Earthquake Cracking Ground

[Julia Kozub]

LocalEffectsTable of ContentsGeologic HazardsUSGS Response to an Urban Earthquake -- Northridge '94The Causes and Effects of Liquefaction, Settlements, and Soil FailuresThe Northridge earthquake caused numerous ground-surface failures acrossa broad area of southern California. Failures occurred from the southern parts of Los Angeles County to the KernCounty line on the north, and from coastal Ventura County on the west to the southeastern parts of Los AngelesCounty. These failures included zones of ground fissures and extensional cracking, lateral displacements, settlementswith vertical displacements, and compressive deformation in the form of soil and pavement warps and buckles. Suchfeatures are common effects of strong earthquakes and warrant scrutiny because of the severe property losses theyinflict. USGS investigators focused on documenting the extent and nature of ground failures, analyzing the impactsof ground failures on the built environment, and conducting field investigations to clarify the mechanisms of failure.

Although ground failures were scattered throughout the Los Angeles region, most occurred nearthe epicenter and above the fault- rupture plane (see p. 13). Failures in artificial fills occurred up to 57 kilometersfrom the epicenter. USGS scientists investigated several areas in great detail to understand the mechanisms ofthe failures.

Ground failure is used here to describe zones of ground cracking,fissuring, and localized horizontal and vertical permanent ground displacement that can form by a variety of mechanismson gently sloping valley floors. Landslides and rockfalls that occur on steep hillside slopes are discussed separatelybeginning on p.44. In general, ground failure may be caused by (1) surface rupture along faults, either as a primaryrupture on the seismogenic fault or as a sympathetic rupture; (2) secondary movement on shallow faults;(3) shaking-induced compaction of natural deposits in sedimentary basins and river valleys, or artificial fills;and (4)liquefaction of loose sandy sediment.

Earthquakes and Ground FailuresWhen large faults rupture and produce earthquakes, they generally deformthe ground surface. Primary surface faulting, such as the 22-kilometer-long surface rupture associated with the1971 San Fernando earthquake, is the direct effect of movement on a seismogenic, or earthquake-producingfault. Rupture on nearby faults induced by the primary event (sympathetic rupture) may also produce surfacefaulting. Earthquakes can also produce secondary features that look similar to primary surface rupture.Primary features related to known or suspected faults can be readily studied by geologists and directly linkedwith earthquake activity on those faults, while secondary features may be difficult to link to activity on a particularfault. However, studies of secondary features can provide information on the effects of earthquake shaking at selectedsitesextremely important information for seismic-hazards evaluation that cannot be directly obtained from studyingseismogenic faults alone.Failures Associated with Faults and FoldsNo evidence of primary surface rupture associated with the seismogenicfault was found. Surface ruptures at Potrero Canyon and Stevenson Ranch were located along the hinge of a foldnear where the seismogenic fault projects to the ground surface, but neither occurrence appears to be associatedwith the seismogenic fault. The Stevenson Ranch ruptures, a 320-meter-long zone of bedding-plane faulting thatdisplaced building pads a maximum of 19 centimeters, appear to be related to folding above the blind thrust fault.In contrast, trenching and strain studies of the 3-kilometer-long belt of ground cracks that formed within PotreroCanyon indicate that they are shallow features formed in sediments along the margins of the valley. Trenching studiesindicated mechanisms of shallow failure by shaking rather than by secondary tectonic deformation.

The most extensive belt of 1994 ground failures overlies the Mission Hills fault zone in the Granada Hills-MissionHills area. Ground cracks displaced the foundations of houses, fractured swimming pools, broke apart sidewalksand streets, and ruptured utility lines within a belt 5 kilometers long and several hundred meters wide. The deformationwas concentrated in certain areas within the belt and formed complex associations of ground and pavement cracks,settlements, compressional features such as buckled pavement and tented sidewalks, and offsets of sidewalks andcurbs. The deformation belt nearly abuts the western end of surface faulting produced by the 1971 San Fernandoearthquake. The area of 1994 ground cracks showed that there had been little permanent surface deformation duringthe 1971 earthquake; however, some minor cracks were reopened in 1994.

This view, taken the morning of the earthquake, looks west across Balboa Blvd. A zone of extensional groundfailure runs approximately from the upper left to the lower right corners of the photograph. Ground failures rupturedtwo major water lines and two gas lines beneath the street. Several minutes after the earthquake, a spark fromthe pickup truck (center) ignited leaking natural gas, and produced a fireball that burned five houses to the groundand seriously damaged a sixth.

