Earthquake Hazards

Earthquake Processes and Effects

The overarching theme of this research is for scientists to discover as much as they can about earthquakes and faulting from field and laboratory observations, and to combine this with geophysical, geological, geochemical, and mathematical (including computational) modeling of earthquake sources and fault zones so as to best improve USGS earthquake hazard assessments.

Overview

San Andreas Fault and its various environments in the Southern California area

The San Andreas Fault and its various environments in the Southern California area, including both mountain building and valley subsidence close together. (Public domain.)

The high level of earthquake activity and the complexity of the fault systems throughout California 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 California to analogous areas in the world with large strike-slip faults, to provide insight into possible past and future earthquakes in the region.

In addition, earthquakes are generated in the laboratory under controlled conditions to learn how they start and what indications there might be that they are about to happen. Also, fault zone materials are carefully tested to learn about the physical and chemical processes that control earthquakes.

The project works in close collaboration with regionally focused projects and with the National Seismic Hazard Mapping (NSHM) project in order to support those efforts at issuing earthquake hazard products. This research strives to increase the quality and impact of these products, and to reduce their uncertainties, through multidisciplinary research aimed at better understanding the earthquake process.

Some of the key scientific questions scientists seek to answer:

  • How is stress loaded onto faults as a function of space and time by both plate motions and other geological processes?
  • How do the stresses redistributed by one earthquake affect the probability of future events?
  • Do identifiable earthquakes recur with some average repeat time and definable variation or is each earthquake unique? How does the structure of faults control the nucleation of small earthquakes and their growth into larger ones and what does this predict about the distribution of sizes of earthquakes we can expect in a region or along a fault?

Tracking Stress Buildup

map showing stressing rate of the crust around California

Stressing rate of the crust around California derived from two decades of geodetic measurements. (Public domain.)

The constant plate tectonic motions between the Pacific and North American plates guarantees that the crust in the western US is continually building up stress. The image of crustal velocities provided by extensive GPS coverage reveals where these velocities change rapidly over short distances, demanding that the intervening crustal rock stretch and build up stress over time. Such a map of the stress reveals two main lines where stress is concentrated: The San Andreas fault zone and the Eastern California Shear Zone. These zones have experienced numerous earthquakes over the century and a half that earthquakes have been historically observed.

The mechanism of stress buildup within these fault zones is uncertain. One hypothesis is that the hot rocks below the upper 15-km-thick layer (the upper crust that has the vast majority of continental earthquakes) flows continually in response to periodic earthquakes, forcing the upper crust to bend with this flow. Another hypothesis is that slip of the deeper continuation of faults, steady slip that doesn’t produce earthquakes but still involves motions across the fault, forces the upper crust around the faults to bend and thus concentrate stress. Both hypotheses are the subject of active research. But the fact remains that high stressing rates observed on the surface likely translate to high stressing rates at the depths (~10 km) where earthquakes typically nucleate, so these stressing rates are a guide to the seismic hazard.

Crustal Deformation

Crustal deformation refers to the changing earth’s surface caused by tectonic forces that are accumulated in the crust and then cause earthquakes. So understanding the details of deformation and its effects on faults is important for figuring out which faults are most likely to produce the next earthquake. There are several hypotheses about how this works, but more data is needed to determine which one is the best.

Crustal deformation is a heavily data driven field. To measure the motions of earth’s surface, the USGS employs a variety of methods, including LIDAR, the Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), creepmeters, and alinement arrays. In parts of the U.S. with few or no historically-recorded major earthquakes or where background seismicity is sparse, geodetic data may provide the only insight into present-day seismic hazard. The motions captured by these diverse measurement techniques provide vital information on:

Map depicting crustal deformation instruments deployed in the San Francisco Bay Area.

Map depicting crustal deformation instruments deployed in the San Francisco Bay Area. (Public domain.)

