lesson plan

EOS 170 – Natural Hazards Lecture Summary

Lecture 7 — Reducing seismic risks

We have spent three lectures studying earthquake hazards ; now it is time to look at how to reduce

their risks. Globally, earthquake impacts are worsening through time, driven by increasing exposure.

The world’s population is rising fast and urbanizing, greatly increasing the number and size of cities

located along known active faults, and efforts to reduce vulnerability are so far lagging behind this

rapid growth in exposure. In thinking of ways to reduce these risks, we will consider three of the pillars

of emergency management — mitigation, preparation, and response.

Earthquake mitigation

“Earthquakes don’t kill people, buildings do”; the best single way to reduce deaths is to prevent buildings

from collapsing. New buildings must be constructed to strict standards (that are actually enforced),

while older buildings that predate regulations may have to undergo expensive seismic retrofits. Building

materials are important: masonry (i.e. bricks and mortar) is especially susceptible to collapse, while

wood and steel framed buildings are preferable for their flexibility and strength. Building design is

equally important; all buildings are designed to withstand vertical forces from their own weight, but

not all the strong horizontal forces produced by large earthquakes. Diagonal braces, corner brackets

and shear walls are low cost methods to prevent buildings from ‘soft first story’ damage. Base isolation

is an effective but expensive way in which to decouple larger buildings from horizontal ground motions

and reduce resonance.

Earthquake preparation

Earthquake prediction means determining the size, location and timing of a future earthquake within

stated limits. Predictions are only deemed significant if they can be shown to be successful beyond

random chance. So far, no proposed prediction methods consistently pass such tests, and indeed they

are generally viewed very sceptically by most earthquake scientists. Most predictions rely on supposed

precursor events, such as foreshocks, that might occur shortly before a large earthquake. Unfortunately,

only a few % of large earthquakes have detectable foreshocks, and those can only be identified as such

after the larger mainshock has occurred (e.g. the July 2019 Ridgecrest sequence in California, covered

in Lecture 6). Earthquake forecasting means determining the probability of an earthquake of a given

magnitude within a specified area over longer timescales, using robust scientific data such as the

locations and lengths of faults (mapped by geologists), rates of strain accumulation (measured with

GPS), and the record of past earthquake activity (determined using paleoseismology). Such forecasts

provide no warning of an imminent earthquake, but they do allow seismic mitigation to be prioritized

in the areas that most need it. Earthquake early warning means assessing (within seconds) the location

and magnitude of an earthquake using dense networks of seismometers surrounding the responsible

fault, and informing nearby communities before the arrival of damaging surface waves (which travel

at ∼2–3 km/sec). This technology, proven in countries such as Japan and Mexico, will likely be rolled out in BC within the coming decade with the help of the UVic/Ocean Networks Canada NEPTUNE

cabled observatory offshore Vancouver Island.

Earthquake response

It is rare for victims to survive more than three days buried in collapsed buildings, so reaching them

quickly is paramount. Unfortunately, authorities and media alike often initially underestimate impacts

from the worst earthquakes, delaying the deployment of expert search and rescue teams. The USGS’

PAGER algorithm estimates the number of fatalities within hours of a large earthquake, based on

the spread of recorded intensities, the population density, and knowledge of local building types and

standards. Often, this algorithm provides a quicker and more accurate estimate of human impacts

than local assessments made ‘on the ground’. Satellites can also help provide a rapid assessment of

damage in an earthquake, by enabling a comparison or correlation of “before” and “after” images.

Authorities should use such data to respond more appropriately in the hours after such an event.

8