2.2 Seismic source characterization

Earthquakes don't just happen randomly. They come from specific places called seismic sources. These can be faults, zones where tectonic plates meet, or even areas inside continents. Scientists use various methods to find and study these sources.

Understanding seismic sources is crucial for assessing earthquake risks. We look at things like where they are, their shape, how fast they move, and how often big quakes happen. This info helps us figure out what kind of shaking we might expect in different areas.

Seismic Source Identification and Characterization

Potential seismic sources

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• Seismic sources encompass various geological features capable of generating earthquakes
  • Faults split into active faults currently producing earthquakes and inactive faults no longer moving
  • Seismogenic zones include large-scale tectonic features generating significant seismic activity
    • Subduction zones where tectonic plates collide (Pacific Ring of Fire)
    • Intraplate regions experiencing earthquakes within continental plates (New Madrid Seismic Zone)
• Characterization methods employ multiple techniques to identify and analyze seismic sources
  • Geological mapping involves field surveys to document surface expressions of faults
  • Remote sensing techniques utilize satellite imagery and LiDAR to detect landscape changes
  • Geophysical surveys measure subsurface properties (seismic reflection, gravity anomalies)
• Key source parameters provide essential information for seismic hazard assessment
  • Location pinpoints the geographical position of the seismic source
  • Geometry describes the three-dimensional shape and orientation of the fault or zone
  • Slip rate quantifies the average movement along a fault over time (mm/year)
  • Recurrence interval estimates the time between major earthquakes on a specific source
• Tectonic settings influence the distribution and behavior of seismic sources
  • Plate boundaries concentrate seismic activity along zones of tectonic interaction (San Andreas Fault)
  • Continental interiors experience less frequent but potentially large earthquakes (New Madrid Seismic Zone)

Geometry of seismic sources

• Fault geometry characterizes the three-dimensional structure of seismic sources
  • Strike measures the compass direction of the fault line at the Earth's surface
  • Dip indicates the angle of the fault plane relative to the horizontal
  • Length represents the horizontal extent of the fault (can range from meters to hundreds of km)
  • Width describes the down-dip dimension of the fault plane
• Slip rate estimation employs various methods to quantify fault movement
  • Geodetic measurements use GPS and InSAR to track surface deformation over time
  • Paleoseismic studies analyze geological evidence of past earthquakes (fault trenching)
  • Historical earthquake data provides information on recent seismic activity and fault behavior
• Maximum magnitude potential assesses the largest possible earthquake a source can generate
  • Empirical relationships link fault characteristics to potential earthquake magnitudes
    • Fault length vs magnitude correlations (longer faults typically produce larger earthquakes)
    • Rupture area vs magnitude scaling (larger rupture areas associated with higher magnitudes)
  • Seismic moment calculation estimates the energy released during an earthquake
    • $M_0 = \mu AD$ where $\mu$ is shear modulus, $A$ is rupture area, and $D$ is average slip
• Magnitude scales provide standardized measures of earthquake size
  • Moment magnitude ($M_w$) offers a physically meaningful measure of earthquake energy
  • Relationship to seismic moment: $M_w = \frac{2}{3} \log_{10}(M_0) - 10.7$ connects $M_w$ to $M_0$

Uncertainty and Hazard Analysis

Uncertainty in source parameters

• Types of uncertainty affect the reliability of seismic source characterization
  • Aleatory variability stems from inherent randomness in earthquake processes
  • Epistemic uncertainty arises from incomplete knowledge or model limitations
• Sources of uncertainty impact the accuracy of seismic source parameters
  • Incomplete historical records limit our understanding of long-term seismic patterns
  • Limited paleoseismic data creates gaps in our knowledge of prehistoric earthquakes
  • Measurement errors introduce inaccuracies in fault geometry and slip rate estimates
• Uncertainty quantification methods aim to assess and represent parameter uncertainties
  • Logic trees incorporate alternative models and parameter values with assigned weights
  • Monte Carlo simulations generate numerous scenarios to explore parameter space
• Sensitivity analysis evaluates the impact of parameter variations on hazard results
  • Helps identify which source parameters most strongly influence hazard estimates
• Expert elicitation combines knowledge from multiple experts to constrain uncertainties
  • Structured process to gather and synthesize expert opinions on seismic source parameters

Incorporation of source characterization

• Deterministic Seismic Hazard Analysis (DSHA) focuses on specific earthquake scenarios
  • Scenario-based approach considers individual earthquake events
  • Selection of controlling earthquake identifies the most impactful event for a site
  • Ground motion prediction equations estimate shaking intensity at a site
• Probabilistic Seismic Hazard Analysis (PSHA) accounts for all possible earthquake scenarios
  • Incorporates the full range of potential earthquakes and their probabilities
  • Earthquake recurrence relationships describe frequency-magnitude distributions
    • Gutenberg-Richter law: $\log N = a - bM$ relates earthquake frequency to magnitude
  • Integration of source characterization combines fault and area source models
    • Fault sources represent known active faults with specific geometries
    • Area sources account for distributed seismicity in regions without mapped faults
  • Hazard curves plot annual exceedance probability against ground motion intensity
• Seismic hazard maps provide spatial representations of hazard levels across regions
  • Useful for building code development and regional planning (USGS National Seismic Hazard Maps)
• Time-dependent vs time-independent models consider earthquake timing
  • Time-dependent models account for earthquake cycles and stress accumulation
  • Time-independent models assume constant earthquake probability over time
• Site-specific hazard assessments tailor analyses to local conditions
  • Incorporate local geological conditions (soil type, basin effects) to refine hazard estimates