Seismogenic Thickness Research Articles

Variations in Seismogenic Thickness Along the Central Alpine Fault, New Zealand, Revealed by a Decade's Relocated Microseismicity

AbstractThe Alpine Fault is an oblique strike‐slip fault that is known to fail in large magnitude (M7–8) earthquakes, yet it is currently seismically quiescent. We examine the low‐magnitude earthquake activity occurring along the central portion of the Alpine Fault using seismic data from five temporary seismic networks deployed for various lengths of time between late 2008 and early 2017. Starting from continuous seismic data, we detect earthquake arrivals and construct the longest and most extensive microearthquake catalog for the central Alpine Fault region to date, containing 9,111 earthquakes. This enables us to study the distribution and characteristics of the seismicity in unprecedented detail. Earthquake locations are constrained by high‐quality automatic and manual picks, and we perform relocations using waveform cross‐correlation to better constrain hypocenters. We have derived a new local magnitude scale calibrated by Mw values. Magnitudes range between ML−1.2 and 4.6, and our catalog is complete above ML 1.1. Earthquakes mainly occur southeast of the Alpine Fault (in the hanging wall) and exhibit low magnitudes. We observe a lack of seismicity beneath Aoraki/Mount Cook, which we associate with high uplift rates and high heat flow. Seismogenic cutoff depths vary along the strike of the Alpine Fault from 8 km, beneath the highest topography, to 20 km in the adjacent areas.

The 2016 Mw 6.5 Nura earthquake in the Trans Alai range, northern Pamir: Possible rupture on a back-thrust fault constrained by Sentinel-1A radar interferometry

On June 26, 2016 an Mw 6.5 earthquake occurred in the Trans-Alai Range, about 37 km west of a 2008 Mw 6.7 earthquake, both near the town of Nura, China. We use Interferometric Synthetic Aperture Radar (InSAR) images from Sentinel-1A to image coseismic surface displacements from the most recent Nura earthquake. We invert the interferograms for both fault geometry and slip distribution. We find that the InSAR data can be described by either a steeply north-dipping fault or shallowly south-dipping fault, although only when slip on the south-dipping fault is constrained to be blind. The InSAR misfit for the north-dipping fault plane is slightly lower, and the north-dipping fault better describes the gradient of deformation in the near-field. We prefer the north-dipping fault geometry (~67° dip and 266° strike) as the source for the 2016 Nura earthquake, not only due to the better fit to the InSAR, but the resulting coseismic slip model is more consistent with the focal mechanism solutions and regional seismogenic thickness; however, it is important to reiterate that the InSAR data cannot uniquely determine which model is the definitive representation of the Nura earthquake. The maximum inferred coseismic slip is about 0.75 m at ~14 km depth beneath the Trans Alai range, with the largest slip in the depth range of about 8–20 km. Slip does not reach the surface, and is dominated by thrust displacement with a right-lateral strike-slip component. The geodetic moment of our preferred slip model is 6.61 × 1018 N·m, equivalent to Mw 6.5. The surface projection of our preferred fault is to the south of the Pamir Frontal Thrust (PFT), and we argue it is a back-thrust of the listric PFT, suggesting that the Trans Alai range is a ‘pop-up’ structure.

Crustal rheology from focal depths in the North China Basin

In order to investigate earthquakes in the lower crust of North China Basin (NCB), we develop a new method of resolving very accurately the focal depth for local earthquakes in the sedimentary regions by using P and S to P converted wave (Sp) at the sedimentary interface. Theoretical analysis shows clearly that the travel-time difference between the Sp and P wave almost linearly correlates with the focal depth. This finding provides tight constraints on the depth. With this method, we obtain well-constrained depths of 44 events in the NCB, with uncertainties in the depth of about 2 km. Such a fine resolution can have great potential in asking questions regarding the crustal rheology. The depth distribution shows abundance of earthquakes in depth interval of ∼20 km, with some events in the lower crust, but also reveals the absence of seismicity deeper than 25 km. We find a good fit between the depth-frequency distribution in this region and the Yield Strength Envelope (YSE) in the Baikal Rift Systems (BRS). We infer that, the seismogenic thickness is ∼25 km in the NCB and the main deformation mechanism is brittle fracture. We further hypothesize that: (1) the rheological layering of dominant rheology in the NCB is similar to that of the BRS, which can be explained with a quartz rheology at 0–10 km depth and a diabase rheology at 10–35 km depth; (2) the temperature is moderate in the seismogenic zone of crust. We emphasize that many accurately resolved earthquake locations can shed light on the local nature of the crustal rheology, and this strategic method can be employed in other sedimentary regions, which are seismically active.

