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Correlation of porosity variations and rheological transitions on the southern Cascadia megathrust

Nature Geoscience volume 14, pages 341–348 (2021)Cite this article

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Abstract

The unknown onshore extent of megathrust earthquake rupture in the Cascadia subduction zone represents a key uncertainty in earthquake hazard for the Pacific Northwest that is governed by the physical state and mechanical properties of the plate interface. The Cascadia plate interface is segmented into an interseismically locked zone located primarily offshore that is expected to rupture in large earthquakes, a region of aseismic slow slip at greater depth, and an intervening transition zone of uncertain rupture potential. Here we image the evolution of the ratio of seismic compressional to shear wave velocities from the locked zone to the transition zone, which is related to changes in fluid content of the plate boundary zone, using a dense onshore–offshore seismic dataset from southernmost Cascadia. The locked zone shows evidence of high fluid content implying a high porosity, yet the downdip transition zone shows an order of magnitude lower porosity. This strong variation is consistent with models that contain a ductile region between the earthquake rupture and slow slip zones that would inhibit onshore propagation of future large earthquake ruptures and hence reduce seismic hazard.

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Fig. 1: Tectonic background of the Cascadia subduction zone and map view of seismic events and stations in our study area.
Fig. 2: Cross-sections (AA′–DD′) of the Vp/Vs models from the synthetic test and real data inversion.
Fig. 3: Downdip variations in interseismic coupling and Vp/Vs.
Fig. 4: Seismic constraints on the stress state of the locked plate interface.
Fig. 5: Summary schematic showing observations and interpretations.

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Data availability

The CI and XQ seismic waveform data used for this study are available at the IRIS Data Management Center (https://doi.org/10.7914/SN/7D_2011 and https://doi.org/10.7914/SN/XQ_2007). The NCEDC seismic waveform data used for this study are available via the NCEDC official website http://ncedc.org.

Code availability

The seismic tomography software package tomoTD is available upon request.

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Acknowledgements

We thank J. Zhu and R. Evans for their efforts in helping us compare our results with unpublished MT models and constructive feedback on the manuscript. We thank P. Segall and N. Beeler for helpful discussions. We thank F. Waldhauser for supplying his NCEDC earthquake catalogue phase data and waveform cross-correlation data. We thank M. Bostock and A. Plourde for supplying their LFE phase data. We thank D. Shelly, J. Hardebeck and S. Hickman for their very helpful comments and suggestions. We thank J. Gao for his help on preparing Extended Data Fig. 8c. H.G. was supported by the China Scholarship Council award 201706340120, NSF EAR award 1520690 and the National Natural Science Foundation of China under grants U1839205 and 41861134009. J.J.M. was supported by NSF EAR award 1520690. H.Z. was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences under grant XDB 41000000, the National Natural Science Foundation of China under grants U1839205 and 41861134009 and the Fundamental Research Funds for the Central Universities under grant WK2080000144.

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Authors and Affiliations

  1. Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, China

    Hao Guo & Haijiang Zhang

  2. Department of Geoscience, University of Wisconsin-Madison, Madison, WI, USA

    Hao Guo

  3. US Geological Survey, Moffett Field, CA, USA

    Jeffrey J. McGuire

  4. CAS Center for Excellence in Deep Earth Science, Guangzhou, China

    Haijiang Zhang

  5. Mengcheng National Geophysical Observatory, University of Science and Technology of China, Hefei, China

    Haijiang Zhang

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Contributions

J.J.M. and H.Z. supervised the study. H.G. and J.J.M. conducted the seismic data analysis and designed the seismic tomographic inversion. H.G. and H.Z. developed the tomoTD method and software. H.G. conducted the seismic tomographic inversion and seismic model resolution tests. All the authors contributed to the interpretation of results and the writing of the manuscript.

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Correspondence to Hao Guo, Jeffrey J. McGuire or Haijiang Zhang.

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Extended data

Extended Data Fig. 1 Model parameterization, initial earthquake locations, initial velocity models, and the trade-off curve analysis for selecting regularization parameters.

(a–c) Initial earthquake locations (black dots) and tomographic grid nodes (white dots) in three directions. Green lines represent the axes of X = 0 and Y = 0 with their arrows pointing to the positive directions. Red triangles represent stations. (d–g) Vp model from the active-source experiment72 and initial Vp, Vs, and Vp/Vs models at the cross-section of Y = 10 km. (h–j) Trade-off curve analysis. (h) Selecting optimal damping value. Lines represent the inversions using smoothing values of 10 (bottom), 100 (middle), and 400 (top). To simplify the analysis for the selection of the damping, the smoothing values of Vp and Vs models are set to be same. Dots on each line represent the inversions using different damping values as labelled. Because the damping is applied to both the event location and velocity model parameters, the norm of all solutions is used for the analysis. The damping value we selected is 500. (i) Selecting optimal smoothing value of the Vp model. Lines represent the inversions using Vs model smoothing values of 10 (bottom), 60 (middle), and 150 (top). Dots on each line represent the inversions using different smoothing values of the Vp model as labelled. All the inversions use the selected optimal damping value. The norm of P-wave slowness parameters and the norm of P-wave data residual are used for the analysis. The optimal smoothing value of the Vp model is 60. (j) Selecting optimal smoothing value of the Vs model. Dots on the line represent the inversions using different smoothing values of the Vs model as labelled. All the inversions use the selected optimal damping and Vp model smoothing values. The norm of S-wave slowness parameters and the norm of S-wave data residual are used for the analysis. The optimal smoothing value of the Vs model is 60. For our final tomographic inversion, the smoothing values of the Vp and Vs models are slightly higher (80) and the damping values are varied around 500 for different iterations to maintain the reasonable condition number of the inversion system.

