Quake
Quake may refer to: For other uses, see Quake (disambiguation).Seismology
Earthquake Fundamentals
An earthquake is the sudden release of energy in the Earth's lithosphere that generates seismic waves, causing the ground to shake. This energy release typically occurs when accumulated stress along a fault overcomes frictional resistance, allowing rock masses to slip past one another.[1][2] The primary cause of most earthquakes is the movement of tectonic plates, which are rigid segments of the Earth's crust that interact at boundaries. These movements build elastic strain in the rocks until it is suddenly released, a process explained by the elastic rebound theory developed by Harry Fielding Reid following his analysis of the 1906 San Francisco earthquake. In this theory, rocks deform elastically under stress and then snap back to their original shape, propagating seismic waves. Earthquakes often occur along specific fault types, such as strike-slip faults where blocks slide horizontally past each other (e.g., the San Andreas Fault) or thrust faults where one block is pushed up over another.[3][4] Earthquakes are classified into several types based on their origins. Tectonic earthquakes, the most common, result from plate boundary interactions and account for about 90% of global seismic energy release. Volcanic earthquakes arise from magma movement or pressure changes within volcanoes. Induced earthquakes are triggered by human activities, such as fluid injection during hydraulic fracturing (fracking) or wastewater disposal, which increase pore pressure on faults. Less common are collapse earthquakes, caused by the sudden subsidence of underground structures like mine roofs or volcanic calderas, and explosion earthquakes, generated by rapid energy releases from nuclear tests or volcanic blasts. Most earthquakes worldwide occur at tectonic plate boundaries, where about 80% of seismic activity is concentrated, including subduction zones, spreading ridges, and transform faults. For example, the 1906 San Francisco earthquake, a magnitude 7.9 event on a strike-slip fault, ruptured approximately 477 kilometers of the San Andreas Fault, demonstrating the scale of tectonic quakes at plate margins.[5][6][7][8] Seismic waves produced by earthquakes travel through the Earth and are categorized into body waves that propagate internally. Primary waves, or P-waves, are compressional waves that alternately squeeze and expand rock in the direction of travel, moving the fastest (up to 8 km/s in the crust) and arriving first at seismic stations. Secondary waves, or S-waves, are shear waves that cause rock to oscillate perpendicular to their propagation direction, traveling slower (up to 4.5 km/s in the crust) and thus arriving after P-waves, but carrying more destructive energy due to their shearing motion.[2][9]Seismic Measurement and Impacts
Seismographs and accelerometers are primary instruments for measuring earthquakes. Seismographs detect and record ground motion from seismic waves, producing seismograms that capture displacement, velocity, or acceleration over time. Accelerometers, often integrated into modern seismic networks, measure the acceleration of the ground, which is crucial for assessing structural response during strong shaking. These tools form the backbone of global seismic monitoring networks operated by organizations like the United States Geological Survey (USGS).[10] Earthquake magnitude is quantified using scales that estimate the total energy released. The local magnitude scale, developed by Charles F. Richter in 1935 and commonly called the Richter scale, is calculated asM_L = \log_{10} A + \text{correction terms},
where A is the maximum amplitude of seismic waves (in millimeters) recorded on a Wood-Anderson seismograph at a standard distance of 100 km, and corrections account for distance and site effects. This logarithmic scale means each whole-number increase in M_L represents about 10 times greater amplitude and roughly 31 times more energy release. For larger events, the moment magnitude scale (M_w) provides a more reliable measure, especially for magnitudes above 7, and is defined as
M_w = \frac{2}{3} \log_{10} M_0 - 6.0,
where M_0 is the seismic moment (in Newton-meters), a physical quantity representing the rigidity of the rocks, the area of the fault that slipped, and the average slip distance. Introduced by Hanks and Kanamori in 1979, this scale does not saturate at high magnitudes and is now the standard for global catalogs.[11][12] In contrast, earthquake intensity assesses local effects rather than overall size. The Modified Mercalli Intensity (MMI) scale, an adaptation of earlier versions from the early 20th century, rates shaking on a 12-point Roman numeral scale (I to XII) based on observed human perceptions, structural damage, and environmental changes. For example, MMI IV indicates light shaking felt by many indoors, while MMI IX causes heavy damage to well-built structures. This scale is particularly useful for mapping spatial variations in shaking during an event, as intensity decreases with distance from the epicenter.[13] Earthquakes produce several primary physical impacts. Ground shaking, the most widespread effect, vibrates structures and can lead to collapse if intensities exceed design limits. Surface rupture occurs when the fault breaks through the ground, displacing land by meters along the fault trace. Liquefaction happens in saturated, loose soils during intense shaking, causing the ground to lose strength and behave like a liquid, often resulting in building settlement or tilting. Landslides and rockfalls are triggered on steep slopes, exacerbating damage in hilly or mountainous regions. These hazards are most severe near the epicenter and in vulnerable terrains.[14][15] Societal impacts include loss of life, injuries, and substantial economic costs from direct damage and indirect disruptions. Casualties arise mainly from collapsing buildings, tsunamis, or secondary effects like fires, with global annual deaths averaging approximately 13,000 over recent decades, though varying widely from a few thousand to over 60,000 in years with major events.[16] Economic losses encompass repairs to infrastructure, lost productivity, and long-term recovery; for instance, the 2011 Tōhoku earthquake in Japan, with a moment magnitude of 9.0, resulted in over 15,000 deaths and approximately $235 billion in damages, the costliest natural disaster on record;[17] more recently, the 2023 Kahramanmaraş earthquake sequence in Turkey and Syria (maximum moment magnitude 7.8) resulted in over 59,000 deaths and an estimated $150 billion in damages (as of 2023), underscoring the continued global threat.[18] Approximately 500,000 earthquakes are detectable worldwide each year by modern instruments, but only about 100 cause significant societal impacts, such as widespread damage or fatalities.[19] Mitigation strategies focus on reducing these impacts through engineering and technology. Building codes, such as those in the International Building Code adopted in seismic-prone areas, require structures to resist specified ground accelerations, incorporating features like base isolation and shear walls. Early warning systems detect initial seismic waves and alert populations seconds to minutes before strong shaking arrives; the USGS-operated ShakeAlert system in the western United States, for example, uses a network of over 1,500 sensors (as of 2024) to issue public alerts via apps and infrastructure controls, potentially averting injuries and economic losses estimated in billions.[20][21][22]