Recurrence Interval Of Earthquakes Research Articles

Paleoseismic Investigation of the Thousand Springs Fault, Northwestern Basin and Range, Oregon

ABSTRACT Earthquake recurrence intervals, surface-rupture extents, and interactions between faults provide insight into how faults behave and are critical for seismic hazard mitigation and earthquake forecasting. Investigating the paleoseismology of spatially related faults can reveal strain distribution and whether faults rupture as a system or independently. Summer Lake basin, a graben in the northwestern Basin and Range with four active faults (three of which have prior paleoseismic investigations), provides an opportunity to investigate fault interactions. To expand the paleoseismic record, two trenches were excavated across the previously undocumented Thousand Springs fault, exposing a normal fault zone that offsets a sequence of deep- to shallow-water lake sediments, sand dunes containing reworked Mazama ash, and other Cascades-sourced tephra. Tephra units were correlated to known units by their physical characteristics, stratigraphic sequence, glass chemistry, and two new radiocarbon dates from the uppermost lake sediments. Using trench exposures, measured vertical separations through auguring, colluvial wedges, and extrapolated offsets based on a constant sedimentation rate, we identified at least five surface-rupturing earthquakes with a total offset of 3.4 + 2/−1 m in the past ∼65 ka. The oldest event (EH5) occurred at 63.8 ± 1.5 ka, event horizon 4 at 36.2 ± 12.7 ka (which could be more than one event), and event horizon 3 at 24.6 ± 0.3 ka. Event horizon 2, a warping event at our site, is likely more than one event and occurred between 7.5 and 10 ka; and the most recent event (EH1+), most likely more than one event, occurred between 3.3 and 7.7 ka. Several events correlate, within error, with events on other faults in the Summer Lake basin, suggesting that (1) the faults generally rupture together as a system, (2) the most recent earthquake may have ruptured all faults in the region, and (3) fault rupture is influenced by the rapid regression of Lake Chewaucan (∼13 ka).

The Stress State before the MS 6.8 Luding Earthquake on 5 September 2022 in Sichuan, China: A Retrospective View Based on the b-Value

On 5 September 2022 (BJT), Luding, located in southwestern Sichuan Province, China, experienced an MS 6.8 earthquake. This earthquake occurred within the historical rupture zone of the 1786 MS 7.75 event, part of the southern section of the Xianshui He Fault belt. Given the average 155-year recurrence interval for strong earthquakes in this area, the 236 years since the last event made this earthquake somewhat expected. However, prior to this event, we did not detect any anomalies indicating low surface b-values, which are often indicative of a high-stress state in the source area before strong earthquakes, as highlighted by numerous studies. Our research focused on the northern section of the eastern boundary of the Sichuan–Yunnan sub-block, encompassing the Xianshui He, Anning He, Zemu He, and Daliang Shan fault belts. We meticulously located earthquakes of ML ≥ 1.5 from 2009 to May 2022. The catalog was divided into two periods: 2009–2014 and 2015–May 2022. Using an AIC-constraint method, we analyzed the changes in b-values (Δb) in the latter period compared to the former. Our findings revealed a significant abnormal Δb zone (Δb < −0.3), with a radius of approximately 50 km, when ΔAIC ≥ 2 was selected. Intriguingly, the epicenter of the recent Luding MS 6.8 earthquake fell within this abnormal zone. Furthermore, we calculated the b-value cross-section for the southern section of the Xianshui He fault belt using a directory of precisely located small earthquakes. This revealed that the location, scale, and shape of the abnormally low-b-value area corresponded with the large displacement co-seismic area of the main earthquake, affirming the b-value’s effectiveness in identifying asperities. The b-value’s temporal evolution prior to the mainshock exhibited a nearly decade-long continuous decrease, signifying a long-term stress-loading process akin to that observed before many strong earthquakes. The b-value anomalies observed from different profiles before the Luding earthquake underline the necessity of a comprehensive, multi-dimensional analysis of such anomalies. Finally, our analysis indicates that nine earthquakes with MS ≥ 6.5, including the Luding MS 6.8 event, have contributed to increased Coulomb Failure Stress change (ΔCFS) in the Daofu (DF)–Kangding (KD) section of the Xianshui He fault belt and the northern section of the Anning He fault belt south of Shimian (SM), with amplitudes surpassing the 0.01 MPa threshold. This suggests the potential for strong earthquakes in these zones.

