Geological Sciences Lecture Series

                                                 Wednesday, March 27th at 1pm
                                                                        CSL 422

Yuval Levy & Te-Yang Yeh

Yuval Levy

Geophysics Joint Doctoral Candidate

SDSU / UCSD

Title:

Structural Modeling of the Western Transverse Ranges: An Imbricated Thrust Ramp Architecture

Bio:

After my military service and a year traveling South America I went to the Hebrew University of Jerusalem. Got my Bachelor’s and Master’s in geology, while working in the Geological Survey of Israel. For my master’s my focus was on rockfalls. I worked for a short time in copper exploration after my Master’s and then joined the program here. 

Abstract

Active fold-and-thrust belts can potentially accommodate large magnitude earthquakes; understanding the structure in such regions has both societal and scientific importance. Recent studies provide evidence for large earthquakes in the Western Transverse Ranges (WTR) of California. However, the diverse set of conflicting structural models for this region highlights the lack of understanding of the subsurface geometry of faults. A more robust structural model is required to assess the seismic risk of the WTR. Toward this goal, we incorporate a full range of geologic observations, as well as vertical motions from uplifted fluvial and marine terraces, as constraints on our kinematic forward modeling of the first order structure of the WTR. Using fault-related folding methods, we predict the geometry and sense of slip of the major faults at depth, and use these structures to model the evolution of the WTR since the late Pliocene. The forward modeling predictions are in good agreement with the observed geology and deformation. Our results suggest that the WTR comprises a southward verging imbricate thrust system, with the dominant faults dipping as a ramp-ramp to the north that steepen as they shoal from ~20˚ degrees at depth to ~60˚ near the surface. By including a full range of observations in our forward modeling efforts, we address fundamental questions regarding the structural geometry and kinematics of the region, which allows for a better understanding of earthquake and tsunami potential.

 

Te-Yang Yeh 

Geophysics Joint Doctoral Candidate

SDSU / UCSD

Title:

Modelling Broadband Seismic Wave Propagation of North Korean Nuclear Explosions

 Bio:

 Te-Yang is currently a PhD student of the SDSU/UCSD Geophysics Joint Doctoral Program. His research focuses on large scale simulation of broadband seismic wave propagation in complex media. His thesis addresses the generation and attenuation of primary seismic phases and its applications to the discrimination between natural earthquakes and explosions. Te-Yang received his B.S. and M.S. in Geophysics at National Central University in Taiwan.  

Abstract

 With the recent advancement in high-performance computers (HPC), regional-scale broadband wave propagation simulations are now feasible. To understand the generation and attenuation of seismic phases that are routinely examined for seismic event discrimination (such as Pn, Pg, Sn, and Lg), we carried out 0-4 Hz wave propagation simulations for the 2009 North Korean nuclear test in the SALSA3D velocity model developed by Sandia and LANL (Begnaud, 2015). We found that it is essential to introduce small-scale heterogeneities from appropriate statistical distributions in the crust to model broadband observed waveform envelopes at regional distances (~500 km). Root-mean-square amplitudes were computed in narrow frequency bands for both the observed and simulated seismic phases. At station INCN (473km away from the source), we found that explosive sources result in higher Pn/Lg, Pg/Lg, and Pn/Sn ratios then the pure double-couple source, which is consistent with the observations. The result of array analysis at KSRK array also confirms that the crustal small-scale heterogeneities with parameters constrained by our study are needed to reproduce the coherence from the data. We found that scattering behavior appears to be associated with source types (e.g., P-S versus S-P scattering), indicating that the predominant S wave train from the double-couple source is very efficiently generating converted S-P scattered waves after Pn. In addition to the 2009 event, the same method was used to model broadband records in Japan of the 2017 mb=6.3 event at NKNTS (~1200 km) to better understand the leakage of Lg energy at the transition between the continental and oceanic crustal layers, as well as the formation of the long-period Rayleigh waves widely observed at stations in Japan.