Earthquakes are sudden ground vibrations caused by the release of accumulated strain energy along tectonic faults or volcanic conduits within the Earth’s lithosphere. As tectonic plates interact at convergent, divergent, and transform boundaries, frictional locking builds stress until sudden slippage generates seismic waves. Understanding earthquake mechanics including focus depths, wave propagation, and magnitude scales is vital for hazard mitigation, structural engineering, and geological science. These multiple-choice questions explore core seismological concepts, fault types, and the underlying physical causes of seismic events.
1. What is the primary physical cause of tectonic earthquakes?
Correct Answer: a) Sudden release of stored strain energy along faults in the Earth's crust
Explanation:
Earthquakes are typically caused by the sudden rupture and slip of rock along a fault line, releasing accumulated elastic strain energy in the form of seismic waves.
2. What scientific theory explains how tectonic stress builds up and is released during an earthquake?
Correct Answer: b) Elastic rebound theory
Explanation:
Proposed by H.F. Reid after the 1906 San Francisco earthquake, the elastic rebound theory explains how rocks bend under stress until friction is overcome, snapping back into place during rupture.
3. What is the point within the Earth where an earthquake rupture originates called?
Correct Answer: b) Focus (Hypocenter)
Explanation:
The hypocenter, or focus, is the exact subterranean point where fault slip begins and seismic energy is first released.
4. What term refers to the geographic point on the Earth's surface directly above an earthquake's focus?
Correct Answer: b) Epicenter
Explanation:
The epicenter is the surface location directly vertical to the subterranean focus where the earthquake started.
5. Which seismic waves are the fastest-moving body waves that can travel through both solids and liquids?
Correct Answer: a) P-waves (Primary waves)
Explanation:
Primary (P) waves are compressional push-pull body waves that travel faster than any other seismic wave and propagate through both solid rock and liquid layers.
6. Why do S-waves (Secondary waves) fail to pass through the Earth's outer core?
Correct Answer: a) Because the outer core is entirely liquid and S-waves are shear waves that require a rigid medium
Explanation:
S-waves are transverse shear waves that cannot be transmitted through fluids (liquids or gases), which proves that the outer core is liquid.
7. Which type of seismic waves travels strictly along the Earth's outer surface and causes the most severe ground destruction?
Correct Answer: c) Surface waves (Rayleigh and Love waves)
Explanation:
Surface waves travel along Earth's outer skin and attenuate more slowly with distance than body waves, carrying rolling and shearing motion responsible for heavy structural damage.
8. What instrument is used to detect, record, and measure seismic waves generated by earthquakes?
Correct Answer: b) Seismograph
Explanation:
A seismograph is an instrument that detects ground motion and records it onto a seismogram as a function of time.
9. How many seismograph stations are minimally required to determine the exact epicenter of an earthquake using triangulation?
Correct Answer: c) Three
Explanation:
Using arrival time differences between P- and S-waves from three separate stations provides three distance circles whose unique intersection pinpoints the epicenter.
10. Which scale measures the total energy released by an earthquake at its source?
Correct Answer: b) Moment magnitude scale
Explanation:
The moment magnitude scale calculates total energy release based on seismic moment, fault rupture area, and rock rigidity, replacing the older local Richter scale for large events.
11. What does the Modified Mercalli intensity scale measure?
Correct Answer: c) Human perception of shaking and the severity of structural damage at specific locations
Explanation:
Mercalli intensity grades an earthquake from I to XII based on subjective human experiences, felt effects, and observed structural damage.
12. What type of fault is created by compressional stress, where the hanging wall moves upward relative to the footwall?
Correct Answer: b) Reverse (thrust) fault
Explanation:
Reverse and thrust faults occur under regional compression, pushing the hanging wall block up and over the footwall.
13. Which fault type is produced by tensional stress, where the hanging wall moves downward relative to the footwall?
Correct Answer: a) Normal fault
Explanation:
Normal faults develop where tectonic forces pull the crust apart (tension), causing the hanging wall to drop down.
14. The San Andreas Fault in California is a prominent example of which type of fault?
Correct Answer: c) Right-lateral strike-slip fault
Explanation:
The San Andreas Fault is a transform boundary feature where plates slide past one another horizontally; standing on one side shows the opposite side shifting to the right.