Geophysical constraints on continental rejuvenation in central China: Implications for outward growth of the Tibetan Plateau

Continental rejuvenation results from the tectonic reactivation of crustal structures and lithospheric reworking by mantle flow. Geochemical observations and field mapping have traditionally provided the primary evidence for the secular evolution of crustal composition and tectonic processes during continental rejuvenation. Nonetheless, the impact of continental rejuvenation on the observed present-day strain rate and orogenic-scale lithospheric structure has not been well constrained. The pre-existing E-W−trending Central China Orogenic Belt has been overprinted by the N-S−trending Central Longitudinal Seismic Belt and constitutes the intracontinental West Qinling Syntaxis in central China, where the tectonic setting changes eastward from contraction to extension. Combining updated global positioning system data and high-resolution crustal seismic tomography, we reveal a modern continental rejuvenation process within the West Qinling Syntaxis in central China. The northward extrusion of the Tibetan Plateau’s weak lithospheric layer (middle-lower crust and lithospheric mantle) of southwestern China relative to the rigid Sichuan Basin/Ordos Block of the eastern West Qinling Syntaxis results in regional dextral shearing that shapes the Central Longitudinal Seismic Belt and defines the eastern Tibetan Plateau margin. The pre-existing E-W−trending Central China Orogenic Belt has been preserved above the brittle-ductile transition zone, and the northward movement of the deep lithospheric layer drives the deformation of the upper crust in the West Qinling Syntaxis. Our results, along with previous studies, suggest the presence of an intracontinental lithospheric interchange structure in central China. The continental rejuvenation of the West Qinling Syntaxis results from a combination of fault reactivation in the upper crust (Stage I, Eocene−Oligocene) and reworking of the deep lithosphere (Stage II, middle−late Miocene) related to the plateau-wide shift in stress accommodation ultimately driven by the redistribution of mass outward from the central Tibetan Plateau. At present, the transition zone between the high- and low-velocity anomalies along the Central Longitudinal Seismic Belt not only shapes the landscape boundary but controls the size and recurrence interval of earthquakes within the West Qinling Syntaxis in central China.

Late Quaternary activity of the NW Cardrona Fault, Otago, New Zealand

ABSTRACT We use new paleoseismic data and lidar to reassess late Quaternary activity of the NW (northwest) Cardrona Fault, a ∼60 km long range-bounding fault in Otago. Paleoseismic investigations of the NW Cardrona Fault were conducted in the 1980s, but findings were limited by a paucity of materials suitable for dating. Here, re-exposure and re-assessment of a 1980s trench at Macdonalds Creek provide stratigraphic evidence for two surface rupturing earthquakes on the NW Cardrona Fault. Through Optically Stimulated Luminescence dating and OxCal modelling, we estimate these earthquakes occurred at (mean ± 2 standard deviations, σ) 20.5 ± 7.8 ka and 8.7 ± 6.6 ka. The timings of these earthquakes are consistent with those proposed from re-interpretation of another 1980s trench on the NW Cardrona Fault in the Kawarau River valley, ∼25 km farther south. The slip rate and recurrence interval derived from the Macdonalds Creek trench data are 0.13 ± 0.05 mm/yr and 11,800 ± 5100 years (2σ error) respectively. These estimates imply lower slip rates and longer earthquake recurrence intervals on the NW Cardrona Fault than previously assessed.

Strong earthquake recurrence interval in the southern segment of the Red River Fault, southwestern China

Along the southeastern margin of the Tibetan Plateau, an important boundary fault, known as the Red River Fault (RRF), formed during the extrusion of mantle and lower crustal materials beneath the Tibetan Plateau. The characteristics of RRF activity are important for understanding the tectonic evolution of the southeastern margin of the Tibetan Plateau. There is less knowledge about the strong earthquake recurrence interval in the southern segment of the RRF than about its northern segment. In this study, based on the characteristics of the active tectonic landform, three paleoseismic trenches were excavated in Adipo Village north of Honghe County in the southern segment of the RRF. In these trenches, three paleoseismic events that occurred in the middle and late Holocene were identified. According to the 14C dating results, these events were constrained to 785–504 BC, 26–492 AD, and 1,415–1857 AD. The average earthquake recurrence interval was determined to be approximately 960–1,320 a.

