Showing papers on "Fault (geology) published in 2013"

Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake sequence

Abstract: Significant earthquakes are increasingly occurring within the continental interior of the United States, including five of moment magnitude (Mw) ≥ 5.0 in 2011 alone. Concurrently, the volume of fluid injected into the subsurface related to the production of unconventional resources continues to rise. Here we identify the largest earthquake potentially related to injection, an Mw 5.7 earthquake in November 2011 in Oklahoma. The earthquake was felt in at least 17 states and caused damage in the epicentral region. It occurred in a sequence, with 2 earthquakes of Mw 5.0 and a prolific sequence of aftershocks. We use the aftershocks to illuminate the faults that ruptured in the sequence, and show that the tip of the initial rupture plane is within ∼200 m of active injection wells and within ∼1 km of the surface; 30% of early aftershocks occur within the sedimentary section. Subsurface data indicate that fluid was injected into effectively sealed compartments, and we interpret that a net fluid volume increase after 18 yr of injection lowered effective stress on reservoir-bounding faults. Significantly, this case indicates that decades-long lags between the commencement of fluid injection and the onset of induced earthquakes are possible, and modifies our common criteria for fluid-induced events. The progressive rupture of three fault planes in this sequence suggests that stress changes from the initial rupture triggered the successive earthquakes, including one larger than the first.

Induced seismicity associated with fluid injection into a deep well in Youngstown, Ohio

Abstract: [1] Over 109 small earthquakes (Mw 0.4–3.9) were detected during January 2011 to February 2012 in the Youngstown, Ohio area, where there were no known earthquakes in the past. These shocks were close to a deep fluid injection well. The 14 month seismicity included six felt earthquakes and culminated with a Mw 3.9 shock on 31 December 2011. Among the 109 shocks, 12 events greater than Mw 1.8 were detected by regional network and accurately relocated, whereas 97 small earthquakes (0.4 < Mw < 1.8) were detected by the waveform correlation detector. Accurately located earthquakes were along a subsurface fault trending ENE-WSW—consistent with the focal mechanism of the main shock and occurred at depths 3.5–4.0 km in the Precambrian basement. We conclude that the recent earthquakes in Youngstown, Ohio were induced by the fluid injection at a deep injection well due to increased pore pressure along the preexisting subsurface faults located close to the wellbore. We found that the seismicity initiated at the eastern end of the subsurface fault—close to the injection point, and migrated toward the west—away from the wellbore, indicating that the expanding high fluid pressure front increased the pore pressure along its path and progressively triggered the earthquakes. We observe that several periods of quiescence of seismicity follow the minima in injection volumes and pressure, which may indicate that the earthquakes were directly caused by the pressure buildup and stopped when pressure dropped.

The Nature of Seismicity Patterns Before Large Earthquakes

Abstract: Various seismicity patterns before major earthquakes have been reported in the literature. They include foreshocks (broad sense), preseismic quiescence, precursory swarms, and doughnut patterns. Although many earthquakes are preceded by all, or some, of these patterns, their detail differ significantly from event to event. In order to examine the details of seismicity patterns on as uniform a basis as possible, we made space-time plots of seismicity for many large earthquakes by using the NOAA and JMA catalogs. Among various seismicity patterns, preseismic quiescence appears most common, the case for the 1978 Oaxaca earthquake being the most prominent. Although the nature of other patterns varies from event to event, a common physical mechanism may be responsible for these patterns; details of the pattern are probably controlled by the tectonic environment (fault geometry, strain rate) and the heterogeneity of the fault plane. Here a simple asperity model is introduced to explain these seismicity patterns. In this model, a fault plane with an asperity is divided into a number of subfaults. The subfaults within the asperity are, on the average, stronger than those in the surrounding weak zone. As the tectonic stress increases, the subfaults in the weak zone break in the form of background small earthquakes. If the frequency distribution of the strength of the subfaults has a sharp peak, a precursory swarm occurs. By this time, most of the subfaults in the weak zone are broken and the fault plane becomes seismically quiet. As the tectonic stress increases further, eventually the asperity breaks and sympathetic displacement occurs on the entire fault zone in the form of the main shock. Foreshocks do or do not occur depending upon the distribution of the strength of the subfaults within the asperity. Since the spatio-temporal change in the stress on the fault plane is most likely to dictate the change in seismicity patterns, detailed analysis of seismicity patterns would provide a most direct clue to the state of stress in the fault zone. However, because of the large variation from event to event, seismicity pattern alone is not a definitive tool for earthquake prediction; measurements of other physical parameters such as the spectra, the mechanism and the wave forms of the background events should be made concurrently.

