The seismic microzonation process in Long Beach relies on an array of borehole seismometers and surface accelerographs deployed across the city’s distinct geological units, from the Mesa terrace deposits to the deeper Los Angeles Basin sediments. These instruments record ambient noise and small earthquakes over extended periods, feeding data into a geophysical model that maps shear-wave velocity profiles down to 30 meters and deeper. Long Beach’s position straddling the Newport-Inglewood Fault and its proximity to the Palos Verdes Fault Zone mean that standard code values often underestimate site amplification. For critical structures in the downtown shoreline area, we combine MASW surveys with downhole seismic testing to refine the site classification beyond a simple Site Class D or E, capturing impedance contrasts that a single Vs30 value would miss.
A single Vs30 value for a Long Beach coastal site can mask a low-velocity layer that doubles spectral acceleration at the structure’s fundamental period.
Common questions
What is the typical cost range for a seismic microzonation study on a single parcel in Long Beach?
For a site-specific seismic microzonation study on a single lot in Long Beach, costs typically fall between US$4,400 and US$18,010 depending on the number of boreholes required, the depth of investigation, the complexity of the geophysical array, and whether nonlinear time history analyses are needed. A small commercial site with one deep boring and MASW lines will be at the lower end, while a multi-acre development requiring 2D basin response modeling and deaggregation of multiple fault sources will approach the upper range.
Does a seismic microzonation study replace the standard geotechnical investigation for a building permit in Long Beach?
No, it does not replace the standard geotechnical investigation. The microzonation study provides the site-specific ground motion parameters—design response spectrum and acceleration time histories—that the structural engineer uses for seismic design. The geotechnical investigation addresses bearing capacity, settlement, grading, and foundation recommendations. Both reports are submitted to the City of Long Beach Building and Safety Bureau as complementary documents for permit review.
How is the shear-wave velocity profile determined for a Long Beach site with deep alluvial sediments?
We determine Vs profiles through a combination of surface wave methods (MASW or MAM arrays with 24- to 48-channel seismographs) and downhole seismic testing in a cased borehole. For Long Beach sites where the alluvium extends beyond 100 feet, the downhole method provides point measurements at 5-foot intervals to the base of the borehole, while the surface wave data constrain the deeper velocity structure to several hundred feet. The combined inversion yields a Vs profile that satisfies the ASCE 7-22 requirement for characterization to rock with Vs ≥ 1,000 m/s.
What return period ground motions are required for a school or essential facility in Long Beach?
For Risk Category IV structures such as schools, hospitals, and fire stations in Long Beach, the California Building Code and ASCE 7-22 require design for the Maximum Considered Earthquake (MCE₂) ground motion with a 2,475-year return period and a deterministic cap based on the characteristic earthquake magnitude of the Newport-Inglewood Fault. The site-specific analysis must also evaluate the 975-year return period motion for operational performance criteria. The microzonation report provides both sets of spectra so the structural engineer can meet the enhanced performance objectives required by the Division of the State Architect for essential facilities.
