GLG 110

Part 2

Chapter 7

Earthquakes and Related Phenomena

7.1 Introduction to Earthquakes

            Large earthquakes can be very destructive and cost many lives

            Most US quakes occur on the west coast, some exceptions exist

7.2 Earthquake Processes

            Faults and Fault Movement

                        Faults are fractures along which movement takes place

                        Rocks fail if the stress exceeds the strength of the rock

                        Faults are the seismic source of earthquakes

                        Fault Types:

                                    Strike-slip faults:

- right-lateral strike-slip fault

- left-lateral strike-slip fault

Dip-slip faults:

- normal fault (detachment fault)

- reverse fault (thrust fault)

buried faults: underneath surface

                        Fault Zones and Fault Segments:

                                    Faults usually occur in fault zones, not just one fault

                                    Long faults are usually segmented

                                    The segment is most important to evaluate the hazard

                                    Paleoseimicity is also important (past earthquakes)

                        Fault Activity:

                                    Active faults: moved in past 10,000 years

                                    Partially active: moved more than 10,000 years ago

                                    Inactive fault: moved more than 1.65 million years ago

                                    Slip rate: ratio of slip and time (displacement over time)

                                    Recurrence interval determined by:

- paleoseismic data: average timing of past quakes

- slip rate

- seismicity: average time interval between events

                        Methods of Estimating Fault Activity:

                                    Paleoseismicity: investigate landforms (Fig. 7.6)

                                    Prehistoric quakes: use radiocarbon dating if possible

                        Tectonic Creep:

                                    Gradual displacement, no earthquakes

                                    Causes structural damage, no fatalities

            Seismic Waves and Ground Shaking

                        Focus and Epicenter:

                                    Focus: point within earth where rupture starts

                                    Epicenter: point on earth surface directly above focus

                        Types of Seismic Waves: (Fig. 7.8)

                                    Body waves:

- p-waves: compressional, fast, move through everything

- s-waves: shear, slower, only through solids

Surface waves:

- Love waves: complex, horizontal movement

- Rayleigh waves: complex, rolling movement

                        The Effect of Wave Frequency on Shaking:

                                    Frequency: number of waves passing a point (measured in hertz)

                                    Period: time between two wave crests

                                    Body waves have higher frequencies

                                    Surface waves have lower frequencies

                                    Buildings have their own frequency (low buildings have higher frequency)

                                    Material amplification: amplification of shaking due to sediment nature and composition

                                    Directivity: is another amplification effect due to fault movement direction

            Comparing Earthquakes

                        Each year about one million earthquakes are felt by people

                        Only a few are felt by people close to the quake

                        Magnitude is a measure of energy released vs. intensity (an evaluation of the impact on people and structures)

                        Earthquake Magnitude:

                                    Richter magnitude:

- amount of energy released by quake

- measured by seismograph (makes seismogram)

- standard distance from quake is 100 km

- plotted on logarithmic scale

- from one magnitude to next: increase in energy is 30-fold

Moment magnitude:

- modern scale, close to Richter magnitude

- more accurate, better physics

- applicable over a wider range of ground motions

                        Earthquake Intensity:

                                    Qualitative way to compare quakes

                                    Modified Mercalli Scale: 12 steps of intensity based on severity of shaking

                        Ground Acceleration During Earthquakes:

                                    Acceleration: change in horizontal or vertical velocity during quake

                                    Measured in relation to gravity, unit is “g”

            The Earthquake Cycle

                        Based on observations following the 1906 San Francisco quake

                        Elastic Rebound:

                                    Strain is deformation as a result of stress

                                    Elastic strain: deformation which is not permanent (rubber band)

                                    Elastic rebound: breaking of material due to stress exceeding elastic limit of material

                        Stages of the Earthquake Cycle:

                                    Three to four stages exist:

- 1. seismic inactivity (just after a major quake)

- 2. increased seismicity (with some small quakes locally)

- 3. foreshocks (local and small); may not be present

- 4. major quake

                        The Dilatancy-diffusion Method:

                                    Controversial model

                                    Water content of rocks is important because it lowers shear strength

                                    For details on this method see textbook p. 178

            Earthquakes Caused by Human Activity

                        Major ways of human-induced quakes:

- loading crust (e.g. dams, reservoirs)

- disposal of waste deep in earth

- underground nuclear explosions

                        Reservoir-induced Seismicity

                                    Small tremors occur during several years after dam construction

                        Deep Waste Disposal

                                    Injection of waste into deep wells causes quakes (e.g. Denver 62-65)

                        Nuclear Explosions

                                    Release of natural tectonic strain after explosion possible (e.g. Nevada)

                                    Speculations about use of nuclear explosions to prevent major quakes

7.3 Effects of Earthquakes

            Shaking and Ground Rupture

                        Widespread surface rupture and displacements

                        Damage to structures and other property

            Liquefaction

                        Transformation of water-saturated material to a liquid

                        Buildings sink into ground, buried objects rise to surface

            Landslides

                        Shaking triggers landslides (e.g. Rightwood, CA) in hilly regions

            Fires

                        Breaking of electric lines and gas lines (e.g. 1906 San Francisco)

            Tsunami

                        Sea wave triggered by submarine quake (e.g. 2004 Indian Ocean)

            Regional Changes in Land Elevation

                        Vertical deformation of land by quakes (subsidence, uplift)

7.4 Earthquake Risk and Earthquake Prediction

            Long-term prediction:

                        Probability that a given quake will occur in a certain time frame

            Short-term prediction:

                        Specific time and location of next quake

                        Not effective today!

            Estimation of Seismic Risk (Fig. 7.24)

                        Relative risk is shown: areas where certain quakes have occurred

            Conditional Probabilities for Future Earthquakes

                        Estimates of probability of a certain quake occurring along a certain fault segment within a specified time period

            Short-Term Prediction

                        Like weather forecasting:

- looking for deformation of ground surface

- patterns and frequency of quakes

- strange animal behavior

                        Preseismic Uplift and Subsidence:

                                    Fast and unusual change in topography

                        Seismic Gaps:

                                    Time periods and areas without quakes

                        Anomalous Animal Behavior:

                                    Dogs, horses, birds exhibiting weird behavior for no apparent reason

Towards Earthquake Prediction

                        We are still a long way away from quake prediction!

                        Short-range predictions are the toughest to make

                        “Where the ground has broken before, it will break again!”

7.5 The Response to Earthquake Hazards

            Earthquake Hazard-Reduction Programs

                        Goals are:

- understand earthquake sources (fault mechanics)

- determines quake potential (e.g. active regions)

- predict effects of quakes (e.g. ground rupture)

- apply research results (e.g. to people, structures, etc.)

            Earthquakes and Critical Facilities

e.g. schools, hospitals, police and fire departments, dams, power plants

            Adjustments to Earthquake Activity

                        Structural Protection:  stricter building codes

                        Land-use Planning: placing critical structures away from faults

                        Increased insurance and relief measures: to help adjustments after quakes

            Earthquake Warning Systems

                        Usually time difference (between quake and first shaking) is in seconds

                        This is not a prediction system, but a warning system!

                        System would be relatively cheap

            Perception of the Earthquake Hazard

                        Too may people underestimate quake hazard

                        Mental distress is often significant (especially in children)

Review questions are on page 204.