Resonant seismic and microseismic ground motion of the Cascadia subduction zone accretionary prism and implications for seismic velocity

Fault slip behavior during episodic tremor and slow slip (ETS) events, which occur at the deep extension of subduction zone megathrust faults, is believed to be related to cyclic fluid processes that necessitate fluctuations in pore-fluid pressures. In most subduction zones, a layer of anomalously low seismic wave velocities [low-velocity layer (LVL)] is observed in the vicinity of ETS and suggests high pore-fluid pressures that weaken the megathrust. Using repeated seismic scattering observations in the Cascadia subduction zone, we observe a change in the seismic velocity associated with the LVL after ETS events, which we interpret as a response to fluctuations in pore-fluid pressure. These results provide direct evidence of megathrust fault-valve processes during ETS.

Download Full-text

Ground-Motion Attenuation Equations for Earthquakes on the Cascadia Subduction Zone

Earthquake Spectra ◽

10.1193/1.1585626 ◽

1991 ◽

Vol 7 (2) ◽

pp. 201-236 ◽

Cited By ~ 54

Author(s):

C. B. Crouse

Keyword(s):

Data Base ◽

Subduction Zone ◽

Ground Motion ◽

Pacific Northwest ◽

Puget Sound ◽

Response Spectra ◽

Cascadia Subduction Zone ◽

Regression Equations ◽

Median Response ◽

The Pacific

An extensive ground-motion data base was compiled for earthquakes occurring in subduction zones considered representative of the Cascadia subduction zone in the Pacific Northwest. The attenuation characteristics of horizontal peak ground accelerations (PGA) and 5 percent damped pseudovelocity (PSV) were studied for various subsets of the total data base. These data suggested that the PGA tend to saturate at small source-to-site distances and large magnitudes. When unprocessed data were added to the data base, the attenuation of PGA with distance was found to be greater than the attenuation observed for the processed data only, a result which was attributed to the selection of only the stronger motion records for processing. The results of the data analysis were used to establish the proper form of regression equations for estimating PGA and PSV at firm-soil sites in the Pacific Northwest. A total of 697 PGA components and 235 PSV components were selected for the regressions. The resulting equation for estimating PGA in gals was ln (PGA) = 6.36 + 1.76M − 2.73 ln (R + 1.58 exp (0.608M) + 0.00916h, σ=0.773 where M is moment magnitude, R is center-of-energy-release distance in km, h is focal depth in km, and σ is the standard error of ln (PGA). Although σ was relatively large, the residuals from the regressions appeared to decrease with increasing M and R. The results of the PSV regressions showed that the M coefficient and the coefficient of the f(R, M) attenuation term generally increased with period, which is consistent with regression results reported by others. The regression equations were reasonably accurate in predicting the response spectra of accelerograms recorded at Olympia and Seattle, Washington during the 1949 and 1965 Puget Sound earthquakes, but overestimated the spectra of the weaker motions recorded at Tacoma and Portland during the latter event. The median response spectra predicted by these equations for a Washington Coastal Ranges site were similar to the spectra computed by Heaton and Hartzell based on their simulations of ground motions from hypothetical giant earthquakes (M = 9.0 and 9.5) in the Pacific Northwest.

Download Full-text

Velocity and Density Models Incorporating the Cascadia Subduction Zone for 3D Earthquake Ground Motion Simulations

Open-File Report ◽

10.3133/ofr20071348 ◽

2007 ◽

Cited By ~ 13

Author(s):

William J. Stephenson

Keyword(s):

Subduction Zone ◽

Ground Motion ◽

Cascadia Subduction Zone ◽

Earthquake Ground Motion ◽

Ground Motion Simulations

Download Full-text

IODP Expedition 328: Early Results of Cascadia Subduction Zone ACORK Observatory

Scientific Drilling ◽

10.5194/sd-13-12-2012 ◽

2012 ◽

Vol 13 ◽

pp. 12-18 ◽

Cited By ~ 2

Author(s):

E. Davis ◽

M. Heesemann ◽

Keyword(s):

Subduction Zone ◽

Gas Hydrate ◽

Accretionary Prism ◽

Cascadia Subduction Zone ◽

Ocean Drilling Program ◽

Integrated Ocean Drilling Program ◽

Early Results ◽

Large Phase ◽

Four Levels ◽

Hydrate Stability

Integrated Ocean Drilling Program (IODP) Expedition 328 was devoted to the installation of an "Advanced CORK" (Circulation Obviation Retrofit Kit) in the Cascadia subduction zone accretionary prism to observe the physical state and properties of the formation as they are influenced by long-term and episodic deformation and by gas hydrate accumulation. Pressures are monitored at four levels on the outside of a standard 10 3/4-inch casing string, two above and two below the base of the gas-hydrate stability zone at 230 mbsf (m below seafloor). The casing was sealed at the bottom, leaving the inside open down to 302 mbsf for installation of a tilt meter, seismometer, and thermistor cable (scheduled for 2013). The initial data, recovered in July 2011, document an initially smooth recovery from the drilling perturbation followed by what may be a sequence of hole-collapse events. Pressure at the deepest screen is roughly 40 kPa above hydrostatic; higher pressures (80 kPa) are observed at the two screens close to the level of hydrate stability. Tidal variations at the deepest screen are in phase with ocean tides, and define a loading efficiency of 0.6, which is reasonable in light of the consolidation state of the for-mation (porosity ~0.5). Tidal signals near the level of gas hydrate stability display large phase lags, probably as a consequence of hydraulic diffusion stimulated by the large contrast in interstitial fluid compressibility at the gas-hydrate boundary. The degree of isolation among the screens, the anticipated good coupling, and the estimated strain-to-pressure conversion efficiency (~5 kPa μstrain<sup>−1</sup>) indicate that this installation will serve well to host a variety of hydrologic, seismic, and geodynamic experiments. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.13.02.2011" target="_blank">10.2204/iodp.sd.13.02.2011</a>

Download Full-text