Entire Plate Boundary Research Articles

Inverting Geodetic Strain Rates for Slip Deficit Rate in Complex Deforming Zones: An Application to the New Zealand Plate Boundary

AbstractThe potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike‐slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike‐slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting.

Core surprise: What's inside a plate boundary?

Despite the fact that 90% of global seismicity occurs at plate boundary faults, our understanding of their internal structure is lacking. It’s not easy to see inside a plate boundary fault – typically composed of a high-strain fault core surrounded by a fractured damage zone – and when we can, it often requires expensive drilling projects that yield limited information on the internal structure of the whole fault. Understanding the internal structure of large faults is crucial, because their chemical and mechanical properties control how and where earthquakes rupture, nucleate and propagate. This in turn limits the size of the earthquake or the amount of radiated seismic energy, and consequently the severity of surface damage. The 1999 magnitude 7.7 earthquake along the Chelungpu plate boundary fault, for example – the second deadliest earthquake in Taiwan’s recorded history – saw significant variations in slip and ground motion at different locations along the fault which resulted in large local variations in casualties and damage. Subsequent field investigations related these variations to changes in the fault’s structure (i.e., clay width, geometry), which in turn controlled how the fault moved.

New hypothesis to explain Quaternary forearc deformation and the variety of plate boundary earthquakes along the Suruga\u2013Nankai Trough by oblique subduction of undulations on the Philippine Sea Plate

Plate-boundary earthquakes of magnitude 8 or greater along the Suruga–Nankai Trough subduction zone have repeated at intervals of 90–150 years, but with widely varying magnitudes and rupture areas. We propose, based on geologic data on crustal movements of the forearc wedge, that these earthquake variations are controlled by two separate locked zones in the deeper part of the plate boundary. Long-wavelength topographic undulations, composed of alternating zones of uplift and subsidence along the forearc wedge, are associated with 2000 to 3000 m of vertical relief that has accumulated during Quaternary time. We suggest that this crustal deformation in the forearc wedge is caused not by stress loading and release during earthquake cycles, but rather by vertical displacements of the plate boundary caused by the westward movement of undulations in the obliquely subducting slab of the Philippine Sea Plate as it subducts beneath Southwest Japan. Dating of emergent marine shell fossil assemblages shows that the Kii Mountains, an uplift zone at the midpoint of the trough, has undergone uplift events at intervals of 400–600 year, and the latest event of those occurred when the 1707 Hoei earthquake ruptured the entire plate boundary along the trough. We infer that the plate boundary under the Kii Mountains is a locked zone and that slips of this zone, which accompanied ruptures of the entire plate boundary, caused uplift of the mountains by decreasing the plate boundary depth. A similar locked zone is inferred under the uplift zone of the Akaishi Mountains, along the eastern margin of the trough, and the 1854 Ansei earthquake pair was presumably caused by the slip of this zone. During the two 1854 events, the locked zone under the Kii Mountains presumably restricted the rupture propagation to the eastern half of the trough, then a rupture of the western half of the trough followed within 32 h. These locked zones are inferred to slip independently every few hundred years and determine the major patterns of characteristic ruptures along the Suruga–Nankai Trough.

Plate boundary localization, slip-rates and rupture segmentation of the Queen Charlotte Fault based on submarine tectonic geomorphology

