Multifault Ruptures Research Articles

Probabilistic seismic hazard analysis for the Dominican Republic

The Dominican Republic experiences moderate to high seismic hazard mostly caused by oblique convergence at the Caribbean/North American plate boundary that manifests as subduction zones, less-pronounced subduction-like trenches with thrust faulting, long strike-slip faults parallel to the plate boundary, and onshore deformation. Historical earthquakes have damaged the Dominican Republic’s large cities and those in neighboring Haiti, once requiring relocation. Given this, the Dominican Republic joined the “Training and Communication for Earthquake Risk Assessment” (TREQ) project funded by the United States Agency for International Development, which aimed to increase earthquake risk assessment capacity in Latin American cities. The TREQ project was the basis for developing an openly available probabilistic seismic hazard model for the Dominican Republic. The input model was developed from two main datasets: a homogenized earthquake catalog and an active faults database that combines results of recent local projects with a global database. The seismic source characterization used these to constrain source geometries and occurrence rates for active shallow crustal earthquakes, subduction interfaces and subduction-like thrusts, and intraslab earthquakes. Shallow crustal earthquakes, including those on subduction-like thrusts, are modeled by smoothed seismicity and fault sources, the latter using pre-defined geometries that permit multi-fault ruptures. Seismicity on the Puerto Rico Trench subduction interface is modeled as a fault source, while intraslab sources use pre-defined gridded ruptures inside the intraslab volume. The source characterization applies epistemic uncertainties to modeling assumptions affecting occurrence rates and maximum magnitudes. The ground motion characterization used residual analyses from past regional projects as a basis, updating some components with more recent ground motion models. Computed hazard results reinforce those from recent studies in terms of geographical hazard patterns and levels. For 475-year return periods, peak ground acceleration (PGA) in Santiago de los Caballeros reaches 0.50 g, controlled by the Septentrional Fault, while all tectonic region types contribute to the PGA 0.31 g computed for Santo Domingo.

The role of heterogeneous stress in earthquake cycle models of the Hikurangi-Kermadec subduction zone

Summary Seismic and tsunami hazard modelling and preparedness are challenged by uncertainties in the earthquake source process. Important parameters such as the recurrence interval of earthquakes of a given magnitude at a particular location, the probability of multi-fault rupture, earthquake clustering, rupture directivity and slip distribution are often poorly constrained. Physics-based earthquake simulators, such as RSQSim, offer a means of probing uncertainties in these parameters by generating long-term catalogues of earthquake ruptures on a system of known faults. The fault initial stress state in these simulations is typically prescribed as a single uniform value, which can promote characteristic earthquake behaviours and reduce variability in modelled events. Here, we test the role of spatial heterogeneity in the distribution of the initial stresses and frictional properties on earthquake cycle simulations. We focus on the Hikurangi-Kermadec subduction zone, which may produce Mw > 9.0 earthquakes and likely poses a major hazard and risk to Aotearoa New Zealand. We explore RSQSim simulations of Hikurangi-Kermadec subduction earthquake cycles in which we vary the rate and state coefficients (a and b). The results are compared with the magnitude-frequency distribution (MFD) of the instrumental earthquake catalogue and with empirical slip scaling laws from global earthquakes. Our results suggest stress heterogeneity produces more realistic and less characteristic synthetic catalogues, making them particularly well suited for hazard and risk assessment. We further find that the initial stress effects are dominated by the initial effective normal stresses, since the normal stresses evolve more slowly than the shear stresses. A heterogeneous stress model with a constant pore-fluid pressure ratio and a constant state coefficient (b) of 0.003 produces the best fit to MFDs and empirical scaling laws, while the model with variable frictional properties produces the best fit to earthquake depth distribution and empirical scaling laws. This model is our preferred initial stress state and frictional property settings for earthquake modelling of the Hikurangi-Kermadec subduction interface. Introducing heterogeneity of other parameters within RSQSim (e.g. friction coefficient, reference slip rate, characteristic distance, initial state variable, etc) could further improve the applicability of the synthetic earthquake catalogues to seismic hazard problems and form the focus of future research.

Fault size-dependent fracture energy explains multiscale seismicity and cascading earthquakes.

Earthquakes vary in size over many orders of magnitude, often rupturing in complex multifault and multievent sequences. Despite the large number of observed earthquakes, the scaling of the earthquake energy budget remains enigmatic. We propose that fundamentally different fracture processes govern small and large earthquakes. We combined seismological observations with physics-based earthquake models, finding that both dynamic weakening and restrengthening effects are non-negligible in the energy budget of small earthquakes. We established a linear scaling relationship between fracture energy and fault size and a break in scaling with slip. We applied this scaling using supercomputing and unveiled large dynamic rupture earthquake cascades involving >700 multiscale fractures within a fault damage zone. We provide a simple explanation for seismicity across all scales with implications for comprehending earthquake genesis and multifault rupture cascades.

