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Faults & earthquakes

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At least 1,585 records · Page 88Linked to original sources

USGS2019

Distributed fault slip in the eastern California shear zone: Adding pieces to the puzzle near Barstow, California

We investigate the dextral Lockhart and Mt. General faults, which are among four active structures in the northwestern portion of the eastern California shear zone (ECSZ). Early mapping depicts the Lockhart and Mt. General faults as discontinuous fault traces that continue northwest of the Lenwood Fault. Recent work indicates that the Lenwood Fault slips at ~0.2-1.0 mm/yr over the past ~8 ka and 0.8 ± 0.2 mm/yr since ~37 ± 7 ka. We reconstruct the record of fault slip for the Lockhart and Mt. General faults using high-resolution Structure-from-Motion built topography, field observations, geochronology, and gravity data. Geomorphic offsets along a Holocene-active trace of the Lockhart Fault indicate dextral displacement between ~4 and 6 m. A feldspar infrared stimulated luminescence (IRSL) age implies surface abandonment and at least one earthquake after 3540 ± 880 ka (2σ). The implied Holocene fault slip rate on the Lockhart Fault is between ~0.9 and 2.3 mm/yr. Holocene-active traces of the 19-km-long Mt. General Fault are marked by southwest-facing scarps and dextral offsets of ~4–5 m on alluvial fans, with down-to-the-southwest vertical offset of ~0.3 m. Summing dextral displacements across subparallel fault strands yields a maximum of ~7–8 m. A feldspar IRSL age indicates deposition of the alluvial fans since 11,380 ± 1700 ka (2σ). This results in a Holocene slip ~0.3–0.6 mm/yr, possibly ranging up to 1.0 mm/yr. Taken together, these observations imply a net Holocene dextral slip rate for active faults in Hinkley Valley at 1.2–3.3 mm/yr―higher than expected given published fault slip rates along-strike to the southeast.

California↗

USGS2009

Geometry of the Nojima fault at Nojima-Hirabayashi, Japan - I. A simple damage structure inferred from borehole core permeability

The 1995 Kobe (Hyogo-ken Nanbu) earthquake, M = 7.2, ruptured the Nojima fault in southwest Japan. We have studied core samples taken from two scientific drillholes that crossed the fault zone SW of the epicentral region on Awaji Island. The shallower hole, drilled by the Geological Survey of Japan (GSJ), was started 75 m to the SE of the surface trace of the Nojima fault and crossed the fault at a depth of 624 m. A deeper hole, drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) was started 302 m to the SE of the fault and crossed fault strands below a depth of 1140 m. We have measured strength and matrix permeability of core samples taken from these two drillholes. We find a strong correlation between permeability and proximity to the fault zone shear axes. The half-width of the high permeability zone (approximately 15 to 25 m) is in good agreement with the fault zone width inferred from trapped seismic wave analysis and other evidence. The fault zone core or shear axis contains clays with permeabilities of approximately 0.1 to 1 microdarcy at 50 MPa effective confining pressure (10 to 30 microdarcy at in situ pressures). Within a few meters of the fault zone core, the rock is highly fractured but has sustained little net shear. Matrix permeability of this zone is approximately 30 to 60 microdarcy at 50 MPa effective confining pressure (300 to 1000 microdarcy at in situ pressures). Outside this damage zone, matrix permeability drops below 0.01 microdarcy. The clay-rich core material has the lowest strength with a coefficient of friction of approximately 0.55. Shear strength increases with distance from the shear axis. These permeability and strength observations reveal a simple fault zone structure with a relatively weak fine-grained core surrounded by a damage zone of fractured rock. In this case, the damage zone will act as a high-permeability conduit for vertical and horizontal flow in the plane of the fault. The fine-grained core region, however, will impede fluid flow across the fault.

