Focal Mechanism Explorer - Educational Beachball Tool

Strike °

Direction of fault trace from north (0-360°)

Dip °

Angle of fault plane from horizontal (0-90°)

Rake °

Direction of slip on fault plane (-180 to 180°)

Source Type (Moment Tensor)

ISO (Isotropic) %

Volume change: +explosion / −implosion

CLVD %

Compensated linear vector dipole

DC (Double-Couple) 100%

DC = 100 − |ISO| − |CLVD| (derived)

Depth (affects station takeoff angles) km

Depth:

0 33 70 150 300 500 700km

Fault Type

Strike-slip

Magnitude

5.0

P-axis (compression)

T-axis (tension)

About Fault Types

Select a preset to learn about different fault types and see their characteristic beachball patterns.

Minimum Magnitude

Time Range

Station Name

Azimuth (°) - Direction from earthquake to station

Epicentral Distance (°)

First Motion

FDSN Data Center

Earthquake Location (click map or select earthquake)

Search Radius

Network Filter (optional)

Upload a CSV file with station data. Required columns:


                            name,azimuth,distance,polarity

                            ANMO,45.2,30.5,up

                            HRV,120.8,25.3,down

Or with coordinates (azimuth/distance calculated):


                            name,lat,lon,polarity

                            ANMO,34.95,-106.46,up

Stations (0)

No stations added yet

Depth: 10 km | Legend: ● UP ○ DOWN ◐ ?

Takeoff angles use a simplified 1-D model. For research-grade polarity analysis, use ray-tracing through IASP91 or AK135.

Fault Cross-Section

■ Hanging Wall ■ Footwall

Earthquake Map Tectonic Plates

Understanding Focal Mechanisms

Click to set strike

Strike-slip

The beachball diagram shows the orientation of fault motion during an earthquake. Black quadrants represent compression (P-wave first motion pushes away), while white quadrants represent tension (P-wave first motion pulls toward source).

Which Plane is the Fault?

●

Nodal Plane 1:

Strike: 0° | Dip: 45° | Rake: 0°

○

Nodal Plane 2 (Auxiliary):

Strike: 90° | Dip: 90° | Rake: 180°

Click to select the fault plane. The cross-section and block diagram will update. How is Plane 2 calculated? →

Test Your Knowledge: Focal Mechanism Quiz →

P and T Axes

P-axis (blue): Maximum compression direction

T-axis (red): Maximum tension direction

The P and T axes bisect the angles between the nodal planes. Their orientations reveal the regional stress field that caused the earthquake.

Source Type (Hudson Plot)

Click on the diagram to set ISO/CLVD values

Source Type Examples

Moment Tensor

Eigenvalues

Key Concepts

Double-couple: Most earthquakes occur on planar faults and produce this characteristic four-quadrant pattern.

Auxiliary plane: Each beachball has two possible fault planes - additional data (aftershocks, surface rupture) determines which actually slipped.

P-Wave First Motion

Stations in compression quadrants see initial UP motion. Stations in tension quadrants see initial DOWN motion.

Famous Examples

Welcome to Focal Mechanism Explorer