10. Earthquakes and Faults

 Earthquakes

The Bermuda Seamount is situated near the middle of the oceanic portion of the North American  tectonic plate (Figure 2b, Chapter 2) – one of the seven primary plates which form the Earth’s outer layer, or crust. These plates “float” on the semi-molten underlying layer, known as the mantle. The shifting and jostling of the tectonic plates against each other, at plate boundaries,  causes deformation of the crust (folding and faulting), earthquakes and volcanism.

Because crustal disturbances  generally diminish away from plate boundaries, it was for many years reasoned (LA1,HA1,ME2,HE2) that the Bermuda Seamount is a stable edifice largely uninfluenced by tectonic forces. However since the late 20th Century this assumed stability has increasingly been questioned (PE2,RO1,BO1,VA5,RO4).

Evidence that tectonic forces exist in the vicinity of Bermuda is provided by unexpectedly high earthquake, or seismic, activity (ZO1,VO1). Local geologist Dr. Martin Brewer has catalogued a total of 56 earthquakes affecting Bermuda over the last 350 years (BR2). One in 1664 was described in a church record as “a great and fearful Earthquake which did shake Churches and Houses, Yea and the hearts of man too” (Bermuda Government archives). This 1664 earthquake was considered by Dr. Brewer to have qualified as “destructive”, with an estimated Richter magnitude of ~ 6.3.

Faults

An earthquake is the outcome of the sudden slippage of rock-against-rock along  a fracture. Evidence of such rock slippage in the field is provided by the displacement of geological layers along a fracture, such that the layers on opposite sides of the fracture are no longer aligned. Where such movement has been demonstrated,  the fracture is more specifically termed a “fault”. An earthquake is the manifestation of released energy as the result of slippage along a fault plane.

An example of the association of faults and earthquakes, with which most people are familiar, is found along San Andreas Fault in California – at the junction of the Pacific tectonic plate and the North America tectonic plate. Any such tectonically active region that exhibits significant faults in its bedrock, will have experienced earthquakes, or at least tremors,  at some point in its geological history.

A degree of tectonic instability at Bermuda in modern times is evident from the previously mentioned record of earthquakes. Equivalent or, arguably much greater, instability in the geological past is witnessed by faults, which are extant in Bermuda’s Pleistocene limestone bedrock (RO4). These faults are incontrovertible where there is  displacement of a palaeosol (Figures 10a – 10e). Where, on the other hand, there is no palaeosol, faults can be easily overlooked or are equivocal. This has perpetuated the under-reporting of faulting – a significant geological phenomenon – on Bermuda.

Fault Wilkinsons
Figure 10a. Faults in Wilkinson Quarry, Hamilton Parish. Two faults are revealed through displacement of a palaeosol. The palaeosol at position A is “downthrown”  relative to that at position A’, similar relative displacement has occurred at B versus B’. The amount of displacement on the faults is 0.5 m (A – A’) and 2 m (B – B’) respectively.

A fault is defined by its orientation and by the relative directions of slippage of the rock on either side of the fault. From this information it can be determined if the fault was caused by tension (pulling forces) or compression (pushing forces) within the bedrock. Until 2016 (RO4) any mention, in the geological literature, of faults on Bermuda had been very brief, and certainly there had been been no attempt to catalogue them or detail their characteristics.

While some of Bermuda’s known faults are “normal” (Figure 10a) and are therefore the outcome of tension, the majority are “reverse faults” in which the rock on one side of the fault has over-ridden that on the other side  as a result of compression (Figures 10b -10e). In one case, there is even an associated drag fold (Figure 10c). Such occurrences challenge the assertion that passive  gravitational mechanisms such as cave collapse could have been responsible for fault generation in Bermuda.

Faulting on Bermuda represents the release of stress that had built-up in the bedrock, which arguably may be attributed to plate tectonics (RO4) – specifically sea-floor spreading at the Mid-Atlantic Ridge. The westward push, created by the upwelling of magma (new crust) at the oceanic ridge, generates compressive stresses within the North Atlantic plate on which the Bermuda seamount rides. Even so, there are other isostatic or volcanic explanations for Bermuda’s faults which are worthy of consideration (see later).

 

 

 Figures 10b, 10c, 10d and 10e (Click on images to enlarge)

Fractures

Other evidence that Bermuda has been subjected to episodic instability  comes from rock fractures which can range from mere cracks, or joints (Figure 10f), to 0.75 m (2.5 ft) wide fissures (Figure 10g). Joints and fissures are not as remarkable as faults, but they are much more numerous and more readily traced across rock surfaces than are faults. It has therefore been possible to map their orientation and in some areas establish a pattern of alignment (HA4).

IMG_1867
Figure 10f. Joints at Spittal Pond West, Smith’s Parish. Known as the “Checkerboard”, these fractures, or joints, in the Belmont Formation are aligned approximately SW-NE and SE-NW, which is consistent with the orientation of prominent fracture systems in other parts of Bermuda.  (Scale: White 1 m rule provides scale).
OLYMPUS DIGITAL CAMERA
Figure 10g. Large fracture at Ferry Point, St. George’s Island. A soil-filled fissure in the Rocky Bay Formation. (Yellow 1 m rule, spanning the fissure, provides scale).

The two principal fracture alignments on Bermuda – approximately SW-NE and SE-NW –  were considered by Scheidegger (SC1)  to reflect tectonic stress trajectories in the North American tectonic plate.  It could be coincidental, but long straight stretches of Bermuda’s coastline including the South Shore, the east and north coastlines of St George’s Island and Abbots Cliff in Harrington Sound parallel these orientations. Certainly fracture planes have locally been exploited by coastal erosion but the extent to which this has affected coastal retreat on a large scale is hard to assess.

An alternative explanation for fracturing and faulting

An alternative explanation, to tectonism,  for faulting and fracturing on Bermuda, which is likely to be as unwelcome as the faulting itself, is volcanic reactivation during the Pleistocene Epoch. Such an event would have caused initial expansion (differential uplift) of the seamount as magma was injected  and ultimate  contraction (differential subsidence) as  the magma cooled. Associated fracturing and faulting  within the Bermuda Seamount,  may well have propagated into the limestone cap.  There would not have been any eruption of magma at the surface,  otherwise we would observe exposed deposits of lava and ash contemporaneous with the Pleistocene limestones.

Age-data from samples of volcanic rock retrieved from boreholes could be used to identify Pleistocene volcanic activity, but these are presently far too limited to do so. An existing age measurement of approximately 34 million years reliably identifies only one episode of magma intrusion (RE1,VO1), but by no means rules out others. In fact, existing age-data are so limited that they  provides little more than the broadest understanding of the volcanic history of the Bermuda Seamount; despite the impression given by some accounts of the subject. Further discussion of the evidence for volcanic reactivation can be found at the end of  Chapter 2.