Pure and Applied Geophysics↗

USGS2012

Modification of wave-cut and faulting-controlled landforms

From a casual observation that the form of degraded fault scarps resembles the error function, this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms. The morphologic observations can be analyzed either in the form of one or more cross-strike elevation profiles or in the form of the slope-offset plot, a point plot of maximum scarp slope versus scarp offset. Working with either or both of these data representations for nine geologic structures, which range in age from 3 to 400 ka B.P. and in offset from 1 to 50 m, we apply analytical solutions for the vertical initial value scarp, the vertical continuous offset scarp, and the finite slope, initial value scarp. The model calculations are intrinsically ambiguous, yielding as the final answer only the product κ t (in the case of the initial value problem) or the product κ A −1 (in the case of the repeated faulting problem); here t is the age of a single scarp-forming event, 2 A is the vertical slip rate, and κ is the “mass diffusivity.” A single profile across three sea cliffs along the Santa Cruz, California, coast is analyzed as three separate initial value problems. A reasonably constrained age for the sea cliff standing above the Highway 1 platform returns κ = 11 GKG (1 GKG = 1 m 2 /ka). With this κ, we can date the two older sea cliffs. In fact, we do the converse: age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same κ as for the lower sea cliff. We treat a single profile of the Raymond fault in Pasadena/San Marino in terms of the repeated faulting problem; for it the uplift rate of R. Crook and others yields κ = 16 GKG. The very substantial preexisting offset across the Raymond fault must have been buried/leveled some 230 ka B.P., when the modern topography began to form. Our analysis of the Lake Bonneville shoreline scarps reveals a dependence of κ t on 2a, suggestive of nonlinear modification processes. This appearance is treated with the finite slope initial value scarp model to determine κ=1.1 GKG for the Lake Bonneville shoreline scarps. The suggestion of M. N. Machette that approximately 100,000-year-old, meter-high scarps are “unobservable” in weakly consolidated alluvial terranes of the Basin and Range and Rio Grande Rift Valley provinces can be formulated as κ ≳ 1 GKG. The coincidence between this inequality and the Lake Bonneville shoreline κ is striking, and it suggests that the value of κ = 1 GKG may be generally applicable, as a good first approximation, to the modification of alluvial terranes within the semiarid regions of the western United States. The Lake Bonneville shoreline κ is the basis for dating four sets of fault scarps in west-central Utah. The Drum Mountains fault scarps can be modeled in several different circumstances, but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3.6 to 5.7 ka B.P. The younger age is associated with quite low initial slope angles (25°). The other three sets of fault scarps show no evidence for finite initial value slopes. Fault scarps along the eastern base of the Fish Springs Range are very young, 3 ka B.P. We estimate the age of fault scarps along the western flank of the Oquirrh Mountains to be 32 ka B.P., which meets the weak geologic constraint that they be older than the Lake Bonneville shoreline. Fault scarps along the northeastern margin of the Sheeprock Mountains are even older, 53 ka B.P. An intriguing consequence of our single-event analysis of these scarps is that an 11.5-m offset occurred in a single earthquake.

Journal of Geophysical Research Solid Earth↗

USGS2017

Neotectonics of interior Alaska and the late Quaternary slip rate along the Denali fault system

The neotectonics of southern Alaska (USA) are characterized by a several hundred kilometers–wide zone of dextral transpressional that spans the Alaska Range. The Denali fault system is the largest active strike-slip fault system in interior Alaska, and it produced a M w 7.9 earthquake in 2002. To evaluate the late Quaternary slip rate on the Denali fault system, we collected samples for cosmogenic surface exposure dating from surfaces offset by the fault system. This study includes data from 107 samples at 19 sites, including 7 sites we previously reported, as well as an estimated slip rate at another site. We utilize the interpreted surface ages to provide estimated slip rates. These new slip rate data confirm that the highest late Quaternary slip rate is ∼13 mm/yr on the central Denali fault near its intersection with the eastern Denali and the Totschunda faults, with decreasing slip rate both to the east and west. The slip rate decreases westward along the central and western parts of the Denali fault system to 5 mm/yr over a length of ∼575 km. An additional site on the eastern Denali fault near Kluane Lake, Yukon, implies a slip rate of ∼2 mm/yr, based on geological considerations. The Totschunda fault has a maximum slip rate of ∼9 mm/yr. The Denali fault system is transpressional and there are active thrust faults on both the north and south sides of it. We explore four geometric models for southern Alaska tectonics to explain the slip rates along the Denali fault system and the active fault geometries: rotation, indentation, extrusion, and a combination of the three. We conclude that all three end-member models have strengths and shortcomings, and a combination of rotation, indentation, and extrusion best explains the slip rate observations.

