Yagoda-Biran et al (2010) examined a paleo landslide known as the Fishing Dock Landslide which is located 2 km. north of Tiberias . They estimated that a minimum Peak Horizontal Ground acceleration (PGA) of between 0.15 and 0.5 g was required for an earthquake to have initiated the slide. The lower the water table at the time of the earthquake, the higher the PGA required to initiate the slide. For groundwater levels at the time of the study, the minimum PGA is 0.5 g. 0.15 g represents a condition where the soil mass is fully saturated up to the level of the ground surface - which the authors note is probably unrealistic.

Discussion

2 D Model

The geometry of the slope model shown above is based on a profile suggested by Keisar and Peferbaum (2001).

Soil Properties

The slide itself is part of the Pliocene Bira formation which is composed of marls, chalks, and some clay. In parts, this formation is unconformably overlain by the Cover Basalt Formation (Saltzman, 1964). Mechanical properties of the sheared materials were determined by tests performed at commercial labs in Israel as reported by Saltzman (2005). Consolidated drained (CD) direct shear and consolidated undrained (CU) triaxial shear tests were performed. Results of the tests are summarized in the table below:

Test	Water Content	Cohesion (C)	Friction Angle (Φ)
Consolidated Drained (CD)	Natural Water Content	400 kPa	46º
Consolidated Drained (CD)	Saturated	35 kPa	42º
Consolidated Undrained (CU)	Saturated	86 kPa	33º

Static Analysis

A Factor of Safety (FS) for the slope was estimated first to determine if the slope was stable under aseismic conditions. The method of slices by Morgenstern and Price (1965) was used which divides the soil model into vertical slices and determines Factor of Safety as a ratio from a sum of moment balances performed on each vertical slice. This accounts for changes in topography and lateral changes in unit thicknesses and elevation of the water table. The generalized equation is shown below.

FS = Σ M_r / Σ M_d

where

M_r = Resisting Moment
M_d = Driving Moment

Resisting Moment is the ability of the soil mass to avoid moving. The driving moment is the forces such as gravity and water saturated layers pushing the soil mass down slope. FS less than 1 indicates that the slope is unstable. FS greater than 1 indicates that the slope is stable. Yagoda-Biran et al (2010) evaluated FS for a variety of scenarios and came up with a range of FS values from 1.35 to 4.9 - indicating that the slope is aseismically stable - but, in some cases, not by much. In fact, the authors report that recent rejuvenation of the slide is what prompted their study. The commercial software Slope/W™ was used to perform the Static Analysis.

Pseudo-Static Analysis

The authors simulated a pseudo-static, horizontal, inertia force as the additional external load required to trigger sliding in their slope model. Yagoda-Biran et al (2010) proposed that only the CU tests simulated the rapid shear loading and increased pore pressures that would develop during an earthquake and suggested that CD tests be ignored because they would only be representative of a slide that developed under slow rates of shearing where increases in pore pressure had time to dissipate. The results for FS and horizontal PGA for five different scenarios are presented in the table below where the two tests that the authors suggest are relevant are bolded. Minimum PGA to initiate a slide at the Fishing Dock landslide was determined to vary between 0.15 and 0.5 g where the elevation of the water table is the driving factor in determining how high the PGA must be before the slope would fail. The higher the water table, the less stable the slope which results in a lower PGA required to induce sliding. For ground water levels at the time of the study, the minimum PGA to induce sliding is estimated at 0.5 g. 0.15 g to 0.5 g translates into local intensities of 6.5 to 8.2 respectively when using Wald et al (1999) for the conversion.

Scenario	Water Content	Soil Test Data	GWT Level	FS	Minimum PGA (g)
A	Natural Water Content	CD	Recent Level	4.9	1.61
B1	Saturated	CD	Recent Level	2.8	0.85
B2	Saturated	CD	at surface	1.52	0.28
C1	Saturated	CU	Recent Level	2.3	0.5
C2	Saturated	CU	at surface	1.35	0.15

Effect	Location	Image(s)	Description	Intensity
Landslide	Fishing Dock Fig. 8 - Aerial view of Fishing Dock landslide scar. Yagoda-Biran et al (2010)		The authors simulated a pseudo-static, horizontal, inertia force as the additional external load required to trigger sliding in their slope model. Yagoda-Biran et al (2010) proposed that only the CU tests simulated the rapid shear loading and increased pore pressures that would develop during an earthquake and suggested that CD tests be ignored because they would only be representative of a slide that developed under slow rates of shearing where increases in pore pressure had time to dissipate. The results for FS and horizontal PGA for five different scenarios are presented in the table below where the two tests that the authors suggest are relevant are bolded. Minimum PGA to initiate a slide at the Fishing Dock landslide was determined to vary between 0.15 and 0.5 g where the elevation of the water table is the driving factor in determining how high the PGA must be before the slope would fail. The higher the water table, the less stable the slope which results in a lower PGA required to induce sliding. For ground water levels at the time of the study, the minimum PGA to induce sliding is estimated at 0.5 g. 0.15 g to 0.5 g translates into local intensities of 6.5 to 8.2 respectively when using Wald et al (1999) for the conversion. Scenario Water Content Soil Test Data GWT Level FS Minimum PGA (g) A Natural Water Content CD Recent Level 4.9 1.61 B1 Saturated CD Recent Level 2.8 0.85 B2 Saturated CD at surface 1.52 0.28 C1 Saturated CU Recent Level 2.3 0.5 C2 Saturated CU at surface 1.35 0.15	6.5-8.2

Estimated Intensity is roughly between 6.5 and 8.

Variable	Input	Units	Notes
PGA		g	Peak Horizontal Ground Acceleration
Variable	Output - Site Effect not considered	Units	Notes
I		unitless	Conversion from PGA to Intensity using Wald et al (1999)

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