Primary surface ruptures of the great Himalayan earthquakes in 1934 and 1255

Abstract: It is unclear where plate boundary thrusts generate giant rather than great earthquakes. Along the Himalayas, the source sizes and recurrence times of large seismic events are particularly uncertain, since no surface signatures were found for those that shook the range in the twentieth century. Here we challenge the consensus that these events remained blind and did not rupture the surface. We use geomorphological mapping of fluvial deposits, palaeo-seismological logging of river-cut cliffs and trench walls, and modelling of calibrated 14C ages, to show that the Mw 8.2 Bihar–Nepal earthquake on 15 January 1934 did break the surface: traces of the rupture are clear along at least 150 km of the Main Frontal Thrust fault in Nepal, between 85° 50′ and 87° 20′ E. Furthermore, we date collapse wedges in the Sir Valley and find that the 7 June AD 1255 earthquake, an event that devastated Kathmandu and mortally wounded the Nepalese King Abhaya Malla, also ruptured the surface along this stretch of the mega-thrust. Thus, in the past 1,000 years, two great earthquakes, 679 years apart, rather than one giant eleventh-century AD event, contributed to the frontal uplift of young river terraces in eastern Nepal. The rare surface expression of these earthquakes implies that surface ruptures of other reputedly blind great Himalayan events might exist. The recurrence times of great Himalayan earthquakes are difficult to assess because they rarely rupture the surface. Field mapping and 14C dating of offset fluvial deposits are used to identify two great Himalayan quakes that ruptured the surface along the main plate boundary fault in AD 1255 and 1934.

Earthquakes Triggered by Hydraulic Fracturing in South‐Central Oklahoma

Abstract: In January 2011, a sequence of earthquakes occurred in close proximity to a well, which was being hydraulically fractured in south‐central Oklahoma. The hydraulic fracturing of the Picket Unit B Well 4–18 occurred from 16 January 2011 18:43 through 22 January 16:54 UTC. This vertical well penetrated into the mature Eola‐Robberson oil field. Earthquakes were identified by cross correlating template waveforms from manually identified earthquakes and cross correlating these templates through the entire operation period of the Earthscope USArray Transportable Array (TA) station X34A. This produced a series of 116 earthquakes, which occurred from 17 January 2011 19:06 through 23 January 3:13 UTC with no other similar earthquakes identified at other times prior to or post‐hydraulic fracturing. The identified earthquakes range in local magnitude ( M L) from 0.6 to 2.9, with 16 earthquakes M L 2 or greater and a b ‐value of 0.98. There is a strong temporal correlation between hydraulic fracturing and earthquakes. This correlation is strengthened because hydraulic fracturing operations ceased for ∼2 days due to bad weather, and earthquakes can be observed to cease during this period and resume after hydraulic fracturing had resumed. Earthquakes were relocated using cross‐correlated phase arrivals and bootstrap iterations of hypoDD. Locations were well constrained for 86 earthquakes. These earthquake locations clearly delineate a fault which strikes ∼166°, subparallel to the mapped minor fault sets in the area, and dips steeply to the west. The earthquakes appear to have occurred at shallow depths from ∼2 to 3 km and within ∼2.5 km horizontally of the well. The first earthquake occurred ∼24 hrs after hydraulic fracturing began at the well. This delay is consistent with the diffusion of pore pressure in the subsurface over a distance of ∼2 km. Online Material: Results from bootstrap hypoDD relocations using cross‐correlation phase arrivals.

