of 20
1.
Introduction
The Los Angeles (LA) Basin is a deep sedimentary structure whose evolution can be roughly characterized by an
initial subsidence and extensional phase during the establishment of the North America - Pacific plate boundary
associated with the opening of the Gulf of California and the rotation of the Transverse Ranges in the Miocene.
This was followed by a period of transpression (Ingersoll & Rumelhart,
1999
), and the generation of a substantial
network of thrust faults within the basin (Wright,
1991
). In its current state, the basin contains both active strike-
slip faults (e.g., the Newport-Inglewood fault, Whittier-Elsinore fault) and an imbricated stack of blind thrust
faults (e.g., the Elysian Park faults, Puente Hills thrust), all of which accommodate the transpressional motion of
the basin. These faults contribute to local seismic hazard both by providing source surfaces for earthquakes and
by controlling local path effects by shaping the basin geometry (Plesch et al.,
2007
). The evolutionary history
of the LA basin, with ample opportunity to produce and bury organic material during extension followed by the
establishment of stratigraphic traps during compression, has allowed LA to be a leading producer of oil in the
United States (US), helping to fuel a large rise in population during the mid-20
th
century. Development took place
predominantly on the soft sediments of the main LA, San Fernando, San Gabriel and San Bernardino basins.
As a consequence, LA is both one of the largest and most economically important cities in the US, while also
Abstract
The proliferation of dense arrays promises to improve our ability to image geological structures
at the scales necessary for accurate assessment of seismic hazard. However, combining the resulting local
high-resolution tomography with existing regional models presents an ongoing challenge. We developed a
framework based on the level-set method that infers where local data provide meaningful constraints beyond
those found in regional models - for example the Community Velocity Models (CVMs) of southern California.
This technique defines a volume within which updates are made to a reference CVM, with the boundary of the
volume being part of the inversion rather than explicitly defined. By penalizing the complexity of the boundary,
a minimal update that sufficiently explains the data is achieved. To test this framework, we use data from
the Community Seismic Network, a dense permanent urban deployment. We inverted Love wave dispersion
and amplification data, from the Mw 6.4 and 7.1 2019 Ridgecrest earthquakes. We invert for an update to
CVM-S4.26 using the Tikhonov Ensemble Sampling scheme, a highly efficient derivative-free approximate
Bayesian method. We find the data are best explained by a deepening of the Los Angeles Basin with its deepest
part south of downtown Los Angeles, along with a steeper northeastern basin wall. This result offers new
progress toward the parsimonious incorporation of detailed local basin models within regional reference models
utilizing an objective framework and highlights the importance of accurate basin models when accounting for
the amplification of surface waves in the high-rise building response band.
Plain Language Summary
Los Angeles is a major city of the United States that is at high risk of
damage due to earthquakes, due to the large number of nearby active faults and its location on a deep bowl of
weak rock, which tends to amplify earthquake damage. We use a large number of instruments located in Los
Angeles district schools to make measurements of earthquakes that occurred near Ridgecrest, California in
July 2019. These earthquakes generated a type of energy that is particularly useful for studying the structures
responsible for amplification of earthquakes. Using this data, we applied a new imaging technique to create a
local model of the northeast Los Angeles basin at higher resolution than had been previously available. Our
imaging technique appropriately balances information from previous, lower resolution inversions with the new
data obtained in this study.
MUIR ET AL.
© 2022. American Geophysical Union.
All Rights Reserved.
Parsimonious Velocity Inversion Applied to the Los Angeles
Basin, CA
Jack B. Muir
1,2
, Robert W. Clayton
1
, Victor C. Tsai
3
, and Quentin Brissaud
4
1
Seismological Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena,
CA, USA,
2
Research School of Earth Sciences, Australian National University, Acton, ACT, Australia,
3
Department of Earth,
Environmental and Planetary Sciences, Brown University, Providence, RI, USA,
4
NORSAR, Oslo, Norway
Key Points:
We generate a new velocity model of
the northeastern Los Angeles Basin
using data from the Community
Seismic Network
Using a level-set framework, we
parsimoniously balance the existing
Community Velocity Models with
new data constraints
The new model indicates a steeper and
deeper basin underneath downtown
Los Angeles, significantly amplifying
4–6 s Love waves
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
J. B. Muir,
jmuir@caltech.edu
Citation:
Muir, J. B., Clayton, R. W., Tsai, V. C.,
& Brissaud, Q. (2022). Parsimonious
velocity inversion applied to the
Los Angeles Basin, CA.
Journal of
Geophysical Research: Solid Earth
,
127
, e2021JB023103.
https://doi.
org/10.1029/2021JB023103
Received 24 AUG 2021
Accepted 18 JAN 2022
Author Contributions:
Conceptualization:
Jack B. Muir, Robert
W. Clayton, Victor C. Tsai
Data curation:
Robert W. Clayton
Formal analysis:
Jack B. Muir, Quentin
Brissaud
Funding acquisition:
Robert W. Clayton,
Victor C. Tsai
Investigation:
Jack B. Muir
Methodology:
Jack B. Muir
Project Administration:
Jack B. Muir
Resources:
Robert W. Clayton
Software:
Jack B. Muir
Supervision:
Robert W. Clayton, Victor
C. Tsai
Visualization:
Jack B. Muir, Quentin
Brissaud
Writing – original draft:
Jack B. Muir
Writing – review & editing:
Robert W.
