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Description
The San Miguel fault zone has been the most seismically active fault in peninsular Baja California this century, with six earthquakes over M6 in a sequence in 1954 and 1956. The 1956 mainshock (M6.8) resulted in a 20 km surface rupture with up to a meter of dextral slip. Total slip across the fault zone, however, is less than 1 kilometer. Paleoseismic studies along the San Miguel fault, in the southern portion of the San Miguel fault zone, have determined: 1) an erosional remnant or ridgeline is horizontally displaced 20 ± 3 m but displays essentially no overall vertical slip. Soils developed in the ridge deposits indicate an age of 50-190 ka, thereby suggesting a long term slip rate of 0.1-0.5 mm/yr; 2) three-dimensional trenching resolved that slip in the 1956 earthquake at this site was about 1.15 m, similar to but slightly higher than observations taken shortly after the 1956 earthquake. Dividing the observed slip by the long-term slip rate, a long term average recurrence interval of 6 +11/-4 ka is suggested; 3) the penultimate event, represented by fissuring on secondary fault strands, occurred about 600 years ago, much more recently than would be predicted by dividing the 1956 slip into the long-term slip rate; 4) a trench across the fault in Pleistocene deposits yielded evidence for only the 1956 and penultimate events, with no other Holocene activity as suggested by the lack of post-Pleistocene vertical separation greater than that expressed from these recent events; and 5) a late Pleistocene channel is dextrally offset about 7.5 m, with only about 30 cm of vertical slip. This observation, along with the horizontal displacement of the ridgeline, indicates that the late Quaternary activity of the fault has been principally strike-slip. These data lead to the conclusion that either the penultimate earthquake was smaller than the 1956 earthquake or that there have been two closely spaced events (temporal clustering of seismic activity) following a relatively long hiatus in large earthquake activity (no prior Holocene surface ruptures). Furthermore, the historical level of earthquake activity experienced along the San Miguel fault zone seems incongruent with its long-term activity, thereby also supporting the notion of clustered activity. In summary, the San Miguel fault zone appears to be relatively insignificant in the transfer of slip across the Baja California peninsula whereas the Agua Blanca fault appears to dominate slip by accommodating about 95 % of this slip. Thus, it is doubtful that geodetic studies across the San Miguel fault will be able to resolve the ongoing deformation, in spite of its historical activity.
