Making the San Andreas Fault at the Mendocino Triple Junction:


Figure 1: Location Map of FAME experiment. Triangles and diamonds are BB seismograph stations from FAME (red), the Berkeley Digital Seismic Network (blue diamonds), and the Transportable Array (black). Line A-A’ in red marks the cross sections shown in Figure 2.  From Liu et al., 2012, EPSL.


We fielded the 75 element broadband array known as the FlexArray Mendocino Experiment (FAME) in the Mendocino Triple Junction Region, Northern California (Figure 1).  the project was funded by NSF EarthScope Grants EAR 0642474 & 0746379 with Gene Humphreys (University of Oregon) and Richard Allen (UC Berkeley).  FAME followed an extensive NSF Continental Dynamics active source experiment in this region during 1993-1994 led by Anne Tréhu, of Oregon State University  (Tréhu et al., 1994, EOS). In addition to the 75 FAME stations we used 22 USArray TA and 14 Berkeley Digital Seismic Network stations.

The broadband experiment imaged the abrupt shallowing of LAB in going along strike from Gorda subduction into the ‘Slab Gap’ under the northernmost San Andreas transform system. We also , imaged structure in the mantle wedge under the Cascadia subduction zone, and the crustal structure of the Lassen and Mount Shasta Cascade volcanoes (Figure 2).


Figure 2


Figure 2a) Active source refraction model with superimposed low-fold reflection section. Bright reflections appear at the top of the subducting Gorda oceanic crust (~Shot Points 905-911, see Beaudoin et al., 1995, Geology), and at basaltic dykes in the San Andreas transform regime found under the Lake Pillsbury region (SP 902-903, see Levander et al, 1998, Geology). 2b) Conventional Ps receiver function CCP stack of broadband data along the same profile showing the Southern Edge of the Gorda Slab (SEDGE). 2c) Vs model determined from the joint inversion of Ps receiver functions and Rayleigh wave phase velocities. 2d) 3D Kirchhoff-Approximate generalized Radon Transform image.  The SEDGE is imaged at ~80 km  (2c and 2d). The LAB shallows from ~65 km under the Gorda plate to ~30-35 km under Lake Pillsbury and the Clear Lake volcanic field. From Liu et al., 2012, GJI.

  1. Beaudoin, B.C., N. Godfrey, S.L. Klemperer, C. Lendl, A.M. Trehu, T.J. Henstock, A. Levander, J.E. Holl, A.S. Meltzer, J.H. Luetgert, and W.D. Mooney, 1996, The transition from slab to slabless: Results from the 1993 Mendocino Triple Junction seismic experiment, Geology, 24, 195-199.
  2. Levander, A., T.J. Henstock, A.S. Meltzer, B.C. Beaudoin, A.M. Trehu, and S.L. Klemperer, 1998, Fluids in the lower crust following Mendocino Triple Junction migration: Active basaltic intrusion? Geology, 26, 171-174
  3. Liu, K., Levander, A., Zhai, Y., Porritt, R.W. and Allen, R.M., 2012. Asthenospheric flow and lithospheric evolution near the Mendocino Triple Junction. Earth and Planetary Science Letters323, pp.60-71.
  4. Liu, K. and Levander, A., 2012. Three-dimensional Kirchhoff-approximate generalized Radon transform imaging using teleseismic P-to-S scattered waves. Geophysical Journal International, p.ggs073.
  5. Tréhu, A.M., and the Mendocino Working Group, 1995, Pulling the rug out from under California: Seismic images of the Mendocino triple junction region, EOS, 76, 369, 380-381.