Alaska↗

USGS2002

Delineation of faulting and basin geometry along a seismic reflection transect in urbanized San Bernardino Valley, California

Fourteen kilometers of continuous, shallow seismic reflection data acquired through the urbanized San Bernardino Valley, California, have revealed numerous faults between the San Jacinto and San Andreas faults as well as a complex pattern of downdropped and uplifted blocks. These data also indicate that the Loma Linda fault continues northeastward at least 4.5 km beyond its last mapped location on the southern edge of the valley and to within at least 2 km of downtown San Bernardino. Previously undetected faults within the valley northeast of the San Jacinto fault are also imaged, including the inferred western extension of the Banning fault and several unnamed faults. The Rialto-Colton fault is interpreted southwest of the San Jacinto fault. The seismic data image the top of the crystalline basement complex across 70% of the profile length and show that the basement has an overall dip of roughly 10?? southwest between Perris Hill and the San Jacinto fault. Gravity and aeromagnetic data corroborate the interpreted location of the San Jacinto fault and better constrain the basin depth along the seismic profile to be as deep as 1.7 km. These data also corroborate other fault locations and the general dip of the basement surface. At least 1.2 km of apparent vertical displacement on the basement is observed across the San Jacinto fault at the profile location. The basin geometry delineated by these data was used to generate modeled ground motions that show peak horizontal amplifications of 2-3.5 above bedrock response in the 0.05- to 1.0-Hz frequency band, which is consistent with recorded earthquake data in the valley.

Bulletin of the Seismological Society of America↗

USGS1979

Clastic dikes of Heart Mountain fault breccia, northwestern Wyoming, and their significance

Structural features in northwestern Wyoming indicate that the Heart Mountain fault movement was an extremely rapid, cataclysmic event that created a large volume of carbonate fault breccia derived entirely from the lower part of the upper plate. After fault movement had ceased, much of the carbonate fault breccia, here called calcibreccia, lay loose on the resulting surface of tectonic denudation. Before this unconsolidated calcibreccia could be removed by erosion, it was buried beneath a cover of Tertiary volcanic rocks: the Wapiti Formation, composed of volcanic breccia, poorly sorted volcanic breccia mudflows, and lava flows, and clearly shown in many places by inter lensing and intermixing of the calcibreccia with basal volcanic rocks. As the weight of volcanic overburden increased, the unstable water-saturated calcibreccia became mobile and semifluid and was injected upward as dikes into the overlying volcanic rocks and to a lesser extent into rocks of the upper plate. In some places the lowermost part of the volcanic overburden appears to have flowed with the calcibreccia to form dike like bodies of mixed volcanic rock and calcibreccia. One calcibreccia dike even contains carbonized wood, presumably incorporated into unconsolidated calcibreccia on the surface of tectonic denudation and covered by volcanic rocks before moving upward with the dike. Angular xenoliths of Precambrian rocks, enclosed in another calcibreccia dike and in an adjoining dikelike mass of volcanic rock as well, are believed to have been torn from the walls of a vent and incorporated into the basal part of the Wapiti Formation overlying the clastic carbonate rock on the fault surface. Subsequently, some of these xenoliths were incorporated into the calcibreccia during the process of dike intrusion. Throughout the Heart Mountain fault area, the basal part of the upper-plate blocks or masses are brecciated, irrespective of the size of the blocks, more intensely at the base and in places extending upward for several tens of meters. North of Republic Mountain a small 25-m-high upper-plate mass, brecciated to some degree throughout, apparently moved some distance along the Heart Mountain fault as brecciated rock. Calcibreccia dikes intrude upward from the underlying 2 m of fault breccia into the lower part of the mass and also from its top into the overlying volcanic rocks; an earthquake-related mechanism most likely accounts for the observed features of this deformed body. Calcibreccia dikes are more common within the bedding-plane phase of the Heart Mountain fault but also occur in its transgressive and former land-surface phases. Evidence that the Wapiti Formation almost immediately buried loose, unconsolidated fault breccia that was the source of the dike rock strongly suggests a rapid volcanic deposition over the area in which clastic dikes occur, which is at least 75 km long. Clastic dikes were injected into both the upper-plate and the volcanic rocks at about the same time, after movement on the Heart Mouuntain fault had ceased, and therefore do not indicate a fluid-flotation mechanism for the Heart Mountain fault. The difference between contacts of the clastic dikes with both indurated and unconsolidated country rock is useful in field mapping at localities where it is difficult to distinguish between volcanic rocks of the Cathedral Cliffs and Lamar River Formations, and the Wapiti Formation. Thus, calcibreccia dikes in the Cathedral Cliffs and Lamar River Formations show a sharp contact because the country rock solidified prior to fault movement, whereas calcibreccia dikes in the Wapiti Formation in many instances show a transitional or semifluid contact because the country rock was still unconsolidated or semifluid at the time of dike injection.

