Earthquake Effects and Seismic Study

Earthquake Engineering: Topic 1 - Introduction

• Earthquake effects are categorized as ground failure, indirect effects, and ground shaking.
• Ground failure includes surface rupture, ground subsidence, ground cracking, soil liquefaction, and landslides.
• Indirect effects encompass tsunamis, seiches (tidal waves in enclosed bodies of water), and fires.
• Ground shaking damage is often comparable to, or greater than, other effects.

Ground Failures

• Ground failures are a part of geotechnical earthquake engineering.
• They involve the movement of the ground surface at locations with geological fissures or weakness in the earth's crust (faults).
• Faults can slip slowly (fault creep) or suddenly (earthquake).
• Surface faulting occurs when the relative movement of rocks on either side of a fault breaks through to the surface.
• This type of ground failure often follows a pre-existing fault line.

Ground Subsidence

• Ground subsidence happens when loose soil rearranges and settles into a denser state during earthquake vibrations.
• Compaction from the vibrations can lead to substantial ground surface settlement.

Ground Cracking

• Ground cracking is frequently observed along the edges of ground subsidence.
• It can also result from slope failure or liquefaction.
• These processes cause the ground to lose support, sink, and fracture into fissures, scarps, horsts, and grabens.

Soil Liquefaction

• Soil liquefaction occurs when loose, saturated granular soils change from a solid to a liquid state.
• This happens when the shear strength is lost.
• Loose saturated sands and nonplastic silts are most vulnerable to soil liquefaction.
• In rare instances, gravel and clay can also liquefy.
• Poor drainage in loose soils is the cause of increased pore water pressure which results from the vibratory effect of seismic waves compressing the soil.

Soil Liquefaction (Illustrative Example)

• In soil liquefaction, load shifts from the soil to the pore water pressure, effectively reducing or eliminating strength.
• A slurry (soil-water mixture) can form and flow vertically to the surface, creating craters or sand boils.

Indirect Effects of Earthquakes

• Tsunamis are devastating long-period sea waves generated by vertical movement of the seafloor during earthquakes.
• They travel at extremely high speeds (over 500 mph) and are challenging to detect due to their small height to wavelength ratio (crest-to-trough height) and hundreds of miles long wavelengths in open ocean.
• Seiches are earthquake-induced waves in enclosed bodies of water such as lakes, reservoirs, or bays.
• They are caused when long-period seismic waves resonate with oscillations of the enclosed water.
• Fires are frequently a terrifying indirect effect of earthquakes because surviving trapped individuals in collapsed buildings often perish from the debris fires.

Ground Shaking

• Ground shaking is a primary cause of earthquake damage.
• Where the intensity of shaking is low, the effects from other events (e.g., landslides, tsunamis) may be negligible.
• Earthquake magnitude, location, and the characteristics of subsurface materials beneath a structure influence the shaking intensity and duration.
• Ground shaking is a major factor in earthquake engineering and design.

Types of Earthquakes

• The majority of damaging earthquakes are caused by surface fault ruptures from tectonic plate movement.
• Other sources of earthquakes can include man-made events (e.g., explosions) and volcanic activities.
• Volcanic activity can also cause earthquakes because of the shifting magma beneath a volcano.

Man-Made Earthquakes

• Man-made earthquakes are generally smaller in magnitude than other earthquake types.
• However, man-made seismic events can sometimes increase stress at plate boundaries, leading to more significant tectonic earthquakes.
• Explosions, particularly nuclear explosions, are a major cause.
• Landslides and collapse of mine/cavern roofs also produce man-made earthquakes.
• Large reservoirs also generate seismic activity from the increased weight and water pressure on existing faults.
• Hydraulic fracturing (fracking) for oil and gas extraction is controversial in relation to the increased seismic activity observed.

Volcanic Earthquakes

• Volcanic earthquakes originate from the same energy source as tectonic earthquakes—the heat from the Earth's core.
• Volcanic activity causes seismic waves when subterranean magma movement changes pressure in the surrounding rock.
• Volcanic earthquakes generate limited area shaking.
• The damage from volcanic earthquakes is typically smaller than that from tectonic earthquakes.

Tectonic Earthquakes

• Tectonic earthquakes occur when segments of the Earth's crust suddenly shift.
• Our planet's crust is segmented into tectonic plates floating on a liquid layer (mantle).
• Plate formation occurred approximately 1 to 2 billion years ago.
• Plate motions are constantly shaping continents and landmasses.
• Movement between plate boundaries produces stress that is released by rocks fracturing, resulting in an earthquake.
• Energy release creates seismic waves, and seismic waves cause ground shaking. The intensity and duration of shaking depend on the earthquake magnitude, location, and the local subsurface materials. Faults have the tendency to displace about 2 inches per year.
• Earthquakes can occur in a variety of locations around the globe.
• The epicenter is located directly over the focus, also known as the hypocenter, or origin of the earthquake.
• Seismology is the study of seismic activity.