Lithospheric deformation beneath the San Gabriel Mountains in the Southern California Transverse Ranges

High-resolution tomographic images from Los Angeles Region Seismic Experiment (LARSE) array and southern California Seismic Network (SCSN) teleseismic data suggest that the entire lithosphere below the San Gabriel Mountains and San Andreas fault in the Transverse Ranges has thickened in a narrow, vertical sheet. P wave travel time inversions of the combined data support the presence of the well-documented upper mantle high-velocity anomaly that extends ∼200 km into the mantle under the northernmost Los Angeles basin and Transverse Ranges, and is associated with mantle downwelling due to oblique convergence. We find that the high-velocity, high-density upper mantle anomaly comprises a 60–80 km wide sheet of mantle material that lies directly below a substantial crustal root in the San Gabriel Mountains. The velocity perturbations are as large as 3% in the anomaly, corresponding to a ∼2% density increase. The tomographic images suggest that deformation in the ductile lower crust and mantle lithosphere may be partially coupled mechanically and thermally if the thickening is occurring together in response to convergence and that it may be a local compressional feature

Modeling core fluid motions and the drift of magnetic field patterns at the CMB by use of topography obtained by seismic inversion

The thermal wind equations, in which the Coriolis force is balanced by pressure gradients and horizontal density gradients rather than by Lorentz forces, are used to describe patterns of magnetic field drift associated with core fluid motions near the core-mantle boundary (CMB). The advection of magnetic field may be due in part to the flow driven by such horizontal temperature gradients, just as East-West air flow is driven by North-South temperature gradients in the Earth's atmosphere. It is argued that this flow may be concentrated in a shell near the CMB, and the horizontal temperature gradients are expected to be directly proportional to horizontal gradients in CMB topography, the lowest harmonics of which are approximately constrained in seismology. Part of the zonal drift is then associated with the 1=2, m=0 harmonic of CMB topography, and anticyclones are attached to topographic highs (thermal highs). Comparison of our derived flow pattern with those determined purely by magnetic field observations provides tentative support for our model

Three-dimensional lithospheric structure below the New Zealand Southern Alps

Uppermost mantle seismic structure below the Southern Alps in South Island, New Zealand, is investigated by teleseismic P wave travel time residual inversion. The three-dimensional tomographic images show a near-vertical, high-velocity (2–4%) structure in the uppermost mantle that directly underlies thickened crust along the NNESSW axis of the Southern Alps. The center of the high-velocity anomaly lies to the east of the Alpine fault which bounds Pacific and Australian plate rocks. The oblique collision of these plates resulted in the uplift of the Southern Alps during the past 5–7 m.y. Also, a high-velocity anomaly (3–5%) corresponding to the Hikurangi subduction zone lies to the northeast of the Southern Alps anomaly, and low-velocity anomalies (-3%) underlying parts of northwestern and southern South Island may be signatures of late Tertiary extension and volcanism. The data consist of teleseismic arrival times from the New Zealand National Seismograph Network and arrival times recorded during the 1995–1996 Southern Alps Passive Seismic Experiment. Crustal heterogeneity was accounted for by back projecting the rays through an independently obtained three-dimensional crustal velocity and Moho depth model. The Southern Alps uppermost mantle velocity anomalies are most simply explained by lithospheric thickening below the center of convergence accompanied by thinning and asthenospheric upwelling adjacent to the region of convergence

Report for passive data acquired in the 1998-1999 Los Angeles Region Seismic Experiment II: a transect from Santa Monica Bay to the Westernmost Mojave Desert

Between October, 1998 and April, 1999, 83 seismic stations were installed in the greater western Los Angeles, California area to record teleseismic, regional, and local earthquakes. The near-linear 93-km long array extended between Santa Monica Bay and the western Mojave Desert, through the epicentral region of the Northridge earthquake. The goals of the experiment were to determine crustal thickness below the western Transverse Ranges, San Fernando Valley basin, and western Mojave Desert, measure anistropy along the line with special emphasis on the San Andreas fault region, evaluate the potential for future strong ground shaking at sites in the basins, and determine the kinematic relationship between crustal and uppermost mantle deformation by three-dimensional tomographic inversion using regional network data combined with the array data. The stations consisted of three-component, broadband and short-period seismometers, and timing was controlled by Global Positioning System (GPS) receivers. The array recorded 165 Gb of raw waveform data in continuous (25 sps) and triggered (50 sps) streams. Approximately 144 teleseismic earthquakes with magnitudes ≥ 5.5, and 2025 local earthquakes with magnitude ≥ 2.0 were recorded. Preliminary results from three-dimensional teleseismic traveltime inversion tomography indicate that uppermost mantle seismic anomalies strongly correlate with thickened crust in the Transverse Ranges suggesting that the width of the compressional region and convergence rate control the location of deformation more than the San Andreas shear zone does