Linking fault behavior over many earthquake cycles to individual earthquake behavior is a primary goal in tectonic geomorphology, particularly across an entire plate boundary. Here, we examine the 1150-km-long, right-lateral Queen Charlotte-Fairweather fault system using comprehensive multibeam bathymetry data acquired along the Queen Charlotte Fault (QCF) offshore southeastern Alaska and western British Columbia. Fine-scale analysis of tectonic geomorphology allowed us to identify and reconstruct 184 strike-slip piercing points over a 630 km stretch of the QCF. Age constraints from glacial recession and offshore sedimentation patterns yield a consistent slip-rate of ∼50–57 mm/yr since ∼17–12 ka, the fastest rate for a continent-ocean strike-slip fault on Earth. These slip-rates equal or exceed estimates of Pacific-North America (PA-NA) relative motion from global plate reconstructions, indicating that PA-NA motion is highly localized. The QCF cuts the seafloor along a narrow and unusually straight trace for its entire length and multiple fault traces are observed only at local step-overs. The geometry and behavior of the QCF over many earthquake cycles is simple and typical of mature faults with relatively homogeneous stress fields. Since the QCF is the primary PA-NA plate boundary, we used the trace of the QCF to define the small circle path for relative plate motion and computed the associated Euler pole. Predicted along-strike obliquity variations based on the new pole agree with observed tectonic geomorphology and suggest that previous global plate reconstructions overestimated the degree of oblique convergence along the QCF. We also find that subtle, long-wavelength (75–150 km) bends and discrete step-overs appear to define the endpoints of M>7 earthquakes, suggesting that obliquity and resultant fault geometry may control rupture segmentation and asperity development. Lastly, the agreement between predicted obliquity and tectonic geomorphology along the entire length of QCF compelled a reevaluation of regional tectonic models. In the north, the eastern Yakatat Terrane appears to be translating northwest with the Pacific plate, and slip transferred from the QCF to the Fairweather Fault results in ∼20 mm/yr of convergence along the southern St. Elias mountains. In the south, we predict a reduced rate of convergence along the QCF west of Haida Gwaii (∼5–6 mm/yr of shortening, on average) relative to previous studies. Our results support a model for transpression and strike-slip partitioning along the edge of a hot and weak Pacific Plate, leading to crustal thickening and growth of the Queen Charlotte Terrace to the west of Haida Gwaii.

Polya Counting Theory Applied to Combination of Edge Conditions for Generally Shaped Isotropic Plates

Structural behaviors of plate components, such as internal stress, deflection, buckling and dynamic response, are important in the structural design of aerospace, mechanical, civil and other industries. These behaviors are known to be affected not only by plate shapes and material properties but also by edge conditions. Any one of the three classical edge conditions in bending, namely free, simply supported and clamped edges, may be used to model the constraint along an edge of plates. Along the entre boundary with plural edges, there exist a wide variety of combinations in the entire plate boundary, each giving different values of structural responses. For counting the total number of possible combinations, the present paper considers Polya counting theory in combinatorial mathematics. For various plate shapes, formulas are derived for counting exact numbers in combination. In some examples, such combinations are confirmed in the figures by a trial and error approach.

Relationship between outer forearc subsidence and plate boundary kinematics along the Northeast Japan convergent margin

Tectonic erosion along convergent plate boundaries, whereby removal of upper plate material along the subduction zone interface drives kilometer‐scale outer forearc subsidence, has been purported to explain the evolution of nearly half the world's subduction margins, including part of the history of northeast Japan. Here, we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern Japan. A synthesis of newly updated analyses of outer forearc subsidence, the timing and kinematics of upper plate deformation, and the history of plate convergence along the Japan trench demonstrate that the onset of rapid fore‐arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the Sea of Japan and with an acceleration in convergence rate at the trench. In Plio‐Quaternary time, relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate. The coincidence of these changes across the forearc, arc, backarc system appears to require an explanation at the scale of the entire plate boundary. Similar observations along other western Pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule, rather than the exception. We suggest that a significant component of forearc subsidence at the northeast Japan margin is not the consequence of basal tectonic erosion, but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics. If correct, this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins.

Quantifying potential tsunami hazard in the Puysegur subduction zone, south of New Zealand

SUMMARY Studies of subduction zone seismogenesis and tsunami potential, particularly of large subduction zones, have recently seen a resurgence after the great 2004 earthquake and tsunami offshore of Sumatra, yet these global studies have generally neglected the tsunami potential of small subduction zones such as the Puysegur subduction zone, south of New Zealand. Here, we study one such relatively small subduction zone by analysing the historical seismicity over the entire plate boundary region south of New Zealand, using these data to determine the seismic moment deficit of the subduction zone over the past ∼100 yr. Our calculations indicate unreleased moment equivalent to a magnitude Mw 8.3 earthquake, suggesting this subduction zone has the potential to host a great, tsunamigenic event. We model this tsunami hazard and find that a tsunami caused by a great earthquake on the Puysegur subduction zone would pose threats to the coasts of southern and western South Island, New Zealand, Tasmania and southeastern Australia, nearly 2000 km distant.