New paleoseismic events reveal decamillenial recurrence time for large earthquakes (M ≥ 7) along the Yuguang Graben Fault in North China

Paleoseismic studies are critical for the assessment and prediction of large earthquakes (M ≥ 7). These studies involve excavating and analyzing evidence from surface–rupturing earthquakes. However, they often face limitations in spatiotemporal resolution. The Yuguang Graben, located in the northern Shanxi Grabens of North China, contains an abundance of Quaternary loess that can be utilized for extending paleoseismic records. Fine-grained deposits of loess contrast sharply with gravel deposits of the colluvial wedge, making the wedge shape apparent. To enhance our understanding of earthquake behavior in the Yuguang Graben, we excavated a trench on terrace T2 of the Songzhikou segment of the Yuguang Graben Fault (YGF). We used LiDAR to obtain a high–resolution orthophoto of the trench wall. Through OSL dating of colluvial wedges and loess, we identified at least four large paleoseismic events with a magnitude of M ≥ 7 that occurred along the YGF. These events took place at >77.6 ± 5.1, 63.9 ± 5.1, 51.0 ± 4.3, and 37.3 ± 0.3 kyr, respectively. Combined with previous paleoseismic studies, we identified seven paleoseismic events since the late Pleistocene. These findings reveal that earthquakes occurring on YGF follow a quasiperiodic pattern with an average recurrence interval of about 12 ± 3 kyr. The elapsed time of the most recent earthquake near the recurrence interval suggests an increased risk of a large earthquake along the YGF. We also analyzed earthquake clusters generated by neighboring faults in the active northeastern boundary of the Ordos block. Our analysis indicated that fault interactions can affect the recurrence interval of earthquakes along single faults, while regional seismic activity tends to concentrate in time during a seismically active period. These observations underscore the importance of expanding paleoseismic records and exploring regional seismic hazards to gain a more comprehensive understanding of earthquakes.

The footprint of a historical paleoearthquake: the sixth-century-CE event in the European western Southern Alps

Abstract. Low-deformation regions are characterized by long earthquake recurrence intervals. Here, it is fundamental to extend back the record of past events as much as possible to properly assess seismic hazards. Evidence from single sites or proxies may be not compelling, whereas we obtain a more substantial picture from the integration of paleo- and archeoseismic evidence at multiple sites, eventually supplemented with historical chronicles. In the city of Como (N Italy), we perform stratigraphic and sedimentological analyses on the sedimentary sequences at Via Manzoni and we document earthquake archeological effects at the Roman baths by means of structure from motion and field surveys. Radiocarbon dating and chronological constraints from the archeological site allow us to bracket the time of occurrence of the deformations to the sixth century CE. We interpret the observed deformations as due to earthquake ground shaking and provide constraints on the lower threshold for the triggering of such evidence. We move toward a regional view to infer possible relevant seismic sources by exploiting a dataset of published paleoseismic evidence in Swiss and N Italy lakes. We perform an inverse grid search to identify the magnitude and location of an earthquake that can explain all the positive and negative evidence consistent with the time interval of the event dated at Como. Our results show that an earthquake (minimum Mw 6.32) with epicenter located at the border between Italy and Switzerland may account for all the observed effects; a similar event in the sixth century CE has not been documented so far by historical sources. Our study calls for the need to refine the characterization of the local seismic hazard, especially considering that this region seems unprepared to face the effects of an earthquake size similar to the one inferred for the sixth-century-CE event.

