Earthquake Hazards

Faults and Earthquake Geology

To understand the risk that different areas of the U.S. face for earthquake hazards, we need to know where faults are and how they behave. We know a fault exists only if it has produced an earthquake or it has left a recognizable mark on the earth’s surface. Once a fault has been identified, the next step is to determine how it behaves.

    Overview

    Scientists are studying faults and their behaviors in various regions of the U.S. and in other parts of the world. Specific areas of study in the U.S. are divided into these regions: Alaska, the Pacific Northwest, Calfornia, the Intermountain West, and Central and Eastern U.S.

    Peter Haeussler measuring the offset of a crevasse

    Peter Haeussler prepares to measure the offset of a crevasse on the Canwell Glacier, Alaska, USA. Photo by Peter Haeussler, USGS, November 9, 2002. (Public domain.)

    Alaska

    Alaska has more large earthquakes than the rest of the United States combined. More than three-quarters of the state’s population live in an areas that can experience a magnitude 7 earthquake. Moreover, all the state's infrastructure centers are located in seismically vulnerable areas. The trans-Alaska pipeline transports about 17% of the Nation’s crude oil, and there are significant federal land holdings and military facilities in earthquake-prone regions. A trans-Alaska natural gas pipeline is likely to be constructed in the next 5-10 years, and all the proposed routes traverse or parallel active fault traces. The ability to prepare for and mitigate the effects of future earthquakes is critical to maintaining the economic health of the region and Nation. In Alaska scientists are investigating the processes of earthquake generation on major fault systems throughout the state and along the southern margin, which will increase our general understanding of fault systems that can generate large earthquakes. This work is also examining relationships between the faults and active volcanoes. The scientific work is helping to develop a chronology of ancient earthquakes on different parts of some of the major fault systems in Alaska. A better understanding of earthquake hazards in Alaska is vital to the economic health and well being of Alaska.

    Major faults in the Pacific Northwest urban corridor map

    Major faults in the Pacific Northwest urban corridor (Public domain.)

    Pacific Northwest

    Scientists are conducting investigations across the Pacific Northwest with a strong emphasis on understanding the earthquake hazards in the heavily populated urban corridor from Eugene, Oregon to Vancouver, British Columbia and across the Yakima Fold and Thrust belt, where significant infrastructure is located. In this area, where the oceanic tectonic plate is diving under the continental plate, hazards can come from:

    • earthquakes occurring within the shallow continental crust
    • earthquakes within the subducting oceanic slab
    • earthquakes along the interface between the subducting oceanic slab and the overlying continental crust
    • tsumanis from local and distant sources

    Scientists are characterizing the hazards posed by these three earthquake source zones, as well as the hazards posed by volcanoes, with the goal of helping the region develop effective mitigation strategies.

    In urban areas of the Pacific Northwest, research is helping to develop products, such as seismic hazard maps, that are used to design and implement effective earthquake mitigation strategies. These products are based on extensive new geologic, geophysical, and seismological investigations that document active crustal and interplate faults and prehistoric earthquake magnitudes and recurrence intervals, and estimate site response and amplification.

    Research objectives in the Pacific Northwest include:

    • contribute to the development of detailed seismic hazard maps for the Seattle and Portland areas that incorporate site response, basin effects, and rupture directivity
    • document the location, geometry, and slip rates of active crustal faults in the Puget Lowland
    • develop a database of the chronology and magnitude of large prehistoric crustal and interplate earthquakes and tsunamis in the Pacific Northwest
    • formulate models for the Pacific Northwest, especially the Seattle and Portland areas, that show wave propagation and ground motion for large scenario earthquakes from diverse sources
    • incorporate new geological, geophysical, and seismological data in regional and national seismic hazard maps. The paleoseismic results of this project will be incorporated into the Quaternary fault database. The geophysical results will be incorporated into representative community velocity models.

    San Francisco Bay Area

    Earthquakes and Faults in the San Francisco Bay Area map

    From "Earthquakes and Faults in the San Francisco Bay Area (1970-2003)", Scientific Investigations Map 2848. (Public domain.)

    The San Francisco Bay Area has the highest density of active faults of any urban area in the Nation. The probability of one or more large (M6.7) urban earthquakes in the next 30 years is high, estimated at 62%. The ability to prepare for and mitigate the effects of these future earthquakes is critical to maintaining the economic and social fabric of the region. This can be accomplished through an increased understanding and characterization of the timing, size, and location of past earthquakes, which are based on paleoseismic information in addition to LiDAR. LiDAR is a remote sensing technology that measures distance by illuminating a target with a laser and analyzing the reflected light. This method is able to “see through” heavily vegetated and forested areas to the ground surface and provide topographic data in an area that would otherwise be difficult to access and collect this information. Paleoseismic and earthquake geologic studies of historical surface ruptures in a range of tectonic environments also provide critical data for evaluating general aspects of fault behavior and input into decisions on the national seismic hazard map.

