Geological & Seismic Hazards

Local Geologic Conditions

Monterey Park lies within a geologic region referred to as the Los Angeles Basin. The geology forming the basin is complex, comprised on several mountain ranges and hill formations and intervening valleys. Monterey Park's distinctive Repetto Hills form part of the San Gabriel Valley's southern boundary and provide evidence of significant geologic structures. Geologic formations underlying the city consist largely of ancient marine and river deposits characterized by sandy and clay like soils.

On the level ground in northeast Monterey Park, these soil types do not pose any significant development constraints. In hillside areas, however, the soils can be unstable and susceptible to sliding. Beginning with the first hillside development in the 1920s, builders have encountered challenges and the need to utilize careful grading and slope stabilization techniques. Despite such efforts, the city has experienced several episodes of slope failure during significant rainstorms and as a result of earthquakes. Past experiences led the city to implement ever more stringent grading and slope stabilization regulations, so that slope failure no longer represents a significant public safety concern.

Seismic Hazards

Southern California lies on the edge of the Pacific Plate, one of the many jigsaw puzzle pieces that fit together to comprise the Earth's crust. The constant shifting, pushing, and shoving of these crustal plates, together with the complex interfacing of many varied geologic structures, create ruptures and crustal weaknesses geologists term faults. Movement along a fault releases stored energy and tension, thereby producing earthquakes.

Faults and Earthquakes

Monterey Park lies within a region with several active faults and therefore is subject to the risks and hazards associated with earthquakes. Figure SCS-2 (PDF) shows the geographic relationship of the city to surrounding active and potentially active faults. No active faults have been identified at the ground surface within city limits, nor have any Alquist-Priolo Earthquake Fault zones[1] been designated. However, the city overlies a number of blind thrust faults. The faults are referred to as blind because they do not intercept the ground surface and therefore cannot be detected visually. These northwest dipping low, angle faults have been named the Puente Hills thrust, the Elysian Park Figure SCS-2 Earthquake faults thrust, and the East Los Angeles thrust (shallowest to deepest). The faults are capable of movement which could produce substantial ground shaking.

As discussed above, ground shaking potential can be expressed qualitatively using the Modified Mercalli Scale or quantitatively by the PHGA (peak horizontal ground acceleration). The PGHA value is calculated based upon the so-called maximum credible earthquake, or the seismic event considered likely to occur on an active fault affecting the city. In Monterey Park, the Los Angeles segment of the Puente Hills blind thrust fault represents the controlling force for calculating the PGHA. Assuming a magnitude 6.5 earthquake on this fault, the best PGHA estimate is 0.5 g for loose soils (alluvium) and 0.55 g for bedrock. This level of ground shaking translates to an approximate Modified Mercalli Scale intensity of IX for the entire city.

Earthquakes Along Faults

A major earthquake produced along any of the regional fault systems shown on Figure SCS-2 also has the potential to produce strong ground shaking in Monterey Park. Experience from the Whittier Narrows (1987) and Northridge (1994) earthquakes has shown that ridge top locations in the city and locations near the margins of alluvial basins may be susceptible to elevated levels of ground shaking.

Seismic risks associated with both regional fault systems and the local blind thrust faults underlying Monterey Park emphasize the need to ensure that all new development projects and the retrofit of existing structures, incorporate appropriate design features to guard against widespread property damage and loss of life in the event of an earthquake.

Seismic-related Hazards

Local geologic conditions can create additional hazards associated with seismic activity. Unstable soils on steep slopes may fail under the stress of a tremor. In locations where high groundwater levels interact with loose, unconsolidated soils, a condition called liquefaction can occur, whereby such soils lose cohesion - and their ability to support structures - when subjected to strong ground motion.

Historically, hillsides in Monterey Park have experienced slope failure due to earthquakes. In particular, steep hillslopes along Abajo Drive failed as a result of the 1987 Whittier Earthquake and have continued to present concerns and threats to private properties and public streets. Liquefaction does not represent a hazard in Monterey Park because groundwater levels are low. Figure SCS-3 (PDF) shows areas susceptible to seismically induced liquefaction and landslides in Monterey Park.

Prior adoption of stringent grading and building codes and slope landscaping/maintenance requirements addressed these problems. In addition, the city requires soils studies and remediation for any problem prior to issuance of permits.

Footnote

The Alquist-Priolo Earthquake Fault Zoning Act (Public Resources Code, Section 2621, et seq.) was adopted by the State legislature to provide for the mapping of surface traces of known active faults and to restrict development within so-called Alquist-Priolo Earthquake Fault zones.