Lesson 3 Flashcards
2 ways to measure earthquake size
- Intensity
- Magnitude
is a non-instrumental perceptibility measure of damage to structures, ground surface effects (e.g. fractures, cracks and landslides) and human reactions to earthquake shaking.
Intensity
it is a descriptive method which has been traditionally used to establish earthquake size, especially for pre-instrumental events.
Intensity
the levels are represented by _______________ and each degree of intensity provides a qualitative description of earthquake effects.
Roman Numerals
Most common intensity scales:
- Mercalli-Cancani-Seiberg (MCS) - 12-level scale
- Modified Mercalli (MM) - 12-level scale
- Medvedev-Sponheuer-Karnik (MSK) - 12-level scale
- European Macroseismic Scale (EMS) - 12-level scale
- Japanese Meteoral Agency (JMA) - 7-level scale
The first intensity scale
Rossi-Forel Scale
______________ are usually obtained from interviews of observers after the event.
Earthquake intensities
Intensity scales are used to plot contour lines of equal intensity or “isoseisms” “isoseismal” then generates an ____________.
Isoseismal Map
provide approximate distributions of damage and the extent of ground shaking and shows how the intensity decreases, or attenuates, with increasing epicentral distance.
Intensity maps
The intensity is generally greatest in the vicinity of the __________
Epicenter
- is a quantitative measure of earthquake size and fault dimensions.
- based on the maximum amplitudes of body or surface seismic waves.
- an instrumental, quantitative and objective scale.
Magnitude
The first attempt to define magnitude scales were made in _________ by _________ and in __________ by ___________ in the 1930s.
Japan-Wadati
California-Ritcher
scales are ___________ because they measure amplitudes of seismic waves with different properties.
frequency-dependent
Magnitudes are calculated from earthquakes recorded by an instrument called
seismograph
Instruments that measure and record, as a function of time, motions of the ground, including those of seismic waves.
Seismometer/Seismographs
Number of seismic stations in the Philippines (as of Dec. 2020)
108
Seismograms can provide
information on
- epicenter location
- magnitude of earthquake
- source properties
The most commonly used instruments for engineering purposes are strong motion
accelerographs/accelerometers
These instruments record the acceleration time history of ground motion at a site, called an
accelerogram
the resulting corrected acceleration record can be used by engineers to obtain __________ and ___________ by appropriate integration.
ground velocity and ground displacement
Most common magnitudes scales:
i. Local (or Richter) magnitude(ML)
ii. Body wave magnitude (mb)
iii. Surface wave magnitude (MS)
iv. Moment magnitude (MW)
In 1935, Charles Richter use a _______________________ to define a magnitude scale for shallow, local (epicentral distances less than about 600km (375miles) earthquakes in southern California
Wood-Anderson seismometer
This scale is suitable for deep earthquakes that have few surface waves.
Body wave magnitude (mb)
is a worldwide magnitude scale based on the amplitude of the first few cycles of P-waves which are not strongly influenced by the focal depth
Body wave magnitude (mb)
is a measure of the amplitudes of LR-waves with a period of 20seconds, i.e. wavelength of about 60km, which are common for very distant earthquakes,
Surface wave magnitude (MS)
accounts for the mechanism of shear that takes place at earthquake sources and not related to any wavelength.
Moment magnitude (MW)
A phenomenon wherein as the total amount of energy released during an earthquake increases, however, the ground-shaking characteristics do not necessarily increase at the same rate.
Saturation
