Mary Lou Zoback

First‐ and second‐order patterns of stress in the lithosphere: The World Stress Map Project

To date, more than 7300 in situ stress orientations have been compiled as part of the World Stress Map project. Of these, over 4400 are considered reliable tectonic stress indicators, recording horizontal stress orientations to within <±25°. Remarkably good correlation is observed between stress orientations deduced from in situ stress measurements and geologic observations made in the upper 1–2 km, well bore breakouts extending to 4–5 km depth and earthquake focal mechanisms to depths of ∼20 km. Regionally uniform stress orientations and relative magnitudes permit definition of broad-scale regional stress patterns often extending 20–200 times the approximately 20–25 km thickness of the upper brittle lithosphere. The “first-order” midplate stress fields are believed to be largely the result of compressional forces applied at plate boundaries, primarily ridge push and continental collision. The orientation of the intraplate stress field is thus largely controlled by the geometry of the plate boundaries. There is no evidence of large lateral stress gradients (as evidenced by lateral variations in stress regime) which would be expected across large plates if simple resistive or driving basal drag tractions (parallel or antiparallel to absolute motion) controlled the intraplate stress field. Intraplate areas of active extension are generally associated with regions of high topography: western U.S. Cordillera, high Andes, Tibetan plateau, western Indian Ocean plateau. Buoyancy stresses related to crustal thickening and/or lithospheric thinning in these regions dominate the intraplate compressional stress field due to plate-driving forces. These buoyancy forces are just one of several categories of “second-order” stresses, or local perturbations, that can be identified once the first-order stress patterns are recognized. These second-order stress fields can often be associated with specific geologic or tectonic features, for example, lithospheric flexure, lateral strength contrasts, as well as the lateral density contrasts which give rise to buoyancy forces. These second-order stress patterns typically have wavelengths ranging from 5 to 10+ times the thickness of the brittle upper lithosphere. A two-dimensional analysis of the amount of rotation of regional horizontal stress orientations due to a superimposed local stress constrains the ratio of the magnitude of the horizontal regional stress differences to the local uniaxial stress. For a detectable rotation of 15°, the local horizontal uniaxial stress must be at least twice the magnitude of the regional horizontal stress differences. Examples of local rotations of SHmax orientations include a 75°–85° rotation on the northeastern Canadian continental shelf possibly related to margin-normal extension derived from sediment-loading flexural stresses, a 50°–60° rotation within the East African rift relative to western Africa due to extensional buoyancy forces caused by lithospheric thinning, and an approximately 90° rotation along the northern margin of the Paleozoic Amazonas rift in central Brazil. In this final example, this rotation is hypothesized as being due to deviatoric compression oriented normal to the rift axis resulting from local lithospheric support of a dense mass in the lower crust beneath the rift (“rift pillow”). Estimates of the magnitudes of first-order (plate boundary force-derived) regional stress differences computed from modeling the source of observed local stress rotations magnitudes can be compared with regional stress differences based on the frictional strength of the crust (i.e., “Byerlees law”) assuming hydrostatic pore pressure. The examples given here are too few to provide a definitive evaluation of the direct applicability of Byerlees law to the upper brittle part of the lithosphere, particularly in view of uncertainties such as pore pressure and relative magnitude of the intermediate principal stresses. Nonetheless, the observed rotations all indicate that the magnitude of the local horizontal uniaxial stresses must be 1–2.5+ times the magnitude of the regional first-order horizontal stress differences and suggest that careful evaluation of such local rotations may be a powerful technique for constraining the in situ magnitude stress differences in the upper, brittle part of the lithosphere.

New Evidence on the State of Stress of the San Andreas Fault System

Contemporary in situ tectonic stress indicators along the San Andreas fault system in central California show northeast-directed horizontal compression that is nearly perpendicular to the strike of the fault. Such compression explains recent uplift of the Coast Ranges and the numerous active reverse faults and folds that trend nearly parallel to the San Andreas and that are otherwise unexplainable in terms of strike-slip deformation. Fault-normal crustal compression in central California is proposed to result from the extremely low shear strength of the San Andreas and the slightly convergent relative motion between the Pacific and North American plates. Preliminary in situ stress data from the Cajon Pass scientific drill hole (located 3.6 kilometers northeast of the San Andreas in southern California near San Bernardino, California) are also consistent with a weak fault, as they show no right-lateral shear stress at ∼2-kilometer depth on planes parallel to the San Andreas fault.

