7.3 Measuring earthquakes: magnitude and intensity scales

Earthquakes pack a powerful punch, but how do we measure their might? This section dives into the scales used to quantify earthquake strength and impact. We'll explore the difference between magnitude and intensity, two key concepts for understanding seismic events.

From the classic Richter scale to the modern Moment Magnitude Scale, we'll unpack how scientists measure earthquake energy. We'll also look at the Modified Mercalli Intensity Scale, which assesses earthquake effects on people and structures in specific locations.

Earthquake Magnitude and Intensity

Quantitative vs Qualitative Measures

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• Earthquake magnitude measures energy released at the source on a logarithmic scale
  • Objective measurement determined by seismograph readings
  • Represented by a single number for each earthquake
• Earthquake intensity assesses effects at specific locations based on observed damage
  • Subjective assessment varying across different areas for the same event
  • Can have multiple values depending on location relative to epicenter
• Relationship between magnitude and intensity not linear
  • Factors affecting intensity include distance from epicenter, local geology, building construction

Applications and Importance

• Understanding both concepts crucial for comprehensive hazard assessment
• Magnitude used for:
  • Comparing overall size of different earthquakes
  • Estimating potential energy release in future events
• Intensity used for:
  • Assessing localized impacts and damage patterns
  • Informing emergency response and recovery efforts
• Combined analysis enables:
  • More accurate risk mitigation strategies
  • Improved building code development
  • Enhanced public education and preparedness

Richter vs Moment Magnitude Scales

Richter Scale Characteristics

• Developed by Charles Richter in 1935
• First widely used magnitude scale for earthquakes
• Based on amplitude of largest seismic wave on seismogram
• Calculated using simple logarithmic formula
• Limited in accurately measuring large earthquakes (above magnitude 6.5-7.0)
  • Scale saturation occurs due to physical limitations of measurement method

Moment Magnitude Scale (MMS) Advancements

• Introduced in 1970s to address Richter scale limitations
• Measures total energy released by earthquake
• Based on seismic moment considering:
  • Area of fault rupture
  • Average amount of slip
  • Rock rigidity
• Accurately measures earthquakes of any size
• Involves more complex calculations than Richter scale
• Largely replaced Richter scale in scientific and media reporting

Similarities and Differences

• Both scales logarithmic
  • Each whole number represents ~31.6-fold increase in energy release
• MMS provides greater accuracy across wider range of earthquake sizes
• Richter scale still used for quick initial estimates and historical comparisons
• MMS values often similar to Richter scale for earthquakes below magnitude 6.5
• Example comparison:
  • 2011 Tohoku earthquake: Richter scale ~8.1, MMS 9.0-9.1
  • 1906 San Francisco earthquake: Richter scale ~8.3, MMS 7.9

Modified Mercalli Intensity Scale

Scale Structure and Application

• 12-point scale measuring intensity of ground shaking and damage at specific locations
• Ranges from I (not felt) to XII (total destruction)
• Each level described by specific observed effects on:
  • People (felt sensations)
  • Objects (movement or damage)
  • Buildings (structural impacts)
  • Environment (changes in landscape)
• Intensity levels represented by Roman numerals
• Particularly useful for:
  • Assessing impacts in areas without extensive seismograph networks
  • Analyzing historical earthquakes before modern instrumentation

Intensity Mapping and Analysis

• Intensity maps visualize distribution of earthquake effects across region
• Highlight areas of amplified shaking due to local geological conditions
• Created using:
  • Field surveys
  • Eyewitness reports
  • Social media data
  • Remote sensing techniques
• Example: 1994 Northridge earthquake
  • Magnitude 6.7, but produced MMI IX in some areas due to local amplification

Importance in Earthquake Studies

• Crucial for:
  • Earthquake engineering (designing structures to withstand specific intensities)
  • Urban planning (zoning based on potential shaking intensity)
  • Emergency response preparation (allocating resources to high-risk areas)
• Complements magnitude data for comprehensive earthquake impact assessment
• Helps correlate historical accounts with modern scientific measurements

Interpreting Seismograms

Seismogram Components and Wave Types

• Graphical representations of ground motion recorded by seismographs
• Display amplitude and frequency of seismic waves over time
• Three main types of seismic waves visible:
  • P-waves (primary): Fastest, compress and expand material
  • S-waves (secondary): Slower, cause side-to-side motion
  • Surface waves: Slowest, largest amplitude, cause most damage
• Time difference between P-wave and S-wave arrivals (S-P interval) used to calculate distance to epicenter

Epicenter Location Techniques

• Requires data from at least three seismograph stations
• Uses principle of trilateration
• Process:
  1. Calculate distance to epicenter for each station using S-P interval
  2. Draw circle around each station with radius equal to calculated distance
  3. Find intersection point of three circles to determine epicenter
• Modern techniques employ computer algorithms analyzing data from numerous stations
  • Improves accuracy and reduces uncertainty
  • Example: Global Seismographic Network uses over 150 stations worldwide

Advanced Seismogram Analysis

• Depth of earthquake focus estimated using:
  • Angle of incidence of seismic waves
  • Arrival times at multiple stations
• Focal mechanism solutions derived from seismogram data
  • Provide information on fault orientation and slip direction
• Magnitude determination involves:
  • Measuring maximum wave amplitudes
  • Applying corrections for distance and local geology
• Example: 2010 Haiti earthquake
  • Rapid analysis of seismograms enabled quick magnitude estimate (7.0) and location
  • Crucial for initiating immediate international response efforts