186 research outputs found
Los Angeles Basin Passive Seismic Experiment: subsurface imaging in a densely populated urban setting
Shallow subsurface structures can sometimes focus seismic waves produced by earthquakes, resulting in enhanced earthquake damage. For example, the large amount of damage
that occurred in Santa Monica after the Northridge earthquake may have been caused by focusing of seismic energy
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 ā¼200 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 60ā80 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 ā¼2% 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
The UCLA Factor building seismic array: monitoring structural state of health
The development of robust designs in seismometer hardware and software is making it more feasible to densely instrument civil structures on a permanent basis in order to study their states of health. The 17-story UCLA Factor building contains one of those cutting-edge structural arrays, recording building vibrations continuously
at high sample rates. It is one of only a handful of buildings in the U.S. permanently instrumented on every floor, providing us with information about how a common
class of urban structures, mid-rise moment-frame steel buildings, will respond to strong ground shaking and how the response changes as the building is damaged. For
example, structural stiffness undoubtedly decreased when welded beam-column connections extensively fractured in numerous moment-frame steel buildings during the
1994 Northridge earthquake. Unfortunately, there are no seismic records from buildings with this type of damage. However, we anticipate that changes should be observable through analysis of vibration data for a well-instrumented building
A damage detection method for instrumented civil structures using prerecorded Greenās functions and cross-correlation
Automated damage detection methods have application to instrumented structures that are
susceptible to types of damage that are difficult or costly to detect. The presented method has
application to the detection of brittle fracture of welded beam-column connections in steel
moment-resisting frames (MRFs), where locations of potential structural damage are known a priori.
The method makes use of a prerecorded catalog of Greenās function templates and a cross-correlation
method to detect the occurrence, location, and time of structural damage in an instrumented building.
Unlike existing methods, the method is designed to recognize and use mechanical waves radiated by
the original brittle fracture event, where the event is not known to have occurred with certainty and the
resulting damage may not be visible. An experimental study is conducted to provide insight into
applying the method to a building. A tap test is performed on a small-scale steel frame to test whether
cross-correlation techniques and catalogued Greenās function templates can be used to identify the
occurrence and location of an assumed-unknown event. Results support the idea of using a
nondestructive force to characterize the building response to high-frequency dynamic failure such as
weld fracture
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 (2ā4%) 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 5ā7 m.y. Also, a high-velocity anomaly (3ā5%) 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 1995ā1996 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
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
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 ā„ 5.5, and 2025 local earthquakes with magnitude ā„ 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
Damage Detection by Template Matching of Scattered Waves
A method based on template matching is presented to detect and locate damage in buildings following severe shaking by an earthquake. The templates are constructed by finiteāelement simulations of a suite of damage scenarios, with the solutions evaluated at the location (and orientation) of each sensor in the structure. The damage detection is carried out by crossācorrelating the templates with recordings acquired from earthquakes. A dense distributed network of sensors is important for detecting anomalies in the presence of ambient noise. The cross correlation of the templates with themselves provides a measure of the resolution of the damage location
Parametric Estimation of Wave Dispersion for System Identification of Building Structures
The linear-elastic response of a building structure subjected to an earthquake base excitation can be approximated as the response of a continuous, spatially inhomogenous, dispersive, viscoelastic solid subjected to vertically incident plane shear waves. The frequency-dependent phase velocity and attenuation of seismic energy at different wavelengths, together with the inertial properties of the multilayer solid characterize the response of the building structure. The objective of this study is to identify the structural system by estimating the parameters that characterize the propagation of seismic waves in an equivalent multilayer viscoelastic solid. To pursue this objective, first, the measured dynamic responses of a building structure are used to derive the frequency response functions (FRFs) of the floor absolute acceleration with respect to the base excitation using a seismic interferometry approach. The FRFs obtained from the measured structural responses are then compared with the FRFs estimated using analytical models for one-dimensional shear wave propagation in a multilayer Kelvin-Voigt dispersive medium. Through a recursive Bayesian estimation approach, the parameters characterizing the phase velocity and damping ratio of the multilayer medium are estimated. This study provides a step forward in seismic interferometric identification of building structures by proposing a new method for parametric estimation of shear wave velocity and damping dispersion at the story level of a building structure. The estimated shear wave velocities before and after a damage-inducing event can be used to identify permanent loss of effective lateral stiffness of the building structure at the story level, thus can provide an alternative method for structural health monitoring and damage identification
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