Isolating the Structural Contributions to the Pn Shadow Zone of the Sierra Nevada, California

Session: Full-Waveform Inversion: Recent Advances and Applications [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Multiple seismic experiments have revealed a Pn shadow zone in seismic record sections across the Sierra Nevada. For earthquakes on the west side of the Sierra, Pn arrivals on the east side of the mountain range are delayed, the Pn phase has anomalously low amplitude and, in some cases, no discernible Pn arrival at all. Additionally, in the case of earthquakes on the west side the range, Pn often appears with low apparent velocities (~7.4 km/s) on stations within the mountains followed by high (~8.6 km/s) apparent velocities east of the range.

In order to constrain the source of this shadow zone, we have developed a set of simple velocity models and systematically vary a characteristic feature of the structure in each model. These models and their variations are then used to produce synthetic seismograms using a finite difference full waveform modeling algorithm. In particular, we examine simple variations in Moho geometry, Moho gradient and slightly more complex variations in lithospheric foundering geometries. This approach allows us to establish a set of “control parameters” and to learn the effect that each parameter has on synthetic record sections.

We present results of these tests along with some qualitative conclusions to assess how the various parameters can potentially play a role in the overall velocity structure of the Sierra Nevada. Notably, we demonstrate how the steepness and location of a ramp in the Moho can influence the nick point location of a change in apparent velocity. A ramp of shallower steepness will have a constant apparent velocity over a greater set of stations. We also observe that the Moho gradient can change arrival times but also mute out the features of the Moho’s structure and the complexity and ambiguity of possible differences in lithospheric foundering geometries.

Presenting Author: Kyren R. Bogolub

Authors