Description
Trenches excavated across the Glen Ivy North strand of the Elsinore fault at Glen Ivy Marsh yielded clear evidence of two earthquakes - one historic and one prehistoric. Evidence of a third and possibly a fourth event were also discovered. The marsh stratigraphy consists of predominantly horizontal silt, sand, and gravel layers separated by carbonaceous, organic peat layers. Evidence of a seismic event bracketed by datable peat layers constrains the timing of the event. Horizontal excavation along two ca.1300 A.D. fault strands exposed gravel/sand contacts laterally offset approximately 48 cm. Folds in the surface of the excavation required a minimum of 38.5 cm of offset along the two faults to create the shortening of this surface. A total offset for the ca.1300 A.D. event then, is 87.5 cm. Due to ductile deformation of the unconsolidated sediments in the marsh and the likelihood that some amount of lateral slip also occurred on the main fault during this event, 87.5 cm is considered to be a minimum for this event. Displacement of peats 8 and below, but not 7 in vertical exposures of one of the ca. 1300 A.D. fault strands, constrains the timing of this event. A man-made, pre-1910 cement flume, laterally offset approximately 35 cm, was found along with a 1914 terracotta pipe and a ca.1940's cement pipe in this area. The latter two are vertically displaced at the fault 75 and 40 cm respectively, but are not laterally offset. Subsidence along the fault, due to local ground water removal from a nearby well, is proposed as the reason for the vertical displacement. After-slip from the 1910 event and aseismic creep are discounted ·because they should be proportional to the movement on the fault in the past. A slip per event of 35 - 88 cm is calculated for this portion of the Elsinore fault based on the lateral offset along the ca.1300 A.D. breaks and the pre-1910 cement flume. However, the lateral slip associated with the ca.1300 A.D. event may have been much larger than that measured and therefore the magnitude of the earthquake may, also, have been much larger than the M6 May 15, 1910 event. A minimum slip rate of 0.58 - 1.47 mm/yr is calculated for the Elsinore fault at Glen Ivy Marsh based on the combined lateral offset of the cement flume and ca.1300 A.D. fault strand features. The large amount of ductile deformation demonstrated for the pre-1910 cement flume suggests up to 70 - of the total lateral offset across the fault may not be represented on individual fault ruptures. Applying this to the measured offset on the ca.1300 A.D. fault strands, then, a slip rate of up to 2.75 mm/yr may be calculated. Addition of another undiscovered event increases the calculated slip rate accordingly. Addition of one extra event indicates a recurrence interval of approximately 300 years for large (M6 -7) ground-breaking earthquakes. Addition of two events decreases the recurrence interval to approximately 200 years.