15. What is fault creep?
Correct Answer: b) Slow, continuous, non-catastrophic slippage along a fault without major earthquakes
Explanation:
Fault creep is steady, smooth displacement along a fault plane that relieves stress gradually without locking up to cause large earthquakes.
16. What phenomenon occurs when water-saturated, unconsolidated sediment temporarily loses its strength and behaves like a liquid during strong ground shaking?
Correct Answer: b) Liquefaction
Explanation:
Liquefaction happens when seismic shaking increases pore water pressure in loose saturated sand and silt, causing the ground to lose load-bearing capacity and behave like a fluid.
17. What is a tsunami?
Correct Answer: b) A series of massive ocean waves caused by the displacement of water from submarine earthquakes, landslides, or volcanic eruptions
Explanation:
Tsunamis are long-wavelength ocean waves triggered by sudden vertical displacement of the seafloor, typically during undersea megathrust earthquakes.
18. Why are tsunamis barely noticeable in the open deep ocean but become towering walls of water near shallow coastal areas?
Correct Answer: b: Their wavelength shortens and wave height increases as friction with the shallow seafloor slows down the front of the wave
Explanation:
As tsunami waves enter shallow coastal waters, friction slows the wave base, causing energy to compress upward, drastically increasing wave height (shoaling).
19. What is the 'Ring of Fire'?
Correct Answer: b) A major seismic and volcanic belt circling the margins of the Pacific Ocean basin
Explanation:
The Ring of Fire is a horseshoe-shaped basin perimeter characterized by active subduction zones, accounting for the vast majority of Earth's earthquakes and volcanic eruptions.
20. What is the primary tectonic setting for deep-focus earthquakes (depths greater than 300 km)?
Correct Answer: c) Subduction zones (Benioff zones)
Explanation:
Deep earthquakes occur exclusively in subduction zones where cold, brittle oceanic lithosphere plunges deep into the mantle along dipping Benioff seismic zones.
21. What are aftershocks?
Correct Answer: b) Smaller earthquakes that follow a major mainshock as the fault adjusts and settles along the ruptured zone
Explanation:
Aftershocks represent continuing readjustments and localized stress redistributions along or near a fault plane following a large mainshock.
22. What is seismic tomography?
Correct Answer: a) A technique using seismic wave velocities from earthquakes to create 3D images of the Earth's interior
Explanation:
Seismic tomography acts like a medical CT scan for the Earth, using variations in seismic wave speeds to image mantle plumes, slabs, and core structures.
23. Which of the following describes a blind fault?
Correct Answer: b) A fault that does not break the Earth's surface, remaining hidden underground
Explanation:
Blind faults terminate below the Earth's surface without a surface rupture, making them difficult to detect prior to generating unexpected earthquakes.
24. What is induced seismicity?
Correct Answer: a) Earthquakes caused directly by human activities such as wastewater disposal, fracking, or reservoir impoundment
Explanation:
Induced seismicity refers to minor to moderate earthquakes triggered by anthropogenic alterations of subsurface pore pressure or stress.
25. What is the primary purpose of seismic retrofitting in earthquake-prone regions?
Correct Answer: b) To reinforce older buildings and infrastructure so they can better withstand ground shaking
Explanation:
Seismic retrofitting upgrades existing structures with shear walls, base isolators, and steel bracing to prevent collapse during severe shaking.
26. What are base isolators in earthquake-resistant building design?
Correct Answer: b) Flexible pads or bearings placed between a building and its foundation to decouple ground motion from the structure
Explanation:
Base isolators absorb and isolate lateral ground movements, allowing the foundation to move while keeping the building superstructure relatively stable.
27. What is a seismic shadow zone?
Correct Answer: a) An area on the Earth's surface where no direct seismic waves from a given earthquake are detected
Explanation:
Shadow zones occur because P-waves are refracted by the core and S-waves are blocked by the liquid outer core, leaving a specific band on Earth's surface without direct wave arrivals.
28. What seismic wave shadow zone spans angular distances from 103° to 142° from an earthquake epicenter?
Correct Answer: b) P-wave shadow zone
Explanation:
The P-wave shadow zone (103° to 142°) is caused by the refraction of P-waves as they enter the slower-velocity liquid outer core.
