Earthquakes and seismic site classes

Earthquakes are sudden releases of strain energy stored in rocks, almost always by slip on faults. Most earthquakes cause no damage, but large ones near populated areas can collapse buildings and bridges, trigger landslides, liquefy saturated sands, rupture the ground surface and generate tsunamis. Engineering geology contributes to earthquake engineering in three main ways: by identifying earthquake sources and their activity, by characterizing the ground at a site so that the shaking can be predicted, and by identifying secondary hazards such as liquefaction and earthquake-triggered landslides. This lesson covers the geological side of earthquakes and the site classification that feeds building code seismic design, using Kestrel's hospital expansion as the example.

How earthquakes happen

Stress builds up slowly in the crust as plates move. Faults are locked by friction, and the rock on either side deforms elastically until the stress exceeds the frictional strength and the fault slips suddenly, releasing stored energy as seismic waves. This elastic rebound model, developed after the 1906 San Francisco earthquake, still describes the basic process. The point on the fault where rupture begins is the hypocentre or focus, and the point on the surface directly above it is the epicentre. Rupture then spreads along the fault, sometimes for hundreds of kilometres in the largest events.

Earthquakes are grouped by setting. Interface or megathrust earthquakes occur on the boundary between a subducting plate and the overriding plate and can exceed magnitude 9. Intraslab earthquakes occur within the subducting plate at depths of tens of kilometres and produce widespread moderate shaking. Crustal earthquakes occur on faults in the upper crust of the overriding plate or within stable continental interiors; they are shallower, so even moderate magnitudes can cause intense local shaking.

Seismic waves

Earthquakes radiate several kinds of waves. Body waves travel through the Earth's interior: P waves are compressional and fastest, and S waves are shear waves, slower, and usually carry the strongest shaking for buildings. Surface waves, called Love and Rayleigh waves, travel along the surface, arrive later and are slower still, and can dominate shaking at large distances and in deep basins. The time difference between the arrival of P and S waves at a seismograph increases with distance from the source, so readings from three or more stations locate the epicentre.

For site characterization, the most important property of the ground is its shear wave velocity, \(V_s\). Soft soils have low shear wave velocities, roughly 100 to 200 m/s, while hard rock has velocities above 1,500 m/s. When seismic waves pass from stiff rock into softer soil near the surface, they slow down and their amplitude increases, and resonance in the soil layer can amplify shaking at particular frequencies. The 1985 Mexico City earthquake, where buildings on soft lake clays far from the source were devastated, is a classic example of site amplification.

Magnitude and intensity

Magnitude measures the size of an earthquake at its source; intensity measures the effects of shaking at a particular place. Early magnitude scales, such as Richter's local magnitude, were based on the amplitude of waves on seismographs and saturate for large earthquakes. Today seismologists use moment magnitude \(M_w\), based on the seismic moment: the rigidity of the rock times the fault area times the average slip. Each whole unit of magnitude corresponds to about 32 times more energy released and about 10 times greater wave amplitude on the original scales, so a magnitude 7 earthquake releases about 1,000 times more energy than a magnitude 5.

Intensity is described with the Modified Mercalli Intensity scale, from I (not felt) to XII (total destruction), based on observed effects on people, buildings and the ground. A single earthquake produces a range of intensities that generally decrease with distance from the source but are higher on soft ground. Instrumental measures of shaking, such as peak ground acceleration and spectral acceleration, are what engineers use for design.

Earthquake hazard in Canada and the United States

Natural Resources Canada monitors earthquakes through the Canadian National Seismograph Network and produces the national seismic hazard model used in the National Building Code of Canada. The highest hazard is in southwestern British Columbia, from the Cascadia subduction zone and crustal and intraslab sources, and along the coast to Haida Gwaii and the Yukon. Moderate hazard zones include the St. Lawrence and Ottawa valleys, the Charlevoix region of Quebec, and parts of the Arctic. The 2020 edition of the National Building Code adopted the sixth-generation seismic hazard model, with hazard values provided for a range of site conditions. Kamloops has much lower hazard than Vancouver, but a hospital is a post-disaster building with the highest importance category, so its design must still account carefully for shaking.

Site classification

Building codes account for local ground conditions through site classes, which are based mainly on the average shear wave velocity of the top 30 m of the ground, written \(V_{s30}\). The average is a time-weighted harmonic average: the total depth divided by the total travel time of a shear wave through the layers.

Where shear wave velocity is not measured, codes allow site class to be estimated from the average standard penetration test blow count or the average undrained shear strength, but measured velocities are preferred and may lead to less conservative designs. Shear wave velocity is measured by downhole or crosshole seismic tests in boreholes, by the seismic cone penetration test, or by surface methods such as multichannel analysis of surface waves (MASW).

