STA/LTA Earthquake Detection

Harvard EPS55

Short-Term Average / Long-Term Average ratio method - an adaptive threshold approach that adjusts to changing noise conditions

1.0s
10s
3.0
Ready
0
Total Detections
0
True Positives
0
False Positives
0
Missed Events

How STA/LTA Works

STA/LTA uses two moving averages to create an adaptive detection threshold:

Ratio = STA / LTA
if Ratio > Threshold → Detection!
STA
1-5 sec
÷
LTA
10-60 sec

STA (Short-Term Average): Captures sudden amplitude changes
LTA (Long-Term Average): Tracks background noise level
Ratio: Spikes when signal emerges from noise

Advantages Over Simple Threshold

✓ Adaptive to noise conditions: LTA tracks changing background
✓ Better sensitivity: Can detect smaller events
✓ Automatic adjustment: Works in varying noise environments
✓ Reduced false triggers: Less sensitive to gradual changes
✓ Standard method: Widely used in seismic networks

The ratio approach means detection threshold automatically adjusts - high during noisy periods, low during quiet times.

Limitations & Challenges

✗ Parameter tuning: STA, LTA, and ratio need optimization
✗ Still amplitude-based: Can't distinguish event types
✗ Delayed detection: LTA needs time to stabilize
✗ Aftershock problems: LTA elevated after large events
✗ Computational cost: Continuous averaging required

Common Issues:
• Cultural noise can still trigger if sustained
• Multiple events can merge if too close
• Requires careful window size selection