REGIONAL PATTERNS OF TECTONIC STRESS IN EUROPE

Nearly 1500 stress orientation determinations are now available for Europe. The data come from earthquake focal mechanisms, overcoring measurements, well bore breakouts, hydraulic fracturing measurements, and young fault slip studies and sample the stress field from the surface to seismogenic depths. Three distinct regional patterns of maximum compressive horizontal stress (SHmax) orientation can be defined from these data: a consistent NW to NNW SHmax stress orientation in western Europe; a WNW-ESE SHmax orientation in Scandinavia, similar to western Europe but with a larger variability of SHmax orientations; and a consistent E-W SHmax orientation and N-S extension in the Aegean Sea and western Anatolia. The different stress fields can be attributed to plate-driving forces acting on the boundaries of the Eurasian plate, locally modified by lithospheric properties in different regions. On average, the orientation of maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe and is rotated 17° clockwise from the direction of absolute plate motion. The uniformly oriented stress field in western Europe coincides with thin to medium lithospheric thickness (approximately 50–90 km) and high heat flow values (>80 m W/m2). In western Europe a predominance of strike-slip focal mechanisms implies that the intermediate principal stress is vertical. The more irregular horizontal stress orientations in Scandinavia coincide with thick continental lithosphere (110–170 km) and low heat flow (<50 m W/m2). The cold thick lithosphere in this region may result in lower mean stresses associated with far-field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography. The stress field of the Aegean Sea and western Anatolia is consistent with N-S extension in a back arc setting behind the Hellenic trench subduction zone. The stress field is influenced in places by regional geologic structures, e.g., in the Western Alps, where SHmax directions show a slight tendency toward a radial stress pattern. Not all major geologic structures, however, appear to affect the SHmax orientation, e.g., in the vicinity of the Rhine rift system horizontal stress orientations are continuous.

Cainozoic evolution of the state of stress and style of tectonism of the Basin and Range province of the western United States

Cainozoic evolution of the modern plate boundary along the western United States from subduction to a predominantly transform boundary coincided with a change from compressional to extensional deformation in the western United States. Extensions tectonism responsible for the modern Basin and Range province appears to represent a unique late-stage episode of a much longer period of extension initiated in an ‘in tra -arc ’ setting contemporaneously with calc-alkaline magmatism. Basin-range extension is distinguished from early extension on the basis of angular unconformities, differences in fault trends and spacing, and associated magmatism (basaltic). Prebasin-range extension (i.e. extension preceding the break-up of the region into ranges resembling the modern ones) was under way locally by at least 30 Ma and is now recognized by faulted and highly tilted strata exposed in uplifted range blocks, by large regions of the crust underlain by passively emplaced subvolcanic batholiths, and by the thickness and distribution of stratigraphic units. Locally, high strain rates that accompanied early extensions of as much as 50-100 % are implied. Data on preferentially orientated dyke swarms and fault slip vectors indicate a strikingly uniform WSW -ENE least principal stress orientation in the period ca. 20-10 Ma, during this early extension. The change from early extension to basin-range style faulting of the upper 15 km of crust, which resulted in broadly spaced ranges (25-35 km crest-crest spacing), was time-transgressive and probably not abrupt; locally both types occurred concurrently. Southern Basin and Range block faulting occurred largely in the period 13-10 Ma, in response to a stress field orientated similarly to that responsible for the early extension. In contrast, northern Basin and Range block faulting developed after 10 Ma and continues to the present in response to a stress field orientated approximately 45° clockwise to the earlier stress field. This modern stress field, with a WNW -ESE to E-W directed least principal stress, characterizes the entire modern Basin and Range province and Rio Grande rift region. The 45° change in least principal stress orientation is consistent with superposition of dextral shear associated with the development of the San Andreas transform fault. Inclusion of pre-basin-range extension may help resolve the discrepancy between estimates of 15-30% for basin-range block faulting and total extension estimates of 100-300 % for the Basin and Range province.

