Niloufar Abolfathian

Jet Propulsion Laboratory

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Yehuda Ben‐Zion

Southern California University for Professional Studies

Patricia Martínez‐Garzón

Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

K. M. Sreejith

Indian Space Research Organisation

Christopher Johnson

Los Alamos National Laboratory

M. Simons

California Institute of Technology

M. C. M. Jasir

Indian Space Research Organisation

Ritesh Agrawal

Indian Space Research Organisation

Marco Bohnhoff

Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

E. J. Fielding

Jet Propulsion Laboratory

Grzegorz Kwiatek

Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

Cooperative Institutions

Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

Freie Universität Berlin

Indian Space Research Organisation

University of Southern California

California Institute of Technology

Jet Propulsion Laboratory

University of California, Los Angeles

University of Potsdam

Helmholtz Association of German Research Centres

National Institute of Oceanography

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A refined methodology for stress inversions of earthquake focal mechanisms

Journal of Geophysical Research Solid Earth (2016)

Patricia Martínez‐Garzón Yehuda Ben‐Zion Niloufar Abolfathian Grzegorz Kwiatek Marco Bohnhoff

Abstract We develop an improved methodology for reliable high‐resolution inversions of focal mechanisms to background stress field orientation and stress ratio R in two or three dimensions. The earthquake catalog is declustered to remove events likely affected strongly by local stress interactions rather than reflecting the large‐scale background stress field. The declustered data are discretized with the k ‐means algorithm into groups containing a number of focal mechanisms between a minimum number N min and 2 N min . Synthetic tests indicate that N min ≈ 30 provides stable inversion results under different stress regimes and noise conditions when R ≈ 0.5, while N min ≈ 45 is needed for R near 0 or 1. Additional synthetic tests compare the performance of selecting the fault plane of each focal mechanism using (a) the plane with lowest misfit angle between the slip vector from the focal mechanism and shear traction from the stress tensor and (b) the plane with highest instability coefficient representing proximity to the optimally oriented fault for given stress field and friction coefficient. The instability criterion is found to provide more accurate inversion results under all tested stress regimes, stress ratios, and noise conditions. The refined inversion methodology combines selecting fault planes using the instability criterion iteratively with a damped simultaneous inversion of different focal mechanism groups. Results characterizing neighborhoods of discretized domains merged during the damped inversion provide high‐resolution information independent of the discretization. Some aspects of the methodology are illustrated with focal mechanism data from the San Jacinto Fault Zone in Southern California.

Focal mechanism

Fault plane

Stress field

10.1002/2016jb013493

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Citations (91)

Numerical Simulations of Stress Variations with Depth in a Model for the San Jacinto Fault Zone

AGU Fall Meeting Abstracts (2018)

Niloufar Abolfathian Christopher Johnson Yehuda Ben‐Zion

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Strain Accumulation and Release in the Himalaya from ALOS-2 InSAR Analysis

IGARSS 2022 - 2022 IEEE International Geoscience and Remote Sensing Symposium (2023)

Niloufar Abolfathian E. J. Fielding K. M. Sreejith M. C. M. Jasir Ritesh Agrawal

We study the active Himalayan deformation via examining the interseismic, coseismic, and postseismic phases, exploring the strain on the Main Himalayan Thrust due to Indian and Eurasian Plate convergence. We examine postseismic deformations resulting from the 2015 M7.8 Gorkha Earthquake, employing InSAR data, ALOS-2 L-band, from the JAXA ALOS-2 SAR satellite. InSAR in the Himalayas is challenging due to the extreme topographic relief, steep slopes, vegetation and snow cover, and L-band is an advantage. We focus on the phase unwrapping, topographic, tropospheric, and ionospheric corrections to improve the InSAR time-series. The final interseismic results provide information on the evolution of the strain field. Initial results from the 2015-2019 Gorkha postseismic analysis are consistent with afterslip down-dip from the 2015 rupture and absence of afterslip on shallower megathrust. This approach sets the stage for future analyses of data from the forthcoming NISAR mission, with enhanced ionospheric and tropospheric corrections.