The Role of Clay in Limiting Frictional Healing in Fault Gouges

AbstractFrictional healing is fundamental to the seismic cycle and plays a role in the energy balance, dynamics, and recurrence interval of earthquakes and slow slip events. Although the healing behavior of quartz has been studied extensively, the role of clay content is less understood. We tested synthetic mixtures of quartz and smectite (10%–100% smectite) in a double‐direct shear configuration to measure frictional healing. We show that the magnitude and rate of healing decreases systematically with higher clay content (from 0.008/decade at 10% smectite to 0.002/decade at 100% smectite). Healing scales with both the magnitude of stress relaxation during holds and layer‐normal compaction of the gouge. We suggest this reflects the alignment of clay minerals, leading to saturation of the real area of contact that limits restrengthening during holds. The low healing rates of clay‐rich faults—together with rate‐neutral to rate‐strengthening friction—should promote frequent, small failures or stable sliding.

Paleoseismic Earthquake Recurrence Interval Derivation for the 2022 Revision of the New Zealand National Seismic Hazard Model

Abstract Recurrence intervals of ground-surface rupturing earthquakes are one of numerous datasets used to constrain the rates of fault ruptures in the 2022 revision of the New Zealand National Seismic Hazard Model (NZ NSHM 2022). Paleoearthquake timing and single-event displacement (SED) data in the New Zealand Paleoseismic Site Database version 1.0 alongside geologic and geodetic slip rates from the New Zealand Community Fault Model version 1.0 and NZ NSHM 2022 Geodetic Deformation Model were used to estimate recurrence intervals on faults across New Zealand for inclusion in the NZ NSHM 2022. Past earthquake timings were fit with lognormal, exponential, and Brownian Passage Time recurrence models to derive probability density functions (PDFs) of mean recurrence interval (MRI) in a Bayesian framework. At some sites, SED and slip-rate (SR) data were used to estimate PDFs of MRI; and at sites where timings, slip rate, and displacement data are available, the timings-based and slip-based PDFs were combined to develop tighter constraints on MRI. Using these approaches, we produce a database of maximum-likelihood MRIs and their uncertainties for 80 sites across New Zealand. The resulting recurrence interval dataset is publicly available and is the largest such dataset in New Zealand to date. It provides a valuable resource for future seismic hazard modeling and highlights areas that would benefit from future study.

SLIP SOURCE MODEL OF THE 1995 NEFTEGORSK EARTHQUAKE (NORTH SAKHALIN) FROM GEODETIC DATA

The May 27, 1995 Mw=7.0 Neftegorsk earthquake occurred in the north of Sakhalin Island, rupturing the Upper Piltun fault, a secondary feature of the main Hokkaido-Sakhalin regional fault zone. The fault geometry, coseismic slip model, and Coulomb stress changes in the earthquake focal area were calculated based on a finite fault modeling. We used near-field coseismic offsets at 24 points obtained by comparison between predating triangulation and GPS observations, which were collected before and after the earthquake. Our slip distribution model shows two major slip patches. Larger slip asperity (amplitude up to 6.36 m) was characterized by right-lateral strike-slip movements, which correspond to focal mechanism of the earthquake, whereas the northern segment has reverse fault mechanism with maximum slip of 2.64 m. The fault length and width, average slip and stress drop values are estimated at 78 km, 28 km, 1.91 m and 11.3 MPa, respectively. The estimated release moment is approximately 7.49×1019 N∙m equal to Mw=7.2, which is larger than that reported by the USGS and GCMT but consistent with the values reported by other researchers. The coseismic Coulomb stress changes enhanced the stress by more than 10 MPa on the southern segment of the Gyrgylaninsky fault and middle section of the Hokkaido–Sakhalin fault. Seismic risks on the nearest faults cannot be ignored in the future despite the fact that the earthquake with a magnitude of 5.8 occurred in 2010 near the Gyrgylaninsky fault. The recent GPS rates in the surroundings of the Neftegorsk surface rupture mean that the recurrence interval for similar earthquakes may be more than a thousand years.