Professional Paper↗

USGS2024

Updating the crustal fault model for the 2023 National Seismic Hazard Model for Alaska

We present the crustal fault model for Alaska, based on geologic observations, as a primary input for the 2023 revision of the U.S. Geological Survey National Seismic Hazard Model. We update the 2013 Alaska Quaternary fault and fold database to produce a simplified model of 105 fault sections and four fault zone polygons with basic geologic parameters including slip sense and rate. Significant updates include the following: (1) a slip rate of ∼53 mm/year on the Queen Charlotte Fault indicating it accommodates all of the plate boundary motion; (2) quantified slip rates on megathrust splay faults in the southern Prince William Sound region and near Kodiak Island; (3) improved details of structures in the Chugach-St. Elias orogen; (4) revision of the Castle Mountain Fault from right-lateral slip to a predominantly reverse fault; (5) improved Interior Alaska tectonic models that clarify relationships between the Denali, Totschunda, and thrust faults on both sides of the Alaska Range; (6) identified large earthquake sources in the eastern Brooks Range; and (7) omission of the Chatham Strait section of the Denali Fault. The fault model underscores that the collision of the Yakutat microplate is the dominant driver of active crustal faulting in most of Alaska.

Alaska, Yukon↗

USGS2009

Premonitory acoustic emissions and stick-slip in natural and smooth-faulted Westerly granite

A stick-slip event was induced in a cylindrical sample of Westerly granite containing a preexisting natural fault by loading at constant confining pressure of 150 MPa. Continuously recorded acoustic emission (AE) data and computer tomography (CT)-generated images of the fault plane were combined to provide a detailed examination of microscale processes operating on the fault. The dynamic stick-slip event, considered to be a laboratory analog of an earthquake, generated an ultrasonic signal that was recorded as a large-amplitude AE event. First arrivals of this event were inverted to determine the nucleation site of slip, which is associated with a geometric asperity on the fault surface. CT images and AE locations suggest that a variety of asperities existed in the sample because of the intersection of branch or splay faults with the main fault. This experiment is compared with a stick-slip experiment on a sample prepared with a smooth, artificial saw-cut fault surface. Nearly a thousand times more AE were observed for the natural fault, which has a higher friction coefficient (0.78 compared to 0.53) and larger shear stress drop (140 compared to 68 MPa). However at the measured resolution, the ultrasonic signal emitted during slip initiation does not vary significantly between the two experiments, suggesting a similar dynamic rupture process. We propose that the natural faulted sample under triaxial compression provides a good laboratory analogue for a field-scale fault system in terms of the presence of asperities, fault surface heterogeneity, and interaction of branching faults. ?? 2009.

Journal of Geophysical Research B: Solid Earth↗

USGS2011

Seismic swarm associated with the 2008 eruption of Kasatochi Volcano, Alaska: earthquake locations and source parameters

