“Depth 10 km” What gives?

Automated systems often declare seismic events with fixed depths. In SeisComP, for example, scautoloc fixes the depth at 10 km when it determines that the depth resolution is poor.

What gives?

Well, automated processing is all about the P’s: P distribution, P proximity and P density. Robust picking on other phases (e.g., the S) continues to elude the automated processing community. The result of this P-centric world is that when there are lots of well distributed P-waves close to the source and spaced with good azimuthal coverage, automated processing systems can determine the depth. When one or more of these elements is missing, then depth resolution is often poor, a fixed depth is assigned and a spurious or misleading depth avoided.

Here are a few real-life examples (click for larger images):

Example 1: The importance of epicenter location with respect to network
Example 1a: Event epicenter outside of network; Depth 10 km

SeisComP3: depth fixed at 10 km by scautoloc. Event location outside of network.

SeisComP3: depth fixed at 10 km by scautoloc. Event epicenter outside of network.

Example 1b: Event epicenter inside of network; Depth 45 km

SeisComP3: depth fixed at 10 km by scautoloc. Event within network.

SeisComP3: depth of 45 km assigned by scautoloc. Event epicenter inside network.

Example 2: The importance of stations local to the epicenter
Example 2a: No stations local to epicenter; Depth 10 km

Good azimuthal coverage, no local stations

Good azimuthal coverage + no stations local to epicenter = Depth 10 km

Example 2b: Stations local to epicenter; Depth 422 km

Good azimuthal coverage,  stations close to epicenter

Good azimuthal coverage + stations local to epicenter = Depth given

Example 3: The importance of good azimuthal coverage
Example 3a: Poor azimuthal coverage; Depth 10 km

Example of poor azimuthal coverage.

Example of poor azimuthal coverage.

Example 3b: Great azimuthal coverage; Depth 48 km

Example of poor azimuthal coverage.

Example of poor azimuthal coverage.

– Branden

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