An Asperity Model of Large Earthquake Sequences

Abstract: The variation in maximum rupture extent of large shallow earthquakes in circum-Pacific subduction zones is interpreted in the context of the asperity model of stress distribution on the fault plane. Comparison of the historic record of large earthquakes in different zones indicates that four fundamental categories of behavior are observed. These are: (1) the Chile-type regular occurrence of great ruptures spanning more than 500 km; (2) the Aleutians-type variation in rupture extent with occasional ruptures up to 500 km long, and temporal clustering of large events; (3) the Kurile-type repeated failure over a limited zone of 100–300 km length in isolated events; and (4) the Marianas-type absence of large earthquakes. Southern Chile, Alaska, Southern Kamchatka, and possibly the Central Aleutians are grouped in the first category. The Rat Island portion of the Aleutians, Colombia, Southwest Japan, and the Solomon Islands zones demonstrate the temporal variation of rupture length and multiple earthquake sequences that characterize category 2. The New Hebrides and Middle America have earthquake clustering on a more moderate scale, and are intermediate between categories 2 and 3. Category 3 includes the Kurile Islands, Northeast Japan, Peru and Central Chile. Zones lacking large earthquakes (category 4) include the Marianas, Izu-Bonin, and large portions of Tonga-Kermadec. By loosely grouping each subduction zone into these categories and comparing the general range in behavior with a simple fault model, which is used in a numerical simulation, the parameters governing large earthquake development are clarified. Interpretation of the four categories in terms of asperity distribution and interaction permits some inferences of the nature of stress distribution in particular zones. Two factors appear to dominate in the development of large earthquake failure zones; the nature and degree of coupling on the fault contact, and the extent of lateral segmentation of the subduction zone by transverse stress barriers. Strong coupling and uniform stress distribution on the fault plane produces larger events, whereas more heterogeneous stress distributions produce smaller ruptures and temporal variation in rupture length. Segmentation of the subduction zone that may result in stress barriers affecting rupture length is produced by subduction of transverse structures such as aseismic ridges, and is reflected by submarine canyons and geometric variations in trench configuration.

Low-temperature thermochronometry along the Kunlun and Haiyuan Faults, NE Tibetan Plateau: Evidence for kinematic change during late-stage orogenesis

Abstract: The Tibetan Plateau is a prime example of a collisional orogen with widespread strike-slip faults whose age and tectonic significance remain controversial. We present new low-temperature thermochronometry to date periods of exhumation associated with Kunlun and Haiyuan faulting, two major strike-slip faults within the northeastern margin of Tibet. Apatite and zircon (U-Th)/He and apatite fission-track ages, which record exhumation from ~2 to 6 km crustal depths, provide minimum bounds on fault timing. Results from Kunlun samples show increased exhumation rates along the western fault segment at circa 12–8 Ma with a possible earlier phase of motion from ~30–20 Ma, along the central fault segment at circa 20–15 Ma, and along the eastern fault segment at circa 8–5 Ma. Combined with previous studies, our results suggest that motion along the Haiyuan fault may have occurred as early as ~15 Ma along the western/central fault segment before initiating at least by 10–8 Ma along the eastern fault tip. We relate an ~250 km wide zone of transpressional shear to synchronous Kunlun and Haiyuan fault motion and suggest that the present-day configuration of active faults along the northeastern margin of Tibet was likely established since middle Miocene time. We interpret the onset of transpression to relate to the progressive confinement of Tibet against rigid crustal blocks to the north and expansion of crustal thickening to the east during the later stages of orogen development.

Continuous permeability measurements record healing inside the Wenchuan earthquake fault zone.

Abstract: Permeability controls fluid flow in fault zones and is a proxy for rock damage after an earthquake. We used the tidal response of water level in a deep borehole to track permeability for 18 months in the damage zone of the causative fault of the 2008 moment magnitude 7.9 Wenchuan earthquake. The unusually high measured hydraulic diffusivity of 2.4 × 10−2 square meters per second implies a major role for water circulation in the fault zone. For most of the observation period, the permeability decreased rapidly as the fault healed. The trend was interrupted by abrupt permeability increases attributable to shaking from remote earthquakes. These direct measurements of the fault zone reveal a process of punctuated recovery as healing and damage interact in the aftermath of a major earthquake.

Structure and composition of the plate-boundary slip zone for the 2011 Tohoku-Oki earthquake.

