Earthquakes and Environment

Environmental Geology Chapter 6 EARTHQUAKES and ENVIRONMENT • Violent ground-shaking phenomenon by the sudden release of strain energy stored in rocks. This is one of the most catastrophic and devastating hazards Millions of people killed and billions of dollars in damage by catastrophic earthquakes. The United States Geological Survey (USGS) estimated about 1 million quakes annually. • Most earthquakes concentrated along plate boundaries, and nearly all catastrophic earthquakes are shallow earthquakes • Divergent plate boundary: Shallow earthquakes • Transform plate boundary: Shallow to intermediate earthquakes Convergent plate boundary: Wide zone of shallow, intermediate, and deep earthquakes; 80% of seismic energy released along the earthquake zone around the Pacific rim. • Earthquakes Can Occur Away From Plate Boundaries (e.g. the New Madrid Fault Zone in the Mississippi Valley) • Human induced earthquakes Ø Much smaller magnitude Ø Reservoir-induced earthquakes (added weight and water pressure-new reservoirs) Ø Injection of fluids into fractured rocks (e.g. deep waste disposal) Ø Underground nuclear explosions • Earthquake Focus and Epicenter. Focus – where the rocks break underground. Epicenter – on Earth’s surface directly above the focus • Fault Types • Normal fault – hanging wall moves down relative to footwall. These are caused by tensional stress – “pulling apart” and are typically related to divergent plate boundaries (e.g. mid-ocean ridges and rift valleys). Normal faulting has createdthe mountains and valleys of the Basin and Range “province” of Arizona and Nevada (including the Phoenix area) • Reverse fault – hanging wall moves up relative to footwall. These are caused by compressive stress and typically are typically related to convergent plate boundaries (e.g. fold and thrust belts-mountain ranges like the Himalaya and Appalachian Mountains) • Strike-slip faults – movement is side to side, with the other side of the fault moving right (right lateral) or left (left lateral) relative the side you are standing on. These are caused by shear stress and are typically related to transform boundaries (e.g. sea floor fracture zone along the mid-ocean ridges, the San Andreas Fault and Anatolian Fault (Turkey) zones). The San Andreas Fault is a right lateral strike-slip fault zone. • Fault-related tectonic creep: gradual displacement-slow semi-steady motion; earthquakes are mostly not felt. • Seismic waves travel outward from the focus, and arrival times at seismic stations allow us to determine the epicenter • Directivity - Intensity of Shaking increases in the direction of fault rupture • Activity vs. Active or Inactive ? - A fault is active if movement has occurred within the last 10,000 yrs. • Earthquake’s Seismic Waves • P-wave: Compressional waves, travel fastest through all physical states of media. They arrive first at seismic stations and have a “longitudinal” motion – “push-pull”. • S-wave: Shear waves, travel slower than P-wave, arriving 2nd at seismic stations, but faster than surface waves, only propagates through solid materials. They have a transverse motion – “up-down”. • Surface waves: Moving along the Earth’s surface, travels slowest and arrive last at seismic stations, but causing most of the damage. They travel in a rolling motion. • Measuring Seismic Waves – We measure them with a Seismograph or seismometer which measures the amplitude of seismic waves: ground vibration. We use the first arrival of seismic waves and determine the time of earthquake. The distance to epicenter from a seismograph is based on the difference in arrival time between P-waves and S-waves. • Finding an earthquake epicenter – a minimum of 3 stations are needed to “triangulate” the surface location (epicenter). • Material Amplification - Seismic waves travel differently through different rock materials, and propagate faster through dense and solid rocks. The intensity (amplitude of vertical movement) of ground shaking is more severe in unconsolidated materials - “weaker” or “looser” materials such as fine-grained sediments, and damage can be more severe if wet. The seismic energy is also attenuated more and propagated less distance in unconsolidated materials. • Primary effects of Earthquakes: • Ground shaking, tilting, land elevation changes and ground rupture • • • Loss of life and collapse of infrastructure • Landslides, liquefaction (“sloshing” around of loose, wet sediments), and tsunamis (seismic sea waves) • Shallower focus earthquakes produce more damage than deep-focus quakes, because energy has been spread out more by the time it reaches the surface Secondary effects • Fires, floods, and diseases (e.g. from spores in dust or water pollution) Earthquakes in Arizona - Damaging earthquakes in AZ are unusual. We are not on anot at active plate boundary. However, we do have quakes associated with Basin and Range faulting (tensional stresses in crust). The valleys and mountain ranges in NM, AZ, NV, and CA produced by