Chapters 1 - U.S. Fish and Wildlife Service

Refuge Resources
Case Road Pond and along the western edge of and to the southeast of the 7 th Street Pond appear
to be overlain with Bolsa silt loam, drained. This soil type generally occurs on large alluvial fans.
A geotechnical investigation conducted in association with the Port of Long Beach restoration
project for the Forrestal, Case Road, and 7 th Street Pond restoration sites found that there was
considerable variation in soil conditions within and between each of the three sites. Generally,
subsurface conditions consisted of clean fine to coarse sand to clayey silts and silty clays (Moffatt &
Nichol, Engineers 1987). The Refuge office and native plant garden are located primarily on
Myford sandy loam, which is formed from sandy sediments and generally occurs on broad terraces
(USDA 1978).
Anaheim Bay is part of a physiographic region known as Sunset Gap, which is mostly flat and
typical of southern California’s coastal floodplains. However, within this flat coastal plain is Hog
Island, a natural upland area that is part of a dissected scarp of the Newport-Inglewood Fault
(Lane and Woods 1975). This fault runs parallel to the coast and acts as a hydraulic barrier to
lateral ground water movement.
The active Newport-Inglewood Fault Rupture Hazard Zone is a system of right-lateral strike-slip
faults that runs northwest through the Refuge (refer to Figure 4-9). This fault zone is associated
with the San Andreas system. The estimated potential size of an earthquake along the Newport-
Inglewood fault zone ranges from magnitude 6.0 to 7.4. The last major earthquake on this fault,
the Long Beach earthquake, was a magnitude 6.3 quake that occurred in March 1933. Another
fault zone in the vicinity of the Refuge is the Palos Verdes Fault Zone, which lies 8.5 miles offshore
to the southwest.
A serious earthquake hazard exists from the proximity of these fault lines to the Refuge. The
resulting damage could be exacerbated by potential liquefaction near the coast. Liquefaction
occurs when saturated soils develop a fluid consistency. Liquefied sediment loses strength and
may fail, potentially causing damage to buildings, bridges, walls, and other structures.
Historically, liquefaction-induced ground failure has been a major cause of earthquake damage in
southern California (DMG 1998). During the 1971 San Fernando and 1994 Northridge
earthquakes, significant damage to roads, utility pipelines, buildings, and other structures was
caused by liquefaction-induced ground displacement.
The Seismic Hazards Mapping Act of 1990 directs the California Department of Conservation
(DOC), Division of Mines and Geology (DMG) to delineate Seismic Hazard Zones (DMG 1998).
The map produced by DMG for the Seal Beach quadrangle shows that the entire Refuge is within a
liquefaction zone, that is, an area “where historic occurrence of liquefaction, or local geological,
geotechnical and groundwater conditions indicate a potential for permanent ground displacements
such that mitigation...would be required” (DMG 1998).
Another factor affecting the Refuge is subsidence. Subsidence of shallow marine sediments can
occur as a result of groundwater extraction, oil extraction, or tectonic activity. Monitoring studies
conducted between 1968 and 1994 have documented both subsidence and rebound in the vicinity of
the Refuge (U.S. Navy 2011). These studies were conducted due to concerns over possible impacts
to habitat quality on the Refuge as a result of subsidence. Subsidence alters the natural elevations
within the marsh plain which can have significant adverse effects on the vegetative communities
within the marsh, particularly cordgrass-dominated salt marsh habitat (USFWS 1987). As the
natural elevations are lowered, the depth of the tides relative to the height of the cordgrass
increases. This situation can adversely affect the health of the vegetation and the recovery of the
light-footed clapper rail, which relies on this vegetation for cover and nesting.
Draft Comprehensive Conservation Plan/Environmental Assessment 4-13