Megathrust roughness and structural complexity are thought to be controls on earthquake slip at subduction zones because they result in heterogeneity in shear strength and resolved stress. However, because active megathrust faults are difficult to observe, the causes and scales of complexity are largely unknown. Here we measured the in situ properties of the megathrust of the Middle America subduction zone in a three-dimensional seismic reflection volume to determine how fault properties vary. We quantify spatial variability in the megathrust roughness, overburden and rock physical properties. Heterogeneity in the megathrust roughness exists at length scales of a few kilometres because the megathrust is dissected by active lower-plate normal faults, which offset the megathrust and renewed fault roughness. Spatial variations in the rock physical properties at the plate interface are characterized by correlation length scales of hundreds of metres. Frontal prism taper, historical seismicity and the variation in earthquake stress drop values local to the megathrust are all affected by the heterogeneity at these length scales. Both geometric and rheological complexities may therefore control the mechanical behaviour of the subduction plate interface, which includes earthquake rupture characteristics.
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The 3D prestack depth-migrated seismic reflection data collected as part of Seismic Project MGL1106 Costa Rica Seismogenesis Project (CRISP) are available in the data repository at http://www-udc.ig.utexas.edu/sdc/ with identifier https://doi.org/10.1594/IEDA/500204. The megathrust reflection geometry, depth beneath sea floor and amplitude for the region shown in Fig. 3 are available at osf.io/3nxgb.
The code used to generate the apparent dip (‘dip-steering’) volume and dip-steered median-filtered data can be accessed at https://github.com/OpendTect/OpendTect.
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Thanks to N. Bangs, K. McIntosh and the crew of the R/V Marcus G. Langseth for their efforts to acquire and process the data. Thanks also to M. Ikari for constructive comments that substantially improved the manuscript and to J. Conrad for feedback on an early version of the manuscript. This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), Discovery Grant RGPIN-2016-04677 (J.D.K.) and the National Science Foundation (NSF) grants OCE‐0851529 and OCE‐0851380.
The authors declare no competing interests.
Peer review information Primary Handling Editor: Stefan Lachowycz.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended Data Fig. 2 Analysis of the effect of the seismic volume attributes on roughness estimates.
a, Effect of dip-steered median filter step-outs on roughness measurements in corrugation-parallel direction. Comparison of calculated power spectral density from the shallow, well-corrugated portion of the megathrust horizon extracted from seismic volumes filtered with difference dip-steered median filter step-outs (inline x crossline). b, Same as A but for corrugation-perpendicular direction. c, Comparison of the megathrust power spectral density roughness calculated in different directions to test if the orientation of the megathrust dataset with respect to the coordinate system affected the length scale at which spectra calculated in perpendicular directions converge. Rotating the data around an axis normal to the mean plane through the dataset changes the number of profiles with different lengths, particularly the number of long profiles, and therefore the number of estimates of the PSD at long wavelengths. Roughness was calculated from profiles taken parallel and perpendicular to three reference directions: down dip and across strike; parallel and perpendicular to the corrugations present at shallow depths; perpendicular and parallel to the ridge axes in the middle portion of the megathrust. Differences in PSD at length scales > 5 km result from the smaller number of the longest profiles following rotation of the data around the Z-axis to the different reference directions. These results show the length scale of convergence is approximately the same in each pair of spectra.
Extended Data Fig. 3 Example of an abandoned megathrust horizon extracted from the footwall at around 19 km landward of the trench.
a, Map of the surface topography. Colors correspond to distance from the mean plane fitted through the region shown. Dip direction indicated by arrow. b, Power spectral density roughness of the abandoned megathrust horizon (bold lines) shown in A. This patch shares similar characteristics to the in-situ megathrust, being anisotropic and smoother in the direction of visible corrugations. Spectra for the corrugated and non-corrugated patches from Fig. 3 in the main text are shown for reference. The roughness of the abandoned horizon is intermediate between the corrugated and weakly corrugated portions of the shallow megathrust.
a–c, Amplitude histograms of the megathrust reflection from different depth intervals. Seismic bandwidth decreases (attenuation of high frequencies) with increasing depth. d, Post-migrated Fresnel zone variation with depth. Fresnel zone size increases with depth.
a, Overview map with bathymetry51 of the study region. Circles represent aftershocks occurring between September 1999 and November 199925. Pink circles indicate earthquakes considered here occurring on the plate interface, for which corner frequencies are calculated using the spectral ratio method. Red stars are the earthquakes that occurred between August 20, 1999 and September 24, 1999, for which corner frequencies are calculated using the spectral ratio method. Yellow stars are the mainshock (big star), and earthquakes (small stars) that occurred between September 9, 1996 and September 23, 1999, for which corner frequencies are calculated using the single spectrum method. Green triangles are the CRSEIZE seismometer locations, and black triangles are RSN (SJS) and IRIS/IDA (JTS) station locations. Dashed black line indicates the location of the cross section below. b, Cross section showing earthquake hypocentral depths. Symbols as in a. Inverted triangles indicate locations of seismic stations.
a–f, Spectral ratio fits for the event pair 4585 (Mw=2.6) and 2554 (Mw=1.9) having CC = 0.8. a. Single spectra amplitude and noise spectra of each event. B. Spectral ratio and the model fit with the final corner frequency estimates. c. and d. Waveforms in the time domain. E. The variance (chi-square misfit) for incremented values of fc1, each value represented by a different color. F. Corresponding fits to the data at different fc1 increments. g–l, Spectral ratio fits for the event pair 4016 (Mw=1.8) and 3378 (Mw=1.3) having CC = 0.7. g. Single spectra amplitude and noise spectra of each event. H. Spectral ratio and the model fit with the final corner frequency estimates. i. and j. Waveforms in the time domain. k. The variance (chi-square misfit) for incremented values of fc1, each value represented by a different color. l. Corresponding fits to the data at different fc1 increments.
Extended Data Fig. 7 Geostatistical analysis of the amplitude field from the corrugated subregion of the shallow megathrust.
γ(h) shown here is the same as in Fig. 3d of the main text. The results clearly show that γ(h) attains a constant value for large h (normalized to 1 here). The curves through the results represent model fits to experimental variograms, which have correlation distances, α, of 485 and 445 m in the two perpendicular reference directions.
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Kirkpatrick, J.D., Edwards, J.H., Verdecchia, A. et al. Subduction megathrust heterogeneity characterized from 3D seismic data. Nat. Geosci. 13, 369–374 (2020). https://doi.org/10.1038/s41561-020-0562-9
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