Geologic and Geophysical Evidence of Repeated Normal Faulting on the Itozawa Fault—the Source of the Mw 6.6 Iwaki Earthquake Triggered by the Mw 9.0 Tohoku-Oki Megathrust Earthquake, Northeast Japan

ABSTRACT The 2011 Mw 9.0 Tohoku-Oki earthquake ruptured the 500 km long and 200 km wide convergent plate margin between the North American and Pacific plates, and changed the crustal stress field and triggered widespread seismic activity in northeast Japan. In particular, many crustal earthquakes struck the southern Fukushima area. The largest inland normal-faulting earthquake with Mw 6.6 occurred on the northwest-trending Yunodake fault and the north-northwest-trending Itozawa fault on 11 April 2011. The coseismic surface ruptures appeared along the previously identified active and presumed active faults. To investigate the near-surface structure of the Itozawa fault, we conducted ground-penetrating radar (GPR) profiling across the fault, and two drilling surveys on its hanging-wall and footwall blocks. The GPR survey line about 50 m long was located near the previous trenching site. The GPR two-way travel-time sections were collected by common-offset modes by 50 and 100 MHz GPR systems. We acquired common midpoint ensembles on both sides of the surface rupture for depth conversion of the time sections. The resulting GPR sections show detailed subsurface structures to a depth of about 7 m. We identified event horizons for the 2011, penultimate, antepenultimate, and preantepenultimate earthquakes, indicating that the Itozawa fault has ruptured repeatedly as a normal fault. The vertical displacements during these earthquake events, obtained from the GPR sections, were about 0.8, 0.3, 0.4, and 0.7 m, respectively. These observations suggest that the coseismic surface offset amount on the Itozawa fault varies greatly. The recurrence intervals of surface-rupturing earthquakes on the Itozawa fault, calculated from the previous research and this study, are much longer than those of giant interplate earthquakes along the Japan trench, such as the 2011 Tohoku-Oki earthquake. We conclude that not every giant earthquake along the Japan trench triggers a rupture of the Itozawa fault.

Holocene Activity of the Wudaoliang–Changshagongma Fault of the Eastern Tibetan Plateau

The Wudaoliang–Changshagongma fault is one of the NW-trending faults located within the southern Bayan Har Block of the Tibetan Plateau in China. In this paper, we used high-resolution imagery and digital elevation model data to study the geomorphological and geological characteristics of the fault. Furthermore, the result also determined the fault trace and estimated the average horizontal slip rate of the fault since the late Quaternary to have been 2.6 ± 0.6 mm/a. This slip rate is approximately equivalent to that of the Awancang, Madoi–Garde, and Dari faults, which are also located within the block. Furthermore, the slip rates of these faults obtained by remote sensing and geological methods are consistent with GPS observations. It indicates that tectonic deformation within the block is continuous and diffuse. Using trenching study results and sedimentary radiocarbon dating, we identified four paleoearthquake events that occurred at 42,378–32,975, 33,935–20,663, 5052–4862, and after 673–628 cal BP, respectively. The recurrence intervals of large earthquakes on the faults within the block are much longer than those of the boundary faults, and the slip rates are also smaller, indicating that faults within the block play a regulatory role in the tectonic deformation of the Bayan Har Block.

Physics-based earthquake simulations of the Anninghe-Zemuhe-Daliangshan-Xiaojiang fault system in Southwestern China

Seismic hazard evaluation is important for urban construction and earthquake disaster prevention and mitigation. However, because of the long recurrence intervals of large earthquakes (hundreds to thousands of years), seismic hazard assessment based on relatively short-term (tens of years) observational seismic catalogs is difficult. Synthetic seismic catalogs provide a useful way to assess long-term seismic hazards. Based on the Coulomb failure criterion, we apply a quasi-static physics-based earthquake simulator (Virtual Quake) in a tectonically complicated region with several major strike-slip faults, namely, the Anninghe, Zemuhe, Daliangshan and Xiaojiang (AZDX) faults, in southwestern China. The slip rates of these major faults are constrained by GPS data, and friction is constrained by laboratory experiments. Considering the stress interactions among the fault system, a synthetic catalog over 20,000 years is simulated. The simulated catalog shows consistency with the observed seismicity in the spatial distribution of large earthquakes, b value and mean seismic rate. Based on the synthetic catalog, the mean recurrence intervals and probabilities of the recurrence interval for strong earthquakes (M ≥ 7) in each fault section are presented in detail. Moreover, qualitative analysis of seismic hazards is conducted for the AZDX fault region. Our results provide a helpful index to evaluate seismic hazards in such a complicated region. Plain language summaryEvaluating seismic hazards is important for building codes, risk prevention and mitigation. Because large earthquakes are rare, modern observed earthquake catalogs are too short to evaluate seismic hazards precisely. Thus, numerical simulations are used to obtain synthetic earthquake catalogs for seismic hazard evaluation. Here, we present a physics-based model of stress accumulation, transfer and release, and simulate an earthquake sequence in the Anninghe-Zemuhe-Daliangshan-Xiaojiang fault system, southwestern China. Thus, we obtain a synthetic catalog over 20,000 years. By comparing the spatial distributions of large earthquakes, b values, and mean seismic rates of the synthetic catalog and observed catalog, we find that the synthetic catalog is consistent with the observed catalog. Based on the synthetic seismic catalog, we summarize the mean strong seismic recurrence interval and the probability of the recurrence interval for each fault section. Qualitative analysis of seismic hazards is conducted for the AZDX fault region. Our results can help to evaluate long-term seismic hazards in a multiple-fault system.