The most intensive ground deformation and damage in the Granada Hills-Mission Hills area occurred near BalboaBlvd., where several zones of tension cracks and a zone of compression (expressed as crumpled pipelines and crushedpavement) formed in fine-grained alluvium. USGS mapping, trenching, and strain studies in this region showed thatthe ground shifted downslope as much as 50 centimeters between the zones of ground extension and compression, andthat the ruptures were shallow features consistent with shaking-induced ground failure.

Subsurface investigations also revealed evidence for a near-surface fault at the southern margin of the deformationzone near Balboa Blvd. This strand of the Mission Hills fault zone apparently was not active during the earthquake,nor does it appear to have been significantly active in the last 10,000 years. It appears to localize ground failureby acting as a ground-water dam in the Balboa Blvd. area. To the east, several zones of ground cracks that coincidewith stream courses filled for housing developments also lie along the fault, as do linear zones of tension cracksthat are observed in bedrock in the Mission Hills. The bedrock cracks are probably from minor landslide movement,although the possibility of fold-related secondary faulting cannot be ruled out entirely.

Lessons Learned

Faults and fold axes appear to be important in localizing ground failure,although the mechanisms that cause ground failure near fault zones can vary. Some secondary tectonic movement associatedwith folding above the blind thrust may have occurred locally, but the effects of these failures are minimal.Cracking in Natural GroundWithin the San Fernando Valley, ground cracks that resembled liquefaction-relatedlateral spreads and settlements were widespread. Liquefaction could not be confirmed, however, because sand didnot vent to the surface. Failures in nonliquefied areas commonly were underlain by both fine-grained sediments(silt and clay) and ground water at depths of less than 10 meters. USGS scientists studied two sites in detailMaldenSt. and Wynne Ave.. Near Malden St., a 500-meter-long by 20-meter-wide zone of tension cracks and settlements brokesewers and water pipelines and damaged the foundations of several homes, streets, curbs, and utilities. Curbs andgutters were displaced 6-10 centimeters, and maximum vertical settlement was about 20 centimeters. At its eastend, the Malden St. zone trended northwesterly and expressed a right-lateral displacement in curbs amounting toabout 17 centimeters. The failures were shallow and resulted from downslope movement of nearly 30 centimeters,consistent with shaking-induced mechanisms of ground failure. Ground failure at the Wynne Ave. site ruptured bothwater and sewer lines, and consisted of a zone of settlement about 150 meters long and 12 meters wide. Permanentvertical offset across cracks ranged from 10 to 20 centimeters.

Lessons Learned

The Northridge earthquake produced widespread ground failures in areas ofthe San Fernando Valley underlain by both ground water and fine-grained sediment (silt and clay) at depths of lessthan 10 meters. This empirical association provides a broader basis for identifying areas subject to ground-failurehazards than by using liquefaction susceptibility. The association of materials and ground-water depths can beused to identify general areas where site-specific studies of potential ground failure may be advisable.Fill FailuresThe most widely distributed ground failures appear to be associated withareas of filled land. Some occurrences were clearly the result of liquefaction, while others lacked direct evidencefor liquefaction. In nearly all cases, however, depth to ground water was less than 10 meters, and any associatednatural deposits were geologically recent.

Fill failures in the San Fernando Valley occurred along the channelized course of Bull Creek, which at manylocalities had been filled for development. Settlements at John F. Kennedy High School and on Odessa Ave. northof Rinaldi St., caused significant damage to structures and buried utilities. Several streets near the epicentralarea exhibited settlement and cracking over known storm drains, and other streets nearby may have been locatedover old filled drainages. A small number of ground failures occurred along the Pacific Coast from Santa Monicato the Port of Los Angeles. These were mainly associated with poorly compacted sandy fills north of the Santa MonicaPier and near Marina Del Rey. In Redondo Beach, a quay wall moved more than 5 meters laterally due to liquefaction.Overall, however, ground failure in the coastal areas was modest. Ground failure concentrated in the southeasternportion of Simi Valley also could be traced to failure of fills, especially fills placed along Simi Arroyo andalong the former courses of its tributaries. Ground failures commonly appeared to involve cracking and differentialsettlement of fills and occasional liquefaction in underlying alluvial deposits. The failures caused significantdamage to structures and buried utilities.

3a8082e126