Structural context of the great Sumatra‐Andaman Islands earthquake

A new three‐dimensional seismic model and relocated regional seismicity are used to illuminate the great Sumatra‐Andaman Islands earthquake of December 26, 2004. The earthquake initiated where the incoming Indian Plate lithosphere is warmest and the dip of the Wadati‐Benioff zone is least steep along the subduction zone extending from the Andaman Trench to the Java Trench. Anomalously high temperatures are observed in the supra‐slab mantle wedge in the Andaman back‐arc. The subducting slab is observed along the entire plate boundary to a depth of at least 200 km. These factors contribute to the location of the initiation of rupture, the strength of seismic coupling, the differential rupture properties between the northern and southern segments of the earthquake, and the cause of convergence in the Andaman segment.

Motion of the Rivera plate since 10 Ma relative to the Pacific and North American plates and the mantle

To better understand the influence of Rivera plate kinematics on the geodynamic evolution of western Mexico, we use more than 1400 crossings of seafloor spreading magnetic lineations along the Pacific–Rivera rise and northern Mathematician ridge to solve for rotations of the Rivera plate relative to the underlying mantle and the Pacific and North American plates at 14 times since 9.9 Ma. Our comparison of magnetic anomaly crossings from the undeformed Pacific plate to their counterparts on the Rivera plate indicates that significant areas of the Rivera plate have deformed since 9.9 Ma. Dextral shear along the southern edge of the plate from 3.3–2.2 Ma during a regional plate boundary reorganization deformed the Rivera plate farther into its interior than previously recognized. In addition, seafloor located north of two rupture zones within the Rivera plate sutured to North America after 1.5 Ma. Anomaly crossings from these two deformed regions thus cannot be used to reconstruct motion of the Rivera plate. Finite rotations that best reconstruct Pacific plate anomaly crossings onto their undeformed counterparts on the Rivera plate yield stage spreading rates that decrease gradually by 10% between 10 and 3.6 Ma, decrease rapidly by 20% after ∼3.6 Ma, and recover after 1 Ma. The slowdown in Pacific–Rivera seafloor spreading at 3.6 Ma coincided with the onset of dextral shear across the then-incipient southern boundary of the Rivera plate with the Pacific plate. The available evidence indicates that the Rivera plate has been an independent microplate since at least 10 Ma, contrary to published assertions that it fragmented from the Cocos plate at ∼5 Ma. Motion of the Rivera plate relative to North America has changed significantly since 10 Ma, in concert with significant changes in Pacific–Rivera motion. A significant and robust feature of Rivera–North America motion not previously recognized is the cessation of margin-normal convergence and thus subduction from 2.6 to 1.0 Ma along the entire plate boundary, followed by a resumption of trench-normal subduction along the southern half of the Rivera–North America plate boundary after 1.0 Ma. Motion of the Rivera plate relative to the underlying mantle since 10 Ma has oscillated between periods of landward motion and seaward motion. The evidence suggests that the torque exerted by slab pull on this young and hot oceanic plate is either minimal or is effectively counterbalanced by forces that resist its motion.

Deformation kinematics in the western United States determined from Quaternary fault slip rates and recent geodetic data

We estimate the horizontal velocity gradient tensor field from Quaternary fault slip rates, and recent Global Positioning System (GPS) and very long baseline interferometry (VLBI) velocity solutions in the western United States transform plate boundary zone. The total velocity obtained from the Quaternary fault slip rate data across the entire plate boundary is within 1 mm/yr of the NUVEL‐1A predicted Pacific (PA)‐North American (NA) plate motion velocity, but directions are 5°–6° anticlockwise of directions given by NUVEL‐1A. The total velocity obtained from inversion of recent geodetic data is 2°–3° anticlockwise from the NUVEL‐1A NA‐PA velocity, but the difference between the two is not significant at the 95% confidence level. The discrepancy between the total PA‐NA motion obtained from the geological data and NUVEL‐1A indicates that a marginally significant amount of NE‐SW shortening (possibly as much as 5 mm/yr) is missing overall in the geologic data. Shortening may occur in the long‐term in the offshore and coastal areas of California where such shortening is required in the shorter‐term geodetic solution. The seismic moment released in the last 148 years is ∼59% of the total moment release rate expected from long‐term strain rate field (including both seismic and aseismic deformation) derived from the inversion of geological data with NUVEL‐1A far‐field PA‐NA motion constraints. The accumulated strain in the areas containing the southern San Andreas fault‐San Jacinto fault, the San Francisco Bay area, and the area containing the Ventura basin in the Western Transverse Ranges in the last 148 years is the equivalent to that which could be released by an Mw>7.0 earthquake in each 50 × 100 × 15 (km3) crustal volume if strain is to be released seismically in these areas.