29. What is a megathrust earthquake?
Correct Answer: b) A massive earthquake occurring at subduction zones where one tectonic plate is thrust beneath another
Explanation:
Megathrust earthquakes happen at subduction zone interfaces and are responsible for the planet's largest historical earthquakes (e.g., Chile 1960, Alaska 1964, Sumatra 2004).
30. What is an earthquake swarms?
Correct Answer: a) A series of minor earthquakes occurring in a specific area over a short period without a distinct mainshock
Explanation:
Earthquake swarms consist of numerous small tremors clustered in time and space, often associated with magma movement or geothermal activity rather than a single main fault rupture.
31. How does the amplitude of ground motion change on the Richter scale for every increase of 1 whole number?
Correct Answer: b) It increases by a factor of 10
Explanation:
The Richter and moment magnitude scales are logarithmic; each whole number increase represents a tenfold increase in measured wave amplitude.
32. Approximately how much more energy is released for every increase of 1.0 on the moment magnitude scale?
Correct Answer: c) About 32 times more energy
Explanation:
While wave amplitude increases tenfold per magnitude step, the energy released increases by a factor of roughly 31.6 (commonly rounded to 32) per magnitude unit.
33. What is a seismic gap?
Correct Answer: b) A segment of an active fault zone that has not experienced a major earthquake in a long time, accumulating high stress
Explanation:
Seismic gaps are quiet segments along otherwise active faults where stress is continually building, making them prime candidates for future significant earthquakes.
34. What is dilatancy in the context of earthquake prediction research?
Correct Answer: a) The expansion of rock volume due to micro-fracturing under mounting tectonic stress prior to an earthquake
Explanation:
Dilatancy refers to the micro-cracking and volumetric swelling of stressed rocks before failure, which alters seismic wave velocities and groundwater levels.
35. What is a tsunami warning system primarily reliant upon for deep-ocean detection?
Correct Answer: b) Deep-ocean bottom pressure sensors (DART buoys) linked to surface buoys and satellite communication
Explanation:
Deep-ocean Assessment and Reporting of Tsunamis (DART) systems measure pressure changes of passing tsunami waves across the open ocean, providing rapid real-time alerts.
36. What are foreshocks?
Correct Answer: a) Smaller earthquakes that sometimes precede a larger main earthquake in the same area
Explanation:
Foreshocks are minor tremors preceding a major mainshock, though distinguishing them from ordinary background seismicity in real-time remains difficult.
37. What does the term 'hypocentral depth' refer to in earthquake seismology?
Correct Answer: a) The vertical distance from the Earth's surface down to the earthquake focus
Explanation:
Hypocentral depth measures how deep underground the earthquake rupture initiated, classifying events as shallow-focus, intermediate, or deep-focus.
38. Which of the following describes Rayleigh waves?
Correct Answer: a) Surface waves that roll along the ground like waves on the ocean surface, moving the ground up-and-down and side-to-side
Explanation:
Rayleigh waves produce an elliptical rolling motion of the ground, similar to ocean surface waves.
39. Which of the following describes Love waves?
Correct Answer: a) Surface waves that cause horizontal shearing motion perpendicular to the direction of wave travel
Explanation:
Love waves move the ground side-to-side horizontally, which is particularly destructive to building foundations.
40. What is the Mohorovičić discontinuity (Moho)?
Correct Answer: a) The boundary between the Earth's crust and the underlying mantle
Explanation:
The Moho marks the compositional boundary where seismic wave velocities increase abruptly as waves pass from the crust into denser mantle rocks.
41. What is earthquake early warning (EEW)?
Correct Answer: b) A technological system that detects initial P-waves and issues automated alerts seconds before destructive S-waves and surface waves arrive
Explanation:
Because P-waves travel faster than damaging S-waves and surface waves, EEW networks detect them instantly and send automated warnings to shut down transit, gas lines, and alarms.
42. What is a blind thrust earthquake?
Correct Answer: a) An earthquake occurring on a concealed reverse fault that does not break the Earth's surface
Explanation:
Blind thrust faults generate earthquakes without surface expression, posing severe hidden hazards in urban basins like Los Angeles.
43. What is the primary factor determining how long an earthquake rupture lasts?
Correct Answer: a) The physical length of the fault segment that ruptures
Explanation:
Larger magnitude earthquakes involve longer fault ruptures; for instance, a great M9 earthquake can involve a rupture zone hundreds of kilometers long lasting several minutes.