Under the 2020 National Building Code, the designer can either assign a site class and use the hazard values for that class, or use the measured \(V_{s30}\) directly through a site designation based on it. Using a measured value avoids the jumps in design forces that occur at class boundaries and rewards the effort of measuring velocities.

# Vs30 by harmonic averaging and site class lookup for both codes
layers = [(4, 170), (8, 230), (9, 380), (9, 520)]   # (thickness m, Vs m/s)
assert sum(d for d, _ in layers) == 30
t = sum(d / v for d, v in layers)
vs30 = 30 / t
arith = sum(d * v for d, v in layers) / 30
print(f"Vs30 = {vs30:.0f} m/s ({vs30/0.3048:.0f} ft/s); thickness-weighted arithmetic mean = {arith:.0f} m/s")

nbc = [(180, "E"), (360, "D"), (760, "C"), (1500, "B"), (1e9, "A")]
asce_fts = [(500, "E"), (700, "DE"), (1000, "D"), (1450, "CD"), (2100, "C"), (3000, "BC"), (5000, "B"), (1e9, "A")]
print("NBC site class:", next(c for lim, c in nbc if vs30 < lim))
print("ASCE 7-22 site class:", next(c for lim, c in asce_fts if vs30 / 0.3048 <= lim))

# Energy ratio between magnitudes
for m1, m2 in ((5, 6), (5, 7), (6, 9)):
    print(f"M{m2} releases about {10 ** (1.5 * (m2 - m1)):,.0f} times the energy of M{m1}")

Vs30 = 302 m/s (991 ft/s); thickness-weighted arithmetic mean = 354 m/s
NBC site class: D
ASCE 7-22 site class: D
M6 releases about 32 times the energy of M5
M7 releases about 1,000 times the energy of M5
M9 releases about 31,623 times the energy of M6

Secondary earthquake hazards

Ground shaking is the main hazard, but several geological effects of earthquakes cause severe damage.

• Liquefaction: loose, saturated sands and silty sands can lose strength during shaking as pore pressures rise, behaving temporarily like a liquid. Liquefaction causes sand boils, settlement, tilting of buildings, flotation of buried tanks and pipes, and lateral spreading of gently sloping ground toward rivers and shorelines. Young, loose, water-deposited sands, such as river deltas, beach deposits and hydraulic fills, are most susceptible. Codes generally require a site-specific evaluation for liquefiable soils, which places them in site class F.
• Earthquake-triggered landslides: shaking adds inertial forces to slopes and can trigger thousands of rockfalls and slides in mountainous terrain, and flowslides in sensitive clays.
• Surface fault rupture: where a fault breaks the surface, offsets of metres can tear apart structures, pipelines and roads that cross it. Avoidance by setbacks from mapped active faults is the main mitigation.
• Tsunami: large subduction earthquakes and earthquake-triggered submarine landslides can generate tsunamis that inundate coastal areas within minutes to hours.
• Basin and topographic effects: deep sedimentary basins can trap and amplify long-period waves, and ridge crests can amplify shaking.

Paleoseismology: reading the prehistoric record

Instrumental records of earthquakes go back only about a century, and written records only a few centuries in most of North America. That is far too short to capture the largest, rarest earthquakes, which may recur every few hundred or few thousand years. Paleoseismology fills the gap by finding geological evidence of prehistoric earthquakes. Trenches dug across faults reveal offset soil layers that can be dated. Along the Cascadia coast, buried marsh soils and drowned forests show sudden land subsidence during great earthquakes, and sand layers washed inland record tsunamis. Turbidite deposits in deep-sea channels, triggered by earthquake shaking of the continental slope, extend the Cascadia record back about ten thousand years and suggest dozens of great earthquakes in that time.

The date of the last Cascadia great earthquake, January 1700, was established by combining tree-ring dating of drowned trees in Washington with written records of an unexplained tsunami that struck the coast of Japan. This kind of geological detective work underpins the hazard models in both countries' building codes, and it is a reminder that a lack of historical earthquakes in a region does not prove a lack of earthquake hazard.

The role of the engineering geologist

For a project such as the hospital, the engineering geologist's tasks include reviewing regional seismicity and nearby faults, confirming that no active fault crosses the site, measuring or estimating shear wave velocities and assigning the site class or Vs30, assessing liquefaction potential and slope stability under earthquake loading, and identifying any need for a site-specific ground motion study. At Kestrel's hospital site, the measured profile gave site class D, the dense gravel and till were judged not liquefiable, and the loose surface sand above the water table was not a concern, so the structural engineers could proceed with code-based design.