INVESTIGATION OF LITHOSPHERIC STRUCTURE IN MONGOLIA: INSIGHTS FROM INSAR OBSERVATIONS AND MODELLING

Abstract. The western Mongolia is a seismically active intracontinental region, with ongoing tectonic deformation and widespread seismicity related to the far-field effects of India-Eurasia collision. During the 20th century, four earthquakes with the magnitude larger than 8 occurred in the western Mongolia and its surrounding regions, providing a unique opportunity to study the geodynamics of intracontinental tectonic deformations. The 1957 magnitude 8.3 Gobi-Altai earthquake is one of the largest seismic events. The deformation pattern of rupture zone associated with this earthquake is complex, involving left-lateral strike-slip and reverse dip-slip faulting on several distinct geological structures in a 264 × 40 km wide zone. To understand the relationship between the observed postseismic surface deformation and the rheological structure of the upper lithosphere, Interferometric Synthetic Aperture Radar (InSAR) data are used to study the 1957 earthquake. Then we developed a postseismic model in a spherical, radially layered elastic-viscoelastic Earth based on InSAR results, and further analysed the dominant contribution to the surface deformation. This work is important for understanding not only the regional tectonics, but also the structure and dynamics of the lithosphere. SAR data were acquired from the ERS1/2 and Envisat from 1996 to 2010. Using the Repeat Orbit Interferometry Package (ROI_PAC), 124 postseismic interferograms are produced on four adjacent tracks. By stacking these interferograms, the maximum InSAR line-of-sight deformation rate along the Gobi-Altai fault zone is obtained. The main results are as follows: (1) The maximum InSAR line-of-sight deformation velocity along this large fault zone is about 6 mm/yr; (2) The modelled surface deformation suggests that the viscoelastic relaxation is the most reasonable mechanism to explain the observed surface motion; (3) The optimal model cover the Gobi-Altai seismogenic thickness is 10 km; (4) The lower bound of Maxwell viscosity of lower crust and upper mantle is approximately 9 × 1019 Pa s, and the Maxwell relaxation time corresponding to this viscosity is 95.13 years.

Spatial variation of seismogenic depths of crustal earthquakes in the Taiwan region: Implications for seismic hazard assessment

This paper presents the first whole Taiwan-scale spatial variation of the seismogenic zone using a high-quality crustal seismicity catalog. The seismicity onset and cutoff depths (i.e., seismogenic depths) are determined by the earthquake depth–moment distribution and used to define the upper and lower boundaries of the seismogenic zone, respectively. Together with the published fault geometries and fault area–moment magnitude relations, the depth difference in the onset and cutoff depths (i.e., seismogenic thickness) is used as the fault width to determine the moment magnitudes of potential earthquakes for the major seismogenic faults. Results show that the largest (Mw7.9–8.0) potential earthquake may occur along the Changhua fault in western Taiwan, where the seismic risk is relatively high and seismic hazard mitigation should be a matter of urgent concern. In addition, the first-motion focal mechanism catalog is used to examine the relation between the seismogenic depths and earthquake source parameters. For crustal earthquakes (≤50km), the shallowest onset and cutoff depths are observed for normal and strike-slip events, respectively. This observation is different from the prediction of the conventional continental-rheology model, which states that thrust events have the shallowest cutoff depth. Thus, a more sophisticated rheology model is necessary to explain our observed dependence of the seismogenic depths on faulting types. Meanwhile, for intermediate to large crustal (Mw≥4; depth≤50km) earthquakes, thrust events tend to occur at the bottom region of the seismogenic zone, but normal and strike-slip events distribute at a large depth range.

