VII. Earthquakes A.Introduction B.Source of seismic energy C.Propagation of seismic energy D.Recording earthquakes E.Magnitude scales
Building design could not withstand accelerations $Millions of damage Thousands of people killed San Francisco, 1906
Geology in the News Two die in 6.5 magnitude Earthquake near San Lois Obispo California Earthquake triggers mudslides California, 2003 San Andreas Fault
Earthquake Bam, Iran A Magnitude 6.5 Earthquake hits a stone- and mud- house city of 100,000 in Iran December 26, ,000 Dead 30,000 Refugees US sends aid and releases sanctions Relations improved Photos from AP
Geological Hazards Related to Earthquakes Landslides Volcanoes MudslidesTsunamis
B: Sources of Seismic Energy Elastic Rebound Buildup of elastic energy during elastic strain Sudden release due to slippage along a fault or brittle rupture
Anatomy of an Earthquake Fig Focus: Source of energy Epicenter: Location directly above focus at the surface (ground motion is greatest) Fault Trace: Shows intersection of fault and the surface of the land Fault Scarp: Indicates vertical motion of fault
Propagation of Seismic Energy Body Waves travel through the earths interior (crust, mantle, core) P wave: Compression and expansion of rock S Wave: Shearing motion of particles Fig. 8.42
Propagation of a P-Wave
Propagation of Seismic Energy Body Waves travel through the earth’s interior (crust, mantle, core) P wave: Compression and expansion of rock S Wave: Shearing motion of particles Surface Waves
Propagation of an S-Wave
Propagation of Surface Waves Surface Waves travel along the earth’s surface Love Wave: Lateral movement of the surface Rayleigh Wave: Rolling movement of the surface (similar to an ocean wave)
Propagation of a Surface Wave
Recording Earth Motion Seismograph: An instrument that measures the horizontal or vertical motion of Earth’s surface Seismograms: The plot of the motion
Measuring Velocity of Seismic waves Because the P wave travels faster the the S wave The S-P interval increases with distance Time of Earthquake
Reading a Seismogram Ground motion vs. Time Each tick mark is 1 minute P-S Time interval indicates distance to epicenter First P wave Arrival First S wave Arrival First Surface wave P-S Interval See Fig. 8.44
Time-Distance Relationships Use P-S interval to determine distance to focus See Fig Distance from focus (Kilometers)
Seismic Waves and Velocities P-waves = ( + ¾ ) : Bulk modulus : Mod. of rigidity : Density S-waves = / Body Waves and Surface Waves
Locating the Epicenter Triangulation using 3 seismograph stations Depth can be determined with four or more stations
Finding the Depth of Earthquakes Using 4 or more seismograph stations Seismicity of the Pacific Rim Depth (km) Shallow quakes at mid ocean ridges (<33km) and Oceanic trenches Deep quakes over the subduction zone (>70 km)
Earthquakes, Plate Interior New Madrid Fault Zone Faults activated by crustal warping Bowling Green Fault Largest Earthquake on the N. American Continent
New Madrid Earthquake, 1811 Modified Mercalli Intensity Scale Subjective observations of Damage and Ground motion Is not a quantitative measure
Earthquake Intensity Scales Modified Mercalli Scale Subjective observations of Damage and Ground motion Is not a quantitative measure Richter Scale Indicates Ground Motion Amplitude Logarithmic ( e.g., 6 is ten times stronger than 5 ) Does not directly indicate energy or destruction
Earthquakes around the World
Assessing Risk “Major Quake Likely to Strike San Francisco Bay Region Between 2003 and 2032” Assessing Risks Avoiding Risks Preventing Damage Predicting Impact Geologic Hazards
Seismic Risk Analysis Quake history (statistics) Locations of active faults Competency of surficial materials (soil and rock) Ocean basin source Tsunamis
Solomon Islands Earth Quake and Tsunami
Profiling Earth’s Interior Velocities of seismic waves vs depth Fig & 20
Imaging Earth’s Interior P and S waves are refracted (bent) within the earth S waves do not travel through fluids Fig. 9.21