In 1996 we discivered an unusual set of deep seated listric normal faults on teh washington continental margin. These faults apparently reflect a mid-crustal basal detachment surface upon which Miocene and younger materials are extending seaward.



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Listric Normal Faulting on the Cascadia Margin



Lisa C. McNeill, Chris Goldfinger, LaVerne D. Kulm, Kenneth A. Piper, and Robert S. Yeats


Analysis of multichannel seismic reflection profiles reveals that listric normal faulting is widespread on the northern Oregon and Washington continental shelf and upper slope, suggesting E-W extension in this region.  Fault activity began in the late Miocene and, in some cases, has continued into the Holocene.  Most listric faults sole out into a subhorizontal deecollement coincident with the upper contact of an Eocene to middle Miocene melange and broken formation (MBF), known as the Hoh rock assemblage onshore, whereas other faults penetrate and offset the top of the MBF.  The areal distribution of extensional faulting on the shelf and upper slope is similar to the subsurface distribution of the MBF.  Evidence onshore and on the continental shelf suggests that the MBF is overpressured and mobile.  For listric faults which become subhorizontal at depth, these elevated pore pressures may be sufficient to reduce effective stress and to allow downslope movement of the overlying stratigraphic section along a low-angle (0.1 deg-2.5 deg) detachment coincident with the upper MBF contact.  Mobilization, extension, and unconstrained westward movement of the MBF may also contribute to brittle extension of the overlying sediments.  No Pliocene or Quaternary extensional faults have been identified off the central Oregon or northernmost Washington coast, where the shelf is underlain by the rigid basaltic basement of the Siletzia terrane.  Quaternary extension of the shelf and upper slope is contemporaneous with active accretion and thrust faulting on the lower slope, suggesting that the shelf and upper slope are decoupled from subduction-related compression.



listric normal faulting, Cascadia margin.  Structure map


Figure 1. Map showing locations of listric normal faults mapped primarily with multichannel reflection profiles.


Normal fault, Washington-Cascadia margin











Figure 2. Migrated MCS profile WO- 4058 on the Washington upper continental slope. This profile shows detail of the upper portion of one of the Washington normal faults, including offset of the seafloor and of a shallow slide debris package, prominent growth strata, and stratal rolloverinto the fault zone. Click for lager image.







seismic line showing lisric normal faults, Cascadia margin

Figure 3) (top) E-W migrated multichannel seismic reflection profile A-A' on the central Washington continental shelf and upper slope with (bottom) interpretive line drawing.  See Figure 1 for location.  Three major listric faults, A1, A2 and A3, are crossed by the profile including fault A1 at the head of Grays Canyon, a target of submersible dives in 1994.  Listric faults deform late Miocene to Quaternary sediments with minor deformation of the uppermost melange and broken formation.  Faults A1 and A2 show evidence of recent activity including deformed Holocene sediments, seafloor offset, and methane-derived carbonates resulting from fluid venting.  The listric faults sole out at depth into a decollement close to or at the upper contact of the melange and broken formation.  The faults are characterized by growth strata and rollover folds.  TWTT, two-way travel time.  Vertical exaggeration ~ 2:1 at seafloor.


listric normal faulting, Cascadia margin.  fault scarplistric normal faulting, Cascadia margin.  Structure fault map

Figure 4. (left) Dive video frame grab from the DELTA submersible on dive 3417. This image shows the active tip of one of the acrive normal faults on the Washington shelf. Lasers are 10 cm apart for scale. Light toned gray sediment of the scarp face is thought to be Pleistocene in age, with a the in Holocene drape of olive green sediment. Right panel shows dive location (click for larger image).

