/orphan-analysis - Orphan Earthquake Detection Skill

Purpose

Identify "orphan earthquakes" - seismic events far from any mapped fault - that indicate unmapped active structures. These structures may be sources of historical "dark earthquakes" detectable in speleothem records.

Usage

/orphan-analysis <lat> <lon> [--name NAME] [--radius KM] [--orphan-threshold KM]

Examples:

/orphan-analysis 44.13 8.11 --name "Bàsura Cave"
/orphan-analysis 41.42 31.93 --name "Sofular Cave" --radius 150
/orphan-analysis 16.2 -89.1 --name "Yok Balum" --orphan-threshold 30

Methodology

Validated in Italy

The orphan analysis methodology was validated in the Ligurian Alps:

• 80% orphan rate revealed unmapped NNW-trending fault
• 193+ earthquakes on structure not in ANY database
• 25+ km lineament identified via DEM + microseismicity
• Now primary candidate for 1285/1394 dark earthquakes

Workflow

Step 1: Search Earthquake Catalog

Use earthquake_search MCP tool:

earthquake_search lat=44.13 lon=8.11 radius_km=100 min_magnitude=2.0

Parameters:

• Default radius: 100 km
• Default min_magnitude: 2.0 (for microseismicity)
• Date range: 1900-present (or available)

Step 2: Load Fault Database

Query regional fault database:

Region	Primary Database	File/URL
Italy	DISS v3.3.1	data/fault_databases/diss331_*.geojson
California	SCEC CFM v7.0	data/fault_databases/scec_cfm_*.geojson
Turkey	AFAD	Web query
Global	GEM Global	data/gem_active_faults.geojson

Step 3: Calculate Distances

For each earthquake, calculate distance to nearest mapped fault:

For each earthquake:
    min_distance = infinity
    for each fault_segment:
        d = calc_distance(eq.lat, eq.lon, fault.lat, fault.lon)
        if d < min_distance:
            min_distance = d
            nearest_fault = fault.name

    earthquake.dist_to_fault = min_distance
    earthquake.nearest_fault = nearest_fault

Step 4: Classify Orphans

Apply orphan threshold (default: 50 km):

if dist_to_fault > orphan_threshold:
    earthquake.is_orphan = True
else:
    earthquake.is_orphan = False

Step 5: Cluster Orphans by Azimuth

Group orphan earthquakes by direction from reference point:

Azimuth bins:
- N (337.5-22.5°)
- NE (22.5-67.5°)
- E (67.5-112.5°)
- SE (112.5-157.5°)
- S (157.5-202.5°)
- SW (202.5-247.5°)
- W (247.5-292.5°)
- NW (292.5-337.5°)

Step 6: Generate Report

## Orphan Earthquake Analysis: [Location Name]

**Reference point**: [lat]°N, [lon]°E
**Search radius**: [X] km
**Orphan threshold**: [X] km
**Analysis date**: [date]

---

### Summary Statistics

| Metric | Value |
|--------|-------|
| Total earthquakes | [N] |
| Orphan earthquakes | [N] ([X]%) |
| M4+ orphans | [N] |
| M5+ orphans | [N] |

---

### Orphan Rate Assessment

| Rate | Interpretation |
|------|----------------|
| > 50% | **CRITICAL** - Major unmapped structure likely |
| 30-50% | **HIGH** - Significant database gap |
| 10-30% | **MODERATE** - Some unmapped faults |
| < 10% | **LOW** - Database reasonably complete |

**This region**: [X]% → [INTERPRETATION]

---

### Orphan Clusters by Azimuth

| Direction | Count | Avg Distance | M4+ | M5+ | Structure? |
|-----------|-------|--------------|-----|-----|------------|
| NNW | [N] | [X] km | [N] | [N] | [YES/NO] |
| SW | [N] | [X] km | [N] | [N] | [YES/NO] |
| ... | ... | ... | ... | ... | ... |

---

### Candidate Unmapped Structures

#### Structure 1: [Direction] Lineament

- **Azimuth**: [X]° ([direction])
- **Orphan count**: [N] earthquakes
- **Distance range**: [min]-[max] km from reference
- **Magnitude range**: M[min]-M[max]
- **Largest event**: M[X] on [date]

**Interpretation**: [Description of possible fault]

---

### Mapped Faults in Region

| Fault Name | Strike | Distance | Type |
|------------|--------|----------|------|
| [name] | [X]° | [X] km | [normal/strike-slip/etc.] |

---

### Visualization

[Description of recommended figure showing:]
- Reference point (cave location)
- Mapped faults (from database)
- All earthquakes (color by orphan status)
- Azimuth bins with orphan clusters

Output Files

The skill can generate:

1. Analysis report ([REGION]_ORPHAN_ANALYSIS.md)
2. Earthquake CSV ([region]_earthquakes.csv)
3. Orphan subset CSV ([region]_orphans.csv)
4. Cluster definitions ([region]_clusters.json)

MCP Tools Used

Tool	Purpose
earthquake_search	Search USGS catalog
calc_distance	Calculate earthquake-fault distances
calc_pga	Estimate ground motion (optional)

Example: Italy Bàsura Results

## Orphan Analysis: Bàsura Cave (Italy)

Reference: 44.1275°N, 8.1108°E
Radius: 100 km
Orphan threshold: 50 km

### Summary
- Total earthquakes: 241
- Orphan earthquakes: 193 (80%)
- M4+ orphans: 12
- M5+ orphans: 2

### Orphan Rate: 80% → CRITICAL

### Key Finding: NNW Lineament

- Azimuth: 340° (NNW-SSE)
- 85 orphan earthquakes
- Not in DISS, ITHACA, or EFSM20
- 25+ km linear structure identified
- Primary candidate for 1285/1394 dark earthquakes

Integration with DEM Analysis

After orphan analysis, recommend DEM lineament check:

If orphan_rate > 30%:
    Recommend: /dem-pipeline for visual confirmation

    Look for:
    - Linear drainage patterns aligned with orphan clusters
    - Edge detection features at orphan azimuths
    - Topographic breaks matching cluster directions

Regional Catalogs

Region	Catalog	Coverage	Notes
Italy	INGV ISIDe	1900-present	Excellent
California	USGS ANSS	1900-present	Excellent
Turkey	AFAD	1900-present	Good
Belize	USGS	Limited	Sparse coverage
Brazil	USGS + IAG	Limited	Intraplate, sparse

Important Notes

1. Orphan threshold varies by region - Use 50 km for plate boundaries, 30 km for intraplate
2. Catalog completeness matters - Low magnitude threshold may include noise
3. Fault database gaps exist - USGS has 9-27 year lag
4. Offshore coverage poor - Many faults unmapped submarine
5. Follow up with DEM - Visual confirmation strengthens case