44. What is an asperities on a fault plane?
Correct Answer: b) Rough patches or locked spots along a fault where friction holds rocks together until stress overcomes strength
Explanation:
Asperities are high-friction areas along a fault where stress accumulates until sudden failure triggers an earthquake.
45. What is elastic strain in tectonic plates?
Correct Answer: b) Temporary, reversible bending or stretching of rocks under stress that stores potential energy
Explanation:
Elastic strain is like a coiled spring; rocks deform elastically under stress until the threshold is exceeded, resulting in brittle fracture and elastic rebound.
46. Which of the following seismic scales is currently preferred by seismologists for rating moderate to large earthquakes?
Correct Answer: b) Moment magnitude scale
Explanation:
The moment magnitude scale does not saturate for very large earthquakes like the older Richter scale, making it the global standard for seismology.
47. What is resonant frequency in relation to buildings during an earthquake?
Correct Answer: a) The natural frequency at which a building tends to vibrate when disturbed, which can cause severe damage if matched by earthquake wave frequencies
Explanation:
If the frequency of seismic ground shaking matches a building's natural resonant frequency, constructive interference amplifies swaying, potentially causing structural collapse.
48. What is a tsunami run-up height?
Correct Answer: b) The maximum vertical height above sea level that a tsunami wave reaches as it surges onshore
Explanation:
Run-up height measures how high up a slope or inland a tsunami penetrates, which can far exceed its offshore wave height due to topography.
49. Why do sedimentary basins often experience stronger and longer-lasting shaking than bedrock areas during an earthquake?
Correct Answer: b) Because seismic waves slow down and get trapped/amplified in soft basin sediments (site amplification)
Explanation:
Soft soils and thick sedimentary fills amplify seismic wave amplitudes and trap wave energy, leading to prolonged shaking compared to rigid bedrock foundations.
50. What is the Wadati-Benioff zone?
Correct Answer: a) A dipping planar zone of earthquakes tracing the descent of a subducting tectonic plate into the mantle
Explanation:
Named after Kiyoo Wadati and Hugo Benioff, this seismic zone outlines the position of subducting oceanic slabs beneath island arcs and continental margins.
51. What is paleoseismology?
Correct Answer: a) The study of ancient earthquakes using geological evidence such as offset sediment layers and fault trenches
Explanation:
Paleoseismologists dig trenches across fault traces to examine prehistoric slip events, carbon-dating layers to determine recurrence intervals for major earthquakes.
52. What is seismic wave attenuation?
Correct Answer: b) The gradual decrease in seismic wave energy and amplitude as waves travel away from the source and pass through rocks
Explanation:
Attenuation describes how seismic energy dissipates with distance due to geometric spreading and frictional absorption by rocks.
53. What is a normal fault earthquake typically associated with in terms of tectonic regime?
Correct Answer: b) Crustal extension and rifting (e.g., Basin and Range Province)
Explanation:
Normal fault earthquakes occur where the crust is being stretched or pulled apart, such as mid-ocean ridges and continental rift valleys.
54. What is a reverse fault earthquake typically associated with?
Correct Answer: a) Crustal compression during tectonic plate convergence and mountain building
Explanation:
Reverse and thrust faults accommodate horizontal compression where tectonic plates collide, uplifting mountain belts.
55. What is structural damping in earthquake engineering?
Correct Answer: b) The incorporation of energy-absorbing devices (like hydraulic dampers) into buildings to dissipate seismic kinetic energy
Explanation:
Dampers act like automotive shock absorbers in skyscrapers, converting seismic vibrational energy into heat to reduce structural sway.
56. What is the primary reason why P-waves arrive at seismograph stations before S-waves?
Correct Answer: b) They have a higher propagation velocity than S-waves in any given elastic medium
Explanation:
Because compressional waves travel faster than shear waves in solid rock, P-waves always register first on seismographs.
57. What is a tsunami's typical speed across the deep open ocean?
Correct Answer: c) About 500 to 800 km/h (jetliner speed)
Explanation:
In deep oceanic water, tsunamis travel at immense speeds proportional to the square root of water depth, crossing entire ocean basins in hours.
