We determine upper mantle seismic velocity heterogeneities below Southern California from the inversion of teleseismic travel-time residuals. Teleseismic P-wave arrival times are obtained from three temporary passive experiments and Southern California Seismic Network (SCSN) stations, producing good raypath coverage. The inversion is performed using a damped least-squares conjugate gradient method (LSQR). The inversion model element spacing is 20 km. Before the inversion, the effects of crustal velocity heterogeneities represented by the Southern California Earthquake Center (SCEC) seismic velocity model version 2 are removed from the teleseismic travel times. The P-wave inversion produces a variance reduction of 43%. S-wave velocities are determined from laboratory Vp/Vs ratios. The most prominent features imaged in the results are high P-wave velocities )%3ם( in the uppermost mantle beneath the northern Los Angeles basin, and the previously reported tabular high-velocity anomaly )%3ם( to depths of 200 km beneath the Transverse Ranges, crosscutting the San Andreas fault. We incorporate the upper mantle seismic velocity heterogeneities into the SCEC Southern California reference seismic velocity model. The prior accounting for the crustal velocity heterogeneity demonstrates the utility of the top-down method of the SCEC seismic velocity model development.
Abstract. Recent high-resolution seismic experiments reveal that the crust beneath the San Gabriel Mountains portion of the Transverse Ranges thickens by 10-15 km (contrary to earlier studies). Associated with the Transverse Ranges, there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle. This high-velocity anomaly has previously been interpreted as a lithospheric downwelling. Both lithospheric downwelling and crustal thickening are associated with the oblique convergence of Pacific and North America plates across the San Andreas Fault, though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle. We show by means of numerical experiment that the balance between buoyancy forces that drive deforrnation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling. We use a simple two-layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust. External plate motion drives convergence, which is constrained by boundary conditions to occur within a central convergent zone of specified width. A fundamental transition in the geometry of downwelling is revealed by our experiments. For slow convergence, or low crustal viscosity, downwelling occurs as multiple sheets on the margins of the convergent zone. For fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone. This complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability. In order for a narrow downwelling slab to have formed beneath the Transverse Ranges within the last 5 Myr, the effective lithospheric viscosity of the convergent region is at most about 1020 Pa s.
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 ( ML=2.9), the 3 September 2002 Yorba Linda, California ( ML=4.7), and the 3 November 2002 Central Alaska ( Mw=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.
We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust. In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ϳ25؇ from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingleor wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ϳ33؇ from ϳ7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ϳ20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the mainshock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections.
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