Earthquake and ambient vibration monitoring of the steel frame UCLA Factor building

Dynamic property measurements of the moment-resisting steel-frame University of California, Los Angeles, Factor building are being made to assess how forces are distributed over the building. Fourier amplitude spectra have been calculated from several intervals of ambient vibrations, a 24-hour period of strong winds, and from the 28 March 2003 Encino, California (M_L =2.9), the 3 September 2002 Yorba Linda, California (M_L=4.7), and the 3 November 2002 Central Alaska (M_w=7.9) earthquakes. Measurements made from the ambient vibration records show that the first-mode frequency of horizontal vibration is between 0.55 and 0.6 Hz. The second horizontal mode has a frequency between 1.6 and 1.9 Hz. In contrast, the first-mode frequencies measured from earthquake data are about 0.05 to 0.1 Hz lower than those corresponding to ambient vibration recordings indicating softening of the soil-structure system as amplitudes become larger. The frequencies revert to pre-earthquake levels within five minutes of the Yorba Linda earthquake. Shaking due to strong winds that occurred during the Encino earthquake dominates the frequency decrease, which correlates in time with the duration of the strong winds. The first shear wave recorded from the Encino and Yorba Linda earthquakes takes about 0.4 sec to travel up the 17-story building

Identification of Sparse Damage in Steel-Frame Buildings Using Dense Seismic Array Measurements

There is an unprecedented increase in the number of real-time measurements produced by permanent, dense accelerometer arrays in buildings, an example being the Community Seismic Network. In the present work, damage identification techniques are developed by coupling such datasets with linear and nonlinear finite-element models of buildings. Damage in steel-frame buildings is manifested in localized areas as cracks in beam-column connections or as an average stiffness reduction. High-fidelity linear or nonlinear finite-element models are developed to allow for realistic behavior, including modeling nonlinearities associated with the opening and closing of cracks. L1 regularization techniques and sparse Bayesian learning tools are further developed fully in the time domain to reduce ill-conditioning and account for the sparsity of damage. The effectiveness of the proposed methods in identifying the location and severity of damage is demonstrated using simulated acceleration data from a three-story steel frame building, and a 15-story building in downtown Los Angeles that is fully instrumented

Identification of Sparse Damage in Steel-Frame Buildings Using Dense Seismic Array Measurements

There is an unprecedented increase in the number of real-time measurements produced by permanent, dense accelerometer arrays in buildings, an example being the Community Seismic Network. In the present work, damage identification techniques are developed by coupling such datasets with linear and nonlinear finite-element models of buildings. Damage in steel-frame buildings is manifested in localized areas as cracks in beam-column connections or as an average stiffness reduction. High-fidelity linear or nonlinear finite-element models are developed to allow for realistic behavior, including modeling nonlinearities associated with the opening and closing of cracks. L1 regularization techniques and sparse Bayesian learning tools are further developed fully in the time domain to reduce ill-conditioning and account for the sparsity of damage. The effectiveness of the proposed methods in identifying the location and severity of damage is demonstrated using simulated acceleration data from a three-story steel frame building, and a 15-story building in downtown Los Angeles that is fully instrumented

Anomalous Seismic Amplitudes Measured in the Los Angeles Basin Interpreted as a Basin-Edge Diffraction Catastrophe

The Los Angeles Basin Passive Seismic Experiment (labpse) involved the installation of an array of 18 seismic stations along a line crossing the Los Angeles basin from the foothills of the San Gabriel Mountains through the Puente Hills to the coast. At 3–5 km spacing between stations the array has much higher resolution than the permanent network of stations in southern California. This resolution was found to be important for analyzing the factors that govern the amplitude variation across the basin. We inverted spectra of P- and S-body-wave seismograms from local earthquakes (M_L 2.1–4.8) for site effects, attenuation, and corner frequency factor using a standard model that assumes geometric spreading varying as inverse distance, exponential attenuation, and an ω^2 source model. The S-wave attenuation was separable into basin and bedrock contributions. In addition to the body-wave analysis, S-wave coda were analyzed for coda Q and coda-determined site effects. We find S- wave Q (Q_S) in bedrock is higher than in the basin. High-frequency Q_S is higher than low-frequency Q_S. Coda Q (Q_c) is higher than Q_S. P-wave Q (Q_P) was not separable into basement and bedrock values, so we determined an average value only. The corner frequencies for P and S waves were found to be nearly the same. The standard model fit over 97% of the S-wave data, but data from six clustered events incident along the basin edge within a restricted range of incidence and azimuth angles generated anomalous amplitudes of up to a factor of 5 higher than predicted. We test whether such basin-edge focusing might be modeled by catastrophe theory. After ruling out site, attenuation, and radiation effects, we conclude a caustic modeled as a diffraction catastrophe could explain both the frequency and spatial dependence of the anomalous variation