TITLE:
Decadal and Yearly Forerunning Earthquakes to Cape Mendocino, Northern California, Mainshock of 1992 Magnitude 6.9
AUTHORS:
Lynn R. Sykes
KEYWORDS:
Earthquakes, California, Prediction, Precursors, Subduction, Cascadia, Gorda Plate
JOURNAL NAME:
International Journal of Geosciences,
Vol.13 No.11,
November
17,
2022
ABSTRACT: The area near the Cape Mendocino earthquake of 1992,
magnitude 6.9, was the site of many moderate to large shocks during the
previous decades. It and the Honeydew event of 1991, however, are distinguished
from most earthquakes in the region by their thrust-fault mechanisms. The
magnitude of the 1991 shock was also unusually large for the preceding decades,
Mw 6.1. The mechanisms of most other large events involved strike-slip
faulting. The 1992 mainshock occurred in a volume of space characterized by few
decadal forerunning earthquakes of moderate to large size. Most of those
forerunners took place on the periphery of that volume. The presence of that
zone suggests that it broke previously in a large to great earthquake. Precise locations
indicate that slip in the 1991 and 1992 earthquakes occurred on faults dipping
shallowly to the NE and ENE. They likely took place within the North American
plate above the subduction plate boundary. Their implications for earthquake
forecasting using sparse precursors are discussed.