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Daniel J. Ponti

Publications and source records attributed to Daniel J. Ponti.

22 records · Page 2Linked to original sources
USGS2001

Implications for the formation of the Hollywood Basin from gravity interpretations of the northern Los Angeles Basin, California

Gravity data provide insights on the complex tectonic history and structural development of the northern Los Angeles Basin region. The Hollywood basin appears to be a long (> 12 km), narrow (up to 2 km wide) trough lying between the Santa Monica Mountains and the Wilshire arch. In the deepest parts of the Hollywood basin, the modeled average thickness ranges from roughly 250 m if filled with only Quaternary sediments to approximately 600 m if Pliocene sediments are also present. Interpretations of conflicting drill hole data force us to consider both these scenarios. Because of the marked density contrast between the dense Santa Monica Mountains and the low-density sediments in the Los Angeles Basin, the gravity method is particularly useful in mapping the maximum displacement along the Santa Monica-Hollywood-Raymond fault zone. The gravity-defined Santa Monica–Hollywood fault zone deviates, in places, from the mapped active fault and fold scarps located with boreholes and trenching and by geomorphological mapping by Dolan and others (1997). Our models suggest that the Santa Monica–Hollywood fault zone dips northward approximately 63°. Three structural models are considered for the origin of the Hollywood basin: pull-apart basin, flexural basin, and a basin related to a back limb of a major fold. Although our preferred structural model involves flexure, the available geologic and geophysical data do not preclude contributions to the deepening of the basin from one or both of the other two models. Of particular interest is that the distribution of red-tagged buildings and structures damaged by the Northridge earthquake has a strong spatial correlation with the axis of the Hollywood basin defined by the gravity data. Several explanations for this correlation are explored, but two preferred geologic factors for the amplification of ground motion besides local site effects are (1) focussing of energy by a fault along the axis of the Hollywood basin and (2) focussing effects related to differential refraction of seismic rays across the basin.

California↗
USGS1999

Liquefaction and soil failure during 1994 Northridge earthquake

The 1994 Northridge, Calif., earthquake caused widespread permanent ground deformation on the gently sloping alluvial fan surface of the San Fernando Valley. The ground cracks and distributed deformation damaged both pipelines and surface structures. To evaluate the mechanism of soil failure, detailed subsurface investigations were conducted at four sites. Three sites are underlain by saturated sandy silts with low standard penetration test and cone penetration test values. These soils are similar to those that liquefied during the 1971 San Fernando earthquake, and are shown by widely used empirical relationships to be susceptible to liquefaction. The remaining site is underlain by saturated clay whose undrained shear strength is approximately half the value of the earthquake-induced shear stress at this location. This study demonstrates that the heterogeneous nature of alluvial fan sediments in combination with variations in the ground-water table can be responsible for complex patterns of permanent ground deformation. It may also help to explain some of the spatial variability of strong ground motion observed during the 1994 earthquake.

Journal of Geotechnical and Geoenvironmental Engin↗
USGS1991

Off-fault ground ruptures in the Santa Cruz Mountains, California: Ridge-top spreading versus tectonic extension during the 1989 Loma Prieta earthquake

The M s 7.1 Loma Prieta earthquake of 18 October 1989 produced abundant ground ruptures in an 8 by 4 km area along Summit Road and Skyland Ridge in the Santa Cruz Mountains. Predominantly extensional fissures formed a left-stepping, crudely en echelon pattern along ridges of the hanging-wall block southwest of the San Andreas fault, about 12 km northwest of the epicenter. The fissures are subparallel to the San Andreas fault and appear to be controlled by bedding planes, faults, joints, and other weak zones in the underlying Tertiary sedimentary strata of the hanging-wall block. The pattern of extensional fissures is generally consistent with tectonic extension across the crest of the uplifted hanging-wall block. Also, many displacements in Laurel Creek canyon and along the San Andreas and Sargent faults are consistent with right-lateral reverse faulting inferred for the mainshock. Additional small tensile failures along the axis of the Laurel anticline may reflect growth of the fold during deep-seated compression. However, the larger ridge-top fissures commonly have displacements that are parallel to the north-northeast regional slope directions and appear inconsistent with east-northeast extension expected from this earthquake. Measured cumulative displacements across the ridge crests are at least 35 times larger than that predicted by the geodetically determined surface deformation. These fissures also occur in association with ubiquitous landslide complexes that were reactivated by the earthquake to produce the largest concentration of co-seismic slope failures in the epicentral region. The anomalously large displacements and the apparent slope control of the geometry and displacement of many co-seismic surface ruptures lead us to conclude that gravity is an important driving force in the formation of the ridge-top fissures. Shaking-induced gravitational spreading of ridges and downslope movement may account for 90¿ or more of the observed displacements on the linear fissures. Similar fissures occurred in the same area and elsewhere near the San Andreas fault during the predominantly right-lateral 1906 San Francisco earthquake and suggest that the Loma Prieta ground ruptures may, in large part, be independent of fault kinematics.

California↗