    In the San Francisco Bay Area, they are doing studies to improve the knowledge of the various earthquake sources. They are trying to understand the three-dimensional nature of the fault system and the variation in size and timing of past earthquakes. Scientists are also investigating the faults outside of the Bay Area region that will increase the general understanding of the behavior of large earthquake-generating fault systems like the San Andreas Fault.

    Landsat images of faults

    Faults from Jennings, 1994; Landsat image from Jet Propulsion Laboratory of the California Institute of Technology. (Public domain.)

    Southern California

    Southern California has the highest level of earthquake risk in the United States, with half of the expected financial losses from earthquakes in the Nation expected to occur in southern California. Sitting astride the Pacific - North American plate boundary at the Big Bend of the San Andreas Fault, Southern California has over 300 faults capable of producing magnitude 6 earthquakes. Affecting the more than 20 million inhabitants of the Los Angeles and San Diego metropolitan areas, this complex set of faults presents the greatest urban risk in the United States. All aspects of the earthquake problem can be addressed in Southern California, using the modern earthquake networks that have been developed over the past decade. The high level of earthquake activity and the complexity of the fault systems in the area provides a unique natural laboratory for the study of the physics of earthquakes. Scientists are studying fault interaction by comparing the seismic behavior in southern California to analogous areas in the world with large strike-slip faults, to provide insight into possible past and future earthquakes in the region.

    Intermountain West

    Shear zone and fault-scarp-derived colluvium

    Shear zone and fault-scarp-derived colluvium exposed along the Wasatch fault zone near Draper, Utah. (Credit: Rich Briggs, USGS. Public domain.)

    Motion between the North American plate, the Pacific plate, and the remnant of the Juan de Fuca plate off the coast of the Pacific Northwest, is causing deformation throughout western North America. The majority of this deformation is occurring close to the plate boundaries in California and offshore Oregon and Washington, but significant ongoing deformation extends eastward across the Basin and Range Province and throughout the Intermountain West to the eastern front of the Rocky Mountains. Even though the rates of seismicity are commonly modest throughout the Intermountain West and the adjacent Rocky Mountain region, historical earthquakes (such as the 1983 Borah Peak, and the 1954 Dixie Valley), coupled with evidence of older surface faulting, demonstrates the existence of a significant, but poorly quantified seismic hazard. Also unknown are what effects strong shaking from an earthquake would have in most of the region's urban centers. In order to better quantify the seismic hazard of this InterMountain region, scientists are conducting geological and geophysical studies that contribute to a better understanding of the levels of hazard and risk, particularly in the more populous areas

    Scientists are determining the shapes of the basins and the types and depths of the basin deposits in order to estimate the expected shaking under the urban areas that sit in these basins. They are attacking problems at a variety of scales ranging from detailed studies of individual faults to regional studies that clarify critical details of the tectonic processes operating in the region.

    Central and Eastern US

    Locations of earthquakes ≥ magnitude 2.5 (yellow circles) and locations where subsurface faulting has been detected (red stars).

    Locations of earthquakes ≥ magnitude 2.5 (yellow circles) and locations where subsurface faulting has been detected (red stars). (Public domain.)

    Earthquakes in the Central and Eastern US are low probability, high impact events on enigmatic sources. They can cause widespread damage because of low attenuation rates and an aging building stock not designed to withstand strong earthquake shaking. When coupled with the large population centers in the CEUS (e.g., Memphis, St. Louis, Boston, Charleston, Washington D.C.), these low probability events could possibly result in substantial losses – often as high or higher than in portions of the seismically active West. Scientists are trying to improve our understanding of earthquakes in these areas, such as when have they happened in the past, where and when are they likely to happen in the future, and what will be the effects?

    Earthquake Geology

    USGS scientists study active fault zones by mapping faults, excavating trenches in fault zones, describing and dating sedimentary layers affected by earthquakes, mapping and dating landforms offset by earthquakes, and measuring past and current motion of active faults using alignment arrays, global positioning systems (GPS), and airborne, terrestrial and mobile laser scanning technology. The USGS works in active tectonic areas around the world and provides scientific response to damaging earthquakes.

    The greater San Francisco Bay region is an active tectonic zone that sits at the boundary of two tectonic plates. The generally northward motion of the Pacific Plate relative to the North American Plate is accommodated across a large number of active faults, which are prone to damaging earthquakes. In order to better understand the hazard posed by these faults, USGS scientists document the history of earthquakes on the faults, the rate at which the opposite sides of the faults are slipping past one another, and the manner in which energy of plate motion is released on a given fault.