Abstract: The mechanics of great subduction earthquakes are influenced by the frictional properties, structure, and composition of the plate-boundary fault. We present observations of the structure and composition of the shallow source fault of the 2011 Tohoku-Oki earthquake and tsunami from boreholes drilled by the Integrated Ocean Drilling Program Expedition 343 and 343T. Logging-while-drilling and core-sample observations show a single major plate-boundary fault accommodated the large slip of the Tohoku-Oki earthquake rupture, as well as nearly all the cumulative interplate motion at the drill site. The localization of deformation onto a limited thickness (less than 5 meters) of pelagic clay is the defining characteristic of the shallow earthquake fault, suggesting that the pelagic clay may be a regionally important control on tsunamigenic earthquakes.

Radiography of a normal fault system by 64,000 high-precision earthquake locations: The 2009 L'Aquila (central Italy) case study

Abstract: [1] We studied the anatomy of the fault system where the 2009 L'Aquila earthquake (MW 6.1) nucleated by means of ~64 k high-precision earthquake locations spanning 1 year. Data were analyzed by combining an automatic picking procedure for P and S waves, together with cross-correlation and double-difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. The fault system is composed by two major faults: the high-angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging wall and footwall along with bends and cross fault intersections along the main fault and fault splays. The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and co-seismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cutoff depth as the presence of a rheological (i.e., creeping) discontinuity explaining how normal faults detach at depth.

Fault Zone Architecture and Fluid Flow: Insights from Field Data and Numerical Modeling

Abstract: Fault zones in the upper crust are typically composed of complex fracture networks and discrete zones of comminuted and geochemically altered fault rocks. Determining the patterns and rates of fluid flow in these distinct structural discontinuities is a three-dimensional problem. A series of numerical simulations of fluid flow in a set of three-dimensional discrete fracture network models aids in identifying the primary controlling parameters of fault-related fluid flow, and their interactions, throughout episodic deformation. Four idealized, but geologically realistic, fault zone architectural models are based on fracture data collected along exposures of the Stillwater Fault Zone in Dixie Valley, Nevada and geometric data from a series of normal fault zones in east Greenland. The models are also constrained by an Andersonian model for mechanically compatible fracture networks associated with normal faulting. Fluid flow in individual fault zone components, such as a fault core and damage zone, and full outcrop scale model domains are simulated using a finite element routine. Permeability contrasts between components and permeability anisotropy within components are identified as the major controlling factors in fault-related fluid flow. Additionally, the structural and hydraulic variations in these components are also major controls of flow at the scale of the full model domains. The four models can also be viewed as a set of snapshots in the mechanical evolution of a single fault zone. Changes in the hydraulic parameters within the models mimic the evolution of the permeability structure of each model through a single deformation cycle. The model results demonstrate that small changes in the architecture and hydraulic parameters of individual fault zone components can have very large impacts, up to five orders of magnitude, on the permeability structure of the full model domains. Closure of fracture apertures in each fault zone magnifies the magnitude and orientation of permeability anisotropy in ways that are closely linked to the implicitly modeled deformation. Changes in fault zone architecture can cause major changes in permeability structure that, in turn, significantly impact the magnitude and patterns of fluid flux and solute transport both within and near the fault zone. Inferences derived from the model results are discussed in the context of the mechanical strength of an evolving fault zone, fault zone sealing mechanisms which control the conduit-barrier systematics of a fault zone as a flow system, and how these processes are related to fluid flow in natural fault zones.

Earthquake repeat time and average stress drop

Abstract: Existing data on source parameters of large crustal earthquakes (subduction events are not considered here) over a wide range of repeat times indicate that, for a given magnitude (M_s or M_w), earthquakes with long repeat times have shorter fault lengths than those with short repeat times. A shorter fault length for a given magnitude indicates a larger average stress drop which reflects the average strength of the fault zone. Our result therefore suggests that faults with longer repeat times are stronger than those with shorter repeat times. In terms of an asperity model in which the average strength of a fault zone is determined by the ratio, r_a, of the total area of the asperities (strong spots on a fault plane) to the total area of the fault zone, the above result suggests that r_a is proportional to the repeat time. Our result provides a method to estimate seismic source spectra from the fault length and the repeat time of a potential causative fault.

