When a 6.3-magnitude earthquake rocked the southern New Zealand city of Christchurch and killed at least 65 people early Tuesday, it was the second quake to strike the city within five months. It was weaker in terms of magnitude than the September quake, but the devastation it caused was far greater.

CBC News talked with Prof. Samir Chidiac, a civil engineer at McMaster University in Hamilton, Ont., about earthquake damage to buildings, what engineers do to shore them up and what lessons the New Zealand experience might hold.

This following is edited and abridged excerpts from email and a telephone interview.

CBC News: How can buildings withstand one earthquake and then be more vulnerable when a second quake strikes?

Samir Chidiac: That's a very simple question, but the answer is complicated. To start, we need to divide the buildings into two groups based on their age. This stems from the fact that older buildings were not designed and built to resist seismic loads, whereas modern construction in most countries adheres to code and standards. When we are talking about modern type of construction, we are talking about buildings that are built with engineered materials such as reinforced concrete and steel. These buildings are designed and built in accordance with building codes and standards.

## The Richter scale

The first practical scale for measuring earthquakes was developed by geologist Charles Richter at the California Institute of Technology in the 1930s, and the scale that most scientists use today still bears his name.

(Actually, seismologists use several different, but related, scales. But the Richter scale, denoted by a number called the "magnitude," is the most common. This quantity, which can be read off a seismograph, reflects the amount by which the earth's crust shifts.)

The Richter scale has no lower limit and no maximum. It's a "logarithmic" scale, which means that each one-point increase on the scale represents a 10-fold increase in the magnitude of the quake.

The energy released by an earthquake increases at an even steeper rate, going up by a factor of 32 for each one-point increase in magnitude. Therefore, a quake with magnitude between 2 and 3 is the lowest normally perceptible to humans. A magnitude 5 quake is considered moderate. Worldwide, there are about 1,500 earthquakes of magnitude 5 or higher every year. An earthquake of magnitude 6 or higher is considered major. The largest earthquakes in history have been of about magnitude 9. Major earthquakes release far more energy than any man-made explosion. The 1906 San Francisco earthquake, with a magnitude of 8.3, was approximately one million times as powerful as the atomic bomb dropped on Hiroshima.

On the other side, if you examine buildings that were built early 1900s or before, you will find that they are built with natural material such as stone, timber etc. These buildings tend to have a high resistance to gravity loads but very low resistance to lateral loads such as seismic loads. Some of these heritage buildings have been structurally upgraded to resist seismic loads. For example, the cathedral in Christchurch was able to resist the first earthquake but most likely did sustain some damages.

The question is what happens to these buildings when they are subjected to a second earthquake. The response of these buildings will depend on their resistance to earthquake actions. If the earthquake load exceeded the linear range of the building's structural system, then the building structural system will not possess the same resistance — it has been weakened.

If a second earthquake comes in before we get a chance to assess and carry the proper upgrade, then the overall resistance of the building has not been upgraded to the level it was prior to the first earthquake … in these cases the building will be vulnerable to damage and potentially collapse in the event of an earthquake.

Can there be damage that is completely hidden?

Yes is the quick answer. The assessment of whether a building is safe or not following an earthquake is carried out by experienced and knowledgeable engineers. However, not all damages are visible and it is possible to miss but most likely what can happen is the underestimation of the impact of the damage....

Sometimes, just visual inspection may not be sufficient. Sometimes, they have to go and do some calculations if they know a little bit more about the design. I'm talking now not about a one- or two-storey building, but I'm talking about medium to highrise buildings. The lowrise buildings, certainly, … one can look at it and see what type of damage that has occurred and then decide whether that building needs upgrades or is OK as is, or probably it's better to tear it down and rebuild.

What is the key to ensuring a building can withstand multiple quakes?

Ductility. The building has to have sufficient ductility.

What is that?

It's a measure of the building being able to absorb energy and it is related to its displacement beyond its elastic limit…. Buildings are designed  to absorb the energy without collapsing by undergoing large deformations, what we call nonlinear behaviour.

How could we picture that action?

Ductility is a material property. If we subject a rod made of steel and a second rod made of stone to a tensile load, we observe that the steel rod will undergo large deformation and when it fails, it is not catastrophic, whereas the rod made of stone will experience a very small deformation if any and then without any notice will crack and fail in a brittle and catastrophic manner. Steel rod failed in a ductile manner whereas the stone fails in a brittle manner. The same concept is extended for buildings. However, we compare the overall deformation of the building to its elastic limit and we use the ratio as a measure of the building ductility.

As an engineer, how do you ensure the best ductility possible?

Building codes such as the National Building Code of Canada and material standards such as those developed for concrete and steel by the Canadian Standards Association guide engineers with their design.

Could a building be made to withstand any earthquake?

In theory, yes. Can we afford it - that's another question. Engineers design according to building codes and standards. Codes provide a balance between life safety and economics.

What about seeing two quakes in New Zealand and much more damage the second time around? What lessons are there for us in that?

Historically, it is possible to experience two earthquakes in a short period of time, but it has not been common. Typically, we have an earthquake followed by aftershocks. We have been conditioned to believe that once we have survived an earthquake, we should be OK for some time, with the false expectation that the next earthquake will not occur soon. Unfortunately, nature does not follow our logic. This was also an unfortunate event for Christchurch.

If there is a lesson to be learned from this experience, it is no one can predict when an earthquake will occur and when the second one will follow. Therefore, if a building suffers significant  damage — particularly when you're dealing with public buildings or buildings where you expect to have large number of occupants — I think repair has to be carried out immediately or the use of those buildings has to be more or less restricted until the building has been repaired to the original level. Otherwise, as we've experienced in Christchurch, the results can be catastrophic depending on the severity of the seismic event.