A Northward-propagating Earthquake Sequence in Coastal Southern California?

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The devastating 1999 M 7.4 Izmit, Turkey earthquake was “no surprise” (Reilinger et al. , 2000) because it occurred as part of a sequence of earthquakes that have ruptured nearly the entire length of the strike-slip North Anatolian Fault zone (> 1,000 km) since 1912. The 1999 rupture occurred on a segment previously identified as a seismic gap (Toksoz et al. , 1979) and recently studied to understand mechanisms of stress transfer and earthquake triggering along strike-slip faults (Stein et al. , 1997). We suggest that an analogous rupture sequence spanning the last few centuries may be in its later stages along southern California coastal faults. As we will discuss, recently published fault investigations in the northern Baja California peninsula (Mexico) and coastal southern California (USA) reveal evidence for geologically contemporaneous or sequential earthquakes along a > 300-km-length, predominantly strike-slip seismic zone. This coastal fault zone includes structures previously mapped as the Agua Blanca, Rose Canyon, San Joaquin Hills, and southern Newport-Inglewood Fault zones (Figures 1 and 2). Radiocarbon dating and the historic record indicate that moderate to large earthquakes occurred after A.D. 1640 ± 160, 1523 to 1769, and 1635 to 1855 on the Agua Blanca Fault (Rockwell et al. , 1993), on the Rose Canyon Fault (Rockwell and Murbach, 1999), and in the San Joaquin Hills (Grant et al. , 2002), respectively. Additionally, a moderate to large ( M ≥ 6.5) earthquake is interpreted for the near offshore region in A.D. 1800 based on historical accounts (Toppozada et al. , 1981). Finally, a Mw 6.4 earthquake on the southern Newport-Inglewood Fault zone (NIFZ) followed in 1933 (Barrows, 1974; Hauksson and Gross, 1991) and increased the Coulomb stress on the northern NIFZ in Los Angeles (Stein et al. , 1994). The date of last surface rupture of the …