Chapter 2

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McMaster University
Civil Engineering Infrastructure Technology
Jonathan Sussman

Chapter 2 Earthquakes 2.1 Introduction Earthquakes  result from the rupture of rocks along a fault Fault  a fracture in the earth’s crust Seismic Waves  the energy formed when rocks on opposite sides of the fault move Earthquake Magnitude  Epicentre  the point on the surface directly above the fault rupture  Richter Scale  a decimal number on a scale that represents the size or magnitude of a quake  See Figure 2.3 on pg 33  Today, the most common unit to measure quakes is Moment Magnitude (Mw), which is determined: o From an estimate of the area that ruptured along a fault plane during the quake o The amount of movement or slippage along the fault o The rigidity of the rocks near the focus  Magnitude and Frequency of Earthquakes o See Table 2.3 on page 34 Earthquake Intensity  The intensity depends on magnitude, distance from epicenter, and the nature of the ground at the site  Modified Mercalli Intensity Scale o Measures the degree to which an earthquake affects people, property, and the ground o Usually shown on maps o Based on newspaper articles, reports of damage assessment teams, and questionnaires to residents 2.2 Earthquake Processes Process of Faulting  Compared to sliding two rough boards past each other  two lithospheric plates move past each other  This rupture produces vibrational energy called seismic waves  Fault Types o Strike-Slip Faults: displacements are mainly horizontal o Dip-Slip Faults: displacements are mainly vertical  Classified as a reverse fault or a normal fault (depending on how the earth’s material moves)  Can also be called thrust faults, which are mostly located in deep-sea trenches, where they are low-angle reverse faults o See Figure 2.12 on pg 40  Tectonic Creep o Gradual movement along a fault without accompanying felt earthquakes o Can slowly damage roads, buildings, etc  Seismic Waves o P Waves  compressional or primary waves  Faster than S waves  Can travel through solids, liquids, or gases. o S Waves  shear or secondary waves  Travel through solids  Produce back-and-forth motion o Surface Waves  Form when P and S waves reach the surface o See Figure 2.15 on pg 42 2.3 Earthquake Shaking Important factors that determine the shaking people experience: 1. Earthquake’s magnitude 2. A person’s distance from the focus 3. Local soil and rock conditions Depth of Focus  Seismic waves lose some energy before reaching the surface (AKA: attenuation)  The deeper the source, the less intense the shaking Local Geological Conditions  Dense granitic and metamorphic rocks of the Canadian Shield transmit energy efficiently  Heterogeneous, folded, and faulted crust transmit energy rapidly away from epicenters  Seismic Waves move slower through unconsolidated sediment and material with high water content  Material amplification  increases the amount of ground motion experienced in an earthquake  Local geologic structures can also amplify shaking o Example: synclines and fault-bounded sedimentary basins (See figure 2.2 on pg 46) 2.4 The Earthquake Cycle  Elastic strain drops abruptly after an earthquake and then slowly accumulates before the next event o See figure 2.25 on pg 49  Elastic strain  temporary deformation  If the strain continues, deformed material eventually ruptures Stages 1. Long period of Inactivity along fault segment 2. Accumulated elastic strain produces small earthquakes 3. Foreshocks (small to moderate sized ear
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