normal faulting. Historical damaging quakes have occurred in several areas: • Grand Canyon/Williams/Prescott area [6 min. video] • SE Arizona (Sonoran Earthquake of 1887) [6:44 min] • SW Arizona (Baja Quake – April 2010) [1:50 min] – PHX effects [27 sec] • • Earthquake Magnitude Scale • Richter scale: Based on the amplitude of ground motion, increasing one order in magnitude equals a tenfold (10x) increase in amplitude • Moment magnitude scale - measures the amount of strain energy released. It is based on the amount of fault displacement and is applicable over a wider range of ground motions than Richter scale • Earthquake energy: Increase one order in magnitude, about a 32-times (32x) increase in energy • Earthquake Intensity Scale - Modified Mercalli Scale – There are 12 divisions to the scale, based on a qualitative severity measurement of damages and ground movement. It is based on ground observations, instead of instrument measurements. The scale depends on earthquake’s magnitude, duration, distance from the epicenter, site geological conditions, and conditions of infrastructures (age, building code, etc.) • Stages of earthquake cycle – 1. Inactive and aftershock stage, 2. Stress accumulation stage (elastic strain-rocks bend), 3. Foreshocks and Main shock (major earthquake) occurs as rupture occurs and rocks “rebound” (elastic rebound) as elastic strain is replaced by displacement (fault slip). • Earthquakes can also cycle over time as strain builds up again and again in the same areas. And, they can cycle in “space” as the location of earthquakes along a fault line can change over time, producing “seismic gaps” or areas along a fault with no movement for long periods of time. Ø Earthquake Risks can be estimated based on likelihood of an event of a certain magnitude within a period of time, or on a combination of largest quake likely and average amount of motion per year. There are probabilistic methods (probability of an earthquake happening) for a given magnitude or intensity, estimating the earthquake risk of an area and of a fault segment. Seismic hazard maps can be created to show areas that are at the greatest risk. • Earthquakes may lead to others farther on along active fault zones, however the pattern may be either somewhat regular in time and space or clustered, with centuries of relative quiet in between, further increasing the difficulty of quake prediction • Earthquake Prediction - Earthquake hazard risk mapping can help with long term predictions of when an earthquake might happen (e.g. within the next 30 years). Short-term prediction (forecast) is much less reliable. These short term predictions may be based on a number of things, such as the frequency and distribution pattern of foreshocks, pre-quake deformation of the ground surface: Tilting, elevation changes (e.g. GPS & Laser ranging), the emission of radon gas (from newly fractured rocks), seismic gap along faults (strain build-up), and abnormal animal activities. • Response to Earthquake Hazards - Reducing the risk of earthquakes involves: Developing a better understanding of the source and processes of earthquake, determining earthquake risk potential, predicting effects of earthquakes, applying research results to improve building methods, land use planning, and improved relief/insurance measures . • Response to Earthquake Hazards - Adjustments to earthquake activities that can be made to reduce future impacts include better site selection for critical facilities, structure reinforcement and protection, better land-use regulation and planning, and emergency planning and management: Insurance and relief measures. • Earthquake Warning Systems are technically feasible, but only about a minute warning. Earthquake warning systems may provide seconds to a minute of warning to nearby urban areas once an earthquake has occurred (such systems are currently in use in California and Japan). This is not a prediction tool, and false alarms are possible. – Community and individual emergency preparedness and response plans and microzonation (taking into account local geology and urban development) are currently the best methods to reduce loss of life and property during future damaging earthquakes, which will certainly occur • Perception of the Earthquake Hazard – The public needs to be prepared for the earthquake potential, even psychologically. Pre-earthquake planning is needed and there needs to be a system for post-earthquake emergency response. Better response is needed in terms of engineering structural designs to minimize the hazard risks. • How to be safer during a quake • Move to safer areas (beneath sturdy desk, in doorways of interior walls) • Duck, cover and hold • Everyone in a house should know how to turn off gas • Establish out-of-area contact to call after an earthquake • Have emergency supplies (food, water, first-aid kit, cash) on-hand • Identify elderly or infirmed individuals in local neighborhood to help • Assist others in forming their own plans and supply kits