Earthquake shakes, doesn’t stir Cayman

Although Grand Cayman felt the effects of an earthquake the same magnitude as the costliest natural disaster in United States history, it suffered minimal damage for reasons not entirely known.

The Northridge earthquake that took place just northwest of Los Angeles, California, in January 1994 also measured 6.7 in magnitude.

The magnitude in the Tuesday earthquake has been revised to 6.8.

In contrast to the earthquake felt here Tuesday evening, the Northridge event caused $44 billion worth of damage, killing 57, seriously injuring 1,500, and making more than 22,000 people homeless.

Aside from the differences in density of population and buildings in the two areas, the reasons for the disparity in damage between the same-strength earthquakes in Cayman and Southern California are complex, and to a great extent, uncertain.

Pearse Murphy, a leading structural engineer in the Cayman Islands with Apec Consulting Engineers Ltd, said he did not know why there was so little damage here from Tuesday’s earthquake.

‘We have a consultant seismologist that we use and we’ve asked him that very question,’ he said.

However, one possible reason for the sparseness of damage in Cayman might simply be good building standards, Mr. Murphy indicated.

The Cayman Trough, located just south of Grand Cayman, is thought to mark the boundary between the Caribbean and Americas tectonic plates. As such it is known to be the potential source of earthquakes.

As a result, building codes here in the Cayman Islands reflect the greater hazard of earthquakes here than in other places. Under the Uniform Building Code, which until recently was used to quantify that hazard, Cayman was considered to fall in Zone 2 in a scale that ranged from Zone 0 (lowest hazard) to Zone 4 (highest hazard).

‘We were at the very cusp of Zone 2 and Zone 3, so we were forced to design for the latter to meet (planning) code requirements’ said Mr. Murphy.

Mr. Murphy also suggested that in building to meet code requirements for hurricanes here, it helps structures endure earthquakes as well. ‘Things are just tied together better,’ he said.

Beyond the high standards of building in the Cayman Islands, some of the mystery lies in the science of tectonic plates.

The Cayman Trough is associated with a fault known as a strike-slip fault, which is caused by a horizontal shearing of tectonic plates. The Northridge earthquake was caused by a reverse dip-slip fault, which creates vertical movement. The difference in the direction of movement can have an impact on the level of damage in a particular place of an earthquake.

Another aspect that determines how much damage occurs in the make-up of the underlying geology. Southern California has areas of crust that are much less consolidated than in Cayman, and it is more prone to damaging surface shockwaves as a result.

From an engineering standpoint, Mr. Murphy pointed out that the Cayman earthquake was within the magnitude range that had been expected, even though seismologists estimate that an earthquake of a magnitude of 6.7 has an average recurrence frequency of approximately once every 440 years here.

Mr Murphy indicated that seismologists have calculated the acceleration of the shockwaves with the earthquake here to be approximately 60 percent of the design values used by the Cayman Building Code.

‘It was consistent with the activity and forces the seismologists predicted for our area, so the effects were within expect levels,’ he said.

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