Contemporary kinematics of the western United States determined from earthquake moment tensors, very long baseline interferometry, and GPS observations

Using moment tensors from earthquakes between 1850 and 1995, we determine the horizontal velocity gradient tensor field associated with the seismic deformation in the western U.S. plate boundary zone. The velocity vectors obtained from the integration of the seismic strain rates across the entire plate boundary lie within 5° of the NUVEL‐IA Pacific‐North American plate motion direction. The magnitude of the earthquake‐related velocity is 62% of the NUVEL‐1A total Pacific‐North American plate motion. If earthquake occurrence is a random, Poisson‐like process, then the large formal errors associated with the model velocity estimates are an indication that the 144‐year interval is too short to define the long‐term seismic moment release rate to within an uncertainty less than the deficit between the observed seismic moment release rate and the total long‐term moment release rate associated with both the seismic and aseismic deformation. To spatially resolve the deficit of the earthquake moment release in the last 144 years, we have also estimated the total long‐term horizontal velocity gradient tensor field using seismic strain rate tensors and recent geodetic velocities, both with and without the NUVEL‐1A Pacific‐North American rigid plate motion constraint. We find a systematic difference between the Southern California Earthquake Center (SCEC) geodetic velocity map and NUVEL‐1A North American reference frames with a pole at (38.8°N, 124.2°W, 0.33° m.y.−1). This difference is significant at the 99% confidence level and indicates that 4–7 mm/yr of NE‐SW convergence is absorbed in offshore areas of southern California if both the SCEC velocity map and NUVEL‐1A correctly describe the relative motion between the Pacific and North America plates. Deficit strain rates calculated from comparison of observed seismic strain rates over the last 144 years with long‐term model strain rate tensors are largest in areas east of San Francisco Bay, in the Transverse Ranges, and along the southern San Andreas fault and San Jacinto fault. The deficit in each 100 by 50 km grid area within these regions is equivalent to an accumulation of strain in the last 144 years that would be released by an Mw>7 earthquake. Elsewhere the accumulated strain in the last 144 years is small, except in northern Mojave Desert, along the Garlock fault, and in the region southwest of the southern San Andreas‐San Jacinto fault zone. Within each 100 by 50 km grid area in these regions the accumulated strain is equivalent to that released by an Mw=6.5–7.0 earthquake.

Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi margin, New Zealand

Analysis of seismic reflection profiles, swath bathymetry, side‐scan sonar imagery, and sediment samples reveal the three‐dimensional structure, morphology, and stratigraphic evolution of the central to southern Hikurangi margin accretionary wedge, which is developing in response to thick trench fill sediment and oblique convergence between the Australian and Pacific plates. A seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby, by coccolith and foraminifera biostratigraphy of three core and dredge samples, and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment. Structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds, on the growth and mechanics of frontal accretion under variable convergence obliquity, and on the amounts and rates of horizontal shortening. The data place constraints on the partitioning of geological strain across the entire southern Hikurangi margin. The principal deformation front at the toe of the wedge is discontinuous and represented by right‐stepping thrust faulted and folded ridges up to 1 km high, which develop initially from discontinuous protothrusts. In the central part of the margin near 41°S, where the convergence obliquity is 50°, orthogonal convergence rate is slow (27 mm/yr), and about 75% of the total 4 km of sediment on the Pacific Plate is accreted frontally, the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening. At least 80% of this shortening has occurred within the last 0.4±0.1 m.y. at an average rate of 12±3 mm/yr. This rate indicates that the frontal 30 km of the wedge accounts for about 33–55% of the predicted orthogonal contraction across the entire plate boundary zone. Despite plate convergence obliquity of 50°, rapid frontal accretion has occurred during the late Quaternary with the principal deformation front migrating seaward up to 50 km within the last 0.5 m.y. (i.e., at a rate of 100 km/m.y.). The structural response to this accretion rate has been a reduction in wedge taper and, consequently, internal deformation behind the present deformation front. Near the southwestern termination of the wedge, where there is an along‐the‐margin transition to continental transpressional tectonics, the convergence obliquity increases to >56°, and the orthogonal convergence rate decreases to 22 mm/yr, the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward‐verging folds. These structures have accommodated not more than 0.5 km of horizontal shortening at a rate of < 1 mm/yr, which represents < 5% of the predicted orthogonal shortening across the entire plate boundary in southern North Island. The landward‐vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin, to the northern South Island region where the upper and lower plates are locked or at least very strongly coupled.

