19.9: Detailed Figure Descriptions

19.1

The fault offset from the 1872 Lone Pine earthquake. The area of rounded granitic boulders to the left has more-or-less stayed in position, while the area to the right has dropped down along a normal fault (in which the hanging wall moves down as a response to tectonic extension).

Normal faulting is typical for Basin & Range movements, however Basin & Range tectonics also encompass significant side-to-side movements as well. A common misconception is that the basins in Basin & Range are best thought of as grabens, while the ranges are horsts, as in the horst-graben tectonic model; however, this is too simplistic in that the downdropped blocks also rotate and are displaced by strike-slip movements.

19.1.1

An unreinforced brick wall in a multistory building in downtown Oakland, at the corner of 19th and Webster Streets, in an area noted for its vulnerability to shaking and liquefaction. In other words, this sort of structure is unlikely to survive a major quake. And yet it stands now, unremarked upon except by geology students who recognize the danger, and will continue to be there until the moment when side-to-side shaking moves each brick slightly past the ones above and below, causing the wall to lose its support and crumble to the ground. This is one of thousands of such vulnerable buildings and yet almost nothing is being done to address the hazard.

Anyone below or near this structure when it inevitably collapses will certainly be hurt or killed.

19.1.2

A building in the San Francisco Marina District destroyed by the 1989 Loma Prieta quake. This is a soft-first story building, meaning that the lowest level is the weakest. As you can see in the picture, the lowest level is parking garages (parking being a particularly difficult problem in the Marina District, having a designated parking spot is a practical requirement for living there). The openness of the lowest level significantly weakens the building's ability to respond to earthquakes.

Pictures of this building were so widely circulated following the 1989 quake that it gave the impression of vast, universal damage to the Marina District; however, the reality is that most of these pictures were of this one spectacularly-leaning building.

19.1.3

An extant apartment complex in Oakland built with a soft first story, in order to accommodate parking underneath. Note too that the support columns and walls appear to be constructed of masonry bricks rather than a solid support such as a metal poles. This configuration is less than ideal for a region that will experience major earthquake shaking. We can expect that even moderate side-to-side earthquake movement will weaken these supports, up to the point where the entire second level collapses to the street.

19.2.1

Population density in California. Some of the state has concentrations of high populations, and most of the state (Modoc, Mojave, north coast, central coast, Owen's Valley) has very low populations. Rather than being evenly distributed, high populations are concentrated in just a few places: the greater Los Angeles region, the Sacramento area, and the Bay Area.

19.2.2

Historic seismicity in California, to the same scale as 19.2.1. Note that earthquakes are concentrated in just a few places in California--but that some of these places are the same as the population centers (Los Angeles, SF Bay). This intersection of earthquakes and population is a recipe for disaster. If the state had developed with a more even population distribution, then the earthquake risk would also be evenly distributed; however, in this configuration most everyone in the state lives in seismically dangerous area.

There are a few seismically active areas with small populations, such as the north coast near the Mendocino Triple Junction, the Mammoth region, the southern Sierra Nevada-Owen's Valley area.

19.2.3

The intensity of the 2019 Ridgecrest quake. Coding indicating "severe/violent" shaking corresponding to peak ground accelerations between 40-74% of the gravitational acceleration constant. Areas of "strong" and "moderate" shaking surround the vicinity of Ridgecrest. Even a few tens of kilometers from the epicenter, the shaking drops to "very light" and "weak" scales. This emphasizes how quickly shaking diminishes away from the epicenter, and the importance of having population centers as far away from faults as possible; however, in California, we've done exactly the opposite.

19.2.4

Major faults in Southern California, indicated in red. The San Andreas moves diagonally from northwest to southeast. This emphasizes is just how many faults there are in California; however, keep in mind that there are many more than are shown at the scale of this map. There are truly few places in California where one is very far from recognized faults. Those places tend not to be population centers; rather, the areas of great fault concentrations are also among the most heavily populated in California.

19.2.5

An intersection of population and faults from Figures 19.2.3 and 19.2.4, showing how population centers intersect with recognized faults.

19.2.6

The San Andreas fault system as it branches underneath the Los Angeles basin with thrust faults such as the Sierra Madre and Whittier Narrows. There are many other thrust faults, and probably a significant number that we do not know about yet, but which will announce their arrival by creating massive damage in areas far from the actual San Andreas. This problem of thrust faults is one of the most dangerous aspects about where we have built and where the hazards lie.

19.2.7

The thrust faults mentioned in 19.2.6, to scale. The San Andreas fault is far from population centers. But like the trunk of a tree, it has many branches, and these branches are the main danger for people living in Los Angeles. For instance, the Whittier and Sierra Madre Thrust Faults branch off from the vertical San Andreas and are tens of kilometers away from that main trunk. However, the Whittier and Sierra Madre faults underlie heavily populated regions in the greater Los Angeles basin, meaning that even moderate seismic activity on them will have big consequences for the people living there.

19.2.8

The collapse of the John Muir School in Long Beach from the 1933 Long Beach quake. Note the extensive amount of building facing that has collapses onto the ground level, which had the quake occurred earlier in the day would have been filled with students. So many schools were similarly damaged that it led to the Field Act, one of the first pieces of California legislation meant to reduce the risk from earthquakes.