An energetic seismic swarm accompanied an eruption of Kasatochi Volcano in the central Aleutian volcanic arc in August of 2008. In retrospect, the first earthquakes in the swarm were detected about 1 month prior to the eruption onset. Activity in the swarm quickly intensified less than 48 h prior to the first large explosion and subsequently subsided with decline of eruptive activity. The largest earthquake measured as moment magnitude 5.8, and a dozen additional earthquakes were larger than magnitude 4. The swarm exhibited both tectonic and volcanic characteristics. Its shear failure earthquake features were b value = 0.9, most earthquakes with impulsive P and S arrivals and higher-frequency content, and earthquake faulting parameters consistent with regional tectonic stresses. Its volcanic or fluid-influenced seismicity features were volcanic tremor, large CLVD components in moment tensor solutions, and increasing magnitudes with time. Earthquake location tests suggest that the earthquakes occurred in a distributed volume elongated in the NS direction either directly under the volcano or within 5-10 km south of it. Following the M W 5.8 event, earthquakes occurred in a new crustal volume slightly east and north of the previous earthquakes. The central Aleutian Arc is a tectonically active region with seismicity occurring in the crusts of the Pacific and North American plates in addition to interplate events. We postulate that the Kasatochi seismic swarm was a manifestation of the complex interaction of tectonic and magmatic processes in the Earth's crust. Although magmatic intrusion triggered the earthquakes in the swarm, the earthquakes failed in context of the regional stress field.

Alaska↗

USGS2021

Marine paleoseismic evidence for seismic and aseismic slip along the Hayward-Rodgers Creek fault system in northern San Pablo Bay

Distinguishing between seismic and aseismic fault slip in the geologic record is difficult, yet fundamental to estimating the seismic potential of faults and the likelihood of multi-fault ruptures. We integrated chirp sub-bottom imaging with targeted cross-fault coring and core analyses of sedimentary proxy data to characterize vertical deformation and slip behavior within an extensional fault bend along the Hayward-Rodgers Creek fault system in northern San Pablo Bay. We identified and traced four key seismic horizons (R1–R4), all younger than approximately 1400 CE, that cross the fault and extend throughout the basin. A stratigraphic age model was developed using detailed down-core radiocarbon and radioisotope dating combined with measurements of anthropogenic metal concentrations. The onset of hydraulic mining within the Sierra Nevada in 1852 CE left a clear geochemical and magnetic signature within core samples. This key time horizon was used to calculate a local reservoir correction and reduce uncertainty in radiocarbon age calibration and models. Vertical fault offset of strata younger than the most recent surface-rupturing earthquake on the Hayward fault in 1868 CE suggest near-surface vertical creep is occurring along the fault in northern San Pablo Bay at a rate of approximately 0.4 mm/yr. In addition, we present evidence of at least one and possibly two coseismic events associated with growth strata above horizons R1 and R2, with median event ages estimated to be 1400 CE and 1800 CE, respectively. The timing of both these events overlaps with paleoseismic events on adjacent fault sections, suggesting the possibility of multi-fault rupture.

California↗

USGS2012

Three-dimensional velocity structure, seismicity, and fault structure in the Parkfield region, central California

This study examines the three-dimensional velocity structure in a 60- by 80-km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shot P arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three-dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across-fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4-km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low-velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20-km-long high-velocity slice is imaged northeast of the SAF near Gold Hill. Its along-fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right-lateral strike-slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure.

Journal of Geophysical Research↗

USGS1996

The southern Whidbey Island fault: An active structure in the Puget Lowland, Washington

Information from seismic-reflection profiles, outcrops, boreholes, and potential field surveys is used to interpret the structure and history of the southern Whidbey Island fault in the Puget Lowland of western Washington. This northwest-trending fault comprises a broad (as wide as 6–11 km), steep, northeast-dipping zone that includes several splays with inferred strike-slip, reverse, and thrust displacement. Transpressional deformation along the southern Whidbey Island fault is indicated by along-strike variations in structural style and geometry, positive flower structure, local unconformities, out-of-plane displacements, and juxtaposition of correlative sedimentary units with different histories. The southern Whidbey Island fault represents a segment of a boundary between two major crustal blocks. The Cascade block to the northeast is floored by diverse assemblages of pre-Tertiary rocks; the Coast Range block to the southwest is floored by lower Eocene marine basaltic rocks of the Crescent Formation. The fault probably originated during the early Eocene as a dextral strike-slip fault along the eastern side of a continental-margin rift. Bending of the fault and transpressional deformation began during the late middle Eocene and continues to the present. Oblique convergence and clockwise rotation along the continental margin are the inferred driving forces for ongoing deformation. Evidence for Quaternary movement on the southern Whidbey Island fault includes (1) offset and disrupted upper Quaternary strata imaged on seismic-reflection profiles; (2) borehole data that suggests as much as 420 m of structural relief on the Tertiary-Quaternary boundary in the fault zone; (3) several meters of displacement along exposed faults in upper Quaternary sediments; (4) late Quaternary folds with limb dips of as much as ≈9°; (5) large-scale liquefaction features in upper Quaternary sediments within the fault zone; and (6) minor historical seismicity. The southern Whidbey Island fault should be considered capable of generating large earthquakes (M s ≥7) and represents a potential seismic hazard to residents of the Puget Lowland.