  • The slow ‘background’ tectonic motions between the earth’s plates, thereby constraining the buildup of stress on faults.
  • The offsets across creeping faults such as the Hayward fault, which results in steady motions (typically several millimeters per year) of blocks of crust moving past each other along a common fault boundary.
  • The offsets across a fault during a large earthquake (the coseismic displacements).
  • Rapidly decaying motions that persist for weeks to years after a large earthquake, arising from a combination of continued slip on the fault (‘afterslip’) and possibly its extension into the lower crust and flow of rock in the deeper lower crust and mantle, where the temperature is high enough to permit ductile flow.

Specific problems of interest include:

  • Quantifying fault slip rates, creep rates, and off-fault strain rates as well as the spatial extent of locked and creeping zones.
  • Deformation rates between earthquakes with different recurrence intervals.
  • Comparing models with observed data to assess the accuracy of each.
  • Mapping the distribution of the locked (non-slipping) and freely sliding parts of faults to determine where slip in future earthquakes will occur.
  • Improving earthquake early warning and rapid response using real time high rate GPS data.
  • Investigating the effects of seismic waves travelling across a fault from an earthquake on another fault.

Fault Slip Rates

Earth’s crust typically moves a few millimeters to centimeters per year. In an actively deforming continental region, the crust often behaves like a set of nearly-rigid blocks separated by faults. The velocity within a block is nearly constant, but the velocity may differ substantially from one block to an adjacent one. Whether the transition in velocity across a bounding fault is abrupt or gradual depends on whether the fault is creeping (abrupt transition) or locked (gradual transition). The distinction is important because a gradual transition implies not only that the fault is locked but that it is accumulating stress that will eventually lead to earthquakes.

An example is the horizontal crustal deformation field in the northern San Francisco Bay area. The crust is approximated as three blocks, their boundaries lying along the right-lateral strike-slip San Andreas fault, Macaama fault, and Bartlett Springs fault. Motions across the San Andreas fault, southern Macaama fault, and southern Bartlett Springs fault are gradual, but those across the northern Macaama fault and northern Bartlett Springs fault are abrupt in several areas, implying some degree of fault creep.

A recent study by Murray, Minson, and Svarc employs use a Bayesian technique to interpret the GPS crustal velocity field to estimate deep slip rates and shallow surface creep. Their model indicates a deep slip rate of 20 mm/yr for the San Andreas fault, a deep slip rate of 13 mm/yr and shallow creep rate of 0 to 13 mm/yr on the Maacama fault, and a deep slip rate of 7 mm/yr and shallow creep rate of 0 to 7 mm/yr along the Bartlett Springs fault.

Post-Earthquake Motions

diagram showing afterslip and relaxation processes

Left: Afterslip. Right: Lower crust and upper mantle relaxation. Areas colored yellow either slip or flow after a large earthquake. Both the afterslip and relaxation processes occur deep under the faults (schematic red lines) that originally slipped in the earthquake. UC, Upper Crust; LC, Lower Crust; UM, Upper Mantle. The UC/LC boundary is schematically the brittle-ductile transition. Determining whether afterslip or relaxation occur after an earthquake is an active research area. (Public domain.)

After a large earthquake, the crust does not stop moving. Earthquakes such as the 1994 M6.7 Northridge, California, or 1989 M6.9 Loma Prieta, California, earthquakes are followed by hundreds of aftershocks, some of them damaging. Many aftershocks occur on the causative fault or its extension into the deeper earth. The slip associated with these aftershocks is termed afterslip. It may occur in the form of ordinary earthquakes or as slow slip that is not felt.

Another process that occurs after an earthquake is relaxation of the ductile rock that underlies the uppermost crust. While most earthquakes occur in the uppermost crust because frictional resistance is high (and hence stress builds up over long periods of time), few occur in the deeper crust or underlying mantle because the rock temperature is too high to permit accumulation of stress. Instead, stress is accommodated by flow of the rock. The depth where the transition from brittle friction to ductile flow is called the brittle-ductile transition, and it generally coincides with the maximum depth of earthquakes, typically 10-20 km (Implications of the Depth of Seismicity for the Rupture Extent of Future Earthquakes in the San Francisco Bay Area).

