Introduction
Katz et al (2009) found evidence for five to seven
MW > 6 earthquakes from paleoseismic trenching and an analysis of paleo landslides
around the Sea of Galilee. They dated five of these events to 45, 40, 35, 10, and 5
ka BP using the optically stimulated luminescence (OSL) method.
They detected what may be a landslide evidence for an event younger than 5
ka BP which they suggested might have been
caused by one of the mid 8th century CE earthquakes.
Map and Sections
Chronology
The northern Ein Gev landslide exhibited a multi-phase sliding history where the youngest slide was dated to ca. 5
ka BP.
However, because the youngest slide had a fresh face with no
colluviation
,
Katz et al (2009:289) suggested the possibility that an additional event might have occurred after 5
ka BP
which
might be related to
one of the mid 8th century CE
earthquakes.
Slope stability analysis
Northern Ein Gev lanslide
Slope stability analysis was performed on the northern Ein Gev landslide using the same methodology as was used for the
Fishing Dock Landslide.
Because the northern Ein Gev landslide exhibited a multi-phase sliding history,
the sandstone of the Ein Gev formation (the unit that failed) was mechanically tested in two different states to extract an Initial
Peak Shear Strength and a Residual Strength that would exist after the earliest failure. Test results are listed below:
Sample |
Mechanical State |
Cohesion (kPa) |
Friction Angle |
Factor of Safety from Static Analysis |
Critical Acceleration |
Pristine Rock |
Peak Shear Strength |
376 |
43° |
4.5 |
0.95 g |
Deformed Rock |
Residual Strength |
0 |
38° |
2.8 |
0.37 g |
In order to assess the possibility that a severe rain storm could have caused the observed landslides, additional static analysis was performed at full water saturation
with the water table at ground level. The slopes were found to be stable under these conditions suggesting that the landslides were seismically induced. Paleoseismic trenching
also associated a seismic event at 5
ka BP with a landslide failure.
Critical acceleration to induce sliding was estimated to 0.95 g when the sandstone of the Ein Gev formation
was at Peak Shear Strength and 0.37 g at Residual Strength. This translates into local intensities of 9.2 and 7.7 respectively
when using
Wald et al (1999) for the conversion. Thus minimum
PGA is estimated at 0.37 g and minimum Intensity is estimated
at 7.7 for the presumably seismically induced landsliding events observed in the northern Ein Gev landslide.
Calculator
Notes and Further Reading