    Most faults store energy that may be released during potentially damaging earthquakes. Some faults, however, slip nearly constantly, releasing energy slowly as “creep,” and some faults exhibit a complex mix of “locked” and creeping behavior. Further, some faults are known to be short and segmented and therefore produce only small to moderate earthquakes, while others are long and continuous or may participate in earthquakes with nearby faults, thereby generating very large earthquakes. For example, the magnitude 7.9 San Francisco Earthquake in 1906 ruptured almost 300 miles along the San Andreas Fault.

    Understanding a fault’s slip behavior, as well as its length and connectivity, is important for constraining the magnitude range and frequency of earthquakes that a particular fault is likely to produce. Some faults that pose significant earthquake hazard may not have a clear expression on the Earth’s surface, but may have vertical motion that over time leads to creation of mountains and valleys.

    Paleoseismology in the San Francisco Bay Area and Beyond

    USGS geologists study active faults in California and beyond. Recent investigations conducted by USGS geologists include studying the Denali-Totschunda Fault in Alaska, the Bear River Fault in Wyoming and Utah, and a wide range of international research projects.

    By excavating trenches across active faults, USGS geologists and collaborators are unraveling the history of earthquakes on specific faults. Damaging earthquakes often rupture along a fault up to the ground surface, and, in doing so, offset layered sediments that were deposited by water, wind and down-slope movement. Following an earthquake, new sediment may be deposited across the disturbed land, creating a new undisturbed horizon that is younger than the earthquake.

    Geologists use radiocarbon dating and other methods to learn the age of pre-existing layers affected by ancient earthquakes as well as the new layers deposited after the earthquakes, and, by doing so, constrain a fault’s earthquake history. These methods work best at sites on faults that lie near streams, slopes, ponds and other areas that have frequent sediment deposition.

    Scientists have successfully pieced together the history of earthquakes over the past several hundred to a few thousand years on many active faults. These histories provide insight into the possibility of future damaging earthquakes. Some faults, such as the Hayward fault in the East Bay, have produced large earthquakes at fairly regular intervals over the past few thousand years. The Hayward fault in particular is thought to be ready for the next damaging earthquake, based on our understanding of the history of past earthquakes exposed by paleoseismic trenching.

    Tectonic Geomorphology

    Repeated earthquakes shape the Earth over the millennia and fault zones often have unique and diagnostic landforms caused by the faulting process. These include steep scarps, folds, elongate ridges, sag ponds, offset terraces, and linear valleys, and deflected, offset and uplifted streams. By studying these landforms, USGS geologists uncover the location and pace at which faults deform the Earth’s surface. Determining the rate at which a faults “slips” is a key piece of information for assessing the hazard that a fault presents to people and infrastructure. Scientists use airborne laser mapping elevation data to create remarkable visualizations of the shape of the Earth’s surface, even in areas covered by vegetation.

    The study of landscapes affected by tectonics, often referred to as “tectonic geomorphology,” also provides important clues about seismic hazard associated with areas beyond the well-defined fault traces. The mountain ranges along the California coast are testament to the combination of sliding and squeezing that occurs at the boundary between the Pacific and North American tectonic plates. Mountains grow as a result of many earthquakes that occur over time as one side of a fault moves up relative to the adjacent side, or a large area is bent and warped upward. Some earthquakes, such as the 1989 Loma Prieta earthquake in the Santa Cruz Mountains south of San Francisco, are associated with the growth of mountains. These types of hazards are better understood by studying uplifting landforms such as marine terraces and other sedimentary deposits and by analyzing the patterns of river channel topographic profiles.

    Near-Field Geodesy

    The USGS is at the cutting edge of measuring ongoing deformation of the Earth’s surface, a field known as geodesy. In order to measure the rate at which the Earth’s crust deforms between, during and after earthquakes, precise measurements need to be made along active faults zones. USGS scientists have long established alignment arrays, which are stable markers that cross faults zones and can be measured to determine the rate of slip on the fault zone. USGS scientists, along with collaborators from universities, have established a network of hundreds of alignment arrays across the major faults of northern California.

    USGS scientists are also at the leading edge of utilizing 3D laser scanning to map the Earth’s surface and objects at and near the Earth’s surface in order to quantify the rates and patterns of crustal deformation. This includes using 3D laser scanning technology from tripod mounts as well as mobile platforms (vehicles, backpacks, balloons, etc) that enable measurement of landscapes at centimeter-level precision over large areas. These techniques allow USGS scientists to provide rapid scientific response to damaging earthquakes, and to advance our understanding of the physics of earthquakes and how earthquakes affect the Earth in three-dimensions and through time.

     Earthquake Response

    USGS geologists respond to damaging earthquakes in active tectonic regions of the United States and around the world, rapidly providing critical information to stakeholders. Earthquake response activities include rapid assessment of landscape change (including mapping and measuring the locations and amount of offset caused by faults rupturing the Earth’s surface), identifying ground failure and liquefaction, and mapping landslides. On-the-ground investigations are complemented by airborne surveys, rapid 3D laser scanning, and rapid acquisition and analysis of remote imagery to study inaccessible regions.