Cascadia Normal Faulting Mechanisms

The melange and broken formation appears to control both the normal fault distribution and the timing of faulting, beginning in the late Miocene, following deposition, uplift, and erosion of the middle Miocene MBF.  The MBF appears to decouple the overlying continental shelf sediments, characterized by extensional deformation, from subduction-controlled E-W to NE-SW compressional deformation evident on the lower continental slope.  Two related mechanisms of decoupling are described below, involving, first, detachment of the basinal shelf sediments from the MBF, and secondly, mobilization and extension of the MBF.  The upper contact of the MBF, represented by the middle to late Miocene unconformity and downward transition in acoustic character from well-stratified to discontinuous reflectors, dips very gently west throughout much of the continental shelf (Figure 7), with measured slopes from the midshelf to the shelf break of approximately 0.1 deg-2.5 deg.  These gentle seaward slopes represent the regional dip of this upper contact, ignoring local vertical variations due to faulting and folding.  We hypothesize that such a shallow surface dip may be sufficient to allow unstable gravitational sliding on the upper MBF surface due to low basal friction and consequent detachment of the overlying sediments (Figures 7a and 7b).  This mechanism can be used to explain extension along only the listric normal faults which sole out at depth into the upper MBF contact.  The reduced strength and effective shear stress along a fault plane or detachment associated with high pore fluid pressures has been documented by Hubbert and Rubey [1959] in the theory of low-angle overthrust faulting or gravitational sliding and by Davis et al. [1983] in the Coulomb theory of the critical tapered wedge.  Seaward or downslope dipping listric normal faults also support gravitational sliding as a mechanism of extension.  The upper MBF contact (middle to late Miocene unconformity) thus may act as a detachment [Piper, 1994], separating the mobile MBF and the more rigid post-MBF sediments, along which the late Miocene to Quaternary section moves downslope (Figure 7b).  The listric faults on the Cascadia margin may therefore be similar to growth faults on the Texas coast, where faults flatten at depth into low-density, high fluid pressure shale masses [Bruce, 1973].  Normal faulting at the base of the Guatemalan slope is also thought to be a result of decoupling through elevated pore fluid pressures, as encountered during Leg 84 of the Deep Sea Drilling Project [Aubouin et al., 1982], although this margin is characterized by much steeper terrain.

listric normal faulting, Cascadia margin.  Structure interpretation




Figure 5. Development and mechanisms of listric faulting on the Cascadia outer shelf.  (a) Prior to extension:  melange and broken formation (MBF) deposited at bathyal depths, uplifted, and eroded, and overlying late Miocene sediments deposited.  The upper MBF contact dips gently seaward.  (b) Extensional failure occurring through gravitational collapse along a detachment separating the MBF and overlying sediments.  Elevated pore pressures within the MBF increase the chance of movement on the low-dipping failure plane.  Dip of the melange surface, a = 0.1deg-2.5deg.  (c)  Mobilization and extension of the MBF results in brittle extension of the overlying sediments.  East dipping normal faults also form.





The subhorizontal upper contact of the MBF on the continental shelf and upper slope suggests mobilization and redistribution of this unit, aided by gravitationally driven downslope movement.  The MBF may therefore be undergoing mobilization and extension to the west, where it is apparently unconstrained, with accompanying rigid or brittle extension of the overlying younger deposits (Figures 7a and 7c).  The upper contact may still behave as a detachment, as hypothesized above, but in this case, both the mobile MBF and the overlying brittle section undergo extension, with reduced relative displacement between these two units.  There may be an additional detachment at depth within the MBF, below which no extension occurs, resulting from increases in strength or decreases in pore fluid pressure.  Mobilization and extension of the MBF comprise a preferred explanation for listric faults which do not flatten into a sub-horizontal decollement, but penetrate and offset the MBF unit.  Diapiric intrusions throughout the shelf and evidence of upward movement of the MBF at the shelf edge (Figures 5 and 7c), where the overlying sedimentary load is reduced, point to significant mobilization.  Extension of both the mobile MBF and overlying brittle sediments explains the presence of east dipping and apparently upslope dipping normal faults on the shelf and upper slope (e.g., western end of Figure 5).  These faults are less easily explained by downslope movement on a seaward-dipping detachment.  Extension and thinning of the Hoh beneath the shelf might be expected to result in net subsidence, which contradicts paleobathymetric evidence of net uplift during the period of extension [Rau, 1970; Bergen and Bird, 1972].  This apparent contradiction can, however, be explained by the counteraction of other factors influencing the uplift history of the shelf, including sedimentation rates, sediment underplating, and the variation of subducting slab dip.

