Introduction to Geophysics

Ali Onceloncel@kfupm.edu.

saDepartment of Earth SciencesKFUPM

Earthquake Seismology: Fundamentals 1

Fairly thick layers result in distinct events on travel time graph

Seismic Waves:

As Probes of Earth’s Interior

Travel Time Models for FLAT EARTH

Travel Time Models of earthquakes for FLAT EARTH

Series of thin layers results in rays that emerge at

steeper angles with increasing depth.

Slopes of first arrivals change with distance, indicating the changing velocities with depth.

Numerous layers so thin that the changing velocity with depth is modeled as a function V(Z).

Deep-penetrating rays, having encountered faster velocities, emerge at steeper angles than the shallower rays.

The changing wave front angles result in changing apparent velocity, evident as changing slope on the travel-time graph.

Travel Time Models of Earthquakes for FLAT EARTH

Summary: Travel Time Models

Earth with constant seismic velocity (V). Ray paths are linear because no refraction occurs. The convex-upward form of the graph is simply because measurements are along Earth’s curved surface.

Travel Time Models of Earthquakes for Curved EARTH

Rays bend away from the vertical, leading to steeper emergence angles (hence, higher apparent velocities) than for the constant velocity case in (a).

Likewise, more curvature on the travel-time graph indicates higher apparent velocity than in (a).

Earth model with a gradual increase in velocity with depth, V(z).

Selected ray paths and travel-time curves for body (P,S) and surface waves. As in (b), seismic velocities increase with depth.

Travel Time Models of

Earthquakes for Curved EARTH