Understanding these processes is important for estimating how faults interact with one another through stress transfer, which can change the seismic hazard for years after an earthquake. Part of this understanding comes from better knowledge of the temperature and composition of rocks deep beneath earth’s surface.

illustration of postseismic motions

An example of the postseismic motions occurring over the year following a thrust (Northridge-type) earthquake is illustrated here. The co-seismic motions occur suddenly at the time of the earthquake, the postseismic motions gradually after the earthquake through the relaxation process. The earthquake originally occurs in the upper gray layer (low-temperature rock), overlying a deeper high-temperature region. (Public domain.)

Case Study: M7.1 October 16, 1999 Hector Mine, California, Earthquake

afterslip after 1999 Hector Mine quake

Afterslip up to a few meters occurred during the first few months after the 1999 Hector Mine quake. Most was deep under the northwest branch of the earthquake rupture. (Public domain.)

The Hector Mine earthquake occurred in the central Mojave Desert and produced several meters of slip along faults of total length ~40 km. It was well-recorded by continuous GPS sites of the SCIGN Network that were already in place in the aftermath of the 1992 M7.3 Landers earthquake, as well as new continuous and survey-mode GPS that were installed in the region by scientists soon after the Hector Mine quake.

Both afterslip on thin zones below the causative fault and relaxation of the flowing lower crust and mantle occurred in the months and years after the Hector Mine earthquake.

2.5 years after the 1999 Hector Mine quake quake, almost all regional motions above the ‘background’ tectonic motions are due to flow of rock in the lower crust and upper mantle. This motion can be tracked unambiguously up to 10.46 years after the 1999 Hector Mine quake, at which time the M7.2 April 4, 2010 El Mayor-Cucapah earthquake changed the regional flow pattern. A model of the flow based on the laboratory behavior of wet olivine and mafic granulite compositions (Thatcher and Pollitz, 2008) agrees well with observed motions.

Ground Movements

Measureable permanent ground displacements are produced by shallow earthquakes of magnitude 5 and greater. These displacements are used by seismologists to understand the earthquake source in detail, such as the amount of slip and the type of underground fault which ruptured. This information has been traditionally used to analyze earthquakes long after they occur, but recent work in Earthquake Early Warning may allow such geodetic measurements to be exploited in real time in order to help provide warning of earthquake shaking while the earthquake is in progress.

Steady background motions of Earth’s crust occur as a result of tectonic plate motions. As the Pacific plate slides past the North American plate, they become stuck at the boundary zone between them, which typically has many faults. If these faults are stuck, then there may be no motion across them for tens to hundreds of years, during which time they build up stress until an earthquake occurs. The earthquake relieves the stress, the fault is stuck again, and the cycle of stress buildup and release begins anew. This process has been documented on the Hayward fault and San Andreas fault for the past few thousand years using geologic investigations.

Surface rupture of the 1999 M7.1 Hector Mine earthquake

Surface rupture of the 1999 M7.1 Hector Mine earthquake in the Mojave Desert, California, amounting to several meters of right-lateral slip (Public domain.)

relief map of the western United States with the background velocity field

Relief map of the western United States with the background velocity field (relative to a fixed North American plate) determined from two decades of GPS observations. Measurements have been made by numerous academic and government organizations, including the Plate Boundary Observatory and the USGS. (Public domain.)

Many faults in the San Francisco Bay area are not completely stuck, but instead they undergo fault creep, steady motions along the fault. If these motions proceed as rapidly as the plates slide past each other, then the fault is essentially ‘unstuck’ and no stress builds up. This is the case for portions of the Hayward fault, Calaveras fault, and San Andreas fault.

