A NEOTECTONIC STUDY OF THE SAN MIGUEL-VALLECITOS FAULT, BAJA-CALIFORNIA, MEXICO

The San Miguel strand of the San Miguel-Vallecitos fault zone displace s an alluvial ridge 23(+5)/(-10) m in the vicinity of Dolores Mountain . The same section of fault produced surface rupture during the M6.8 e arthquake of 1956. We estimate a minimum age of the ridge at between 1 10(+150)/(-60) ka by comparing soil development on the offset ridge to two soil chronosequences in southern California. Dividing the measure d offset by the age of the ridge places a maximum range on the fault s lip rate of 0.2(+0.35)/(-0.15) mm/yr. Excavations along and across the fault at the same site revealed at least 80 cm and probably 115 cm of right-lateral offset of a now-buried stream channel deposit. We attri bute the 115-cm offset to the 1956 M6.8 earthquake, a displacement tha t is slightly larger than previously reported for this earthquake. Div iding the 80 to 115 cm of displacement by the calculated fault slip ra te yields an estimated return time for similar-sized events of about 5 .8(+17)/(-4.3) ka. Similarly, dividing the slip expected for rupture o f the entire 160-km length of the San Miguel-Vallecitos fault zone by the fault slip rate yields an estimate of return time of M(w)7.8 earth quakes of about 80 ka. The slip rate determined from this study is at least an order of magnitude less than that contributed by the Agua Bla nca fault and indicates that the San Miguel fault zone transfers less than about 1% of the plate motions. The San Miguel fault shows a compl ex fault trace and registers a small value of cumulative geologic offs et (maximum of 0.6 km). When combining the geological estimates of mag nitude and return time of the largest earthquakes with magnitude-frequ ency data recorded along the San Miguel fault by the RESNOR seismic ne twork during the period 1976 to 1991, we observe that the shape of the magnitude-frequency distribution along the fault may be described by the Gutenberg-Richter relationship Log n = a - bM. In contrast, along- strike-slip faults in southern California that are characterized by or ders of magnitude more cumulative offset and less complex fault traces , we observe that extrapolation of earthquake frequency statistics fro m the instrumental record of seismicity severely underestimates the ra te of occurrence of maximum expected events. We speculate that the rel atively high productivity of small earthquakes along the San Miguel fa ult reflects a more heterogeneous stress field associated with the inc ipient and complex nature of the fault trace.