58. What is fault plane solution (focal mechanism)?
Correct Answer: a) A mathematical method using seismic wave polarities to determine the orientation of fault slip and stress regime during an earthquake
Explanation:
Focal mechanism 'beachball' diagrams illustrate whether an earthquake was caused by normal, reverse, or strike-slip faulting based on first motions recorded globally.
59. What is a seismic hazard map?
Correct Answer: a) A map showing the probability that ground shaking of a certain intensity will occur in a given geographic area over a specified time period
Explanation:
Seismic hazard maps guide building codes and land-use planning by illustrating long-term earthquake probabilities and expected shaking intensities.
60. What is the principal hazard associated with earthquakes in urban areas built on unreinforced masonry?
Correct Answer: b) Building collapse due to brittle walls failing under lateral seismic shaking
Explanation:
Unreinforced brick and masonry buildings lack ductility and tensile strength, making them highly susceptible to catastrophic collapse during moderate to strong shaking.
61. What is the role of fluids injected into deep disposal wells in triggering induced earthquakes?
Correct Answer: a) They lubricate faults and increase pore fluid pressure, reducing frictional resistance and allowing slip
Explanation:
High-pressure fluid injection increases pore pressure in subsurface faults, effectively counteracting normal stress and triggering slip (induced seismicity).
62. What is a slow slip event (silent earthquake)?
Correct Answer: b) A fault slip event that releases energy over days, weeks, or months rather than seconds, producing no detectable high-frequency seismic waves
Explanation:
Slow slip events release tectonic stress gradually without generating traditional seismic waves, detectable only via sensitive GPS geodetic instruments.
63. What is an active fault defined as in geological terms?
Correct Answer: a) A fault that has generated earthquakes within the last 10,000 years (Holocene epoch)
Explanation:
Geologists generally classify a fault as active if it has exhibited movement within the Holocene epoch, indicating a high probability of future activity.
64. What is the primary difference between a seismograph and a seismogram?
Correct Answer: a) A seismograph is the recording instrument, while a seismogram is the output record of ground motion
Explanation:
The seismograph is the physical sensor and recorder device, whereas the seismogram is the squiggly line graph paper or digital file produced by it.
65. What is the typical wavelength of a tsunami in the deep ocean?
Correct Answer: c) 100 to 200 kilometers
Explanation:
Unlike wind waves with wavelengths of tens of meters, tsunamis have enormous wavelengths often exceeding 100 kilometers, though their wave height in deep water is less than a meter.
66. What is a tectonic plate boundary where earthquakes are shallow and frequent but volcanic activity is absent?
Correct Answer: b) Transform fault boundary
Explanation:
Transform boundaries (like the San Andreas Fault) feature shallow earthquakes due to horizontal sliding, but lack the melting processes that generate volcanism.
67. What is the significance of the 1960 Valdivia earthquake in Chile?
Correct Answer: a) It was the largest instrumentally recorded earthquake in human history (Magnitude 9.5)
Explanation:
The 1960 Great Chilean earthquake registered a magnitude of 9.5 on the moment magnitude scale, producing widespread destruction and a trans-Pacific tsunami.
68. What is seismic reflection profiling?
Correct Answer: a) A geophysical method using controlled seismic sources (like dynamite or airguns) to map subsurface rock layers and fault structures
Explanation:
Seismic reflection uses echoes of artificial seismic waves bouncing off underground strata to image fault geometry for oil, gas, and earthquake hazard studies.
69. What is the main danger of secondary fires following a major urban earthquake?
Correct Answer: b) Ruptured gas lines, broken water mains preventing firefighting, and downed electrical wires
Explanation:
Earthquakes frequently sever gas and water lines simultaneously, turning ruptured gas mains into widespread fires that firefighters cannot extinguish due to lack of water pressure.
70. What is an intraplate earthquake?
Correct Answer: a) An earthquake occurring in the middle of a tectonic plate, far from plate boundaries (e.g., New Madrid seismic zone)
Explanation:
Intraplate earthquakes occur within stable continental interiors along reactivated ancient fault zones, often felt over much larger areas than plate-boundary events due to cold, rigid crustal rocks.
71. What is the focal mechanism 'beachball' symbol used for in seismology?
Correct Answer: a) To illustrate the type of faulting (strike-slip, normal, or reverse) and orientation of stress planes for an earthquake
Explanation:
Beachball diagrams graphically represent compressional and dilatational first-motion quadrants of seismic waves to display focal mechanisms.