The seismogenic thickness in Italy: constraints on potential magnitude and seismic hazard

AbstractThe thickness of the seismogenic layer is a key parameter for seismic hazard, since it can be used to constrain the maximum depth of faulting and the potential magnitude. In this study, we compute the seismogenic thickness in the Italian region by defining the lower seismicity cut‐off, using high‐quality hypocentral locations of earthquakes that occurred in the past decade. Along the eastern Alps, the seismogenic thickness is about 12–14 km, laterally homogeneous along the entire south‐verging thrust front. In the Apennines extensional belt, lateral changes in seismogenic thickness are evident, and correlate with changes in the seismic energy released by past earthquakes. The potential magnitude is larger in the southern Apennines where the seismogenic thickness is greater (16–18 km) than in the northern Apennines where it is less (6–10 km) and seismic energy is partially released by the creeping of faults.

Estimates of fault strength from the Variscan foreland of the northern UK

We provide new insights into the long-standing debate regarding fault strength, by studying structures active in the late Carboniferous in the foreland of the Variscan Mountain range in the northern UK. We describe a method to estimate the seismogenic thickness for ancient deformation zones, at the time they were active, based upon the geometry of fault-bounded extensional basins. We then perform calculations to estimate the forces exerted between mountain ranges and their adjacent lowlands in the presence of thermal and compositional effects on the density. We combine these methods to calculate an upper bound on the stresses that could be supported by faults in the Variscan foreland before they began to slip. We find the faults had a low effective coefficient of friction (i.e. 0.02–0.24), and that the reactivated pre-existing faults were at least 30% weaker than unfaulted rock. These results show structural inheritance to be important, and suggest that the faults had a low intrinsic coefficient of friction, high pore-fluid pressures, or both.

Surface rupture of the 1911 Kebin (Chon–Kemin) earthquake, Northern Tien Shan, Kyrgyzstan

Abstract The 1911 Chon–Kemin (Kebin) earthquake culminated c. 30 years of remarkable earthquakes in the northern Tien Shan (Kyrgyzstan and Kazakhstan). Building on prior mapping of the event, we traced its rupture in the field and measured more than 50 offset landforms. Cumulative fault rupture length is >155–195 km along 13 fault patches comprising six sections. The patches are separated by changes of dip magnitude or dip direction, or by 4–10 km-wide stepovers. One <40 km section overlaps and is parallel to the main north-dipping rupture but is 7 km north and dips opposite (south). Both ends of the rupture are along mountain front thrust faults demonstrating late Quaternary activity. We computed the moment from each fault patch using the surface fault traces, dip inferred from the traces, 20 km seismogenic thickness, rigidity of 3.3×10 10 N m −2 and dip slip converted from our observations of the largely reverse sense of motion vertical offsets. The discontinuous patches with c. 3–4 m average slip and peak slip of <14 m yield a seismic moment of 4.6×10 20 Nm ( M w 7.78) to 7.4×10 20 Nm ( M w 7.91). The majority of moment was released along the inner eastern rupture segments. This geological moment is lower by a factor of 1.5 from that determined from teleseismic data.

Strength of the Iberian intraplate lithosphere: Cenozoic deformations and seismicity

Previous studies about the strength of the lithosphere in the center of Iberia fail to resolve the depth of earthquakes because of the rheological uncertainties. Therefore, new contributions are considered (the crustal structure from a density model) and several parameters (tectonic regime, mantle rheology, strain rate) are checked in this paper to properly examine the role of lithospheric strength in the intraplate seismicity and the Cenozoic evolution. The strength distribution with depth, the integrated strength, the effective elastic thickness and the seismogenic thickness have been calculated by a finite element modelling of the lithosphere across the Central System mountain range and the bordering Duero and Madrid sedimentary basins. Only a dry mantle under strike-slip/extension and a strain rate of 10-15 s-1, or under extension and 10-16 s-1, causes a strong lithosphere. The integrated strength and the elastic thickness are lower in the mountain chain than in the basins. This heterogeneity has been maintained since the Cenozoic and determine the mountain uplift and the biharmonic folding of the Iberian lithosphere during the Alpine deformations. The seismogenic thickness bounds the seismic activity in the upper–middle crust, and the decreasing crustal strength from the Duero Basin towards the Madrid Basin is related to a parallel increase in Plio–Quaternary deformations and seismicity. However, elasto–plastic modelling shows that current African–Eurasian convergence is resolved elastically or ductilely, which accounts for the low seismicity recorded in this region.

