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

Abstract

The M w 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 urn) (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. CREEPMETERS AND DATA PROCESSINGThe u.s. Geological Survey (USGS) has maintained a creepmeter array in Parkfield since the 1966 earthquake, increasing the number of sensors over the years to thirteen (Yamashita and Burford, 1973;Schultz, 1989;Schultz et al., 1990). Data from these instruments indicate that interseismic creep reduces in rate from more than 20 mm/year north of Middle Mountain to zero south of Highway 46 (Langbein et al., 1990). Ten of these now record afterslip with a displacement resolution of approximately 20 rm. The three University of Colorado extensometers discussed in this article were installed to capture coseismic slip and afterslip at Parkfield but were abandoned a decade ago due to funding difficulties. With the resumption of USGS funding this year, one was reactivated in April and two others shortly after the Parkfield earthquake. They measure linear displacements 30-60 em below the surface between attachment piers on the flanks of the fault. Each attachment pier consists of steel rods driven to refusal in the form of a buried tripod with a maximum depth of 2 m. The length standards of these original extensometers are solid I-em-diameter invar rods that slide within buried telescopic PVC pipes, although new instrumen...

Space‐time distribution of afterslip following the 2009 L'Aquila earthquake

“…Afterslip has approached the surface but has not completely ruptured the most shallow layer as demonstrated by the steep but continuous displacement gradients along the EP segment (Figure 6). This type of temporal relationship between surface fractures and the main shock has also been observed after the 2004 Parkfield earthquake where direct observations and geodetic measurements have shown that surface slip was delayed (sometimes days after the main shock) depending on the proximity of coseismic fault rupture to the surface [ Bilham , 2005; Rymer et al , 2006]. We thus suggest that a significant, and possibly dominant, contribution to the development of surface fractures along the EP segment have been provided by postseismic slip on the patch C (Figure 13), where slip appears to have migrated immediately after the main shock [ Amoruso and Crescenti , 2009], and which, lately, controlled the development of the hanging wall syncline shown in Figure 6.…”

A silent Mw 4.7 slip event of October 2006 on the Superstition Hills fault, southern California

“…(d) The surface crack was measured over a length of 8 km, about one half of the total fault length. The lower amplitude of the field measurements with respect to the interferometry could indicate that some creep was underestimated in the field because it occurred off the main fault strand or rotation of en‐echelon cracks occurred [ Bilham , 2005]. …”

Probabilistic Estimates of Surface Coseismic Slip and Afterslip for Hayward Fault Earthquakes

“…Furthermore, geodetic models of coseismic slip and afterslip on the fault surface suggest that afterslip started almost immediately after the earthquake rupture, was roughly complementary in spatial distribution to the coseismic slip, and led to a total seismic moment (coseismic slip plus afterslip) that was two to three times larger than the seismic moment for the coseismic slip (Johanson et al, 2006;Johnson et al, 2006;Murray and Langbein, 2006). At the southern end of the rupture, a creepmeter recorded strain associated with the coseismic slip, but surface slip in the form of en echelon cracks did not appear until at least four days after the earthquake (Bilham, 2005).…”