Disaster Resilience: A National Imperative

pled with the increasing frequency of billion-dollar disaster events, such as the recent Hurricane Sandy, highlight some of the challenges to hazards and disaster policy in the United States. American society is also facing challenges to its economic, sociocultural, and environmental systems: The national jobless rate is near historic high values, more than one in six Americans now live in poverty, population migration to the coastal communities continues, and environmental degradation due to development, farming practices, or industrial processes and accidents continues to degrade natural defenses against floods, storm surge, and wildfires. Many of these changes are transformative and long lasting and, coupled with the nation’s inability to act decisively to counteract climate change, portend a future where we are more vulnerable to hazards at multiple scales. Extreme natural events (either unprecedented magnitudes or intensities of natural hazards or the unprecedented consequences of more routine hazards) may become normal occurrences under changing climatic conditions or changes in economic circumstances and social conditions.1,2 Low-probability/high-consequence events and highly improbable ones like earthquakes, pandemics, and other kinds of hazards take on more policy interest as these events become more probable.3-5 From a policy perspective, these unlikely events pose significant risk management challenges, starting with how to encourage investments to lessen the impacts of these disasters (Figure 1). by Susan L. Cutter, Joseph A. Ahearn, Bernard Amadei, Patrick Crawford, Elizabeth A. Eide, Gerald E. Galloway, Jr., Michael F. Goodchild, Howard C. Kunreuther, Meredith Li-Vollmer, Monica Schoch-Spana, Susan C. Scrimshaw, Ellis M. Stanley, Sr., Gene Whitney, and Mary Lou Zoback

Regional geophysics of the Colorado Plateau

Abstract The Colorado Plateau (CP) is a relatively coherent block surrounded on three sides by the extensional block faulted regime of the Basin and Range Province (BRP) and the Rio Grande Rift (RGR). The CP appears to be part of an inter-related system including the Sierra Nevada, BRP, and RGR which has undergone major uplift and extension during the last 20 m.y. The final elevation in any area probably depends upon which processes dominate there. In most geophysical properties the CP is intermediate between the BRP/RGR and the stable platform of the southern Great Plains. However, many BRP/RGR geophysical characteristics appear to extend well inward of the classical Plateau physiographic boundary. Geologically these 50–100 km wide zones of transition are marked by normal faulting and late Tertiary and Quaternary volcanism. The interior of the Plateau is characterized by a 40 km-thick crust, a Pn velocity of about 7.85 km/sec, and an average heat flow of 1.5–1.6 HFU. Available data on the modern stress field in the Plateau interior indicate high horizontal stresses and a stress field oriented differently from that in the surrounding BRP/RGR, inconsistent with the theory that the Plateau is merely an inherited, more coherent subplate subjected to the same stresses as its surroundings. Free-air gravity anomalies on the CP average near zero and imply isostatic equilibrium; however, crustal thickness is insufficient to explain all the elevation, and low density mantle material must be involved. P wave velocity, gravity, and electrical data are compatible with a depth of approximately 80 km to low density, low velocity, hot conductive mantle (= asthenopshere?). These data also suggest a lithospheric thickness of ~120 km for the southern Great Plains, while surface wave data for the BRP suggest an approximately 60 km-thick lithosphere. Low angle subduction in mid-Cenozoic time has been inferred from calc-alkalic magmatic activity throughout a broad region of the western U.S. Subsequent restriction of this activity to the Sierras and westernmost BRP suggest an abrupt switch to steep subduction which occurred by about 20 m.y. ago, probably with rupturing of the low angle slab. Gradual warming and expansion (with phase changes) of the cutoff stagnant shallow slab is suggested as a possible mechanism for regional uplift.

Basin and Range rifting in northern Nevada: Clues from a mid-Miocene rift and its subsequent offsets

A linear rift composed of dike swarms and graben-filling volcanic rocks marks the inception of Basin and Range rifting in northern Nevada 17 to 14 m.y. ago. This mid-Miocene rift, with its associated aeromagnetic anomaly, indicates S68/sup 0/W-N68/sup 0/E (+-5/sup 0/) extension. A small strike-slip (transform) fault at Sawtooth dike, one short element of the rift, confirms the mid-Miocene extension direction. The present extension, based on data throughout the Basin and Range province, is N65/sup 0/W-S65/sup 0/E (+-20/sup 0/), which is consistent with younger fault offsets of the rift. In the Sawtooth dike region this 45/sup 0/ change in direction of extension took place between 15 and 6 m.y. B.P. The Nevada rift is thought to be a part of a linear rift zone roughly 700 km long, which includes feeder dikes of Columbia River basalts and the mid-Miocene location of the Yellowstone hot spot. Extension in this zone was probably perpendicular to a then-active trench along the western plate margin. The clockwise change in extension direction is consistent with a superposition of right-lateral shear along the plate margin when the San Andreas fault was activated.