Measureable motions above the ‘background’ often occur days, months, or even years after an earthquake occurs, even though the causative faults are stuck. This usually happens after magnitude > 7 earthquakes. Such motions continued for several years following the 1999 M7.1 Hector Mine earthquake in the Mojave Desert, California. Such large earthquake impart large stresses into the Earth’s lower crust and mantle, the layer between the crust and the core. The lower crust and mantle have higher temperature than the upper crust (the upper ~15 km), and minerals like quartz will flow at these higher temperatures. As a result, stresses that lead to earthquakes tend to be concentrated in the continental upper crust, but the gradual dissipation of these stresses in the ductile layer will lead to continued crustal motions for years after a large earthquake.

Some ‘earthquakes’ occur without shaking. Scientists often refer to these events as slow earthquakes. Many slow earthquakes occur along the Cascadia subduction zone, where the Juan de fuca plate is plunging beneath the North American plate . Many also occur in the San Francisco Bay area, specifically along the creeping central San Andreas fault.

Surface Motions

Faults are thought to be creeping at depth in the lower crust, where the lack of frictional resistance at the prevailing high temperatures allows steady fault slip and no buildup of stress. Faults may also be creeping at shallower depth in the upper crust (left figure), leading to block-like motions and sharp changes in surface velocity across the fault. Faults are usually locked in the upper crust (right figure), leading to a gradual change in surface velocity across the fault and bending of the upper crust. This bending produces stress buildup that eventually leads to earthquakes.

The horizontal velocity field in the San Francisco Bay area is constrained by continuous GPS (CGPS) and survey-mode GPS (SGPS) measurements. These velocities are almost parallel to the regional faults and decrease from west to east as one approaches the interior of the North American plate. While a gradual transition across the San Andreas Fault indicates that it is locked, the abrupt transition across the southern Hayward fault indicates that it is creeping. This is confirmed by creepmeter measurements across the Hayward fault. Creepmeter measurements across the Hayward fault.

Ground Shaking

map showing total amplification expected Los Angeles region

Two important local geologic factors that affect the level of shaking experienced in earthquakes are (1) the softness of the surface rocks and (2) the thickness of surface sediments. This image of the Los Angeles region combines this information to predict the total amplification expected in future earthquakes from local geologic conditions or site effects. (Public domain.)

The overall objective of this research is to improve the understanding of the damaging ground motions produced in earthquakes in order to develop better methods for seismic hazard assessment and mitigation in urban areas. Past earthquakes have shown that the amplification of motions due to surface-to-bedrock geology, 3D crustal structure, and topography have a major influence on seismic damage and loss in urban areas. Also of significant importance are the details of the rupture process on the fault, and the way a built structure is engineered.

As the waves propagate they are affected by the earth structure, such as changes in elastic properties resulting in effects such as constructive and destructive interference and basin amplification. Near the ground surface, strong shaking can result in nonlinear soil behavior or raise pore fluid pressure causing liquefaction. Likewise, the geometry of a man-made structure, the construction materials, the type of ground, and its anchorage in the ground affect its vulnerability to damage during the shaking. This research aims to understand each of these processes and to work with the seismic engineering community to bring the best estimates of strong ground shaking to engineering practice.

Research includes:

  • investigation of the ground beneath high-impact areas to understand the shaking that would occur in a large earthquake.
  • investigation of the effect on seismic waves of traveling long distances through the earth’s crust, sedimentary basins, mountains, and other crustal features.
  • development of better imaging methods for identifying faults and crustal structure.

Cone Penetration Testing (CPT)

Cone Penetration Testing (CPT) is used to identify subsurface conditions in the upper 100 ft of the subsurface. The USGS CPT uses a 23-ton truck to push a “cone” into the ground. The weight of the truck is partially supported by both the tip of the cone and the sleeve of the cone. The “tip resistance” is determined by the force required to push the tip of the cone and the “sleeve friction” is determined by the force required to push the sleeve through the soil. The “friction ratio” is the ratio between sleeve friction and tip resistance, measured as a percentage. Soil type and thereby resistance to liquefaction can be inferred from these measurements.

close up of CPT cone.

CPT cone. 

