Earthquake Source Property: Seismic Moment

Seismic moment ($M_0$) is a fundamental measure quantifying the size of an earthquake. It represents the physical parameters of the earthquake source itself.

The seismic moment is defined as:

$M_0 = \mu A \bar{u}$

• $\mu$: Shear modulus of the source region rock.
• $A$: Area of the fault rupture.
• $\bar{u}$: Average slip or displacement on the fault.

Key Point: The seismic moment is an intrinsic characteristic of the earthquake source. It does not change based on the distance or direction from the source to the recording station.

Why Other Recorded Properties Vary

The seismic waves recorded at different stations are affected by factors beyond the source itself:

• Amplitude and Duration: The observed amplitude and duration of seismic waves decrease with distance from the source due to geometric spreading (wave energy dispersing over a larger area) and attenuation (energy loss due to absorption in the Earth's materials). The radiation pattern of the specific earthquake source (double-couple) also means wave amplitude varies significantly with direction.
• Seismic Energy: Like amplitude and duration, the energy detected at a station is influenced by distance, attenuation, and the radiation pattern. It is not solely a property of the source.

Since the stations are equidistant and located in cardinal directions, they experience different effects from geometric spreading and the radiation pattern. Therefore, while amplitude, duration, and energy readings will vary between stations, the underlying Seismic moment, representing the source's fundamental size, remains the same.