shear of the fault zone, but in the absence of surface rupture, the creepmeters during early afterslip acted as extensometers, measuring strain rather than fault creep. The serendipitous location of creepmeters at Work Ranch both across and eccentric to the fault zone has provided insight into the development of surface faulting. The u.s. Geological Survey (USGS) has maintained a creepmeter array in Parkfield since the 1966 earthquake, increasing …