GEOL 0820 Ramsey Natural Disasters Spring, 2018 LECTURE #5: Plate Tectonics: Boundaries & Earthquake Science Date: 23 January 2018 I. Reminder: Exam #1 is scheduled for Feb 1st one week from Thursday o I will take a vote at the start of class to see if sliding it to Feb 6 th is preferred be sure to be on time (no one will be allowed to take the exam if they show up more than 15 minutes late!) bring a pencil, eraser, a photo ID, and your PeopleSoft number! we will go over some example questions at the start of next lecture o material covered: first 7 or 8 lectures (including the video) II. Review from Last Class brief history of Plate Tectonics two hypotheses that combined to become the Theory of Plate Tectonics 5 steps of the Scientific Method III. Today: Plate Tectonics three types of plate boundaries: o divergent plates spreading apart small earthquakes example: ocean floors o convergent plates come together subduction zones increased volcanic activity large earthquakes example: South America o transform plates slide past one another moderate earthquakes example: San Andreas fault Page 1/6
how do we know the structure of the Earth? o seismology study of seismic waves as they move through the crust formed by earthquakes (EQ s) can be VERY destructive but can also be used to image the structure of the Earth generalized structure of the Earth o how do we know what is below our feet? we have only drilled down several km s never through the crust 5km thick (ocean) and 35km thick (continents) in contrast, the Earth has a radius of 6,370km o examining the seismic waves as they pass through the Earth waves change speeds when they pass through different rock compositions and density seismologists can model these changes and estimate what the rock types are, how thick they are, and what state they are in (e.g., solid or liquid) IV. Seismology the branch of geology concerned with the study and analysis of EQ's, seismic waves and their propagation through the Earth seismic waves: o energy in the form of waves (causing particle motion) radiating out from a fault rupture (break) o two main types: body waves and surface waves o type 1: Body Waves: pass through the volume of the Earth two sub-types: P-waves, S-waves P (primary) waves: compressional particle motion fastest average speeds in the crust (~ 6km/s) behave similar to sound waves in air can pass through solid, liquid and gas S (secondary/shear) waves: second fastest wave (~ 3.5 km/s) shearing particle motion (side to side mov't with respect to the wave propagation direction) only passes through solids more damaging to structures because of the shearing ground motion o type 2: Surface Waves: confined to the very upper surface of the crust are slower than both P & S waves most damaging to structures two main sub-types: Love-waves, Rayleigh-waves Page 2/6
Love waves: similar to the motion of S-waves Rayleigh waves rotating particle motion Seismic record of an EQ o major compositional zones of the Earth crust (two types: oceanic and continental) 1. continental crust thicker (30-50 km) less dense (granitic rocks) older (some regions up to ~3 billion years old) 2. oceanic crust thinner (~ 5km) more dense (gabbroic rocks) much younger (only up to ~200 million years old) mantle extends from the asthenosphere/lithosphere boundary (Moho) down to the outer core 2900 km some indirect sampling when material (called kimberlites) is brought to the surface from very deep rooted, explosive volcanoes most information comes from seismic profiles relatively uniform composition of rock pressure/temperature increases with depth low-velocity zone (upper 80-300 km) in upper mantle only a few percent molten material critical: this zone allows Plate Tectonics to happen Page 3/6
core (two zones that are VERY different) enriched in iron and nickel compared to the crust/mantle much more dense mostly iron (~ 5% nickel and 1-2% sulfur) 1. inner core solid (caused by the very high pressures) known because of seismic wave behavior 2. outer core liquid (caused by the very high temperatures) known because of seismic wave behavior rotation of this metallic liquid creates the Earth s magnetic field yellow arrow: low-velocity zone (small amount of liquid) both P and S waves slow red ellipse: outer core (all liquid) P wave slows & S wave stops o major mechanical zones of the Earth upper layer (lithosphere) defines a tectonic plate brittle behavior does not flow, hence fractures when stress is applied all of the crust and part of the upper mantle (compositional units) define the lithosphere asthenosphere below plastic behavior flows when stress is applied contains a low-velocity zone some amount of partial melting all mantle material Page 4/6
Earthquakes V. Earthquakes what are earthquakes (EQ s)? o ground movement caused by the release of seismic energy o seismic energy is usually caused by the brittle failure of rocks under stress important hazards to understand: o they kill the most people per year (> 1 million in total over the past century) o commonly strikes without precursors or warning Page 5/6
o 1000's of large EQ's every year only ~ 20 are > M7.0 (known as major M7.0 to M7.9 or great larger than M8.0) account for 90% of the energy released and 80% of all the fatalities o recall: most of the fastest growing cities lie on the Pacific Rim and are threatened by large EQ's VI. Earthquakes & Faults where and how do they occur? o sudden release of stored energy as the result of rapid movement between two blocks (plates) o energy stored because of plate movement and friction release point is the focus or hypocenter energy radiates out in all directions point on the surface directly above the focus is the epicenter fault trace: line where the fault/fracture intersects the surface Earthquakes in CA/NV during one week: http://earthquakes.usgs.gov Page 6/6