Clayton, Victor C. Tsai, Quentin Brissaud
10.1029/2021JB023103
RESEARCH ARTICLE
1 of 20
Journal of Geophysical Research: Solid Earth
MUIR ET AL.
10.1029/2021JB023103
2 of 20
being one of the most exposed to significant earthquake hazard due to the complex fabric of active faults and
ground-motion amplifying sedimentary structures associated with the geology that has allowed its preeminence.
Seismic hazard within the basin is controlled by the locations and potential for slip on the multiple local and
regional faults of southern California, combined with the significant amplifying effect of the basin on ground
motions. The importance of path effects, such as wavefield focusing, multipathing, and basin amplification, on
LA basin ground motions has motivated extensive development of seismic velocity models. The ultimate goal of
these models is to produce accurate synthetic waveforms at frequency ranges relevant to infrastructure and build
-
ing codes within the basin. Early efforts focused on creating rule-based models of southern California (Magistrale
et al.,
1996
,
2000
) using empirically derived velocity laws (Faust,
1951
) in combination with inferred geological
structure obtained by correlating surface outcrops, borehole profiles and potential methods (Wright,
1991
). Since
these initial efforts, regional scale models of southern California have assimilated ever greater quantities of seis
-
mic data, including seismic reflection profiles, receiver functions, and earthquake source locations and mecha
-
nisms. This increase in the amount of data has led to better demarcated boundaries, including faults (Magistrale
et al.,
2000
; Plesch et al.,
2007
), and allowed for more lateral variation of within basin velocity structures by using
geostatistical methods to tie together disparate seismic data (Shaw et al.,
2015
; Süss & Shaw,
2003
). Continued
development of seismic velocity models of southern California has resulted in two widely used reference Com
-
munity Velocity Models (CVMs), CVM-S4.26 (Lee et al.,
2014
, CVM-S hereafter) and CVM-H 15.1.0 (Shaw
et al.,
2015
, CVM-H hereafter), that have incorporated waveform based seismic tomography to further refine the
models. CVM-S and CVM-H broadly agree in the positions, average velocity profile, and geometry of the major
basins of southern California, however in detail they are quite different, with CVM-H containing more explicit
geological information. Figure
1
shows a characteristic cross-section of the LA basin for both models, running
from Catalina Island, across the Inner Borderland to Palos Verdes, then through the main LA basin, San Gabriel
basin and though the Transverse Ranges to the high desert. This profile makes evident the considerably higher
detail present in the CVM-H model due to its construction including explicit geological features (notably includ
-
ing an Inner Borderland basin not present in CVM-S, as seen to the left of profile A–B in Figure
1
), as well as its
significant artifacts associated with changing lateral resolution, as evident in profile marks R1 and R2. In con
-
trast, CVM-S is significantly smoother than CVM-H due to its reliance on waveform-tomography during the final
stages of construction, although several sharp resolution based artifacts are also evident, such as the jagged edges
of the San Gabriel basin. Many features of the seismic wavefield within the LA basin, such as phase arrival times
and P-to-S amplitude ratios, are captured for local events at frequencies of up to 0.2 Hz (Lai et al.,
2020
; Taborda
et al.,
2016
). However, excitations of the basin from the recent large regional Ridgecrest earthquake sequence in
July 2019 have illustrated that ground motion amplification predictions from finite-difference wave propagation
through the SCEC CVM-H and CVM-S models do not accurately predict the observations in the 0.1–1 Hz range
that is relevant for tall buildings within downtown LA (Filippitzis et al.,
2021
), warranting continued close study
of the LA basin velocity model.
Seismic tomography offers the best opportunity for full spatial coverage of the basin at high resolution, especially
when dense seismic arrays are utilized. In the southern and central parts of the basin, the deployment of high-den
-
sity temporary seismic arrays using 10 Hz corner-frequency geophone nodes by the petroleum industry has
enabled considerable exploration of the shallow structure of the basin using ambient-noise derived observables,
such as Rayleigh-wave phase velocities, Rayleigh-wave amplifications, and body-wave travel times (e.g., Bowden
et al.,
2015
; Castellanos et al.,
2020
; Jia & Clayton,
2021
; Lin et al.,
2013
). However, similarly dense industry
deployments have not to date taken place in the northern part of the basin, which encompasses the downtown LA
region, with buildings that are highly susceptible to resonant coupling to the basin. The permanent broadband
southern California Seismic Network (SCSN), while providing a long time series of excellent quality observa
-
tions, has already been incorporated into the CVM reference models and does not provide the spatial resolution
required for the next generation of basin models. A potential alternative data source is the Community Seismic
Network (CSN, Clayton et al.,
2012
,
2020
), a permanent network of three-component micro-electromechanical
system (MEMS) accelerometers, designed to provide real-time strong-ground-motion telemetry in the event of
local earthquakes within the LA basin. The CSN instruments have been designed for inexpensive construction,
utilizing off-the-shelf components, and have a maximum observable acceleration of ±2
g
, in order to fulfill their
primary goal of strong-ground-motion monitoring. As a result, the instrument noise floor is above the amplitude
of ground motions produced by smaller regional earthquakes, and is also above the ambient seismic noise level.
This unfortunately precludes the use of ambient-noise cross-correlation methods on CSN data as these methods
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