EVIDENCE FOR A TEAR IN THE JUAN DE FUCA PLATE IN THE VICINITY OF THE 2001 NISQUALLY EARTHQUAKE

Deep earthquakes within the Juan de Fuca (JdF) plate beneath western Washington define a linear trend that may mark a tear within the subducting plate. This earthquake trend is oriented ENE, in the direction of plate convergence. Earthquakes within the zone reach depths approaching 100 km—20 km deeper than seen elsewhere in the JdF plate. The earthquakes bound the south flank of an arch in the slab and mark an abrupt change in seismicity rate, with many earthquakes to the north and few to the south. All the large historic earthquakes in the Puget Lowland—including the 2001 (M6.8) Nisqually earthquake—have occurred along this linear trend. We interpret the earthquake zone to result primarily from flexural stresses associated with unbending the arch.

The high background seismicity and large earthquakes observed along this zone may also denote localized stress concentrations between contrasting strain segments in the slab. The zone marks an abrupt change in slab dip, from a very shallow dip on the north side (~8°) to a less shallow dip (~15°) on the south side. We hypothesize that the flatter slab to the north resists subduction, promoting strain segmentation between the two regions. Furthermore, an accommodation structure—such as a tear zone—may exist at the segment boundary. We speculate that such a discontinuity would concentrate stress locally, and thus expect future earthquakes to occur in the same vicinity.

Tomographic data depict an abrupt 10-km step in p-wave velocity isocontours (south side down) in the vicinity of the earthquake trend. Geodetic data depict a perturbation in rate of surface elevation change along the trend, with slower subsidence to the north and faster subsidence to the south. These regional elevation changes are consistent with the slab geometry. The contrast in slab pull forces expected from the change in dip should lead to left-lateral, strike-slip focal mechanisms along the segment boundary, allowing a test for whether a through-going tear has yet formed or slips aseismically.