The Los Angeles basin: from shallow hidden faults to its deep kilometric roots

Home to more than 13 million people, the urban area of Los Angeles sits on top of a large-scale deep sedimentary basin that during large earthquakes can considerably amplify the recorded seismic amplitudes. One significant example of this effect is the magnitude 6.7 Northridge in 1994. The repercussions of the seismic amplification during this event were profound, affecting structures, infrastructure, human lives, and the entire Los Angeles region. Thus, the accurate and high-resolution knowledge of the shaking intensity throughout the basin is fundamental for informing building codes and designing earthquake-reliance infrastructure in the necessary locations. Achieving such characterization requires detailed mapping of existing faults and imaging the seismic velocity structures concealed beneath the Earth's surface.

Seismic investigations stand out as indispensable tools for comprehensively characterizing seismic hazards in diverse regions. In this presentation, I will first demonstrate how seismic passive ambient noise recorded on dense arrays can be used to identify preexisting near-surface faults present in heavily urbanized areas. The findings reveal the identification of previously unmapped shallow faults, exhibiting a remarkable correlation with both shallow seismic activity and active survey images.

I will then change the focus in the latter part of the seminar where we delve into the exploration of deep basin structures and the filling sediments that play a crucial role in amplifying seismic waves during earthquakes. Our approach involves imaging the roots of the Los Angeles basin by leveraging millions of P- and S-wave arrival times accumulated over 40 years through the conventional seismic network. We complement this data using two distributed acoustic sensing (DAS) instruments, which recorded hundreds of earthquakes on telecommunication fibers for over a year. The combination of the high-spatial density of DAS with the substantial number of cataloged events from the conventional station dataset allows us to significantly enhance the existing velocity models and capture the basin’s structure at an unprecedented level of detail.

Speaker: Ettore Biondi, CalTech

Room 350/372