Washington↗

USGS2003

An empirical model for earthquake probabilities in the San Francisco Bay region, California, 2002-2031

The moment magnitude M 7.8 earthquake in 1906 profoundly changed the rate of seismic activity over much of northern California. The low rate of seismic activity in the San Francisco Bay region (SFBR) since 1906, relative to that of the preceding 55 yr, is often explained as a stress-shadow effect of the 1906 earthquake. However, existing elastic and visco-elastic models of stress change fail to fully account for the duration of the lowered rate of earthquake activity. We use variations in the rate of earthquakes as a basis for a simple empirical model for estimating the probability of M ≥6.7 earthquakes in the SFBR. The model preserves the relative magnitude distribution of sources predicted by the Working Group on California Earthquake Probabilities' ( WGCEP, 1999 ; WGCEP, 2002 ) model of characterized ruptures on SFBR faults and is consistent with the occurrence of the four M ≥6.7 earthquakes in the region since 1838. When the empirical model is extrapolated 30 yr forward from 2002, it gives a probability of 0.42 for one or more M ≥6.7 in the SFBR. This result is lower than the probability of 0.5 estimated by WGCEP ( 1988 ), lower than the 30-yr Poisson probability of 0.60 obtained by WGCEP ( 1999 ) and WGCEP ( 2002 ), and lower than the 30-yr time-dependent probabilities of 0.67, 0.70, and 0.63 obtained by WGCEP ( 1990 ), WGCEP ( 1999 ), and WGCEP ( 2002 ), respectively, for the occurrence of one or more large earthquakes. This lower probability is consistent with the lack of adequate accounting for the 1906 stress-shadow in these earlier reports. The empirical model represents one possible approach toward accounting for the stress-shadow effect of the 1906 earthquake. However, the discrepancy between our result and those obtained with other modeling methods underscores the fact that the physics controlling the timing of earthquakes is not well understood. Hence, we advise against using the empirical model alone (or any other single probability model) for estimating the earthquake hazard and endorse the use of all credible earthquake probability models for the region, including the empirical model, with appropriate weighting, as was done in WGCEP ( 2002 ).

California↗

USGS2015

Proceedings of the 9th U.S.-Japan natural resources panel for earthquake research

Introduction The UJNR Panel on Earthquake Research promotes advanced study toward a more fundamental understanding of the earthquake process and hazard estimation. The Ninth Joint meeting was extremely beneficial in furthering cooperation and deepening understanding of problems common to both the U.S. and Japan. The meeting included productive exchanges of information on approaches to systematic observation and modeling of earthquake processes. Regarding the earthquake and tsunami of March 2011 off the Pacific coast of Tohoku, the Panel recognizes that further efforts are necessary to achieve our common goal of reducing earthquake risk through close collaboration and focused discussions at the 10th UJNR meeting. We look forward to continued cooperation on issues involving the densification of observation networks and the open exchange of data among scientific communities. We recognize the importance of making information publicly available in a timely manner. We also recognize the importance of information exchange on research policy and strategies, including the frameworks of research organizations. Areas of Cooperation –Specific areas of earthquake research where cooperative research between the U.S. and Japan may lead to significant advancement include, but are not limited to: –Probabilistic earthquake and tsunami hazard estimation, including extraordinarily large earthquakes, both in our respective countries and worldwide, incorporating knowledge of current and past behavior, and physics based computational models; –Real-time information from seismic, geodetic and strain measurements, including borehole strainmeters and seafloor observations using offshore cabled networks; –Technologies for measuring crustal deformation; –Early warning technologies for earthquakes and tsunamis; Studies of recurrence of large and extraordinary large earthquakes using paleoseismic, paleotsunami, geodetic and seismic methods; –Laboratory, theoretical and in situ studies of fault-zone processes; –Studies of episodic tremor and slow slip events using seismic, geodetic, and borehole strain measurements, and simulation techniques; –Systematic studies of earthquake predictability through rigorously evaluated scientific prediction experiments and robust databases; –Studies of near-source ground motions, geological effects and the response of engineered structures. The Panel strongly urges that the appropriate agencies in the U.S. and Japan that are represented on this panel work together with the academic sector to support and coordinate scientific work in these areas of cooperation. The Panel recognizes the importance of promoting the exchange of scientific personnel, exchange of data, and fundamental studies to advance progress in earthquake research. The U.S. and Japan should promote these exchanges throughout the world. The Panel endorses continuation of these activities.

