Unit
- Plate tectonics: Earth's lithosphere is divided into plates that move relative to each other. The interaction at plate boundaries causes stress accumulation and sudden release, generating seismic waves. Understanding plate motions helps predict seismic zones.
- Fault types: Strike-slip, normal, and reverse faults produce different ground motions. Strike-slip faults, like the San Andreas, have horizontal displacement; reverse faults create compression and uplift. Each type influences seismic hazard characterization.
- Earthquake magnitude scales: Richter magnitude measures seismic wave amplitude but saturates for large events. Moment magnitude (Mw) is preferred for large earthquakes because it relates to the seismic moment, accounting for fault area and slip.
- Seismic wave propagation: Body waves (P and S) travel through the Earth's interior, while surface waves (Love and Rayleigh) cause most damage. P waves are compressional; S waves shear material perpendicular to travel direction.
- Attenuation: Seismic waves lose energy as they travel. Distance and geologic conditions reduce ground motion intensity. Attenuation relationships are used in hazard analysis to predict shaking at a site.
- Seismicity and recurrence: Gutenberg-Richter law describes frequency-magnitude distribution, with more small earthquakes than large ones. Recurrence intervals help estimate the likelihood of future events for design.
- Site effects: Local soil conditions amplify or deamplify ground motion. Soft soils may amplify long-period waves, causing resonance in tall buildings. Site classification (A through F) modifies design spectra in codes.
- Historical earthquakes: Studying past events like the 1906 San Francisco or 2011 Tohoku earthquakes provides data on fault behavior, ground motion, and failure patterns. This informs probabilistic hazard models.
- Ground motion parameters: Peak ground acceleration (PGA), velocity (PGV), and displacement (PGD) characterize shaking intensity. These parameters are used in seismic design and hazard mapping. PGA is commonly used for structural design, while PGV correlates better with damage in certain structures, especially pipelines and flexible systems.
- Liquefaction: Saturated loose sands can lose strength during strong shaking, behaving like a liquid. This phenomenon causes ground failure, lateral spreading, and settlement. Liquefaction hazard assessment involves evaluating soil type, groundwater depth, and seismic demand using simplified procedures or advanced numerical modeling.
- Tsunami generation: Subduction zone earthquakes with vertical displacement of the seafloor can generate tsunamis. Tsunami wave height and arrival time depend on earthquake magnitude, depth, and fault geometry. Early warning systems and evacuation plans are critical for coastal community resilience. Seismic engineers consider tsunami loads in design for near-shore structures.
Security Consideration
HIGH RISK
Vulnerability: Lack of understanding of local seismicity can lead to underestimation of design forces.
Mitigation: Always use region-specific hazard maps and update them with new data. Engage geotechnical engineers for site-specific studies.
Reference: ASCE 7-22 Chapter 11
Code Example
Python
import numpy as np
def moment_magnitude(M0):
return (2/3) * np.log10(M0) - 10.7
M0 = 1.0e20 # seismic moment in N.m
Mw = moment_magnitude(M0)
print('Mw:', round(Mw,2))This function calculates moment magnitude from seismic moment. For typical large earthquakes, M0 is on the order of 10^19 to 10^21 N.m.
Quick Revision
- Earthquakes originate from plate tectonic movements and fault slip.
- Moment magnitude is the standard scale for large earthquakes.
- Attenuation reduces ground motion with distance.
- Site effects must be considered for accurate hazard assessment.