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 ∼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

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

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

A damage detection method for instrumented civil structures using prerecorded Green’s 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 Green’s 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 Green’s 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 (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

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 finite‐element 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 cross‐correlating 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

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