Lesly Santos - Post Lecture #7 Document RCSJ (3)

Post Lecture #7 What does the Theory of Plate Tectonics have to do with geospheric hazards (earthquakes, tsunami, volcanoes)? ● Mantle convection drives plate tectonics and deforms Earth's surface, creating volcanic eruptions, earthquakes, tsunamis, and landslides, and sometimes causing great destruction. What is an earthquake? ● Earthquake: ground shaking that occurs when rapid and sudden movement occurs at plate boundaries and faults, result of the release of stress built up at boundaries and faults. This is known as seismic activity. What is elastic rebound? ● Elastic Rebound: Earth's crust deforms the crust at opposing sides of a faults are subjected to shear stress, further deformation their internal rigidity is exceeded, faults create separation with a rupture along the fault, sudden movement releases accumulated energy, crust snaps back almost to their original shape, the energy released through the surroundings in a seismic wave What is a foreshock and aftershock? Are these episodes considered dangerous? Why or why not? ● Foreshock: earthquakes that precede larger earthquakes in the same location, energy release and ground shaking before an earthquake event, this can occur hours before an earthquake event. Aftershock: earthquakes produce further energy release after an earthquake event energy release, this can occur for hours or days after a large earthquake. Unfortunately, foreshocks are usually too small and too close to the time of the main quake to help us know that the main quake is coming. Aftershocks are sometimes just as hazardous as the main quake itself. In fact, aftershocks may be so strong that they're stronger than the main quake. Who was Inge Lehmann and what did she discover? ● Danish seismologist and geophysicist, 1936 she discovered that the Earth has a solid inner core and a molten outer core based on surveying how seismic body waves traveled through Earth’s Interior. What is the difference between the Modified Mercalli Scale and the Richter Scale? Describe both methods. ● While the Mercalli scale describes the intensity of an earthquake based on its observed effects, the Richter scale describes the earthquake's magnitude by measuring the seismic waves that cause the earthquake. The Modified Mercalli Scale measures

earthquake intensity with a humanistic approach. The scale has “levels” with numeral distinctions- each level of intensity has a description based on human observation of an earthquake event. The intensity is determined by the physical observations of people during an earthquake event- this is a qualitative approach to earthquakes, intensity observations include- furniture movement, picture frame movement, cracks and structural damages, tree movement and breakage, etc. The Richter Scale is a mathematical approach to measuring earthquake intensity. The scale measures intensity based on measurements of amplitude from seismograph readings of an earthquake event, the scale logarithmic meaning each number is 10x greater in intensity than the last number. The Richter Scale is a quantitative measurement of earthquake intensity and provides a precise number distinction for how powerful an earthquake is. Magnitude 1-5 are considered low grade earthquakes and 5-10 are considered high grade earthquakes. The Richter Scale is used most often to communicate earthquake intensity to the public. During a seismic event, body waves P (compressional/pressure) and S (shear) occur. Which wave is received first by the seismograph, P-waves or S-waves? What is the S-P interval? ● P waves travel fastest and are the first to arrive from the earthquake. In S or shear waves, rock oscillates perpendicular to the direction of wave propagation. In rock, S waves generally travel about 60% the speed of P waves, and the S wave always arrives after the P wave. Why are seismograph readings and S-P intervals important to understand and utilize? ● Thus the S-P interval tells us the distance to the epicenter from the seismographic station where the earthquake was recorded. Thus, at each station we can draw a circle on a map that has a radius equal to the distance from the epicenter. ● What is building resonance? Why is this important to understand? ● Building Resonance: Structural stability during earthquakes, defined as the number of seconds it takes for the building to naturally vibrate back and forth (frequency signature), the crust also has a specific resonant frequency during earthquakes, if a building is shaken at this frequency, it will oscillate with large amplitude and may be severely damaged or destroyed. If the same building is shaken at either lower or higher frequencies, it will oscillate with smaller amplitudes and may withstand earthquake shaking without significant damage. Visit this USGS Earthquake Database: https://earthquake.usgs.gov/earthquakes/map/ At the time of completion of your Post Lecture, record three earthquake readings from the database. Include the time of the earthquake, magnitude, and the location. ● M 2.5 - 61 km S of Cordova, Alaska ; Time : 2023-11-16 18:23:40 (UTC-05:00) Location :

was USD $550 million. In Hawaii, the tsunami caused 61 deaths, 43 injuries, and USD $23.5 million in damage. Additional damage of USD $1 million, 2 deaths, and 4 injuries resulted on the U.S. west coast from 1-2m waves. The tsunami hit the Pacific coast of Japan almost a day after the earthquake, causing 139 deaths and destroying or washing away almost 3,000 houses in the Hokkaido, Aomori, Iwate, and Fukushima Prefectures. Waves observed in Japan were higher than other adjacent regions nearer to the source due to the directivity of tsunami wave radiation. At least 21 people died in the Philippines due to the tsunami. Define volcanoes. ● Volcanoes are openings or vents where lava, tephra (small rocks debris), and steam erupt onto the Earth's surface. How do volcanoes generally form? ● Many mountains form by folding, faulting, uplift, and erosion of the Earth's crust. Volcanic terrain, however, is built by the slow accumulation of erupted lava and volcanic debris. Subduction allows water from the subducting plate to be driven upward, off the subducting plate and into the mantle wedge. This lowers the melting point of the mantle, and it melts to form magma. This magma will rise and leak into the crust forming a volcano. Molten rock within the subsurface is called magma and molten rock expelled at the surface is called lava . Explain the Volcanic Explosivity Index. What numericals are in this scale and how does this scale measure volcanic explosivity? ● The Volcanic Explosivity Index (VEI) is a scale that describes the size of explosive volcanic eruptions based on magnitude and intensity. The numerical scale (from 0 to 8) is a logarithmic scale, and is generally analogous to the Richter and other magnitude scales for the size of earthquakes. What is an effusive eruption? What hazards coincide with this type of eruption? Provide one example of a location or specific volcano where this type of eruption takes place. ● Effusive eruptions occur when hot, (1200 o C) runny basalt magmas reach the surface. Dissolved gasses escape easily as the magma erupts, forming lava that flows downhill quite easily. Effusive eruptions build up gently-sloping Shield Volcanoes like Hawaii. What is an explosive eruption? What hazards coincide with this type of eruption? Provide one example of a location or specific volcano where this type of eruption takes place. ● Several hazards may affect the area around the volcano, such as lava flows, pyroclastic

