Research ArticleSeismology

A case for historic joint rupture of the San Andreas and San Jacinto faults

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Science Advances  11 Mar 2016:
Vol. 2, no. 3, e1500621
DOI: 10.1126/sciadv.1500621
  • Fig. 1 Records of the 1812 earthquake.

    The SAF and SJF are shown in black; bold lines represent the fault sections modeled here. Paleoseismic sites are listed with their most recent ruptures. On the SAF, Pallett Creek (5), Wrightwood (6, 7), Cajon Creek (8), Pitman Canyon (9, 10), and Burro Flats (14) record the 1812 earthquake. To the north, Elizabeth Lake (11) may possibly record 1812, but Frazier Mountain (12) and Mil Potrero (13) do not. The 1812 rupture did not extend south of San Gorgonio Pass; there is no evidence for slip in this time period at Thousand Palms (15), Coachella (16), and Salt Creek (17). Plunge Creek (18) has no record of this earthquake. On the SJF, Colton (19), Quincy (20), Mystic Lake (21), and Hog Lake (22, 23) show early 1800 ruptures. Missions are marked with dots shaded to indicate reported damage (13).

  • Fig. 2 Finite element mesh for dynamic earthquake rupture models.

    Map view of model geometry for the junction of the SAF and SJF, simplified from the USGS Quaternary Faults Database (37). Surface traces for both faults are marked in blue; this geometry extends vertically to depth. Locations of Missions that reported effects of the 8 December 1812 earthquake are marked with letters: SFR, San Fernando Rey; SG, San Gabriel; SJC, San Juan Capistrano; SLR, San Luis Rey.

  • Fig. 3 Initial on-fault stress conditions.

    Warmer colors indicate regions of higher potential stress drop, which promotes more energetic rupture. The sharp contrast at −10 km along strike on the SAF represents a change in regional stress orientation (30). The other along-strike changes correspond to changes in fault strike. Potential stress drop tapers to zero over the top 3 km of the faults to account for reduced normal stress toward the free surface.

  • Fig. 4 Model slip distributions.

    (A to D) Plots of total slip on the SAF and SJF for model ruptures with nucleation on the SAF at Pallett Creek (A), the SAF in Cajon Pass (B), the SJF in Cajon Pass (C), and the SJF at Mystic Lake (D). Paleoseismic sites are marked with white lines and listed with their surface slip value. The location of initial forced nucleation is indicated with the dashed black circle.

  • Fig. 5 Model ground motions.

    (A to D) Plots of low-frequency (≤1 Hz) ground motions from model ruptures with nucleation on the SAF at Pallett Creek (A), the SAF in Cajon Pass (B), the SJF in Cajon Pass (C), and the SJF at Mystic Lake (D). The faults included in the model are marked by heavy black lines; the finer black lines represent the continuations of the SAF and SJF. The initial nucleation location is marked with a white star. Spanish Missions affected by the 8 December 1812 earthquake are marked with labeled circles, with darker red shading corresponding to more significant damage. Small white circles indicate the location of PBRs near the SAF (44) and SJF (26, 44).

  • Fig. 6 Coulomb stress changes from a model rupture with nucleation on the SJF at Mystic Lake.

    Snapshots of Coulomb stress changes, as resolved onto 80° NE-striking left-reverse faults. The faults included in the model are marked by heavy black lines; the finer black lines represent the continuations of the SAF and SJF. The left-reverse faults of the Transverse Ranges are indicated by orange lines. (A) Rupture has reached the northern end of the SJF and has jumped onto the SAF. (B) Rupture has reached the northern end of the modeled portion of the SAF, and a large stress increase propagates into the Transverse Ranges. (C and D) This stress increase continues to propagate in (C), but is no longer evident in (D), which represents the end-of-rupture state and would be a closest match to a static Coulomb stress change calculation.

  • Table 1 Early 1800 slip at paleoseismic sites.
    SiteSlip
    Pallett Creek (SAF)2–6 m (5)
    Wrightwood (SAF)2.5–4.5 m (6, 7)
    Cajon Creek (SAF)~4 m (8)
    Pitman Canyon (SAF)3–4 m (9, 10)
    Plunge Creek (SAF)No evidence for early 1800 surface rupture (18)
    Burro Flats (SAF)Several centimeters of normal slip on a fault
    perpendicular to SAF (14)
    Colton (SJF)Large liquefaction features; slip not determined (19)
    Quincy (SJF)1.8–3 m (20)
    Mystic Lake (SJF)1.8–3 m (20, 21)
  • Table 2 Physical and computational model input parameters.

    SCEC, Southern California Earthquake Center.

    ParameterValue
    P-wave velocityFrom SCEC CVM-S
    S-wave velocityFrom SCEC CVM-S
    DensityFrom SCEC CVM-S
    Static coefficient of friction0.6
    Dynamic coefficient of friction0.2
    Slip-weakening distance0.4 m
    Vertical principal compressive stress30 MPa
    North-south principal compressive stress36 MPa
    East-west principal compressive stress12 MPa
    SAF regional stress orientationN5E (N of Cajon Pass);
    N15W (S of Cajon Pass)
    SJF regional stress orientationN12E
    Element size~200 m (near field);
    ~400 m (far field)
    Forced nucleation radius3 km

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/3/e1500621/DC1

    Fig. S1. Coulomb stress changes from a model rupture with nucleation on the SJF at Cajon Pass.

    Fig. S2. Coulomb stress changes from a model rupture with nucleation on the SAF at Cajon Pass.

    Fig. S3. Coulomb stress changes from a model rupture with nucleation on the SAF at Pallett Creek.

    Fig. S4. Coulomb stress changes on the SAF.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Coulomb stress changes from a model rupture with nucleation on the SJF at Cajon Pass.
    • Fig. S2. Coulomb stress changes from a model rupture with nucleation on the SAF at Cajon Pass.
    • Fig. S3. Coulomb stress changes from a model rupture with nucleation on the SAF at Pallett Creek.
    • Fig. S4. Coulomb stress changes on the SAF.

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