Open-File Report↗

USGS1982

Performance assessments for radioactive waste repositories; the rate of movement of faults

Performance assessments of mined repositories for radioactive waste require estimates of the likelihood of fault movements and earthquakes that may affect the repository and its surrounding ground water flow system. Some previous assessments have attempted to estimate the rate of formation of new faults; some have relied heavily on historic seismicity or the time of latest movement on faults. More appropriate emphasis is on the identification of faults that have been active or may have been active under the present teconic regime in a broad region and on estimates of the long-term rate of movement of such faults. Faults that have moved under the current stress field, even at low rates, are likely to move again during the time the wastes will remain toxic. A continuum exists for the present rate of movement of faults which ranges from 10 mm per year for obviously active faults along the western margin of the North American plate to as low as 10 -4 mm per year for recently documented faults in the Atlantic Coast province. On the basis of regional consistency in movement rates and constraints imposed by geomorphology, I derive upper bounds for the rates of occurrence of fault offsets for various crustal stress provinces in the conterminous United States. These upper bounds are not meant to substitute for detailed studies of specific faults and seismicity at specific sites. They can help to reduce the considerable uncertainty that attaches to all estimates of future tectonic activity. The principal uncertainty in their estimation is the manner in which total slip across faults is distributed among discrete events especially in regions in which the rate of movement is very low.

Open-File Report↗

USGS1999

The San Andreas fault in the San Francisco Bay region, California: Structure and kinematics of a Young plate boundary

Recently acquired high-resolution aeromagnetic data delineate offset and/or truncated magnetic rock bodies of the Franciscan Complex that define the location and structure of, and total offset across, the San Andreas fault in the San Francisco Bay region. Two distinctive magnetic anomalies caused by ultramafic rocks and metabasalts east of, and truncated at, the San Andreas fault have clear counterparts west of the fault that indicate a total right-lateral offset of only 22 km on the Peninsula segment, the active strand that ruptured in 1906. The location of the Peninsula segment is well defined magnetically on the northern peninsula where it goes offshore, and can be traced along strike an additional ~6 km to the northwest. Just offshore from Lake Merced, the inferred fault trace steps right (northeast) 3 km onto a nearly parallel strand that can be traced magnetically northwest more than 20 km as the linear northeast edge of a magnetic block bounded by the San Andreas fault, the Pilarcitos fault, and the San Gregorio-Hosgri fault zone. This right-stepping strand, the Golden Gate segment, joins the eastern mapped trace of the San Andreas fault at Bolinas Lagoon and projects back onshore to the southeast near Lake Merced. Inversion of detailed gravity data on the San Francisco Peninsula reveals a 3 km wide basin situated between the two strands of the San Andreas fault, floored by Franciscan basement and filled with Plio-Quaternary sedimentary deposits of the Merced and Colma formations. The basin, ~1 km deep at the coast, narrows and becomes thinner to the southeast along the fault over a distance of ~12 km. The length, width, and location of the basin between the two strands are consistent with a pull-apart basin formed behind the right step in the right-lateral strike-slip San Andreas fault system and currently moving southeast with the North American plate. Slight nonparallelism of the two strands bounding the basin (implying a small component of convergence with continued strike-slip movement) may explain the progressive narrowing of the basin to the southeast and the puzzling recent uplift of the Merced Formation in a predominantly extensional (pull-apart basin) setting. The 1906 San Francisco earthquake may have nucleated within the step-over region, and the step-over places a strand of the San Andreas fault 3 km closer to downtown San Francisco than previously thought.