72. What is a seismic wave shadow zone evidence of?
Correct Answer: a) That the Earth's interior is chemically and physically heterogeneous, containing distinct layers like the liquid outer core
Explanation:
The bending and termination of body waves across shadow zones proved to early seismologists that Earth possesses distinct mantle, core, and fluid outer core layers.
73. What is a coseismic crustal deformation?
Correct Answer: a) Sudden permanent warping, uplift, or subsidence of the Earth's surface occurring instantaneously during an earthquake rupture
Explanation:
Coseismic deformation is the rapid surface displacement caused during the seconds to minutes of a major fault slip event.
74. What is the primary limitation of earthquake prediction science today?
Correct Answer: b) Earthquakes do not give reliable, repeatable short-term precursors that allow exact timing, location, and magnitude prediction
Explanation:
While probabilistic forecasting and hazard mapping are advanced, reliable deterministic short-term earthquake prediction remains an elusive scientific goal.
75. What is the Gutenberg-Richter law in seismology?
Correct Answer: a) A statistical relationship showing that smaller earthquakes occur much more frequently than larger earthquakes globally
Explanation:
The Gutenberg-Richter relation describes the inverse logarithmic frequency-magnitude distribution of earthquakes, meaning roughly ten times as many M5 earthquakes happen as M6 earthquakes.
76. What is a tsunami boulder?
Correct Answer: a) A massive rock hurled high inland or onto coastal cliffs by the extreme hydraulic force of a tsunami wave
Explanation:
Tsunami boulders are coastal megaclasts weighing tons that are ripped from shorelines and deposited inland by catastrophic tsunami inundations.
77. What is the hanging wall in fault terminology?
Correct Answer: a) The block of rock that lies immediately above an inclined fault plane
Explanation:
Miners historically called the block hanging above their heads the 'hanging wall' and the block they stood on the 'footwall'.
78. What is the footwall in fault terminology?
Correct Answer: a) The block of rock situated underneath an inclined fault plane
Explanation:
The footwall is the underlying block of rock beneath an inclined fault surface, where miners would place their feet.
79. Which seismic wave arrives second at a recording station?
Correct Answer: b) S-wave
Explanation:
Secondary (S) waves travel slower than P-waves, making them the second body wave to register on seismographs.
80. What is a seismic array?
Correct Answer: a) A network of multiple seismographs deployed in a coordinated geometric pattern to improve signal detection and locate seismic sources accurately
Explanation:
Seismic arrays use signal processing across multiple networked sensors to filter noise and detect subtle seismic events like nuclear tests or micro-earthquakes.
81. What is the primary cause of damage during the 1906 San Francisco earthquake?
Correct Answer: b) Widespread fires that raged out of control for days because water mains were ruptured by the earthquake
Explanation:
While the M7.9 earthquake caused severe shaking damage along the San Andreas Fault, over 80% of San Francisco's destruction was caused by post-earthquake fires.
82. What is crustal shortening?
Correct Answer: a) The horizontal compression and thickening of the Earth's crust at convergent plate boundaries, accommodated by thrust faulting and folding
Explanation:
Crustal shortening occurs during continental collisions where reverse and thrust faults stack crustal blocks upward, forming mountain chains.
83. What is a seismogenic zone?
Correct Answer: a) The brittle crustal depth range where rocks are cold enough to store elastic strain and rupture in earthquakes, rather than deform plastically
Explanation:
At greater depths, high temperatures cause rocks to flow ductilely rather than fracture; the brittle upper region where earthquakes occur is the seismogenic zone.
84. What is the main objective of probabilistic seismic hazard analysis (PSHA)?
Correct Answer: a) To calculate the likelihood that various levels of ground shaking will occur at a site over a given timeframe for engineering design
Explanation:
PSHA integrates fault locations, historical seismicity, and ground motion models to quantify shaking probabilities for building codes and risk management.
85. What is a strike-slip fault characterized by?
Correct Answer: b) Horizontal side-to-side displacement along the fault plane
Explanation:
Strike-slip faults involve predominantly horizontal motion where blocks slide past each other laterally, categorized as right-lateral or left-lateral.
