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.

The landslide deposits are stacked in the form of thrust sheets. Judgement Cliff landslide is a complex rock slide-slump, with a volume of approximately 6.6 x 10 6 m 3, 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). Scarp According to Zans (1959), the landslide occurred in 1692 and was either triggered by the 6 th 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?

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 Making connection with an earthquake Must be able to define the Earthquake parameters

How do we identify landslides that are related to Earthquake events? One must first identify evidence of slope failure - most areas are still 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 slower- moving 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

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 Ness Castle -Arntully Slide

Multiple episodes of failure- slumping and deposition sequence mechanism ~100 ft scarp Slump and episodes of deposition

Fault offset~ 32 inches

Jamaica 1907 Earthquake The Slide Trigger 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