Research ArticleGEOPHYSICS

Systematic deficiency of aftershocks in areas of high coseismic slip for large subduction zone earthquakes

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Science Advances  14 Feb 2018:
Vol. 4, no. 2, eaao3225
DOI: 10.1126/sciadv.aao3225
  • Fig. 1 Schematic illustration of the 1 April 2014 Mw 8.1 North Chile (Iquique) megathrust earthquake sequence.

    The mainshock involved thrust faulting on the plate boundary between the underthrusting Nazca and the overriding South American plates. Locations of Mw ≥ 5.0 interplate and intraplate foreshocks within the preceding 60 days (blue) and aftershocks in the following 14 days (red) are projected on the plate boundary (37). Contour lines indicate an actual mainshock coseismic slip model (38). Bathymetry is from ETOPO1 (39). Vertical exaggeration is ×3.5. Megathrust depth regimes domains A, B, and C from the study of Lay et al. (23) are indicated.

  • Fig. 2 Three examples of master event relocation of aftershocks.

    The maps show the 14-day aftershock sequences at the initial National Earthquake Information Center (NEIC) epicenters (blue dots) and the relocated positions (red dots), connected by black lines. The mainshock NEIC hypocenters are shown by open circles. Loc, located; Reloc, relocated.

  • Fig. 3 Schematic of the measurement of aftershock locations relative to mainshock slip zones.

    Hypocenters for i magnitude ≥ 5.2 aftershocks are projected to each mainshock fault plane, and in-plane minimum distances (Δri) are measured from the slip contour for a given fraction of the peak slip (χ) (colored zones are shown for values of χ = 0.80, 0.50, and 0.15). Negative values are for aftershocks enclosed by the contour, and positive values are external to it. When combining measurements for different events into a composite plot, these distances are normalized by the source dimension Rj, taken as the radius of a circle with an area equivalent to that of the selected slip contour for the jth event.

  • Fig. 4 Examples for three mainshocks and the associated 14-day aftershocks plotted with the calculated distances from the selected slip contour.

    (A to C) Maps of three mainshocks showing all GCMT regional Mw ≥ 5.2 aftershocks, plotted at the relocated epicenters and showing the GCMT focal mechanism. Each mainshock epicentral location is from the NEIC hypocenter indicated by the black focal mechanism. The region(s) within the χ = 0.15 contour of the slip model is (are) plotted by the light pink polygon. The areas in which aftershocks are considered are indicated by the magenta circles. Interplate and intraplate aftershocks are distinguished by the color of the compressional quadrants, red and gray, respectively. The lower right corner of each map shows a lower-hemisphere stereographic plot of the distribution of the compressional (P), tensional (T), and null (B) principal stress axes of the aftershocks with respect to the P, T, and B axes of the mainshock (solid diamonds). Events having focal mechanism solutions with P, T, and B axes all within 30° of the mainshock values are identified as interplate events. (D), (E), and (F) show corresponding histograms of numbers of aftershocks at varying minimum distances from the χ = 0.15 contour. Interplate events are plotted along the positive y axis (red bars), and intraplate events are plotted along the negative y axis (gray bars). Negative values of Δr indicate events within the slip contour. The dashed vertical line marks the position of the reference contour.

  • Fig. 5 GCMT seismicity locations relative to mainshock slip zones.

    Composite distributions of relocated aftershocks in the GCMT catalog with Mw ≥ 5.2 at their minimum distance from the χ = 0.15 contour for (A) all mainshocks, (B) domain A events, and (C) domain B events, with normalization by Rj of both the radial positions, Δri, and the number counts, Nij. The horizontal dashes within the histograms separate contributions from each event. (D), (E), and (F) show the corresponding subset distributions of interplate seismicity (positive bars) and the intraplate seismicity (negative bars). The blue curves superimposed on the interplate seismicity distributions are for a circular rupture area with seismicity decaying proportional to (Δr/R)−0.5 with respect to the reference contour (Δr = 0; both internally and externally) with a base level equal to the average of the observations in the two intervals adjacent to Δr = 0. This matches the basic distribution of the composite sequence well, for Δr/R from −1 to 1, capturing the reduction of seismicity both within the reducing area inside the contour and in the expanding area outside the contour.

