Ambient noise Love wave attenuation tomography for the LASSIE array across the Los Angeles basin

The Los Angeles basin is located within the North America–Pacific plate boundary and contains multiple earthquake faults that threaten greater Los Angeles. Seismic attenuation tomography has the potential to provide important constraints on wave propagation in the basin and to provide supplementary information on structure in the form of the distribution of anelastic properties. On the basis of the amplitude information from seismic interferometry from the linear LASSIE array in the Los Angeles basin, we apply station-triplet attenuation tomography to obtain a 2D depth profile for the attenuation structure of the uppermost 0.6 km. The array crosses four Quaternary faults, three of which are blind. The attenuation tomography resolves strong attenuation (shear attenuation Qs ~ 20) for the fault zones and is consistent with sharp boundaries across them.

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2019 Ridgecrest Earthquake Reveals Areas of Los Angeles That Amplify Shaking of High-Rises

Seismological Research Letters ◽

10.1785/0220200170 ◽

2020 ◽

Vol 91 (6) ◽

pp. 3370-3380

Author(s):

Monica D. Kohler ◽

Filippos Filippitzis ◽

Thomas Heaton ◽

Robert W. Clayton ◽

Richard Guy ◽

...

Keyword(s):

Los Angeles ◽

Strong Motion ◽

Ground Level ◽

Site Amplification ◽

Seismic Network ◽

Los Angeles Basin ◽

High Rise ◽

The North ◽

South Central ◽

San Fernando

Abstract The populace of Los Angeles, California, was startled by shaking from the M 7.1 earthquake that struck the city of Ridgecrest located 200 km to the north on 6 July 2019. Although the earthquake did not cause damage in Los Angeles, the experience in high-rise buildings was frightening in contrast to the shaking felt in short buildings. Observations from 560 ground-level accelerometers reveal large variations in shaking in the Los Angeles basin that occurred for more than 2 min. The observations come from the spatially dense Community Seismic Network (CSN), combined with the sparser Southern California Seismic Network and California Strong Motion Instrumentation Program networks. Site amplification factors for periods of 1, 3, 6, and 8 s are computed as the ratio of each station’s response spectral values combined for the two horizontal directions, relative to the average of three bedrock sites. Spatially coherent behavior in site amplification emerges for periods ≥3 s, and the maximum calculated site amplifications are the largest, by factors of 7, 10, and 8, respectively, for 3, 6, and 8 s periods. The dense CSN observations show that the long-period amplification is clearly, but only partially, correlated with the depth to basement. Sites with the largest amplifications for the long periods (≥3 s) are not close to the deepest portion of the basin. At 6 and 8 s periods, the maximum amplifications occur in the western part of the Los Angeles basin and in the south-central San Fernando Valley sedimentary basin. The observations suggest that the excitation of a hypothetical high-rise located in an area characterized by the largest site amplifications could be four times larger than in a downtown Los Angeles location.

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The Whittier Narrows, California Earthquake of October 1, 1987—Seismology

Earthquake Spectra ◽

10.1193/1.1585464 ◽

1988 ◽

Vol 4 (1) ◽

pp. 43-53 ◽

Cited By ~ 5

Author(s):

L. Jones ◽

E. Hauksson

Keyword(s):

Los Angeles ◽

Los Angeles Basin ◽

The West ◽

Northern Margin ◽

The North ◽

Rupture Area ◽

Largest Aftershock ◽

Large Earthquakes ◽

East West ◽

Strike Slip Movement

The October 1, 1987 Whittier Narrows earthquake ( ML = 5.9) was located at 34° 3.0′N, 118° 4.8′W, at the northwestern end of the Puente Hills. The sequence ruptured a small part, 4 km by 5 km, of a previously unidentified, buried, thrust fault that strikes east-west and dips 25° down to the north. This fault may be part of a large system of thrust faults extending across the entire east-west length of the northern margin of the Los Angeles basin. The focus of the mainshock is deep, at 14 ± 1 km. The largest aftershock ( ML = 5.3) produced mostly strike-slip movement on a steeply dipping, northwest plane, that bounds the mainshock rupture area to the west. Enhancement of the Los Angeles basin seismic network would facilitate investigation of the potential of these faults for moderate-sized or large earthquakes.

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