Collaborative Research: Determining the Velocity and Structure of the Los Angeles Basin From New High-Resolution Data

This project aims to develop a detailed 3D model of the Los Angeles (LA) Basin, critical for accurately estimating earthquake ground motion and assessing seismic hazard for this densely populated area. The LA metropolitan area sits above a deep sedimentary basin that significantly affects the level of ground shaking from local and regional earthquakes.

The basin has a complex tectonic history of extension and compression and is also crosscut by numerous faults. This has resulted in a complicated subsurface structure that leads to significant variation in site amplification of seismic waves across the basin, as evidenced by the recorded ground shaking following the 2019 Ridgecrest earthquake. In the summer of 2022, a temporary 300-node geophone array was deployed across the entire LA basin, providing a seismic data set with uniform and dense coverage for the first time.

In this project, the researchers will analyze these data and combine them with other available seismic and gravity observations in the region to construct a detailed 3D basin model. The model will help explain the level of amplification in different areas of the basin as well as its lateral variations The resulting model may be used to model ground motion for realistic earthquake scenarios, which is vital for evaluating infrastructure preparedness.

In addition, this project will explore the connection between the resulting seismic velocity model with mapped geological features, and the tectonic evolution of the LA Basin. Through the research, the project will support graduate and postdoctoral education, and the scientific findings will contribute to seismic hazard assessment efforts in southern California.

The density of the 300 sensor LA nodal array deployed in 2022 will enable the use of novel passive seismic imaging methods. By extracting Rayleigh and Love surface waves from multi-component ambient noise correlations, it will be possible to measure their velocity dispersion and Rayleigh wave ellipticity in the region. Techniques based on particle motion and apparent slowness will be developed to isolate different modes of surface waves.

Both isotropic as well as radially and azimuthally anisotropic basin structures will be investigated using surface wave measurements. By using receiver function and autocorrelation methods, in addition to surface wave properties, it will be possible to determine crustal discontinuities, including major intra-basin sedimentary interfaces, the bottom of the basin, and the shape of the Moho beneath it.

The use of gravity data will guide the identification of converted and reflected phases and determine the tectonic extension that the basin has experienced, enabling evaluation of the extent of thermal subsidence that has occurred within it. The dataset that will be analyzed is unique in an academic setting for its density, regularity, and completeness of coverage, allowing for the exploration of new methodologies.

These include mapping shallow seismicity to identify possible unknown faults, using reflected surface waves to map the properties of faults within the basin as well as discover new ones, and determining aspects of the stress field through anisotropy. The 3D basin model constructed in this project is expected to be more accurate than the current community velocity models (CVMs), enabling more reliable ground motion predictions for various earthquake rupture scenarios. The new model will be validated through simulations of recent earthquakes.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.