flows, lahars, jökulhlaups and landslides or debris avalanches. Volcanic activity also produces hazards that can affect areas far from the volcano, such as tephra or ash falls, releases of gas and tsunamis. Most explosive eruptions take place at summit craters but the 1980 Mount St. Helens exploded from the side of the mountain. The 79 AD eruption of Mount Vesuvius that destroyed Pompeii was an explosive eruption. Pompeii and nearby Herculaneum were buried beneath 4 to 6 meters of volcanic ash, and 11,000 people died. Define the following volcanic hazards mainly associated with explosive eruptions: Pyroclastic flow - a dense, destructive mass of very hot ash, lava fragments, and gasses ejected explosively from a volcano, typically flowing downslope at very high speed Tephra - ash, rock fragments (all sizes), and particles ejected by a volcanic eruption Lahars - a destructive mudflow on the slopes of a volcano Volcanic Gas - part of tephra, can form massive dark clouds, becomes very thick, blocks sunlight Match the three types of volcanoes with the appropriate characteristics. Stratovolcano A. Large, broad, gradual slopes, basaltic lava, at divergent plate boundaries, mid plate, low explosivity, island builders Shield Volcano B. Steep slopes, andesitic lava, at convergent plate boundaries medium/high explosivity, hazards include pyroclastic flow, lahars, tephra, ash, formed on subduction faults, form near subduction zones Cinder Cone Volcano C. Steep conical hill of loose pyroclastic fragment, cinders, volcanic ash, and scoria that has been built around a volcanic vent, medium explosivity, form on the flanks of stratovolcanoes and shield volcanoes

Stratovolcano Shield volcanoes Cinder Cone Provide all of the factors that occur in the aftermath of volcanic eruptions. ● Volcanic eruptions can also cause secondary events, such as floods, landslides and mudslides, if there is accompanying rain, snow or melting ice. Hot ashes can also start wildfires. Can volcanic eruptions affect global climate? How and why? ● The gasses and dust particles thrown into the atmosphere during volcanic eruptions have influences on climate. Most of the particles spewed from volcanoes cool the planet by shading incoming solar radiation. The cooling effect can last for months to years depending on the characteristics of the eruption.

What occurred during the volcanic eruption of Pompeii 79 AD? Describe the details of this event and its hazards. What do you think about the volcanic preservation of Pompeii? ● Explosions created hot avalanches of rock, ash, and gasses that sped down the volcano's flank. Over two days, Pompeii and nearby Herculaneum were buried by these deadly ashflows. Many of their 20,000 citizens were killed. Today, a million people live within range of the volcano that could again erupted catastrophically. Watch the Yellowstone Supervolcano Smithsonian documentary-directions on PPT. Provide five specific facts from the documentary! Research one major volcanic event that has occurred in recorded history. Please provide where the volcanic eruption occurred, when it occurred, what the VEI reading was, the death toll associated with the event, the financial cost of the event, and the effects in the aftermath of the event. Your answer does not need to be in paragraph form, you may list your information. You may not use one of my examples from the slides! ● Mount Tambora, a volcanic mountain on the northern coast of Sumbawa island, Indonesia , that in April 1815 exploded in the largest volcanic eruption in recorded history. It is now 2,851 meters (9,354 feet) high, having lost much of its top in the 1815 eruption. The volcano remains active; smaller eruptions took place in 1880 and 1967, and episodes of increased seismic activity occurred in 2011, 2012, and 2013. Tambora’s catastrophic eruption began on April 5, 1815, with small tremors and pyroclastic flows . A shattering blast blew the mountain apart on the evening of April 10. The blast, pyroclastic flows, and tsunamis that followed killed at least 10,000 islanders and destroyed the homes of 35,000 more. Before its eruption Mount Tambora was about 4,300 meters (14,000 feet) high. After the eruption ended, a caldera spanning some 6 km (3.7 miles) across remained. Many volcanologists regard the Mount Tambora eruption as the largest and most-destructive volcanic event in recorded history, expelling as much as 150 cubic km (roughly 36 cubic miles) of ash, pumice and other rock, and aerosols — including an estimated 60 megatons of sulfur —into the atmosphere . As that material mixed with atmospheric gasses, it prevented substantial amounts of sunlight from reaching Earth ’s surface, eventually reducing the average global temperature by as much as 3 °C (5.4 °F). The immediate effects were most profound on Sumbawa and surrounding islands. Some 80,000 people perished from disease and famine , since crops could not grow. In 1816, parts of the world as far away as western Europe and eastern North America experienced sporadic periods of heavy snow and killing frost through June, July, and August. Such cold weather events led to crop failures and starvation in