Extension Versus Compressional Deformation

Current extension of the continental shelf and upper slope is contemporaneous with accretion and thrust faulting on the lower slope of the accretionary wedge.  In addition, extensional faulting appears to be contemporaneous with mapped fold structures of C. Goldfinger and L.C. McNeill (manuscript in preparation, 1997) and other workers on the continental shelf.  In the light of the evidence for mobile extension, we have reexamined our earlier mapping and conclude that many of the folds in the vicinity of the normal faults are rollover folds, drape structures, and folds driven by downslope spreading of the MBF.  These structures could be misinterpreted as purely convergence-related structures without the high quality data set used for this study.E-W contractile strain is apparently low on the shelf and upper slope.  We hypothesize that the extensional tectonic regime of this region is isolated by the mobile MBF from the convergence-related E-W to NE-SW compression on the lower slope.  Extension extends seaward to the upper slope, and the prominent bulge may mark the seaward edge of the MBF, and therefore extension, on the central Washington margin.  The midslope area, lying between these two regions of known compression and extension, may act as a transition zone or, more likely, a distinct change from extension to compression is located in this area.  The seaward extent of the MBF is uncertain, and the resolution of available data may prevent the identification of extensional faults on much of the slope.  The thickness and strength of the older MBF are unknown, and therefore the depth to which extension extends is unclear:  a deeper compressional regime may underlie the extending MBF.  The presence of E-W trending folds on the inner continental shelf suggests that N-S compression and E-W extension are operating simultaneously.  An extreme case of decoupling extending to the plate interface (10-15 km beneath the shelf) would have significant implications for the extent of coupling on the subduction zone and hence position and width of the interplate locked zone.  The extent and significance of decoupling induced by the MBF are the subject of further study and cannot be fully addressed in this paper.


Listric normal faulting appears to be the result of  (1) downslope movement along a low-angle deecollement between the uppermost middle Miocene MBF and the overlying basinal sediments and (2) mobilization and extension of the MBF and consequent brittle extension of the overlying sediments.  Miocene and Pliocene uplift of the continental shelf may have resulted in oversteepening of the shelf and further gravitational collapse but was probably not a requirement for extension.  The subsurface distribution of the MBF restricts extension to the Washington and northern Oregon shelf and upper slope.  Contemporaneous compressional tectonics of the lower slope and extensional tectonics of the shelf and upper slope are apparently isolated from each other, with the latter region being decoupled from the E-W compressional forces of convergence by the underlying mobile material.  Such segregation of extensional and compressional regimes on convergent margins is not unique to Cascadia, with similar observations on the Peru, Japan, Costa Rica, and Alaskan margins.  Many N-S trending fold structures previously interpreted as tectonic expressions of convergence-related compression, including rollover folds, drape folds, and hanging wall synclines, can be attributed to listric faulting, with E-W extension being the dominant tectonic style.  We conclude that E-W contractile strain is low on the Washington and northern Oregon shelf and that a transition from extension to compression occurs in the mid slope region, likely coincident with the seaward edge of the MBF (Figure 2a).  The presence of long-term major extensional faults, which displace sediments to depths of 2-3 km or greater throughout much of the northern Cascadia continental shelf and upper slope, is of importance to the current stability of the margin.



McNeill, L.C., Piper, K.A., Goldfinger, C., Kulm, L.D., and Yeats, R.S., 1997, Listric normal faulting on the Cascadia continental margin: Journal of Geophysical Research, v. 102, p. 12,123-12,138.