On Fault Representativeness of Software Fault Injection

Abstract: The injection of software faults in software components to assess the impact of these faults on other components or on the system as a whole, allowing the evaluation of fault tolerance, is relatively new compared to decades of research on hardware fault injection. This paper presents an extensive experimental study (more than 3.8 million individual experiments in three real systems) to evaluate the representativeness of faults injected by a state-of-the-art approach (G-SWFIT). Results show that a significant share (up to 72 percent) of injected faults cannot be considered representative of residual software faults as they are consistently detected by regression tests, and that the representativeness of injected faults is affected by the fault location within the system, resulting in different distributions of representative/nonrepresentative faults across files and functions. Therefore, we propose a new approach to refine the faultload by removing faults that are not representative of residual software faults. This filtering is essential to assure meaningful results and to reduce the cost (in terms of number of faults) of software fault injection campaigns in complex software. The proposed approach is based on classification algorithms, is fully automatic, and can be used for improving fault representativeness of existing software fault injection approaches.

Acoustic emissions document stress changes over many seismic cycles in stick-slip experiments

Abstract: [1] The statistics of large earthquakes commonly involve large uncertainties due to the lack of long-term, robust earthquake recordings. Small-scale seismic events are abundant and can be used to examine variations in fault structure and stress. We report on the connection between stress and microseismic event statistics prior to the possibly smallest earthquakes: those generated in the laboratory. We investigate variations in seismic b value of acoustic emission events during the stress buildup and release on laboratory-created fault zones. We show that b values mirror periodic stress changes that occur during series of stick-slip events, and are correlated with stress over many seismic cycles. Moreover, the amount of b value increase associated with slip events indicates the extent of the corresponding stress drop. Consequently, b value variations can be used to approximate the stress state on a fault: a possible tool for the advancement of time-dependent seismic hazard assessment. Citation: Goebel, T. H. W., D. Schorlemmer, T. W. Becker, G. Dresen, and C. G. Sammis (2013), Acoustic emissions document stress changes over many seismic cycles in stick-slip experiments, Geophys. Res. Lett., 40, 2049–2054, doi:10.1002/grl.50507.

The habitat of fault-generated pseudotachylyte : Presence vs. absence of friction-melt

Abstract: Anticipated frictional dissipation during seismic rupture is such (10-100 MW/ m 2 ) that melting on fault planes should be widespread provided slip is well-localized. However, despite evidence of slip localization throughout the upper crustal seismogenic zone, fault-hosted pseudotachylyte is rare and largely restricted to crystalline host rocks. In such rocks, pseudotachylyte fault-veins inferred to have been through a melt phase (commonly, T ∼ 1200 °C) have typical thicknesses of millimeters to centimeters and occupy low-displacement faults that only occasionally show evidence of reshear. Wall-rock damage adjacent to fault-veins is often remarkably slight but erratic injection veins may extend 40-60 km depth in subduction settings. Their apparent scarcity raises the question as to whether pseudotachylyte is rarely generated (perhaps because of dynamic lowering of shear resistance), or is rarely preserved in recognizable form. Estimates of melting energies for 1-10 mm thick fault-veins (∼ 4-40 MJ/m 2 ) are mostly higher than estimates of seismic fracture energy (0.1-10 MJ/m 2 ) and radiated seismic energy (0.1-10 MJ/m 2 per meter of slip), except for large displacements. Fault-hosted pseudotachylytes thus appear to be the product of high-stress (τ > 100 MPa) rupturing associated with fault initiation in mostly dry, intact crystalline crust.

Do great earthquakes occur on the Alpine fault in central South Island, New Zealand?

Abstract: Geological observations require that episodic slip on the Alpine fault averages to a long-term displacement rate of 2-3 cm/yr. Patterns of seismicity and geodetic strain suggest the fault is locked above a depth of 6-12 km and will probably fail during an earthquake. High pore-fluid pressures in the deeper fault zone are inferred from low seismic P-wave velocity and high electrical conductivity in central South Island, and may limit the seismogenic zone east of the Alpine fault to depths as shallow as 6 km. A simplified dynamic rupture model suggests an episode of aseismic slip at depth may not inhibit later propagation of a fully developed earthquake rupture. Although it is difficult to resolve surface displacement during an ancient earthquake from displacements that occurred in the months and years that immediately surround the event, sufficient data exist to evaluate the extent of the last three Alpine fault ruptures: the 1717 AD event is inferred to have ruptured a 300-500 km length of fault; the 1620 AD event ruptured 200-300 km; and the 1430 AD event ruptured 350-600 km. The geologically estimated moment magnitudes are 7.9 ± 0.3, 7.6 ± 0.3, and 7.9 ± 0.4, respectively. We conclude that large earthquakes (Mw >7) on the Alpine fault will almost certainly occur in future, and it is realistic to expect some great earthquakes (Mw ≥8).