Scaling of maximum observed magnitudes with geometrical and stress properties of strike‐slip faults

AbstractWe test potential scaling between observed maximum earthquake magnitudes along 27 strike‐slip faults with various properties including cumulative displacement, mapped fault length, seismogenic thickness, slip rates, and angle between fault strike and maximum horizontal stress. For 75–80% of the data set, the observed maximum scalar moment scales with the product of seismogenic thickness and either cumulative displacement or mapped fault length. Most faults from this population have slip rates >5 mm/yr (interplate faults), cumulative displacement >10 km, and relatively high angles to the maximum horizontal stress orientation. The remaining 20–25% population involves events at some distance from a plate boundary with slip rate <5 mm/yr, cumulative displacements <10 km, and ≈ 45° to the maximum horizontal stress. These earthquakes have larger magnitudes than the previous population, likely because of larger stress drops. The most likely interpretation of the results is that the maximum rupture length, and hence earthquake magnitudes, correlates with the cumulative displacement and the fault surface length. The results also suggest that progressive fault smoothing may lead to decreasing coseismic stress drops.

Focal depths and mechanisms of shallow earthquakes in the Himalayan–Tibetan region

The complexity of the Himalayan–Tibetan lithospheric deformation is evident from widespread seismicity and diverse focal mechanism solutions. Here we investigate the focal depths and fault plane solutions of 97 moderate and shallow earthquakes in the Himalayan–Tibetan region by modeling teleseismic P-wave and its tailing surface reflections pP and sP. Earthquakes in central Tibet are restricted to the upper crust and originate dominantly by strike-slip faulting, in agreement with the low P-wave velocity layers in the lower crust and the strong S-wave attenuation zones in the uppermost mantle. In northern and southern Tibet, where the Asian and Indian plates descend beneath central Tibet, earthquakes appear to be distributed throughout the thickness of the crust and exhibit dominantly reverse faulting. We incorporate well-estimated focal depths of 127 additional earthquakes from previous studies to estimate the seismogenic thickness (Ts) of the study region. The resulting pattern of Ts is found to be rather flat for central and northeastern Tibet and highly variable along the strike of the Himalayan foreland.

Estimates of [formula omitted] for continental regions using GOCE gravity

Satellite-only gravity fields and surface gravity obtained from altimetric measurements now agree well at wavelengths greater than ∼180 km. Satellite gravity fields can therefore be used to estimate the elastic thickness Te in regions where surface observations are sparse. They are used for this purpose in a number of continental regions, of India, Africa, and Antarctica, where the topography is sufficiently rough, and also in regions of the USA, China, Australia and Siberia, where there are surface measurements. Estimates of Te for Antarctica depend on measurements of ice thickness, which are now available for much of the continent. Values of Te are obtained using two methods: from the admittance between the free air gravity and the topography, and from the coherence between Bouguer gravity anomalies and the topography. The first, but not the second, gives values of Te that are everywhere less than the seismogenic thickness. Where there is sufficient topography, estimates of Te from PreCambrian shields are all greater than 10 km and do not correlate with the lithospheric thickness. They are probably are governed by variations in crustal heat generation rates. Values for regions strongly affected by Phanerozoic tectonics are all less than 7 km, and all such regions are underlain by thin lithosphere.

Assessing infrequent large earthquakes using geomorphology and geodesy: the Malawi Rift

In regions with large, mature fault systems, a characteristic earthquake model may be more appropriate for modelling earthquake occurrence than extrapolating from a short history of small, instrumentally observed earthquakes using the Gutenberg–Richter scaling law. We illustrate how the geomorphology and geodesy of the Malawi Rift, a region with large seismogenic thicknesses, long fault scarps, and slow strain rates, can be used to assess hazard probability levels for large infrequent earthquakes. We estimate potential earthquake size using fault length and recurrence intervals from plate motion velocities and generate a synthetic catalogue of events. Since it is not possible to determine from the geomorphological information if a future rupture will be continuous (7.4 ≤ M W ≤ 8.3 with recurrence intervals of 1,000–4,300 years) or segmented (6.7 ≤ M W ≤ 7.7 with 300–1,900 years), we consider both alternatives separately and also produce a mixed catalogue. We carry out a probabilistic seismic hazard assessment to produce regional- and site-specific hazard estimates. At all return periods and vibration periods, inclusion of fault-derived parameters increases the predicted spectral acceleration above the level predicted from the instrumental catalogue alone, with the most significant changes being in close proximity to the fault systems and the effect being more significant at longer vibration periods. Importantly, the results indicate that standard probabilistic seismic hazard analysis (PSHA) methods using short instrumental records alone tend to underestimate the seismic hazard, especially for the most damaging, extreme magnitude events. For many developing countries in Africa and elsewhere, which are experiencing rapid economic growth and urbanisation, seismic hazard assessments incorporating characteristic earthquake models are critical.

