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Southern California Geology


Crustal Strain and Crustal Structure in Southern California

Crustal Structure Studies

The composition and geologic structure of the Earth's crust beneath southern California-the 30 or so kilometers (20 miles) of rock between the surface landscape and deeper levels of Earth's interior-need to be understood for two reasons:


  • Ultimately, geologic processes that occur in the crust determine the position and orientation of geologic structures like folds and faults observable at the surface, and especially their counterparts in the subsurface that are "blind"-that is, they have not propagated to the surface. These geologic structures are the direct result of region-wide stress forces at work within the Earth's crust, and southern California earthquakes result when long-term accumulated stress is relieved abruptly. Thus, for purposes of understanding and mitigating geologic hazards, crustal-structure studies that clarify the physical nature and organization of deep-crustal materials are beneficial to southern Californians.
  • From a more academic perspective, the long-term geologic history of southern California-the origin of down-warped sedimentary basins, the uplift of mountain belts, the horizontal displacement of crustal blocks by faults of the San Andreas system-clearly is the result of processes at work within Earth's crust. As geologists understand this history better and especially how it has helped shape crustal structure, application of their findings and models will benefit the identification and mitigation of geologic hazards. But even without this immediate applicability, insights gained from how the southern California crust looks and operates benefit our overall understanding of southern California's geologic setting.


So, what kinds of studies can be conducted to investigate what the southern California crust looks like?


  • Geophysical studies (potential-field geophysics): these measure regional variations in the gravity and magnetic fields of southern California and thereby evaluate the distribution of different kinds of geologic materials in the Earth's crust concealed by the near-surface materials

  • Seismic-imaging studies : these use a sound source (explosion, hammer strike, vibrations) to send sound energy into the subsurface. Different geologic materials transmit sound energy at different velocities, and the boundary zones-geologic contacts, faults, transitional boundaries between materials having different acoustic properties-between different geologic materials will be zones where energy waves are refracted or reflected (echoed) back to the surface. There, a string of listening devices (geophones) stretched across the land surface records the echoes. When the numerous echo recordings are processed, synchronized, and analyzed, an image of the layered structure of geologic materials and their orientation along the line of the seismic transect can be developed. This technique also allows geologic structures (faults, folds) to be captured, as well as the depth to high-velocity "basement-rock" materials that lie beneath slow-velocity sedimentary materials.

The Los Angeles Regional Seismic Experiment (LARSE) is an ongoing example where both potential-field geophysics and seismic imaging are being used to understand crustal composition and structure. LARSE is conducted along transects that extend from the offshore southern California Borderland onshore through the Los Angeles Basin and the Transverse Ranges to the Mojave Desert. Use links from the LARSE Homepage to examine the LARSE offshore transect and its connection to the initial onshore transect results. Preliminary online LARSE reports, include:


  • Data report for the 1993 Los Angeles Region Seismic Experiment (LARSE93), Southern California: a passive study from Seal Beach northeastward through the Mojave Desert
  • Multichannel seismic-reflection profiling on the R/V Maurice Ewing during the Los Angeles Region Seismic Experiment (LARSE), California

Ultimately, crustal-structure studies in southern California will contribute to a global model of what the Earth's crust looks like in terms of its composition, structure, and geologic age. The U.S. Geological Survey has a Global Crustal Data Base that is directed toward this effort. Navigate to the Geologic Provinces component of this data base to see how the crust of southern California fits in to the rest of North America; in addition, navigate to a Global Thermotectonic Map that shows the latest crustal-scale thermal (heating) and deformational events to affect southern California and the rest of North America.

Selected References


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This site last updated September 3, 2004 (ps)

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