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Short-term response analysis

Overview​

Short-term seismic response refers to the aftershock sequences that occur following a trigger event (such as a blast or large earthquake). Understanding and quantifying these responses is critical for mine safety and operational planning, particularly for determining exclusion zones and re-entry times.

Response Characteristics​

When a trigger event occurs, the surrounding rock mass typically experiences a period of elevated seismic activity. This response can be characterized in multiple ways:

Spatial Distribution -- Response events cluster around the trigger location, with density decreasing with distance. The spatial extent depends on the trigger magnitude and local stress conditions.

Temporal Decay -- Response activity typically follows a power-law decay pattern, often described by the Modified Omori Law (MOL), where event frequency decreases over time following the trigger.

Magnitude Distribution -- Response events typically follow the Gutenberg-Richter relationship, with many small events and progressively fewer larger events.

Analysis Methods​

Spatial Analysis​

Understanding the spatial distribution of response events helps define exclusion zones:

  • 3D visualisation -- Viewing trigger and response event locations in 3D space reveals the spatial extent and geometry of the response.
  • Distance Normalization -- Normalizing by volume (expanding sphere) reveals whether event density is uniform or concentrated in specific directions.
  • Density Mapping -- 2D and 3D density plots show where response events are most concentrated.

Temporal Analysis​

Temporal analysis helps determine when seismic activity has returned to background levels:

  • Time-after-Trigger Charts -- Plotting event frequency as a function of time after the trigger reveals the decay pattern.
  • Modified Omori Law Fitting -- The MOL provides a quantitative description of the decay: N(t) = K(t+c)^-p, where N is cumulative events, t is time, and p, K, c are fitted parameters.
  • Re-entry Thresholds -- Setting threshold values for various parameters (energy, event count, activity rate) helps determine when conditions are safe for re-entry.

Magnitude Analysis​

Understanding the magnitude distribution of response events:

  • Frequency-Magnitude Relationships -- Plotting event count vs magnitude reveals the b-value and helps identify anomalous behavior.
  • Energy-Moment Relationships -- Examining the relationship between seismic moment and energy provides insights into stress release mechanisms.

Trigger Assessment​

Effective response management requires understanding which triggers produce significant responses:

  • Trigger Characteristics -- Blast size, type, location, and depth all influence response magnitude.
  • Comparative Analysis -- Comparing responses across multiple triggers helps identify patterns and develop exclusion/evacuation criteria.
  • Cumulative Distributions -- Examining cumulative distributions of response parameters helps set appropriate thresholds.

References​

For detailed information on response analysis parameters and methodology, see Tierney et al. 2019.