Research ArticleGEOLOGY

Abundant off-fault seismicity and orthogonal structures in the San Jacinto fault zone

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Science Advances  15 Mar 2017:
Vol. 3, no. 3, e1601946
DOI: 10.1126/sciadv.1601946
  • Fig. 1 Map of the trifurcation area of the SJFZ.

    Historical seismicity is denoted by black dots (20). Events with M > 4.0 that have occurred in this area since 2001 are indicated by blue stars and focal mechanisms. Aftershocks of the 2016 Borrego Springs earthquake are colored by depth. Red stars and focal mechanisms indicate aftershocks with M > 3.0. Stations used are denoted by green triangles. The inset contains a close-up of the detected aftershocks, which delineate numerous strike-slip and normal faults in conjugate orientations. Profiles of seismicity along cross secitons A–A′ and B–B′ are shown in Figs. 2 and 3. SAF, San Andreas fault.

  • Fig. 2 Fault-normal cross section A–A′ detailing seismicity within 5 km.

    Black dots denote the regional network relocated solutions (20), whereas red dots indicate the detected aftershocks of the 2016 Borrego Springs earthquake. Red focal mechanisms correspond to aftershocks with M > 3.0, whereas blue focal mechanisms on the CL, CC, and BR faults are from the 2008 Mw 4.06, 2010 Mw 5.4, and 2013 Mw 4.7 sequences, respectively. The 2016 main shock is indicated by the largest red focal mechanism. All focal mechanisms shown are rear hemisphere projections. The interpreted locations of the CC, CL, and BR fault planes are shown as dashed lines and dip nearly 70° below 10 km. A change in dip must occur at around 10 to 13 km in order for the faults to meet with the surface expression (green triangles). In the regions off the main faults, there are damage zones with intense distributed seismicity that includes persistently dipping lineations.

  • Fig. 3 Fault-parallel cross section B–B′ detailing all seismicity within 5 km.

    Black dots denote the regional network relocated solutions (20), whereas red dots indicate the detected aftershocks of the 2016 Borrego Springs earthquake. Red focal mechanisms correspond to aftershocks with M > 3.0. All focal mechanisms shown are rear hemisphere projections. Numerous dipping structures are visible and are generally normal faulting or left-lateral strike-slip. Focal mechanisms correspond to 2016 aftershocks with M > 3.0. The 2016 Borrego Springs main shock is indicated by the largest focal mechanism.

  • Fig. 4 Aftershock focal mechanisms.

    Most cases are either strike-slip or normal faulting. The diversity of focal mechanisms is observed on a scale of ~100 m. Structures that are oriented fault-normal are typically normal faulting or left-lateral strike slip, whereas fault-parallel events are typically right-lateral strike-slip. Seismicity is colored by depth, according to the scale. Stereonet shows P and T axes of all focal mechanisms. The inset indicates the map’s location within the broader trifurcation area.

  • Fig. 5 Frequency-magnitude distributions of on-fault and off-fault events since 1981.

    The regions for on-fault events are defined by the white dashed lines in Fig. 1. The left panel presents cumulative statistics for various event populations, whereas the right panel shows discrete statistics for on-fault events and the 95% confidence interval (CI). The largest events occur only on the main faults, whereas nearly two-thirds of all events occur off-fault. On-fault events have a b value of 1.15 ± 0.10, whereas off-fault events have a b value of 1.31 ± 0.07. On-fault events with M > 3.0 have a distinctly different b value (0.77 ± 0.25), forming a separate population. These observations suggest that many events are produced on off-fault structures even within the defined on-fault regions and that the primary fault interfaces are likely to be locked. On-fault events with M > 4.0 are outside the 95% CI in the discrete statistics shown in the right panel.

  • Fig. 6 Schematic of the SJFZ trifurcation area.

    Three main right-lateral strike-slip faults are near vertical in the upper 10 km and dip roughly 70° below this depth. In the top 5 km, there is a broad damage zone associated with the ongoing regional deformation. Between the CL and BR faults, there is a seismically active damage zone at depth consisting of interlaced normal and strike-slip faults. The damage zone produces most of the earthquakes in the region and appears to terminate roughly halfway between CL and BR.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/3/e1601946/DC1

    fig. S1. Additional details regarding the detected 2016 Borrego Springs aftershocks.

    fig. S2. Cross section along profile A–A′.

    fig. S3. Map of seismic stations used.

    data set S1. Seismicity catalog of all detections (unrelocated).

    data set S2. Seismicity catalog of hypoDD relocated detections.

    movie S1. Animation of the relocated seismicity in 3D.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. Additional details regarding the detected 2016 Borrego Springs aftershocks.
    • fig. S2. Cross section along profile A–A′.
    • fig. S3. Map of seismic stations used.
    • Legends for data set S1 and S2
    • Legend for movie S1

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    Other Supplementary Material for this manuscript includes the following:

    • data set S1( Microsoft Excel format). Seismicity catalog of all detections (unrelocated).
    • data set S2 (Microsoft Excel format). Seismicity catalog of hypoDD relocated detections.
    • movie S1 (.mov format). Animation of the relocated seismicity in 3D.

    Files in this Data Supplement:

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