Lushan M S7.0 earthquake: A blind reserve-fault event

Abstract: In the epicenter of the Lushan M S7.0 earthquake there are several imbricate active reverse faults lying from northwest to southeast, namely the Gengda-Longdong, Yanjing-Wulong, Shuangshi-Dachuan and Dayi faults. Emergency field investigations have indicated that no apparent earthquake surface rupture zones were located along these active faults or their adjacent areas. Only brittle compressive ruptures in the cement-covered pavements can be seen in Shuangshi, Taiping, Longxing and Longmen Townships, and these ruptures show that a local crustal shortening occurred in the region during the earthquake. Combining spatial distribution of the relocated aftershocks and focal mechanism solutions, it is inferred that the Lushan earthquake is classified as a typical blind reverse-fault earthquake, and it is advised that the relevant departments should pay great attention to other historically un-ruptured segments along the Longmenshan thrust belt and throughout its adjacent areas.

A comparison of landslide susceptibility mapping of the eastern part of the North Anatolian Fault Zone (Turkey) by likelihood-frequency ratio and analytic hierarchy process methods

Abstract: The North Anatolian Fault is known as one of the most active and destructive fault zones which produced many earthquakes with high magnitudes both in historical and instrumental periods. Along this fault zone, the morphology and the lithological features are prone to landslides. Kuzulu landslide, which is located near the North Anatolian Fault Zone, was triggered by snow melting without any precursor, occurred on March 17, 2005. The landslide resulted in 15 deaths and the destruction of about 30 houses at Kuzulu village. There is still a great danger of further landslides in the region. Therefore, it is vitally important to present its environmental impacts and prepare a landslide susceptibility map of the region. In this study, we used likelihood-frequency ratio model and analytical hierarchy process (AHP) to produce landslide susceptibility maps. For this purpose, a detailed landslide inventory map was prepared and the factors chosen that influence landslide occurrence were: lithology, slope gradient, slope aspect, topographical elevation, distance to stream, distance to roads, distance to faults, drainage density and fault density. The ArcGIS package was used to evaluate and analyze all the collected data. At the end of the susceptibility assessment, the area was divided into five susceptibility regions, such as very low, low, moderate, high and very high. The results of the analyses were then verified using the landslide location data and compared with the probability model. For this purpose, an area under curvature (AUC) and the seed cell area index assessments were applied. An AUC value for the likelihood-frequency ratio-based model 0.78 was obtained, whereas the AUC value for the AHP-based model was 0.64. The landslide susceptibility map will help decision makers in site selection and the site-planning process. The map may also be accepted as a basis for landslide risk-management studies to be applied in the study area.

Andean structural control on interseismic coupling in the North Chile subduction zone

Abstract: Segmentation can influence the extent of earthquake rupture and event magnitude: large megathrust earthquakes result from total rupture of relatively continuous segments of the subduction interface. Segmentation is attributed to variations in the frictional properties of the seismogenic zone or to topographic features on the down-going plate. Structures in the overriding plate may also influence segmentation but their importance has been dismissed. Here, we investigate the links between interface segmentation at the North Chile seismic gap and a crustal-scale fault structure in the overriding plate that forms a coastal scarp of about 1 km in height. We use satellite interferometric synthetic aperture radar (InSAR) and Global Positioning System (GPS) data to measure interseismic surface deformation between 2003 and 2009 and compare the deformation with rupture extent during well-documented earthquakes. From these data we infer the degree of coupling and segmentation at depth. We find that along a 500-km-long segment, the base of the strongly coupled seismogenic zone correlates with the line of the surface coastal scarp and follows the outline of the Mejillones Peninsula. This correlation implies that large-scale structures in the overriding plate can influence the frictional properties of the seismogenic zone at depth. We therefore suggest that the occurrence of megathrust earthquakes in northern Chile is controlled by the surface structures that build Andean topography.