International Geology Review↗

USGS1995

Frequency-moment distribution of deep earthquakes; Implications for the seismogenic zone at the bottom of slabs

We present a systematic investigation of the variation with depth of the frequency of earthquake occurrence vs. seismic moment based on 16 years of Harvard Centroid Moment Tensor (CMT) solutions. We analyze depth variations of earthquake size distribution in terms of variations in the absolute value of the slope of the regression of the logarithm of the population vs. seismic moment, a quantity known as the β parameter. The shallowest earthquakes (0–50 km depth) exhibit a well-defined and robust size distribution regime characterized by a discontinuous increase in β with increasing moment. Others have shown that this increase probably represents the effects of a physical limit in the dimensions of the area of seismogenic slip of shallow earthquake sources. The population of deep earthquakes in the depth interval 500–600 km shows two markedly different distributions. The deep earthquakes in the Tonga region feature an initially high β value (0.92) at small moments and a lower β value (0.41) at high moments. In contrast, the size distribution of non-Tonga deep events shows the reverse of those changes (β = 0.41 at low moment and β = 1.17 at higher moment). To help explain these observations, we propose a model of deep seismogenesis that assumes three-dimensional earthquake source regions that vary principally in their transverse dimensions. The two-β segment behavior in the Tonga region and other subduction zones is thought to represent, in part, constraints owing to the threshold of completeness of the CMT catalog and to its short time interval of sampling. We interpret the differences between Tonga and other deep Wadati-Benioff zones as being a consequence of Tonga's markedly higher subduction rate and, hence, its colder thermal structure and presumably thicker region of seismogenesis. We interpret the critical moments at which β values change in terms of variations in the transverse thickness of deep seismogenic zones and estimate that it is about 11 km for the Tonga region and about 3 km for other zones at depths of 500–600 km. These results are generally consistent with deep earthquakes being restricted to wedge-shaped regions of peridotite persisting metastably to as deep as 700 km in old, rapidly descending and hence cold slabs. Failure is thought to occur in metastable peridotite by transformational faulting. Great deep earthquakes present special challenges to any theory of deep earthquakes based on slab thermal structure. For example, a continuing question is how such large events can fit in a thermally controlled seismogenic zone that is diminishing in its transverse dimensions with increasing depth. The very concept of a scale-invariant earthquake size distribution may be inappropriate for these rare events and the unusual settings in which they are found.

Physics of the Earth and Planetary Interiors↗

USGS1977

Search for seismic forerunners to earthquakes in central California

The relatively high seismicity of the San Andreas fault zone in central California provides an excellent opportunity to search for seismic forerunners to moderate earthquakes. Analysis of seismic traveltime and earthquake location data has resulted in the identification of two possible seismic forerunners. The first is a period of apparently late (0.3 sec) P-wave arrival times lasting several weeks preceding one earthquake of magnitude 5.0. The rays for these travel paths passed through — or very close to — the aftershock volume of the subsequent earthquake. The sources for these P-arrival time data were earthquakes in the distance range 20–70 km. Uncertainties in the influence of small changes in the hypocenters of the source earthquakes and in the identification of small P-arrivals raise the possibility that the apparantly delayed arrivals are not the result of a decrease in P-velocity. The second possible precursor is an apparent increase in the average depth of earthquakes preceding two moderate earthquakes. This change might be only apparent, caused by a location bias introduced by a decrease in P-wave velocity, but numerical modeling for realistic possible changes in velocity suggests that the observed effect is more likely a true migration of earthquakes. To carry out this work — involving the manipulation of several thousand earthquake hypocenters and several hundred thousand readings of arrival time — a system of data storage was designed and manipulation programs for a large digital computer have been executed. This system allows, for example, the automatic selection of earthquakes from a specific region, the extraction of all the observed arrival times for these events, and their relocation under a chosen set of assumptions.

California↗