10.1109/igarss52108.2023.10283012

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Variations of stress parameters in the Southern California plate boundary around the South Central Transverse Ranges

Niloufar Abolfathian Patricia Martínez‐Garzón Yehuda Ben‐Zion

We examine stress parameters in Southern California with a focus on the region near the South Central Transverse Ranges (SCTR), using a refined stress inversion methodology to 1981-2017 declustered and aftershocks focal mechanisms independently. Comparison between the associated stress parameters provides information on the local dominant loading. The estimated stress parameters are examined in relation to the regional stress regime and local loadings. Over the regional scale, the Strends towards the NNE and the stress ratios vary from transtensional stress regime near the Eastern California Shear Zone (ECSZ), to shear stress near the SCTR, and towards transpression near the Western Transverse Ranges. Detailed analysis of stress parameters near the SCTR indicates deviations from the regional shear stress. The San Bernardino Mountain area shows S direction towards NNW and transpressional stress components likely associated with the relative motion of the San Andreas Fault and ECSZ. The Cajon Pass and San Gorgonio Pass show transpressional stress regime near the bottom of the seismogenic zones likely associated with the elevated topography. In Crafton Hills, rotation of the principal stress plunges and S direction and transtensional stress regime below ~10 km, along with lower estimated apparent friction coefficient suggest a weak fault possibly associated with deep creep. The results reveal effects of local loadings resolved by the performed multi-scale analysis. The study does not show significant temporal variations of stress variations near the SCTR from the average stress parameters in the analyzed 37 years.

Transpression

10.1002/essoar.10502117.1

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Towards Detailed Characterization of Spatio-temporal Variations in Stress Parameters along the San Jacinto Fault Zone

AGU Fall Meeting Abstracts (2016)

Niloufar Abolfathian Patricia Martínez‐Garzón Yehuda Ben‐Zion

Characterization

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Variations of Stress Parameters in the Southern California Plate Boundary Around the South Central Transverse Ranges

Journal of Geophysical Research Solid Earth (2020)

Niloufar Abolfathian Patricia Martínez‐Garzón Yehuda Ben‐Zion

Abstract We examine the stress field in Southern California with a focus on the region near the South Central Transverse Ranges (SCTR), using a refined stress inversion methodology. We independently invert declustered and aftershocks focal mechanisms from the years 1981 to 2017. Comparisons between the stress field orientations and the stress ratios from these inversions provide information on local sources of the stress field. Regionally, the S Hmax tends to be toward NNE and the stress ratios vary from transtensional stress regime near the Eastern California Shear Zone (ECSZ), to strike‐slip faulting near the SCTR, and toward transpression near the Western Transverse Ranges. Detailed analysis of stress parameters near the SCTR indicates deviations from the regional strike‐slip faulting including the following: (1) The San Bernardino Mountain area shows S Hmax direction toward NNW and transpressional stress components likely associated with the motions of the San Andreas Fault and ECSZ; (2) the Cajon Pass and San Gorgonio Pass show transpressional stress regime near the bottom of the seismogenic zone likely associated with the elevated topography; and (3) in Crafton Hills, rotation of the principal stress plunges and S Hmax direction and transtensional stress regime below ~10 km, along with lower estimated apparent friction coefficient, suggest a weak fault possibly associated with deep creep. The performed multiscale analysis resolves effects of regional and local loadings. The average stress parameters in the analyzed 37 years do not show significant temporal variations of stress field near the SCTR region.

Stress field

Transpression

Focal mechanism

10.1029/2020jb019482

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Citations (7)

Spatiotemporal Variations of Stress and Strain Parameters in the San Jacinto Fault Zone

Pure and Applied Geophysics (2018)

Niloufar Abolfathian Patricia Martínez‐Garzón Yehuda Ben‐Zion

Stress field

Clockwise

Transtension

Focal mechanism

10.1007/s00024-018-2055-y

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Citations (20)