(Credit: Tom Noce, USGS. Public domain.)

CPT cone and 23-ton truck

The cone, when it includes a seismometer, can also be used to predict how local shallow soil conditions can modify shaking. The capacity of local soil conditions to modify shaking is inversely proportional to the velocity of seismic waves near the surface, which can be computed with data recorded with the seismometer. Seismic energy is created manually with a sledgehammer or automatically by a compressed air driven hammer. We measure the time it takes for the seismic energy to travel from the land surface, through the ground, to a seismometer mounted in the cone. The distance to the seismometer divided by the travel time is approximately the average shear-wave velocity. (Public domain.)

Data from the USGS cone, which includes a seismometer, can also be used to predict how local shallow soil conditions can modify shaking. The capacity of local soil conditions to modify shaking is inversely proportional to the shear-wave velocity near the surface, which can be computed with data recorded with the seismometer. Seismic energy is created manually with a sledgehammer or automatically by a compressed air driven hammer. We measure the time it takes for the seismic energy to travel from the land surface, through the ground, to the seismometer mounted in the cone. The distance to the seismometer divided by the travel time of the shear-wave is approximately the average shear-wave velocity.

Output from the cone as it penetrates the soil is digitally recorded by a computer and is collectively known as a sounding. For more information, see USGS Factsheet: Subsurface Exploration with the Cone Penetration Testing Truck.

San Francisco Bay Area Arrays

Portable Seismograph Deployments to Research the Effects of Basins, Topography, and Fault Zones on Seismic Waves

location of portable arrays of seismometers

Portable arrays of seismometers across the San Ramon, Pleasanton, and Livermore Valleys. (Public domain.)

Seismologists have observed that both topographic highs and basins have complex and varying effects on seismic waves. By deploying arrays of seismic recorders our understanding is improved of what specific features have what specific effects on the seismic waves. These studies use basins, ridges, and fault zones in the San Francisco Bay area as a laboratory to explore in greater detail what is happening as seismic waves propagate through them. The results can be used in similar urban areas throughout the world to estimate the expected shaking from both local and more distant earthquakes.

Seismic waves traveling through basins and topography are affected by:

  • The edges of the basin
  • The age of the basin, which is related to the density of the material in the basin
  • Fault zones in these areas
  • The direction of approach of the seismic waves and the shape of the feature
  • The distance between the location of the earthquake and the feature
  • The frequency content of the seismic waves

The basic approach employed in these studies is to record seismic waves produced from both earthquakes and man-made sources such as explosions, and then to use several different mathematical methods and models to try to explain the observed levels of shaking. The portable seismographs are set up in different arrays in different locations in order to record various effects. Our studies are facilitated by using a combination of different analysis methods to reveal the wave type, direction, and speed of the important components of the ground motion. Both basins and topographic highs can trap seismic waves and cause them to be amplified. An understanding of these amplification effects is important for proper evaluation of seismic hazards in urban areas.

East Bay Seismic Experiment

Warren Hall towering above California State University East Bay Campus

Before August 2013 Warren Hall towered above the California State University East Bay Campus, with the city of Hayward and San Francisco Bay in the valley below. The Hayward Fault runs along the base of the hills. (Public domain.)

deployment of hundreds of seismographs

Map showing the deployment of hundreds of seismographs in the city of Hayward that the USGS used to record and observe the implosion of Warren Hall on the campus of California State University East Bay, Hayward. (Public domain.)

California State University East Bay (CSU-EB) imploded Warren Hall in August of 2013, which was located near the active trace of the Hayward Fault. This implosion provided an excellent opportunity to use a “free” seismic source that was practically located on the Hayward Fault. 500 to 600 temporary seismographs were deployed in the Hayward area to capture the seismic signal generated by the implosion.

