Lab 2a - Harmonic amplification of seismic shaking

Lab 2b - Microzonation assessment of amplification potential

Lab 2a - Harmonic amplification of seismic shaking

The objective of this lab is to understand the differences in propagation of body waves (eg. P or S waves) at different wavelengths, and to see how the amount of ground shaking is affected by the relationship between the wavelength of the waves and the natural harmonic frequencies of geological materials. The exercise will be completed during the lab period and should be handed in at the end of the class. We will use the geology classroom (Rm 107/ Bldg. 370) for this lab.

Data from the harmonic amplification experiment: lab02a-data-07.xls

Lab 2b - Microzonation assessment of amplification potential in Nanaimo

In this lab we will be examining a surficial geology map of the Nanaimo area (ie. a map showing the distribution of unconsolidated surficial materials such as glacial deposits) with the objective of making some predictions about the potential for harmonic amplification.

Click here to have a look at the map.

 

Notes to accompany the Surficial Geology map of the Nanaimo area

Surficial Geology involves the mapping and description of the unconsolidated materials that lie on top of bedrock.  These materials include clay, silt, sand, gravel and peat, and, in our region, are generally of glacial, fluvial or marine origin.  They range in thickness from a few centimetres to tens of metres.

The thickness of surficial deposits plays a critical role in the amplification of earthquake waves.  Significant amplification can take place where poorly-consolidated material is several to tens of metres thick.  This map does not show thicknesses, but in the absence of that information, it can be assumed that colluvium – which is largely the broken material derived from weathering – is generally present as layers that are less than a couple of metres thick.  In this region moraine – a.k.a. glacial till – tends to be up to a few metres thick.  Marine and glacio-marine deposits are also typically in this thickness range.  Fluvial and glacio-fluvial deposits (ie. stream deposits) can be much thicker, in some cases as much as tens of metres thick, and organic (peaty) deposits can also be quite thick.

The types of materials shown on the map can be described as follows:

Bedrock

Exposed rock (While there are no areas of exposed rock shown, there is a lot of exposed rock in the Nanaimo area.  The outcrop areas are not shown because they are relatively small, but they mostly lie within the areas marked as colluvium)

Colluvium

The products of erosion of bedrock.  Most areas mapped as colluvium also include extensive areas of exposed bedrock.  Colluvium deposits are largely comprised of relatively coarse gravel and rock fragments.

Fluvial

The deposits of rivers and streams.  Grain sizes vary, but thick layers of sandy deposits are common.  Fluvial deposits are typically poorly-consolidated and loosely packed.

Glacio-fluvial

Similar to fluvial, but deposited by streams that existed during glaciation.  Because there is typically a lot of water flowing around during glaciation, some glaciofluvial deposits can be very thick.

Marine and glacio-marine

Sand, silt and mud deposits formed in marine areas.  Within a few hundred meters of shore these deposits can be sandy and silty, but otherwise they are quite clay-rich.  The existence of marine and glacio-marine deposits on land around Nanaimo is a result of the fact that Vancouver Island was pushed down by a heavy load of glacial ice, and did not rebound fully until a few thousand years after the ice had left.

Moraine

Material moved by glacial ice.  Material is moved on top of, within and beneath an ice sheet, although in many cases the sub-ice material is dominant.  Almost no sorting takes place beneath an ice sheet, so most glacial till is a mixture of material ranging from fine clay up to coarse boulders.  Because of the weight of up to several thousand metres of ice, sub-ice glacial till can be very well compacted – and almost rock-like.

Organic

Vegetative material, primarily from swamps. Vegetation can accumulate to thicknesses of many metres in swampy environments.  The vegetation is typically mixed with silty and sandy sediments, and is not well compacted.