Fly over the Hollywood fault zone

By Times Staff

May 6, 2014

The California Geological Survey released a preliminary map of a section of the Hollywood fault in January. The state may make adjustments to the final map, to be released later this year, after hearing appeals from building owners. When the map is made official, construction will be allowed in the state-designated earthquake fault zone only if owners prove new buildings won’t be on top of the fault. (Update: View the final map here, released in November 2014.)

Existing buildings are not affected by the law unless owners plan substantial renovations.

About the map

The lines in this video depict the California Geological Survey’s preliminary map of the Hollywood fault.

The exact location of the Hollywood fault has not been established in many areas. The lines are the California Geological Survey's estimate of the fault's location, with varying levels of confidence.

The lines were mapped primarily to create an earthquake fault zone under state law. Extensions of the fault could be anywhere in the larger fault zone, bordered by blue and white.

Further studies could later show a different path for the fault. Only detailed underground studies, such as digging a trench, can show whether a fault exists or is absent underneath a specific property.

The video shows fault lines in three colors representing the type of evidence state geologists used to establish their locations.

Areas where state geologists think faulting exists but have no specific evidence for it are shown as gaps in the fault line.

Approximately located fault traces are based on projection from better-located traces or geomorphic features indicative of faulting. They may also be shown where a fault is interpreted from abrupt contrasts between two or more soil features, on or below the surface. These traces are most often interpolated — drawn between two better-defined locations — but are sometimes extrapolated by continuing a better-defined line that has a distinct trend. The error in their location could be greater than the line shown on the map.

Inferred fault traces are based on less-distinct geomorphic features or more widely spaced or ambiguous outcrop contrasts. They are more commonly based on extrapolation of a fault line than interpolation. The true fault pattern may be more complex or discontinuous than shown. Related faulting could occur anywhere within the boundaries of the earthquake fault zone.

Concealed fault traces are usually indicated where a fault is strongly believed to exist but the precise surface location is not possible to know because of overlying young sediments — usually young stream alluvium or an alluvial fan. Concealed traces usually connect better-located fault traces to either side.

Notes on the video

A higher level of certainty, "accurately located," is shown as "approximately located" because only a small stretch of the map falls within it — about 150 feet crossing Camino Palmero Street, north of Franklin Avenue.

The earthquake fault lines and zones were derived by The Times from the California Geological Survey's map. Additional sections of the fault west and east of what is depicted here could be mapped later.

Google Earth's sophisticated three-dimensional rendering is not perfect, and flickering of fault lines and zone boundaries could not be avoided in some areas.

The rendering obscures the fault line where it passes through trees and buildings. In a few segments of the video, portions of the fault line and zone fall outside the field of view due to the fixed angle and elevation of the camera view. Please refer to the state's preliminary fault map for the complete source.

How we did it

The California Geological Survey released its preliminary map of the Hollywood fault and zone overlaid on a 1981 paper map digitized into a PDF. The Times converted the PDF into a TIFF image, a format that can be imported into geographical drawing software (GIS), and affixed latitude and longitude coordinates at key points to align the paper map to modern streets.

With the map's position accurately fixed, The Times manually traced the faults and the fault zone and created a flight path and saved them as GIS files. The files were imported into Google Earth, where styles and colors were applied and a tour was created using the flight path. The tour was rendered as a movie file and imported into motion graphics software to add building and street labels.

The final movie was exported and uploaded to The Times' website where custom movie controls were built to overlay additional information and to enable users to skip to significant points in the video.

Sources: California Geological Survey, Times reporting

Credits: Len De Groot, Armand Emamdjomeh, Rong-Gong Lin II, Matt Moody, Lauren Raab, Doug Smith, Rosanna Xia/Los Angeles Times