Influences of across-strike heterogeneous viscosity on the earthquake cycle in a three-dimensional strike-slip fault model

Geodetic observations have shown that there exist large differences in the viscosity of the deep lithosphere across many large strike-slip faults. Heterogeneity in lithospheric viscosity structure can influence the efficiency of stress transfer and thus may have a significant effect on the earthquake cycle. Until now, how the lateral viscosity variation across strike-slip faults affects the earthquake cycles is still not well understood. Here, we investigate the effects of across-strike viscosity variation on long-term earthquake behaviors with a three-dimensional strike-slip fault model. Our model is a quasi-static model which is controlled by the slip-weakening friction law and power-law rheology. By comparing with the reference case, we find that low viscosity on one side of the fault results in a smaller rupture area but with a higher Coulomb stress drop on the ruptured fault region. In addition, low viscosity also leads to a small Coulomb stress accumulation rate. These combined effects increase the earthquake recurrence interval by approximately 10% and the earthquake moments by about 30% when the low viscosity is related to a geothermal gradient of 30 ​K/km. In addition, across-strike viscosity variation causes asymmetric interseismic ground surface deformation rate. As the viscosity contrast increases, the difference in the interseismic ground surface deformation rate between the two sides of the fault gradually increases, although the asymmetric feature is not pronounced. This asymmetry of interseismic ground deformation rate across a strike-slip fault is supposed to result in asymmetric coseismic deformation if the long-term plate motion velocity is invariant. As a result, this kind of asymmetry of interseismic deformation may influence the evaluation of potential earthquake hazards along large strike-slip faults with lateral viscosity contrast.

Tectonic landform and paleoseismic events of the easternmost Matano fault in Sulawesi, Indonesia

The 190-km-long Matano fault is a major left-lateral strike-slip fault in Sulawesi, Indonesia that accommodates the rapid westward motion of the Pacific plate relative to the Australian plate. The Matano fault has remained unruptured at least in the past two centuries. The lack of geologic information, such as precise active fault traces, paleoseismic history, and slip rates has been a significant barrier to assessing the seismic hazards of the Matano fault. This paper presents the results of tectonic geomorphic and paleoseismic investigations on the 30 km long easternmost portion of the Matano fault. We identified four geometrical fault segments, documented five surface faulting events, and calculated a surface-rupturing earthquake recurrence interval of 200–470 years and a slip rate of 21 ± 9 mm/yr. The timing of the most recent surface-rupturing earthquake was constrained between 1432 and 1819 CE with an estimated Mw ∼ 7.4 and a probable rupture length of ∼110 km. At least 200 years have passed since the latest faulting event, exceeding the shortest recurrence interval. Thus, the next surface-rupturing earthquake is already due. Integrating our interpretations with results from several published studies clarifies that the Matano fault slips slower than the Palu-Koro fault and connects to the South Sula fault in addition to the Tolo thrust.

