Earthquakes - FACTORS OF GROUND SHAKING Flashcards
when was the Tangshan disaster and what magnitude was it?
1976 (Mm 7.5)
what kind of earthquake was the Tangshan disaster?
intraplate earthquake
why was the Tangshan disaster so destructive?
people have no memory of a large earthquake occurring in the area, therefore luling them into a false sense of security
t or f: earthquakes don’t kill people buildings do
T
it is the leading cause of death and property damage in most earthquakes
structural failure
china earthquake when and how strong
2008, magnitude 7.9
when engineers design a structure they take into account the fact that the structure must be able to withstand a range of _____, with _____ being the most important
different forces ; gravoty
t or f: all structures have mass, at a bare minimum they must be strong enough to support their own weight against gravity
T
gravity works in ____ direction, where structures are usually more resistant
vertical
the lack of structural strength in the _____ direction becomes one of critical importance in areas where string earthquakes occur
horizontal / lateral
the types of wave that is most destructive because of movement in lateral direction
surface waves
buildings with crawl spaces or large open areas on the ground floor are inherently weak and prone to ______
cripple-wall failure
the most dangerous types of homes are those constructed of
unreinforced masonry
unreinforced masonry is when walls are usually constructed of brick bound together with mortar
T
reinforced walls have internal support of _____ or _____
woods or steel
this occurs when floors within a building become detached from the columns and fall in a cascading manner
pancaking
mexico city earthquake
1985, magnitude 8.0
concrete columns even if reinforced, can collapse when concrete reach their _____ and literally _____
elastic limit;
explode
factors that affect ground shaking
period, natural vibration frequency, resonance;
focal depth and wave attenuation;
ground amplification
the time (in seconds) it takes for a building to naturally vibrate back and forth
period
t or f: smaller structures have higher period than larger structures
false (shorter period)
period of ground motion is generally controlled by the ground’s _____
stiffness
refers to the vibration of a structure at a fixed frequency
natural vibration frequency
is the number of times the motion is repeated at a set amount of time
frequency
t or f: as building height increases, the natural vibration frequency decreases
T
what is resonance?
natural vibration frequency of a structure matches that of the seismic waves
resonance occurs when _____ of the individual waves ______
amplitude ; combine
what buildings are most susceptible to resonance?
10-20 stories high
t or f: tall skyscrapers are unlikely to experience resonance
t
energy of the resulting seismic waves steadily decreases as they travel away from the focus
wave attenuation
the most dangerous earthquakes are those with a combination of
shallow focus and large magnitude
t or f: it is possible for a relatively shallow, low-magnitude earthquake to generate greater ground motion than a deep, high magnitude earthquake
T
seismic waves experience different amounts of wave attenuation depending on the ________
type of geologic materials they pass through
loose materials and rocks with lower density will _______ from passing seismic waves
absorb more energy
on areas of rigid rocks, seismic waves are able to ______ as they travel farther
retain more of their energy
t or f: when waves undergo less attenuation, they therefore have the potential to cause damage farther from the focus
T
new madrid earthquake series
magnitude 8, 1811-1812
new madrid earthquake was able to retain energy to ring church bells in
Boston, Massachusetts
when seismic waves travel through weaker materials, they _____ and ______ at a faster rate
slow down ; lose energy
when waves travel through weaker material, they slow down causing the ______ to increase, creating a phenomenon called _____
wave amplitude ; ground amplification
t or f: during ground amplification, weaker materials can begin to vibrate at the same frequency as that of seismic waves
T
seismic waves entering a basin would amplify because they are ________ in the sedimentary material
forced to slow down
t or f: areas undergoing ground amplification may have an extended duration of ground shaking
T
seismic waves in sedimentary basin can become trapped and undergo ______ creating a ________ effect
internal reflection ; reverberating effect
the ____ shape of a basin can cause waves to _____ and _____, focusing their energy into localized areas which then experience more intense shaking
convex ; refract and merge