Source-Scaling Relationships for the Simulation of Rupture Geometry within Probabilistic Seismic-Hazard Analysis

Abstract Source‐scaling relations are an integral component of probabilistic seismic‐hazard analysis (PSHA). These models that enable finite‐rupture dimensions to be computed for a given earthquake scenario provide the link between magnitude–frequency distributions and ground‐motion models. However, the development of scaling relations has not traditionally been undertaken in a manner that is consistent with their application within the hazard analysis. The causative problem is that rupture widths saturate as a result of the limited seismogenic thickness in any given region. This saturation has not previously been dealt with in a consistent manner, either in the development of scaling relations or in their typical application. This article approaches the development of source‐scaling relations for shallow crustal earthquakes in active regions in an entirely new way that results in models that are physically consistent and consistent with the assumptions of PSHA. The innovative treatment of the width saturation issue results in new models that feature nonlinear scaling of the expected values, as well as strongly heteroskedastic variances. The first set of correlation coefficients is also provided for the rupture dimensions so that their joint variability can be appropriately accounted for within PSHA. The findings of the article have significant implications for hazard analysis, as the conditional distribution of finite‐rupture distances for a given magnitude differs quite markedly from those suggested by existing approaches.

The effective elastic thickness of the lithosphere in the collision zone between Arabia and Eurasia in Iran

The effective elastic thickness, Te, has been calculated in the collision zone between Arabia and Eurasia in Iran from the wavelet coherence. The wavelet coherence is calculated from Bouguer anomalies and topography data using the isotropic fan wavelet method, and gives Te values between 14.2 and 62.2km. The lower value is found in the Central Iranian Blocks and the East Iranian Belt which are bounded by several large strike-slip faults with lithospheric origin. The higher value occurs in the east of the South Caspian Sea Basin. The resulting Te map shows positive and negative correlation with shear wave velocity and surface heat flow, respectively. A comparison between the seismogenic thickness (Ts) and Te in Iran suggests that Te>Ts. Results of the load ratio in Iran indicate that in most of the study area surface loads are much more prevalent than subsurface loads, except in the Central Iranian Blocks and NW of Iran. Intermediate to low Te values in Iran were inherited from multiple rifting and orogenic activities from Late Precambrian (∼650Ma) to present day which are not only reflected in thin and warm lithosphere but also an increasing seismicity rate.

The UCERF3 Grand Inversion: Solving for the Long-Term Rate of Ruptures in a Fault System

Abstract We present implementation details, testing, and results from a new inversion‐based methodology, known colloquially as the “grand inversion,” developed for the Uniform California Earthquake Rupture Forecast (UCERF3). We employ a parallel simulated annealing algorithm to solve for the long‐term rate of all ruptures that extend through the seismogenic thickness on major mapped faults in California while simultaneously satisfying available slip‐rate, paleoseismic event‐rate, and magnitude‐distribution constraints. The inversion methodology enables the relaxation of fault segmentation and allows for the incorporation of multifault ruptures, which are needed to remove magnitude‐distribution misfits that were present in the previous model, UCERF2. The grand inversion is more objective than past methodologies, as it eliminates the need to prescriptively assign rupture rates. It also provides a means to easily update the model as new data become available. In addition to UCERF3 model results, we present verification of the grand inversion, including sensitivity tests, tuning of equation set weights, convergence metrics, and a synthetic test. These tests demonstrate that while individual rupture rates are poorly resolved by the data, integrated quantities such as magnitude–frequency distributions and, most importantly, hazard metrics, are much more robust.