  • Fig. 6 Examples of aftershock deficiency within the rupture zone.

    Normalized distributions of relocated aftershocks, as shown in Fig. 5, for specific events for χ = 0.15. The green curves with circles are reference distributions for models calculated using the specific rupture area for each event with randomly distributed seismicity with respect to the reference contour (Δr = 0; both internally and externally) normalized so that the sum of the reference values (green) over the first five intervals from −1.0 to 0.25 is equal to the sum of the observations (red) over the same intervals. The dashed curves are SDs in the reference contours based on bootstrap analysis. The gray arrows show the deficiency in aftershocks from the predicted model values for a random distribution of the events within the slip contour. Corresponding plots for 25 events with at least seven total aftershocks are shown in fig. S4.

  • Fig. 7 Aftershock deficiency measurements.

    Estimation of cumulative deficiency of aftershock activity within varying distance intervals from the individual earthquake rupture contours as a function of minimum number of interplate aftershocks considered. While deficiency is observed for all thresholds, it is more pronounced as the event population increases. The results presented in Fig. 6 are for a minimum number of aftershocks of seven.

  • Fig. 8 Schematic of possible aftershock distributions relative to a mainshock slip zone.

    Seismicity is shown by the black dots, and the coseismic slip area is marked by the magenta contour and shaded regions. (A) Randomly distributed aftershocks unaffected by the specific mainshock slip distribution. (B) Aftershock distribution relative to a substantial stress drop mainshock that still has residual stress within the margins of the slip zone. (C) Aftershock distribution relative to a total stress drop mainshock that has no shear stress remaining in the slip zone. (D to F) The corresponding schematic radial distributions of aftershocks relative to the margins of the slip zones for the three cases. The black dashed line is the radius of the slip zone, as used in all previous measurements (that is, Fig. 5), and the green dashed line is the predicted level of possible activity for a random distribution of aftershocks within the slip contour.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/2/eaao3225/DC1

    table S1. Mainshock information.

    fig. S1. Bundle of event sequences for 101 mainshocks.

    fig. S2. Seismicity locations relative to mainshock slip zones for different time windows.

    fig. S3. Seismicity locations relative to mainshock slip zones.

    fig. S4. Bundle of 25 event-specific slip deficiency estimates.

    fig. S5. Cumulative distributions for varying aftershock thresholds.

    fig. S6. GCMT seismicity locations relative to mainshock slip zones.

    fig. S7. Examples of rupture dimension modification with ±20% change in modeled rupture velocity.

    fig. S8. Seismicity locations within the mainshock slip zone.

    fig. S9. Cumulative seismic moment distributions.

  • Supplementary Materials

    This PDF file includes:

    • table S1. Mainshock information.
    • Legend for fig. S1
    • fig. S2. Seismicity locations relative to mainshock slip zones for different time windows.
    • fig. S3. Seismicity locations relative to mainshock slip zones.
    • Legend for fig. S4
    • fig. S5. Cumulative distributions for varying aftershock thresholds.
    • fig. S6. GCMT seismicity locations relative to mainshock slip zones.
    • fig. S7. Examples of rupture dimension modification with ±20% change in modeled rupture velocity.
    • fig. S8. Seismicity locations within the mainshock slip zone.
    • fig. S9. Cumulative seismic moment distributions.

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    Other Supplementary Material for this manuscript includes the following:

    • fig. S1 (.pdf format). Bundle of event sequences for 101 mainshocks.
    • fig. S4 (.pdf format). Bundle of 25 event-specific slip deficiency estimates.

    Files in this Data Supplement:

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