Extended Data Fig. 2 Earthquake and LFE relocations, location uncertainties, and station corrections.

(a) Map view and cross-section of earthquake (black dots) and LFE (red dots) relocations. Triangles represent the stations used to locate LFEs. (b) Location uncertainties of earthquakes (colored dots) and LFEs (colored stars), estimated from bootstrapping analysis. The median values of earthquake location uncertainties in longitude, latitude, and depth directions are 0.071, 0.058, and 0.090 km, respectively. The median values of LFE location uncertainties in longitude, latitude, and depth directions are 0.589, 0.582, and 1.161 km, respectively. (c, d) P-wave and S-wave station corrections from the tomographic inversion for stations (colored triangles) near the triple junction. Only stations with station-pair data are inverted for station corrections. Gray lines in (a) and (b) represent depth contours of the Gorda plate interface model of ref. 30 in 5 km increments. SAF, San Andreas Fault. MFZ, Mendocino Fault. The dark gray line represents the coast. The black sawtooth line represents the deformation front.

Extended Data Fig. 3 Final Vp and Vs models at the cross-sections of AA’-EE’.

(a) Vp model. (b) Vs model. Earthquakes (black dots) and LFEs (green dots) are within 5 and 20 km of the cross-sections, respectively. Solid lines near the surface represent the bathymetry. Solid lines at depth are 2 km above and 4 km below the plate interface model of ref. 30 (dashed lines). Inverted black triangles represent the deformation front and coast. Low-resolution regions, estimated from checkerboard resolution test, are masked to gray.

Extended Data Fig. 4 Checkerboard resolution test.

(a–d) Input and recovered checkerboard Vp and Vs models and the corresponding Vp and Vs model resolvability at the cross-sections of Y = 0, 15, 25, and 45 km. In model resolvability panels, contours of the resolvability value of 0.7 are shown. Note that the ‘X’ in this figure refers to the east-west tomographic coordinate, not the ‘distance’ in the cross-section figures throughout the paper.

Extended Data Fig. 5 Synthetic test for estimating model resolution for the case of increased Vp/Vs plate boundary zone.

(a–d) True (a, c) and recovered (b, d) Vs perturbation model (a, b) and Vp/Vs model (c, d) at the cross-sections of AA’-EE’. (e, f) Map view of the true and recovered Vp/Vs models of the plate boundary zone. The true model contains a 6-km-thick, low Vs, and high Vp/Vs layer below the plate interface.

Extended Data Fig. 6 Synthetic test for estimating model resolution for the case of decreased Vp/Vs plate boundary zone.

(a, b) True (a) and recovered (b) Vp/Vs models at the cross-sections of AA’-EE’. (c, d) Map view of the true (c) and recovered (d) Vp/Vs models of the plate boundary zone. The true model contains a 6-km-thick, high Vs, and low Vp/Vs layer below the plate interface.

Extended Data Fig. 7 Megathrust interseismic coupling models.

(a–c) Interseismic coupling models calculated with different mantle viscosity values (a, 4.4 × 1018 Pa s; b, 4.0 × 1019 Pa s; c, 3.6 × 1020 Pa s), from ref. 6. Curves in the panels below map views show downdip variations in megathrust locking ratio along BB’, CC’, and DD’ and are colored by the Vp/Vs value of plate boundary zone. All the other elements in map views are the same as Fig. 3a, except that the LFEs are shown as orange dots.

Extended Data Fig. 8 Downdip variations in interseismic coupling, Vp/Vs, and resistivity models.

(a) Map view of the megathrust interseismic coupling model. (b–c) Vp/Vs and resistivity model cross-sections along CC’ shown in (a). The resistivity model is taken from Fig. 5 of ref. 25. Blue stations in (a) and (c) represent the MT stations25 within our study area. All the other elements in this figure are the same as those in Fig. 3a,c.

Extended Data Fig. 9 Comparing real data inversion results using tomoDD and tomoTD methods.

Earthquake relocations (dots), Vp, Vs, and Vp/Vs models using (b) tomoDD and (c) tomoTD with catalog data and (d) the differences between tomoTD and tomoDD models at the cross-section of Y = 15 km, starting from the same initial locations and velocity models (a). (e–g) The inversion results using different combinations of catalog differential data. (e) station-pair data only. (f) combining station-pair and event-pair differential data. (g) combining station-pair, event-pair, and double-pair differential data. Earthquakes within 10 km of the cross-section are shown. The line near the surface represents the bathymetry or topography. The gray regions near the surface in Vp/Vs models represent the seawater or air and are adjusted to roughly fit the topographic variation. Triangles represent the deformation front and the coast. The other two lines are 2 km above and 4 km below the plate interface of ref. 30, outlining a 6-km-thick layer at the top of the subducted Gorda plate.

Extended Data Fig. 10 Comparing synthetic data inversion results using tomoDD and tomoTD methods.

Vp, Vs, dVp (difference between the inverted and true Vp models), dVs (difference between the inverted and true Vs models), and Vp/Vs models at the vertical cross-section of Y = 15 km from (b, d) tomoDD and (c, e) tomoTD inversions with (b, c) noise-free and (d, e) noisy synthetic data. (a) Input true models. Initial models for all inversions are the same as the one used in the real data inversion (Extended Data Fig. 9a). The gray regions near the surface in dVp, dVs, and Vp/Vs models represent the seawater or air and are adjusted to roughly fit the topographic variation.

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Guo, H., McGuire, J.J. & Zhang, H. Correlation of porosity variations and rheological transitions on the southern Cascadia megathrust. Nat. Geosci. 14, 341–348 (2021). https://doi.org/10.1038/s41561-021-00740-1

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