The long‐wavelength admittance and effective elastic thickness of the Canadian Shield

AbstractThe strength of the cratonic lithosphere has been controversial. On the one hand, many estimates of effective elastic thickness (Te) greatly exceed the crustal thickness, but on the other the great majority of cratonic earthquakes occur in the upper crust. This implies that the seismogenic thickness of cratons is much smaller than Te, whereas in the ocean basins they are approximately the same, leading to suspicions about the large Te estimates. One region where such estimates have been questioned is the Canadian Shield, where glacial isostatic adjustment (GIA) and mantle convection are thought to contribute to the long‐wavelength undulations of the topography and gravity. To date these have not been included in models used to estimate Te from topography and gravity which conventionally are based only on loading and flexure. Here we devise a theoretical expression for the free‐air (gravity/topography) admittance that includes the effects of GIA and convection as well as flexure and use it to estimate Te over the Canadian Shield. We use wavelet transforms for estimating the observed admittances, after showing that multitaper estimates, which have hitherto been popular for Te studies, have poor resolution at the long wavelengths where GIA and convection predominate, compared to wavelets. Our results suggest that Te over most of the shield exceeds 80 km, with a higher‐Te core near the southwest shore of Hudson Bay. This means that the lack of mantle earthquakes in this craton is simply due to its high strength compared to the applied stresses.

An explanation for the age independence of oceanic elastic thickness estimates from flexural profiles at subduction zones, and implications for continental rheology

Most properties of oceanic lithosphere are widely observed to be dependent on the age of the plate, such as water depth, heat flow, and seismogenic thickness. However, estimates of the ‘effective elastic thickness' of oceanic lithosphere based on the deflection of the plate as it enters a subduction zone show little correlation with the age of the incoming lithosphere. This paradox requires reconciliation if we are to gain a full understanding of the structure, rheology, and behaviour of oceanic lithosphere. Here, we show that the permanent deformation of the plate due to outer-rise faulting, combined with uncertainties in the yield stress of the lithosphere, the in-plane forces transmitted through subduction zones, and the levels of noise in bathymetric and gravity data, prevents simple elastic plate modelling from accurately capturing the underlying rheological structure of the incoming plate. The age-independent estimates of effective elastic thickness obtained by purely elastic plate modelling are therefore not likely to represent the true rheology of the plate, and hence are not expected to correspond to the plate age. Similar effects may apply to estimates of elastic thickness from continental forelands, with implications for our understanding of continental rheology.

Tectonic inheritance of the Indian Shield: New insights from its elastic thickness structure

Abstract A new evaluation of the elastic thickness (Te) structure of the Indian Shield, derived from isotropic fan wavelet methodology, documents spatial variations of lithospheric deformation in different tectonic provinces correlated with episodic tectono-thermal events. The Te variations corroborated by shear velocity, crustal thickness, and seismogenic thickness reveal the heterogeneous rheology of the Indian lithosphere. The thinned, attenuated lithosphere beneath Peninsular India is considered to be the reason for its mechanically weak strength (

Active Pacific North America Plate boundary tectonics as evidenced by seismicity in the oceanic lithosphere offshore Baja California, Mexico

Pacific Ocean crust west of southwest North America was formed by Cenozoic seafloor spreading between the large Pacific Plate and smaller microplates. The eastern limit of this seafloor, the continent–ocean boundary, is the fossil trench along which the microplates subducted and were mostly destroyed in Miocene time. The Pacific–North America Plate boundary motion today is concentrated on continental fault systems well to the east, and this region of oceanic crust is generally thought to be within the rigid Pacific Plate. Yet, the 2012 December 14 M_w 6.3 earthquake that occurred about 275 km west of Ensenada, Baja California, Mexico, is evidence for continued tectonism in this oceanic part of the Pacific Plate. The preferred main shock centroid depth of 20 km was located close to the bottom of the seismogenic thickness of the young oceanic lithosphere. The focal mechanism, derived from both teleseismic P-wave inversion and W-phase analysis of the main shock waveforms, and the 12 aftershocks of M ∼3–4 are consistent with normal faulting on northeast striking nodal planes, which align with surface mapped extensional tectonic trends such as volcanic features in the region. Previous Global Positioning System (GPS) measurements on offshore islands in the California Continental Borderland had detected some distributed Pacific and North America relative plate motion strain that could extend into the epicentral region. The release of this lithospheric strain along existing zones of weakness is a more likely cause of this seismicity than current thermal contraction of the oceanic lithosphere or volcanism. The main shock caused weak to moderate ground shaking in the coastal zones of southern California, USA, and Baja California, Mexico, but the tsunami was negligible.