BAS Seismic Flashcards
The point where the fault first slips…
… is termed the “focus” or “hypocenter.”
A theoretical point on the earth’s surface directly above the focus is termed the “epicenter
Slippage along a fault line deep in the earth’s surface may eventually result in surface faulting,…
… the crack or split on the earth’s surface that provides the layperson’s vision of earthquakes.
Liquefaction:
When sandy, water saturated soils are shaken, the bearing capacity of the soil is reduced as the soil liquefies and flows laterally and vertically. Liquefied soils can produce volcano-like sandboils at the ground surface or flow laterally if the soil is not contained. The ground surface and structures built on shallow foundations can subside several feet or be torn apart as spreading occurs.
Describe the wave types in an earthquake:
First to arrive at the surface is the P or primary wave. In this wave the ground is successively pushed and pulled along the wave front. The effect is of a sharp punch - it feels as if a truck has hit the building. The P wave is followed by the S, secondary or shear wave, which is a lateral motion, back and forth (but sideways to the wave front).
Earthquake motion is recorded by …
…a seismograph, an instrument that records the movement, over time, of a freely supported pendulum within a frame: the instrument may be placed on the ground or within a structure.
In modern seismographs, pendulum movement is converted into electronic signals on tape. Strong-motion seismographs, called accelerometers, are designed to directly record nearby rather than distant ground movement, and they produce a record called an accelerogram.
Instruments are normally placed so as to measure movements along the two horizontal axes as well as one vertical. Three measures are of major interest:
acceleration, velocity, and displacement.
Acceleration is …
…the rate of change of velocity: when multiplied by mass it results in the inertial force that the building must resist. This is a key measure, and forms the basis of the estimation of earthquake forces on buildings: Newton’s Second Law of Motion states in essence, that an inertialforce, F, equals mass (M) multiplied by the acceleration (A).
Acceleration is commonly measured in “g’s”- the acceleration of a free falling body due to the earth’s gravity (approx. 32ft/sec/sec, or 980 cm/sec/sec, or l.Og.).
Velocity, measured in inches or centimeters per second, refers to …
…the rate of ground motion at any time.
Displacement, measured in inches or centime- ters, refers to …
…the distance a particle is removed from its “at rest” position.
The level of acceleration generally taken as sufficient to produce some damage
to weak construction is …
0.10 g
A more significant measure is that of acceleration combined with –1–, which takes into account the impact of earthquake forces over time.
1) duration
Earthquake magnitude is expressed as…
… Richter magnitude based on the scale devised by Professor Charles Richter of the California Institute of Technology in 1935. Richter’s scale is based on the maximum amplitude of certain seismic waves recorded on a standard seismograph at a distance of 100 kilometers from the earthquake epicenter. The scale, however, tells nothing about duration, which may be of great significance in causing damage, nor does it tell anything about frequency content which, in its relationship to the building period, as discussed later, is also of great significance in determining damage. Because the instrument is unlikely to be exactly 100km from the source, Richter developed a method to allow for the diminish- ing of wave amplitude (or “attenuation”) with increased distance, just as the light of a star appears dimmer with distance.
Richter magnitude is logarithmic, meaning…
…Richter magnitude indicates a 10 times increase in wave amplitude. But the energyincreaserepresentedbyeachunitofscaleisestimatedbyseismologists as approximately 31 times. Since Richter magnitude is a measured quantity, the scale is open-ended, but seismologists believe that a Richter magnitude of about 9 represents the largest possible earthquake.
To provide information directly related to local shaking and building damage, –1– scales are used.
1) intensity
intensity scales are used. These scales are based on subjective observation of the effects of the earthquake on buildings, ground and people, hi the United States the most commonly used scale is the ModifiedMercolli (MM)
The MM Scale is a twelve pointscale,I-XII.
