EARTHQUAKE INDUCED LANDSLIDES: An assessment of Ness
Castle- Arntully, St. Thomas, Jamaica
Lyndon Brown, PhD
Earthquake Unit,
University of the West Indies, Mona
Introduction
The Judgment Cliff landslide in the area of Llandewey in St.
Thomas, Jamaica is a classical example of the massive
effects earthquakes may have on steep slopes in a
seismically active area.
This area lies within the Plantain Garden Fault (PGF), the
westward extension of the Enriquillo Plantain Garden Fault
Zone (EPGFZ) that forms the southern boundary of the
Gonave microplate that cuts across the island of Jamaica.
This fault line demarcates areas of steep slopes in the
interior sections of the island. The history of landslides in a
particular area has implications for building placed on these
slopes and the underlying areas lying on the foothills.
Judgement Cliff landslide is a complex
rock slide-slump, with a volume of
approximately 6.6 x 106 m3, located in
the lower Yallahs valley in St.
Thomas. It is one of the largest and
most spectacular landslide landforms
in Jamaica and was first described by
Zans (1959).
The landslide deposits are
stacked in the form of
thrust sheets.
Scarp
According to Zans (1959), the
landslide occurred in 1692 and
was either triggered by the 6th
June Port Royal Earthquake or
immediately followed the
earthquake during a period of
hurricane rainfall
Earthquake Induced Slides
This assessment is aimed at defining the slope
failure mechanism, offset, and the slope failure
displacement episodes associated with landslide.
This research should provides more constraint on
failure mechanism and improving the paleoseismic
study to determine the activity of major faults in
Jamaica.
Objective of this research is to improve seismic
hazard analysis, to design slopes to better withstand
earthquake shaking, improve the history of
earthquake activity and to improve the seismic codes
The causes of landslides are usually related to instabilities
in slopes. It is usually possible to identify one or more
landslide causes and one landslide trigger. The difference
between these two concepts is subtle but important. The
landslide causes are the reasons that a landslide occurred in
that location and at that time. Landslide causes include
geological factors, morphological factors, physical factors
and factors associated with human activity.
Causes may be considered to be factors that made the slope
vulnerable to failure, that predispose the slopes to becoming
unstable. The trigger is the single event that finally initiated
the landslide. Thus, causes combine to make a slope
vulnerable to failure, and the trigger finally initiates the
movement. Landslides can have many causes but can only
have one trigger . Usually, it is relatively easy to determine
the trigger after the landslide has occurred (although it is
generally very difficult to determine the exact nature of
landslide triggers ahead of a movement event).
One of the principal cause of damage in earthquakes is
landslide triggered by strong shaking earthquakes with
magnitudes 4.0 can trigger landslides on susceptible slopes
and earthquakes with magnitudes 6.0 can trigger
widespread land sliding.
A number of very well known landslides can be attributed to
earthquake because they happen right after the event and
has happened within documented historical times- common
examples San Salvador, El Salvador, (2001) the Chi-Chi,
Taiwan (1999),
However, if they do not happen within memorable time how
do we determine if they were earthquake induced?
Making connection with an earthquake
Must be able to define the Earthquake parameters
• Must Give Place, Time, Size, Type (Fault Activity)
• Controlling Factors: These are numerous and not
directly observable, e.g.
Stress (Local,
Regional, Global)
• Strain Accumulated
• Rock Friction and Strength
How do we identify landslides that are related to
Earthquake events?
One must first identify evidence of slope failure - most areas are stil
difficult to simply look and classify.
Still lot of techniques help to identify slope failures: among these
are landform patterns, slumping surface, contour on hill surfaces
may indicate multiple episodes of slope failure or evidence of
slumping
Identify evidence of tectonic movement - fault offset very important
to associate slide activity with a seismic event
Fault activity must be dated to be associate with a particular time
to have value, as ultimately these will better define earthquake
frequency of a particular area and as such create better input for
seismic hazard analysis
A review of data from historical earthquakes indicates that the
landslides they generated can be classified into 14 different types.
These include highly disaggregated and fast-moving falls,
disrupted slides, and avalanches; more coherent and slowermoving slumps, block slides, and earth slides; and lateral spreads
and flows that involve partly to completely liquefied material. Rock
falls, disrupted rock slides, and disrupted slides of earth and
debris are the most abundant types of earthquake-induced
landslides, whereas earth flows, debris flows, and avalanches of
rock, earth, or debris typically transport material the farthest.
(Keefer, 1984)
Near Jamaica Tectonics
Triggers for Slope Failure
Seismicity of Jamaica
Ness Castle- Arntully
Slide
Area located in the foothills of
Blue Mountain,
1800 m (a.s.l),
The area is sparsely populated
Ness Castle
-Arntully Slide
According to local residents landslide occurred as a result of the
1907 earthquake, clear evidence of large surface area of rupture:
possibility of multiple events or may have been initiated by one
event. Slide area: 500m *200m * 150m, -bigger than Judgement
Cliff
~100 ft
scarp
Slump and
episodes of
deposition
Multiple episodes of failure- slumping and deposition sequence
mechanism
Fault offset~ 32 inches
Jamaica 1907 Earthquake
The Slide Trigger- 1907- Magnitude 6.5 , Intensity 1X (in
Kingston)
Documented effects of the 1907 event: 1000 dead, fire,
most building collapsed in Kingston, water main broken,
landslides and slump, localized tsunami as far as Port
Antonio
Epicentre? Is it possible to cause landslides in this area?
Next Step: Dating of this event using radionuclide methods
Conclusion
Still more work needs to be done to identify further offset or
determine the cycles of slope failure associated with earthquake
within this area and surrounding communities, possible effects of
shaking in the area.
Consideration in terms of seismic codes and seismic hazard
analysis.
THANK YOU