Earthquake characteristics

• Geos 218 covers geological disasters and society, focusing on earthquakes
• Unit 5a centers on earthquakes and their study through seismology

Earthquake Waves

• P-waves, S-waves, and Surface waves are types of earthquake waves

Seismology

• Seismology is the study of earthquakes
• Seismographs are instruments used to detect and record ground motion caused by earthquakes
• Seismographs typically consist of a heavy weight suspended from a frame with a pen to record movement on a revolving drum
• Seismograph function: A concrete base moves with the ground, while a heavy weight with a pen remains relatively still, recording movement
• P waves are the fastest seismic waves

Types of Seismic Waves

• P waves (Primary waves) are fast and compressional, causing particles to move in the same direction as the wave
• S waves (Secondary waves) are intermediate in speed and are shear waves, causing particles to move perpendicular to the wave direction
• Surface waves are slow and travel along the Earth's surface, causing both vertical and horizontal motion
• Waves change velocity and direction as they encounter varied density levels in the earth

Travel Time Graph

• P waves are faster than S waves
• P waves arrive before S waves
• The S-P interval is the time difference between the arrival of P and S waves, depending on the distance to the earthquake

Locating Earthquakes

• Finding the distance of an earthquake is possible by analyzing wave arrival times on a seismograph
• Earthquake location requires distance data from at least three seismic stations
• The intersection of circles drawn from three seismic stations determines the earthquake location

Earthquake Frequency

• Magnitude 8.5+ earthquakes occur ~1 every 3 years
• Magnitude 8-8.4 earthquakes occur ~1 each year
• Magnitude 7.5-7.9 earthquakes occur ~1 every 4 months
• Magnitude 7-7.4 earthquakes occur ~1 each month
• Magnitude 6.6-6.9 earthquakes occur ~1 each week
• Magnitude 6-6.5 earthquakes are strong and destructive, occur ~every other day
• Magnitude 5-5.9 earthquakes are moderate and damaging, occur ~1 every 12 hrs
• Magnitude 4-4.9 earthquakes are light, occur ~1 every 90 min
• Magnitude 3-3.9 earthquakes are minor, occur ~1 every 10 min
• Magnitude 2-2.9 earthquakes are very minor, occur ~1 every 90 sec
• Magnitude 0-1.9 earthquakes occur ~1 every 10 sec

Energy Released from Earthquakes

• Magnitude 5 earthquakes release 48 times more energy than magnitude 4
• Magnitude 6 earthquakes release 43 times more energy than magnitude 5
• Magnitude 7 earthquakes release 39 times more energy than magnitude 6
• Magnitude 8 earthquakes release 35 times more energy than magnitude 7
• Most energy is released in ~20 magnitude 7 and larger earthquakes each year

Earthquake Examples

• Northridge earthquake: Magnitude 6.7
• Loma Prieta earthquake: Magnitude 6.9, ~5.4 times the energy of Northridge
• Great San Francisco earthquake: Magnitude 7.8, ~9.25 times the energy of Loma Prieta, and ~50 times Northridge
• Sumatra earthquake: Magnitude 9.3, ~200 times the energy of San Francisco, ~1850 times Loma Prieta, and ~10,000 times Northridge

Foreshocks, Mainshocks, and Aftershocks

• Large earthquakes are part of earthquake series occurring over months or years
• Mainshock: The largest event in a sequence
• Foreshocks: Smaller events that precede the mainshock
• Aftershocks: Smaller events that follow the mainshock
• A large event may be re-classified as a foreshock if followed by an even larger earthquake

Mainshock Aftershocks

• Mainshocks are generally followed by many smaller events defining the rupture area
• Northridge Earthquake (1994): Magnitude 6.7, with 500 aftershocks in the first 24 hours and 3000 with M > 1.5 in first 3 weeks

Earthquake Magnitude

• Richter Magnitude measures the size of an earthquake: a logarithmic scale
• A magnitude 7 earthquake has 10 times larger ground motion than a magnitude 6 earthquake
• A magnitude 7 earthquake releases 30 times more energy than magnitude 6 earthquake
• Largest recorded earthquake: 1960 Chilean earthquake, ~9.8 magnitude, fault length = ~1000 km, with offsets up to 20 meters

Richter Scale

• Richter scale definition: Logarithm of maximum seismic wave amplitude recorded on standard seismogram at 100 km from earthquake, corrected for distance
• Limited in utility for distant, large earthquakes as it doesn't account for short-period waves not raising amplitude for larger events
• Richter Scale Examples: 1906 San Francisco earthquake: magnitude 8.3 and 1964 Alaska earthquake: magnitude 8.3
• Alternative Scale Examples: 1906 San Francisco earthquake magnitude 7.8 and 1964 Alaska earthquake magnitude 9.2 (100 times more energy)

Seismic Moment

• Measures amount of strain energy released by movement along whole rupture surface and is more accurate for big earthquakes
• Seismic moment calculated using rocks' shear strength times rupture area of fault times displacement (slip) on the fault. One magnitude change equates to magnitude per quake.
• Mw = 2/3*log10 M。 – 10.7 (constants are aligned to agree with Richter scale for smaller earthquakes)

Earthquake Rupture Length

• Fault rupture length (and area) greatly influence EQ magnitude
• 0.1 km (100m) long fault rupture → magnitude 4 EQ
• 1 km long fault rupture → magnitude 5 EQ
• 10 km long fault rupture → magnitude 6 EQ
• 100 km long fault rupture → magnitude 7 EQ

Magnitude vs. Intensity

• Magnitude measures the amount of energy released
• Intensity (Modified Mercalli) measures the amount of shaking and damage
• Factors affecting intensity include magnitude, distance from epicenter, geology, building style and duration

Ground Motion

• Buildings generally withstand vertical forces (weight) and are safe during vertical shaking
• Horizontal shaking causes massive damage to buildings
• Acceleration refers to how quickly things change their motion, like stepping on a car's accelerator
• Weakness, weak or older builds are vulnerable to damage from horizontal accelerations of more than 0.1 g, or 0.98 m/s squared.
• In the 1994 Northridge, there was an 1.8 g recorded in Tarzana Hills

EQ Intensity Controls

• Intensity is controlled by magnitude, distance, geology, building style, and duration of shaking
• Modified Mercalli Intensity is qualitative, rating earthquake effects
• Mercalli intensity depends on earthquake magnitude, distance from hypocenter, type of rock or sediment, and steep slopes

Delve into earthquake characteristics: seismic wave types and behavior. Explore methods for locating an epicenter using triangulation. Effects on different ground types are also addressed.