The effort was primarily a collaboration between CSU-EB and the USGS, but other organizations and agencies also conducted additional seismic investigations using the Warren Hall source. From the source, the USGS and CSU-EB recorded the seismic energy as it moved from the source along concentric circles. Because attenuation is generally greater with distance from the source, the concentric circles allowed scientists to measure the same theoretical amplitude (a) in the hard rock of the hills, (b) in the soft rocks/sediments of the valley, (c) within the fault zone, and (d) on the peaks, regardless of the source function. From these measurements, they compared the relative amplification effect of the various geologic terrains. The seismographs were densely spaced so that scientists could also look at the effect of relative amplification within each geologic terrain, which may help to explain why seismic energy can be stronger on one side of a street relative to the other side of the same street.

Along several of the radii extending from Warren Hall, scientists measured seismic velocities (Vp, Vs) in the hills, within the fault zone, and in the valley. These data are very useful in modeling expected ground shaking, and the data can help determine if there are additional faults east and west of the main surface trace. The regional earthquake-monitoring network of seismographs used to locate earthquakes in the Bay Area was turned on before the implosion to record the signal. From these data, scientists can determine how accurately the network located the implosion and provide correction factors, essentially providing a calibration for the network.

Data

2013 East Bay Seismic Experiment (EBSE) — Implosion Data, Hayward, California

Rock Physics Labs

Noel Bartlow loads a granite sample into a pressure vessel

Noel Bartlow loads a granite sample into a pressure vessel at the USGS Rock Physics Laboratory as part of her thesis work at Stanford University on tidal triggering of earthquakes. (Public domain.)

There are currently two main Experimental Rock Physics Laboratories in the Earthquake Science Center in Menlo Park, California. These laboratories specialize in generating earthquakes under controlled conditions to measure the processes that determine how they start and how large they will become. Since most damaging earthquakes originate many miles below the earth’s surface, it is almost impossible to study them directly. Instead, much of what we know about natural earthquakes comes from analyzing the seismic waves that they produce.

In the laboratory, however, we are able to recreate the conditions of high pressure, high temperature and slow stress buildup that faults undergo in the months to years before earthquakes. We also have specialized testing machines that slide rocks against each other at the high speeds (meters per second) that occur during earthquakes. Because testing is carried out in a controlled environment, relevant properties are measured before, during and after the occurrence of laboratory earthquakes. These include strength and frictional behavior of rocks and fault zone materials, the velocity of seismic waves through rock, electrical resistivity measurements, as well as the role of fluids and fluid flow in fault zones.

How is this Data Used?

Information on rock properties is combined with other geophysical observations to improve our models of the earthquake process, such as the timing and magnitude of earthquakes, earthquake triggering, recurrence, rupture propagation, and ground motion. This in turn is necessary to understand earthquake hazards and risk in earthquake-prone areas.

Neogene Deformation History of Western North America and Volcanism in Coastal California

Summary

These movies animate a model we created to both reconstruct the geography of California through time and to aid predictions about how it will change in the future. Western California is cut by a major plate tectonic boundary called the San Andreas transform boundary. This broad boundary is composed of many faults that divide the land into separate pieces called fault blocks. The fault-bounded blocks create earthquakes as they jostle each other while being rafted along as pieces of crust caught within the San Andreas transform boundary. Some faults that compose this boundary moved rapidly millions of years ago but now move slowly or not at all. Other, younger faults are moving rapidly today and are a potential source of damaging earthquakes.

As you watch these movies, you can see faults and the fault blocks they bound start to move at various times in the past. Motion will slow on some blocks as the location of most rapid fault slip (thick orange line) jumps eastward. Each movie frame is a snapshot in time, numbered in millions of years before present (Ma). Watch for the initial fault to form near the base of the continental slope (thin blue line) about 28.5 Ma. A second fault segment forms to the north about 2 million years later, and the two segments grow towards each other and join. About 19 Ma, this initial trace of the San Andreas transform boundary is abandoned and a new trace forms near the present-day coastline. About the same time, a new segment forms near the western coastline of Baja California. The misalignment of these two segments triggers rotation of the Transverse Ranges in central California. About 12.4 Ma, the most active trace of fault system jumps eastward to its present-day location, known as the San Andreas fault. At this time, the fault extends south through the nascent Gulf of California, and Baja California begins to tear away from mainland Mexico. More detailed information can be found in the Journal Abstract and Synopsis below.