Selection of Earthquake Scaling Relationships for Seismic‐Hazard Analysis

Abstract: A fundamentally important but typically abbreviated component of seismic‐hazard analysis is the selection of earthquake scaling relationships. These are typically regressions of historical earthquake datasets, in which magnitude is estimated from parameters such as fault rupture length and area. The mix of historical data from different tectonic environments and the different forms of the regression equations can result in large differences in magnitude estimates for a given fault rupture length or area. We compile a worldwide set of regressions and make a first‐order shortlisting of regressions according to their relevance to a range of tectonic regimes (plate tectonic setting and fault slip type) in existence around the world. Regression relevance is based largely on the geographical distribution, age, and quantity/quality of earthquake data used to develop them. Our compilation is limited to regressions of magnitude (or seismic moment) on fault rupture area or length, and our shortlisted regressions show a large magnitude range (up to a full magnitude unit) for a given rupture length or area across the various tectonic regimes. These large differences in magnitude estimates underline the importance of choosing regressions carefully for seismic‐hazard application in different tectonic environments.

A fluid-driven earthquake swarm on the margin of the Yellowstone caldera

Abstract: [1] Over the past several decades, the Yellowstone caldera has experienced frequent earthquake swarms and repeated cycles of uplift and subsidence, reflecting dynamic volcanic and tectonic processes. Here we examine the detailed spatial-temporal evolution of the 2010 Madison Plateau swarm, which occurred near the northwest boundary of the Yellowstone caldera. To fully explore the evolution of the swarm, we integrated procedures for seismic waveform-based earthquake detection with precise double-difference relative relocation. Using cross correlation of continuous seismic data and waveform templates constructed from cataloged events, we detected and precisely located 8710 earthquakes during the 3 week swarm, nearly 4 times the number of events included in the standard catalog. This high-resolution analysis reveals distinct migration of earthquake activity over the course of the swarm. The swarm initiated abruptly on 17 January 2010 at about 10 km depth and expanded dramatically outward (both shallower and deeper) over time, primarily along a NNW striking, ~55° ENE dipping structure. To explain these characteristics, we hypothesize that the swarm was triggered by the rupture of a zone of confined high-pressure aqueous fluids into a preexisting crustal fault system, prompting release of accumulated stress. The high-pressure fluid injection may have been accommodated by hybrid shear and dilatational failure, as is commonly observed in exhumed hydrothermally affected fault zones. This process has likely occurred repeatedly in Yellowstone as aqueous fluids exsolved from magma migrate into the brittle crust, and it may be a key element in the observed cycles of caldera uplift and subsidence.

Controls on Fault‐Zone Architecture in Poorly Lithified Sediments, Rio Grande Rift, New Mexico: Implications for Fault‐Zone Permeability and Fluid Flow

Abstract: We have studied the geometry and continuity of structures and diagenetic features of a normal growth fault in poorly lithified sediments. Fault-zone width and complexity vary spatially with the grain-size distribution of faulted beds. The fault zone is narrow and structurally simple where it cuts either thick beds with >20% clay and silt, or thin beds that alternate between >20% and ≤20% clay and silt. Where the majority of beds juxtaposed by the fault are ≥80% sand and gravel, and clay beds are thin and rare, the fault zone is wide and structurally complex. In all cases, the fault zone can be divided into three architectural elements. The core includes the primary slip surface(s) and a nearly continuous clay smear 0.3 - 32 cm wide. It is flanked by structurally and lithologically heterogeneous mixed zones, which include material derived from adjacent sediments during fault movement. Mixed zone sediments vary from little deformed to well foliated, tectonically mixed material within which bedding has been destroyed. The mixed zones are bound by damage zones, within which deformation was confined to minor faults and folds. Grain-size and structural variations among these elements lead us to conclude that they have hydrologic significance. In addition, the fault zone is preferentially cemented with respect to adjacent sediments. We use degree of cementation as a proxy for fluid flux, and patterns of cementation as a record of paleo-flow pathways. Extensive sparry calcite cement is typically confined to coarse-grained sediments in the hanging wall (basinward) mixed zone. Steeply plunging, elongate patterns of cement are interpreted to record subvertical groundwater flow at the time of precipitation. As regional flow is inferred to have occurred roughly from the margins to the center of the basin at the time of cementation, these relationships indicate a combination of cross-fault and subvertical, fault-parallel flow.