Tectonic geomorphology of the Ottawa-Bonnechere Graben, Eastern Canada: implications for regional uplift and intraplate seismicity

In intraplate areas where regional tectonic strain is accommodated by the reactivation of pre-existing structures, the level of seismic hazard associated with faults may be underestimated due to the poor surface expression of faults, scattered earthquake distribution, and long earthquake recurrence intervals. The cause(s) of seismicity in eastern Canada remains unresolved. This is partially because surface expressions of faults have been eroded during glacial and deglacial periods and in the Ottawa-Bonnechere Graben (OBG) are undetectable until a seismic event. Morphotectonic analysis has been widely applied to assess relative tectonic activity in various geological settings. To establish whether active uplift is occurring and to investigate the spatial distribution of relative uplift rates, 131 bedrock drainage basins in OBG were analysed. The aim of this was to (1) test the applicability of geomorphic indices for quantifying active deformation, (2) quantify the spatiotemporal distribution of relative uplift rates, and (3) explore the implications for faulting mechanisms, deformation styles, and ultimately regional seismic hazard. We measured valley floor width-to-height ratio ( Vf), basin elongation ratio ( Re), basin hypsometric integral, and normalized channel steepness index ( ksn). The results show that high relative uplift rates exist in all the six bedrock escarpment sections investigated, suggesting that they are presumably related with regional broad wavelength uplift (epeirogeny) caused by a complex interaction between far-field tectonic stress and glacial isostatic adjustment (GIA). Our analysis showed that Vf, Re, and ksn reveals no considerable spatial differences in high relative uplift rates, consistent with the Canadian Base Network GPS uplift rates. Highlights Geomorphic indices are applied in a low relief and low-medium seismicity area. Geomorphic indices reveal a broad wavelength regional uplift and show relative tectonic activity in the OBG. Relatively high geomorphic indices values provide longer term deformation history compared to global positioning system (GPS) records. Landscape formation and river incision is driven by a complex interaction between GIA and far-field tectonic stresses. Relative uplift rate results show similar patterns to published GPS uplift rates. Possible link between relative tectonic activity and current seismicity.

Evidence for surface rupture of the 1939 Erzincan earthquake based on field data and paleoseismology on the Ezinepazarı Fault (North Anatolian Fault Zone, Central Anatolia)

The Ezinepazarı Fault (EPF) is an active segment that restricts the Niksar Basin in the central part of the North Anatolian Fault Zone and has been the source of the devastating Mw 8.0 earthquake in the last 100 years. Traces of the surface rupture of the EPF, which occurred in AD 1939, can still be observed in the field today as a right-lateral strike-slip character. While paleoseismological studies were mostly focused on the western part of the North Anatolian Fault Zone (NAFZ) in previous studies, the studies on the central and eastern parts of the NAFZ are rarely addressed. In this context, trench-based paleoseismology studies were carried out on the surface rupture traces in order to determine both historical/prehistoric earthquakes caused by the fault and the Quaternary period characteristics of the EPF. According to the data obtained, at least two historical earthquake activities have been detected besides the last 1939 earthquake on the EPF. Accordingly, the first and second events occurred in E1: 5871 ± 2300 BP and E2: 1651 ± 128 BP, respectively. When the determined data are compared with the historical earthquake catalogs, it is concluded that Event 2 (E2) can be correlated with the AD 343 Niksar earthquake; thus, the EPF has an irregular earthquake recurrence interval.

Anelastic deformation in the lower crust and its implications for tectonic dynamics in Kyushu, Southwest Japan

Most of the seismicity in inland Kyushu, southwest Japan occur in the localized intraplate shear zones which are Beppu-Shimabara Graben (BSG) and left-lateral shear zone (LSZ). One of the largest fault systems in Japan (Median Tectonic Line—MTL), and subduction zone of the Philippine Sea Plate also exist near Kyushu. These suggest that the island arc, such as Kyushu has a complex deformation process. This study estimated the anelastic strain rate in the lower crust and the interplate coupling rate via inversion analysis using Global Navigation Satellite System data to elucidate the seismotectonics. Deformation in the lower crust and interplate coupling were modeled by cuboid volumetric sources and a back slip model, respectively. To stabilize the solution, a L2 regularization constraint was applied to the anelastic strain rate. Further, constraints on direction and smoothing to the slip deficit rate were applied. These strength of the constraints were determined via the minimum value of the Akaike Bayesian information criterion. The analyses showed that the anelastic strain rate of the BSG and LSZ was ∼0.6 ×10−6 yr−1, and BSG was related to MTL activity. The findings also suggest that this anelastic deformation in the lower crust created a stress concentration zone in the upper crust of BSG and LSZ, with a stress change rate of ∼6–10 kPa·yr−1. The value is consistent with the stress drop and recurrence interval of regional earthquakes, suggesting that the seismicity around BSG and LSZ is strongly influenced by anelastic deformation in the lower crust. It is also suggested that BSG and LSZ development is influenced by slab rollback, and a subducting ridge. Lastly, a comparison between the deviatoric stress field estimated using focal mechanisms and the anelastic strain rate suggests the existence of a shear zone consisting of weak faults south of BSG.