19.2.9

A collapsed freeway section, from the 1994 Northridge quake. The two halves of the elevated section have separated, and one has collapsed to the ground. A great deal of the transportation infrastructure in the Los Angeles area was severely damaged by this quake.

19.3.1

Soil type and associated seismograms from the Oakland-Alameda area during the 1989 Loma Prieta quake. Note that the bedrock shows the least amount of disruption, while the natural sand and gravel shows more, and the artificial fill of soft mud shows the greatest amount of movement. This corresponds to the dangers of living on each soil type.

The center shows the collapse of Interstate 880 in Oakland, which killed 42 of the 63 people who died in this quake.

19.4.1

US mainland earthquake risks, with one significant cluster near New Madrid, MO, and then very little risk outside of California, all of which is listed has having major likelihoods of damaging quakes. This emphasizes that for the contiguous United States, California has an exceptional risk of future seismic danger. Texas, according to this map, has only a 4% change of a damaging earthquake in the next 100 years, while California has a >74% chance.

19.4.2

Bay Area faults with associated percentage likelihoods of creating M_w6.7 or greater quakes by 2043. The two greatest risks come from the Hayward fault (33%) and the San Andreas (22%). Other lesser known faults, such as the San Gregorio fault (shown in green) have only a 6% chance of producing a M_w6.7 or greater quake by 2043.

Figure 19.5.1 Map of the Cascadia Subduction Zone

The seaward edge of the subduction zone, where the subducting plates begin the descent beneath the North American plate extends from Cape Mendocino, south of Eureka into Canada off the coast of Vancouver Island. The stuck or “locked” part of the interface between North America and the subducting plates, which is the fault that breaks in great earthquakes, extends from nearly the southern extent of the seaward edge of the subduction zone in the south to part-way up the coast of Vancouver Island in the north.

Figure 19.5.2 Land Subsidence and Sand Deposit Following a Tsunami

The sequence of events shown begins with a marsh that is present before the earthquake event. The land subsides during the earthquake and a sand-laden tsunami overruns the subsided landscape within minutes to hours after the earthquake. This leaves behind a sand sheet. Centuries after the earthquake the sand sheet becomes buried by tidal silt and eventually the marsh returns. A photograph shows an example of the resulting three layers of sediment: marsh peat, covered by tsunami sand, then tidal silt. The whole sequence is approximately 15 cm (6 inches) thick.

Query Box 19.5.1 Tsunami Hazard Maps Change With Updated Science

The purpose of including this image comparison is to show how tsunami hazard maps can change as scientists refine their understanding of the hazard. Crescent City is intended as an example only and therefore many of the details in these maps, such as street names, etc. are not necessarily relevant to this context. The important information that users should ascertain from the comparison is that more of the area of Crescent City is included in the 2021 tsunami hazard zone than was previously included in the 2010 version. The expanded hazard zone includes areas very near the city's hospital, as well as very near the Elk Valley Rancheria tribal headquarters. There are also some areas that were previously in the hazard zone and are no longer, but this represents a much smaller area overall. Fortunately, the middle school and high school remain safely outside the tsunami hazard zone.

Video 19.5.1 Tsunami Forecast Animation: Cascadia 1700

The video shows a wave propagating across the Pacific Ocean. The wave reaches southern California within 1 to 2 hours, but wave heights there never exceed 3 meters (about 10 feet). The first wave hits Hawaii about 5 hours after the earthquake and tsunami heights in some places in Hawaii are shown as as much as 3 meters (about 10 feet). The first tsunami reaches Japan in about 10 hours, but the maximum tsunami height doesn’t arrive until a few hours later, with maximum heights between 1 and 3 meters (about 3 to 10 feet).

Figure 19.5.4 California Magnitude 4.5 Earthquakes

Earthquakes of magnitude 4.5 or higher, which have occurred in the decade between July 2014 and July 2024 in California and surrounding areas are shown on a map. A few earthquakes occur individually, and some occur in clusters. One large cluster occurs near the Menocino triple junction and along the Mendocino fracture zone, as well as near the Gorda ridge. Most earthquakes on this map are of magnitude smaller than 5.

Figure 19.5.5 Earthquakes in the Juan de Fuca Plate

The tectonics of the Pacific Northwest are described elsewhere in this text (see 3.4: California's Plates and Plate Boundaries and 7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc). This figure is included here to show the location of the Gorda plate, which is located south of the main part of the Juan de Fuca plate and makes up the portion of plate that is subducting at the Cascadia subduction zone beneath California. This figure also shows the locations of earthquakes deeper than 25 km, which are clustered in northern California, the Olympic Peninsula and Vancouver Island (the ends of the subduction zone), and are largely absent from Oregon (the middle of the subduction zone).

Video 19.6.1

An NSF/IRIS description of earthquake prediction and probability, part 1 of 2.

Video 19.6.2

An NSF/IRIS description of earthquake prediction and probability, part 2 of 2.

Video 19.6.3

The long-awaited Parkfield quake shakes the ground.