    Additional activities include quantifying and forecasting ongoing post-seismic deformation, identifying locations where critical infrastructure may be damaged, and providing assessment of ongoing hazards to critical infrastructure.

    USGS is able to provide information quickly to all levels of government, utilities, media and citizens about earthquake effects and ongoing hazards. Observations and data collected immediately following a damaging earthquakes helps scientists determine ongoing hazard and also allows them to better understand earthquake process and effects which then guides assessment of future hazard.

    Salton Seismic Imaging

    Studying Earthquake Hazards and Rifting Processes in the Imperial and Coachella Valleys

    faults in Southern California

    Southern California consists of two of Earth’s plates (the Pacific and North American plates) moving past each other. The boundary between the two plates is quite crooked. Heavy red lines indicate the San Andreas and related faults. As the two plates move past each other along these faults (in the directions of the small white arrows), earthquakes occur. The purple lines indicate locations between these faults where the Earth is being pulled apart, creating a deep valley or even new ocean. Volcanoes and underground magma in these areas create geothermal energy and hot springs (CPG is Cerro Prieto Geothermal area; BSZ is Brawley Seismic Zone and geothermal area). In the Transverse Ranges, where the San Andreas Fault undergoes a “Big Bend,” the plates are pushing against each other (heavy white arrows), building mountains, which are uplifted along thrust faults (the thin red lines with teeth). Thus, mountain building and valley subsidence are occurring very close to each other in this part of southern California. (Public domain.)

    Southern California consists of two of Earth’s plates (the Pacific and North American plates) moving past each other. The boundary between the two plates is quite crooked. Heavy red lines indicate the San Andreas and related faults. As the two plates move past each other along these faults (in the directions of the small white arrows), earthquakes occur. The purple lines indicate locations between these faults where the Earth is being pulled apart, creating a deep valley or even new ocean. Volcanoes and underground magma in these areas create geothermal energy and hot springs (CPG is Cerro Prieto Geothermal area; BSZ is Brawley Seismic Zone and geothermal area). In the Transverse Ranges, where the San Andreas Fault undergoes a “Big Bend,” the plates are pushing against each other (heavy white arrows), building mountains, which are uplifted along thrust faults (the thin red lines with teeth). Thus, mountain building and valley subsidence are occurring very close to each other in this part of southern California.

    The Imperial and Coachella Valleys, located in the Salton Trough, are forming by active plate tectonic processes. From the Imperial Valley southward into the Gulf of California, plate motions are rifting the continent apart. In the Coachella Valley, the plates are sliding past one another along the San Andreas and related faults. These processes build the stunning landscapes of the region, but also produce damaging earthquakes.

    Rupture of the southern section of the San Andreas Fault, from the Coachella Valley to the Mojave Desert, is believed to be the greatest natural hazard that California will face in the near future. With an estimated magnitude between 7.2 and 8.1, such an earthquake would result in violent shaking, loss of life, and disruption of lifelines (freeways, aqueducts, power, petroleum, and communication lines) that will bring much of southern California to a standstill.

    As part of the nation’s effort to avert a catastrophe of this magnitude, a number of projects are underway to more fully understand and mitigate the effects of such an event. One project, funded jointly by the National Science Foundation (NSF) and the U.S. Geological Survey (USGS), is to understand through “seismic imaging” the structure of the Earth surrounding the San Andreas Fault, including the sedimentary basins on which cities are built.

    This project, the Salton Seismic Imaging Project (SSIP), will create images of underground structure and sediments in the Imperial and Coachella Valleys and adjacent mountain ranges to investigate the earthquake hazards they pose to cities in this area. Importantly, the images will determine the underground geometry of the San Andreas Fault, how deep the sediments are, and how fast earthquake energy can travel through the sediments. All of these factors determine how hard the Earth will shake during a major earthquake. If we can better understand how and where earthquakes and strong shaking will occur, then buildings can be better designed or retrofitted to resist damage and collapse, and emergency plans can be prepared.

    Special Earthquakes, Earthquake Sequences, and Fault Zones

    Photograph shows the collapse of Fourth Avenue near C Street, Anchorage, Alaska, in 1964.

    Collapse of Fourth Avenue near C Street, Anchorage, due to earthquake caused landslide. Before the earthquake, the sidewalk at left, which is in the graben, was at street level on the right. The graben subsides 11 feet in response to 14 feet of horizontal movement. Anchorage district, Cook Inlet region, Alaska. 1964. (Public domain.)

    Significant earthquakes provide a wealth of data, and a unique opportunity to observe phenomena that do not occur very often. These earthquakes have been critically examined.