A Test of the Earthquake Gap Hypothesis in Mexico: The Case of the Guerrero Gap

ABSTRACT The seismic gap hypothesis has been widely cited in Mexico to predict the location of future earthquakes. However, no analysis of the outcome of any predictions of the hypothesis in Mexico has been done to date. This work analyzes the outcome of the prediction by Nishenko and Singh (1987a), which is based on probability distribution functions over time in defined segments that allow for a formal evaluation. Specific probabilities were given for 5, 10, and 20 yr after 1986, using the cumulative distribution function. The prediction relies on the precise repeat times of characteristic earthquakes to define the segments, but we show that the catalog the authors use relies on an imprecise definition of characteristic earthquakes. We discuss some of their decisions in building their catalog to explain how we analyze the outcome of the prediction. An unexpected result is that the very catalog the authors use to create the gap hypothesis prediction does not seem to support a narrow recurrence interval and instead seems to suggest large variability in earthquake recurrence intervals along the Mexican subduction zone. We generate null model earthquake catalogs using the average number of earthquakes that occur in the subduction zone and randomly distribute these along the segments according to their relative lengths. We find that the null model performs better than the seismic gap hypothesis prediction. No earthquakes occur in segments with a 70% or higher probability according to NS1987 (there were four such segments in the 20-year time frame), but an Mw 8.0 earthquake occurs in a segment with a less than 16% probability of an earthquake. We conclude that the gap hypothesis performed poorly at predicting earthquakes in Mexico and, in fact, its predictions were worse than predicting earthquakes by chance.

Revised Earthquake Geology Inputs for the Central and Eastern United States and Southeast Canada for the 2023 National Seismic Hazard Model

AbstractIt has been about a decade since updates to seismic and fault sources in the central and eastern United States (CEUS) were last assessed for the 2012 Central and Eastern United States Seismic Source Characterization for nuclear facilities (CEUS-SSCn) and 2014 U.S. Geological Survey National Seismic Hazard Model (NSHM) for the conterminous United States. In advance of the 2023 NSHM update, we created three related geospatial databases to summarize and characterize new fault source information for the CEUS. These include fault section, fault-zone polygon, and earthquake geology (fault slip rate, earthquake recurrence intervals) databases that document updates to fault parameters used in the prior seismic hazard models in this region. The 2012 CEUS-SSCn and 2014 NSHM fault models served as a foundation, as we revised and added fault sources where new published studies documented significant changes to our understanding of fault location, geometry, or activity. We added nine new fault sections that meet the criteria of (1) a length ≥7 km, (2) evidence of recurrent Quaternary tectonic activity, and (3) documentation that is publicly available in a peer-reviewed source. The prior CEUS models only included six fault sections (sources) and 10 fault-zone polygons (previously called repeating large-magnitude earthquake [RLME] polygons). The revised databases include 15 fault sections and 12 fault-zone polygons. Updates to the faults constitute a 150% increase in fault sections, and two additional fault-zone polygons, although some fault-zone polygons differ from RLME polygons used in the prior models. No faults were removed from the past models. Several seismic zones and suspected faults were evaluated but not included in this update due to a lack of information about fault location, geometry, or recurrent Quaternary activity. These updates to the fault sections, fault-zone polygons, and earthquake geology databases will inform fault geometry and activity rates of CEUS sources during the 2023 NSHM implementation.