Animated tectonic reconstruction at regional and close-up scales

North America is kept stationary in these models. Gridded areas highlight larger rigid blocks, non-gridded areas are treated as non-rigid crust. Thick orange lines show the location of the fastest moving faults at any given time. Colored stripes offshore depict age and motion of the oceanic Pacific plate through time.

Abstract

The geologic record of coastal California includes evidence of numerous volcanic centers younger than 30 Ma that do not appear to have erupted in an arc setting. By correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the Pacific plate, we add new constraints to tectonic reconstructions since 30 Ma. Our correlations— such as erupting the Morro Rock-Islay Hill complex south of the Pioneer fracture zone and the Iversen Basalt south of the Mendocino fracture zone—require larger displacements within western North America than advocated by most previous authors. Specifically, we infer at least 315 km of motion between the Sierra Nevada and rigid North America at an azimuth of about N60°W, and at least 515 km between Baja California and rigid North America in a similar direction. A consequence of inferring a large displacement of Baja California is that the Pacific-North American plate boundary must have developed most of its current form prior to 10 Ma. We interpret a slab window developing between Cocos and Monterey plates after 19 Ma that reconstructs under nearly all of the southern California volcanic centers dated at 18-14 Ma. Most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism, followed by slow subsidence of variable but often extended duration, consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere.

Synopsis

Our goal is to determine whether correlating the coastal volcanic rocks with the slab windows has any unreasonable implications that would require revising generally accepted ideas about relative motions in the late Cenozoic. To this end, we define our model for North American deformation more rigorously than in previous studies by specifying finite rotations on a sphere among large numbers of rigid blocks, using the same mathematical description as motions of the major plates. We combine a global circuit of major plate motions, similar to that used by other recent authors, with this new model that describes relative motions within North America using finite rotations. The main advantage of our technique is that the position of a continental fragment relative to the Pacific plate is an exact prediction of the kinematic model; relative positions of volcanic centers and slab windows can be used as direct tests of a set of relative motion parameters. Furthermore, with all of the reconstruction performed by parameterized modeling, internal consistency of the reconstructions is tested much more rigorously than in reconstructions with a substantial component of freehand drafting.

Our model for North America includes a few major blocks generally recognized as mobile but nearly rigid, namely the Colorado Plateau, Sierra Nevada-Great Valley, Baja California, and the Sierra Madre Occidental (see Figure 4 in full publication). Close to the coast, especially in the areas where the volcanic rocks are observed, we keep track of many individual blocks bounded by major to moderate faults. For the inland deformation zones, commonly characterized by basin and range extension, we ignore most of the individual faults and fault blocks, considering only the motion between the major blocks and the North America craton.

Though some of the resulting reconstructed positions differ from what has commonly been published, we find no serious problems with these positions. We see our interpretation of a moderate increase in motion of the Sierra Nevada relative to the Colorado plateau or North America as a refinement of previous work, not requiring serious reinterpretations, and not clearly preferable to potential revisions in the plate circuit. Previous estimates of the original position of Baja California have been bimodal, with a majority advocating about 300 km of motion and a minority advocating about 500 km. Our reconstruction of the past positions of coastal volcanic units from 27 to 12 Ma strongly supports the minority view of at least 500 km of displacement. A direct consequence of this interpretation is that the approximate geometry of the current Pacific-North American plate boundary in California and western Mexico developed at about 10-12 Ma.

Within moderate uncertainties, there is no need to modify either the global plate circuit or the reversal time scale to satisfy the constraints derived from the slab window correlations. Of course, this consistency does not necessarily imply that either our reconstruction, the plate circuit or the time scale is correct, and better testing of the reconstruction will be possible as independent information refines the time scale and plate circuit.

Citation