Gaussian probability estimates for large earthquake occurrence in the Jordan Valley, Dead Sea rift

Abstract A catalog including only large earthquakes in the Jordan Valley segment of the Dead Sea rift was compiled using existing recent catalogs for this region. Large earthquakes in the Jordan Valley did not always break the entire plate boundary and, in particular, the last large earthquake (1927) broke only a southern segment of the valley. Thus, the catalog was divided into northern and southern zones (segments). A conservative approach was used whenever the length of the epicentral area was not well constrained. These data were used to estimate Gaussian conditional probabilities for the occurrence of a large (rupturing the plate boundary) earthquake during a 50-year time window, versus the time elapsed from the last large earthquake. Since the historical data, particularly the location and extent of the epicentral areas are not always accurate, the results can be used for gross estimates of the probabilities and generalized conclusions. In general, the analysis show that for the southern Jordan Valley, conditional probability estimates are low mainly due to the short time which elapsed since the last large earthquake relative to the mean repeat time. In the northern Jordan Valley, acceptable probability estimates are high. In addition the probability that a large earthquake would have occurred by 1986 is close to 100%. Such unusually high values most probably mean that some input parameters are erroneous; either the mean repeat time is much larger and the region was the site of much less seismic activity than indicated in the catalog, or, more probably, the last large earthquake is more recent than generally thought. Whatever the explanation, the various computations clearly indicate a higher probability estimate in the northern than in the southern part of the Jordan Valley. The results stress the need for improved studies of the historical seismic record, particularly stressing accurate determination of date and rupture length of the last large earthquake.

The Azores‐Gibraltar Plate Boundary: Focal mechanisms, depths of earthquakes, and their tectonic implications

We have analyzed the focal mechanisms and depths of 10 moderately sized earthquakes along the Azores‐Gibraltar plate boundary by a variety of methods including formal inversion of the waveform and amplitude of teleseismic P and SH waves, first motion readings, and the identification of depth phases. Our data, together with a compilation of results reported for very large events from the past 30 years and for two recent events in 1983, place new constraints on the present‐day deformation of this boundary separating the Eurasian and African plates. The slow (∼2 to 3 mm/yr) divergent plate motion near the Azores Islands at the western end of the boundary appears to be very similar to that of a typical ridge‐transform‐ridge configuration. A currently aseismic segment with presumed transcurrent motion separates the western end from a broad zone of ocean‐ocean convergence to the east. This convergence zone is characterized by scattered seismicity, complex bathymetry, and large positive geoid and gravity anomalies. The dispersed locations of the earthquakes and the lack of consistency in the orientation of their nodal planes and slip vectors all suggest that a single major plate boundary is not present there. We note that a regional stress field of north‐northwest‐south‐southeast compression can be inferred from the consistent P axis directions. The focal depths for earthquakes in this region reach a maximum of 50±5 km beneath the seafloor. These events are among the deepest oceanic earthquakes which are apparently unrelated to a Benioff zone and their maximum depths are approximately limited by the position of the 600°C isotherm. We interpret the regional stress field and the focal depths as the result of the interaction of two sections of strong, cold oceanic lithosphere with nearly identical thermal structure. As the plate boundary crosses the passive continental margins of Iberia and Africa, the zone of ocean‐ocean convergence becomes that of continental collision. Two unusually deep events, one at 100 and another at 640 km, have occurred beneath southern Spain since the occurrence of the deep Spanish earthquake of 1954 in the same area. The configuration of a seismic upper crust and uppermost mantle straddling an aseismic lower crust resembles the distribution of focal depths in other areas of recent continental convergence. Based on the distribution of focal depths along the entire plate boundary and recent reconstructions of the history of relative motions between Eurasia and Africa, the very deep events beneath southern Spain are likely to have occurred within a detached piece of lithosphere which sank to its present depth during the current phase of plate convergence.