Recent state of stress change in the Walker Lane zone, western Basin and Range province, United States

The NW to north-trending Walker Lane zone (WLZ) is located along the western boundary of the northern Basin and Range province with the Sierra Nevada. This zone is distinguished from the surrounding Basin and Range province on the basis of irregular topography and evidence for both normal and strike-slip Holocene faulting. Inversion of slip vectors from active faults, historic fault offsets, and earthquake focal mechanisms indicate two distinct Quaternary stress regimes within the WLZ, both of which are characterized by a consistent WNW σ3 axis; these are a normal faulting regime with a mean σ3 axis of N85°±9°W and a mean stress ratio (R value) (R=(σ2-σ1)/(σ3-σ1)) of 0.63–0.74 and a younger strike-slip faulting regime with a similar mean σ3 axis (N65° – 70°W) and R values ranging between ∼ 0.1 and 0.2. This younger regime is compatible with historic fault offsets and earthquake focal mechanisms. Both the extensional and strike-slip stress regimes reactivated inherited Mesozoic and Cenozoic structures and also produced new faults. The present-day strike-slip stress regime has produced strike-slip, normal oblique-slip, and normal dip-slip historic faulting. Previous workers have explained the complex interaction of active strike-slip, oblique, and normal faulting in the WLZ as a simple consequence of a single stress state with a consistent WNW σ3 axis and transitional between strike-slip and normal faulting (maximum horizontal stress approximately equal to vertical stress, or R ≈ 0 in both regimes) with minor local fluctuations. The slip data reported here support previous results from Owens Valley that suggest deformation within temporally distinct normal and strike-slip faulting stress regimes with a roughly constant WNW trending σ3 axis (Zoback, 1989). A recent change from a normal faulting to a strike-slip faulting stress regime is indicated by the crosscutting striae on faults in basalts <300,000 years old and is consistent with the dominantly strike-slip earthquake focal mechanisms and the youngest striae observed on faults in Plio-Quaternary deposits. Geologic control on the timing of the change is poor; it is impossible to determine if there has been a single recent absolute change or if there is, rather, an alternating or cyclical variation in stress magnitudes. Our slip data, in particular, the cross-cutting normal and strike-slip striae on the same fault plane, are inconsistent with postulated simple strain partitioning of deformation within a single regional stress field suggested for the WLZ by Wesnousky and Jones [1994]. The location of the WLZ between the deep-seated regional extension of the Basin and Range and the right-lateral strike-slip regional tectonics of the San Andreas fault zone is probably responsible for the complex interaction of tectonic regimes in this transition zone. In early to mid-Tertiary time the WLZ appears to have had a similarly complex deformational history, in this case as a back arc or intra-arc region, accommodating at least part of the right-lateral component of oblique convergence as well as a component of extension.

The northern Nevada rift: Regional tectono-magmatic relations and middle Miocene stress direction

As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500 km from southern Nevada to the Oregon Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that, based on new radiometric dating, erupted between 17 and 14 Ma and probably during an even shorter time interval. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space (at its northern terminus) with the oldest silicic caldera complex along the Yellowstone hot-spot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault. The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse (∼95% volumetrically) off the Columbia River flood basalts in northern Oregon ∼15.5-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The site of the initial breaching of the North America plate by this plume is probably the McDermitt volcanic center at the north end off the rift near the Oregon-Nevada border. The present north-northwest trend of the rift and its internal elements, such as dikes and lava-filled grabens, record the orientation of the arc-normal extensional stress in this back-arc region at the time of emplacement. Paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range and Rio Grande rift provinces.

Stress perturbation associated with the Amazonas and other ancient continental rifts

The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0–2.6 km depth) breakout data and deep (20–45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ∼75° rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80–100 MPa probably most representative of the overall structure. The observed ∼75° stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20–40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.