Empirical Earthquake Source Scaling Relations for Maximum Magnitudes Estimations in Central America

ABSTRACT Central America is a seismically active region where six tectonic plates (North America, Caribbean, Cocos, Nazca, Panama, and South America) interact in a subduction zone with transform faults and two triple points. This complex tectonic setting makes the maximum magnitude—Mmax—estimation a challenging task, with the crustal fault earthquakes being the most damaging in the seismic history of Central America. The empirical source scaling relations (ESSR) allow the Mmax of faults to be determined from rupture parameters. In this study, we use a dataset of well-characterized earthquakes in the region, comprising 64 events from 1972 to 2021 with magnitudes between Mw 4.1 and 7.7. The dataset incorporates records of rupture parameters (length, width, area, slip, and magnitude) and information on the faults and aftershocks associated. This database is an important product in itself, and through its use we determine which global relations fit best to our data via a residual analysis. Moreover, based on the best-quality records, we develop scaling relations for Central America (CA-ESSR) for rupture length, width, and area. These new relations were tested and compared with recent earthquakes, and logic trees are proposed to combine the CA-ESSR and the best-fit global relations. Therefore, we estimate the Mmax for 30 faults using the logic tree for rupture length, considering a total rupture of the fault and multifault scenarios. Our results suggest that in Central America rupture areas larger than other regions are required to generate the same magnitudes. We associate this with the shear modulus (μ), which seems to be lower (∼30% less) than the global mean values for crustal rocks. Furthermore, considering multifault ruptures, we found several fault systems with potential Mmax≥Mw 7.0. These findings contribute to a better understanding of regional seismotectonics and to the efficient characterization of fault rupture models for seismic hazards.

Capturing Directivity in Probabilistic Seismic Hazard Analysis for New Zealand: Challenges, Implications, and a Machine Learning Approach for Implementation

ABSTRACT The proximity of fast-slipping crustal faults to urban areas may result in pulse-like ground motions from rupture directivity, which can contribute to increased levels of damage even for engineered structures. Systematic modeling of directivity within probabilistic seismic hazard analysis (PSHA) remains challenging to implement at the regional scale, despite the availability of directivity models in the literature. In the process of developing the 2022 National Seismic Hazard Model for New Zealand (2022 NSHM), we explored the feasibility and impact of modeling directivity for PSHA at a national scale using the previous generation 2010 NSHM. The results of this analysis allowed us to quantify the impact of directivity on the resulting seismic hazard maps for New Zealand and gain insights into the factors that contribute to the expected increases (and decreases) in ground-motion level. For the 2022 NSHM, the earthquake rupture forecast (ERF) seismogenic source models introduced enormous challenges for directivity modeling due to the abundance of large multisegment or multifault ruptures with complex geometries. To overcome these challenges, we applied a machine learning-based strategy to “overfit” an artificial neural network to capture the distributions of directivity amplification and its variability for each unique rupture in the earthquake rupture forecast. This produces a compact representation of the spatial fields of amplification that are computationally efficient to generate within a complete PSHA calculation for the 2022 NSHM. This flexible and reproducible framework facilitates the implementation of directivity in PSHA at a regional scale for complex ERF source models and opens up the possibility of more complex characterization of epistemic uncertainties for near-source ground motion in practice.

A Comprehensive Fault-System Inversion Approach: Methods and Application to NSHM23

ABSTRACT We present updated inversion-based fault-system solutions for the 2023 update to the National Seismic Hazard Model (NSHM23), standardizing earthquake rate model calculations on crustal faults across the western United States. We build upon the inversion methodology used in the Third Uniform California Earthquake Rupture Forecast (UCERF3) to solve for time-independent rates of earthquakes in an interconnected fault system. The updated model explicitly maps out a wide range of fault recurrence and segmentation behavior (epistemic uncertainty), more completely exploring the solution space of viable models beyond those of UCERF3. We also improve the simulated annealing implementation, greatly increasing computational efficiency (and thus inversion convergence), and introduce an adaptive constraint weight calculation algorithm that helps to mediate between competing constraints. Hazard calculations show that ingredient changes (especially fault and deformation models) are the primary driver of hazard changes between NSHM23 and UCERF3. Updates to the inversion methodology are also consequential near faults in which the slip rate in UCERF3 was poorly fit or was satisfied primarily using large multifault ruptures that are now restricted by explicit b-value and segmentation constraints.

The USGS 2023 Conterminous U.S. Time-Independent Earthquake Rupture Forecast

ABSTRACT We present the 2023 U.S. Geological Survey time-independent earthquake rupture forecast for the conterminous United States, which gives authoritative estimates of the magnitude, location, and time-averaged frequency of potentially damaging earthquakes throughout the region. In addition to updating virtually all model components, a major focus has been to provide a better representation of epistemic uncertainties. For example, we have improved the representation of multifault ruptures, both in terms of allowing more and less fault connectivity than in the previous models, and in sweeping over a broader range of viable models. An unprecedented level of diagnostic information has been provided for assessing the model, and the development was overseen by a 19-member participatory review panel. Although we believe the new model embodies significant improvements and represents the best available science, we also discuss potential model limitations, including the applicability of logic tree branch weights with respect different types of hazard and risk metrics. Future improvements are also discussed, with deformation model enhancements being particularly worthy of pursuit, as well as better representation of sampling errors in the gridded seismicity components. We also plan to add time-dependent components, and assess implications with a wider range of hazard and risk metrics.

Characteristic versus Gutenberg–Richter Nucleation-Based Magnitude–Frequency Distributions in the New Zealand National Seismic Hazard Model 2022

Abstract Probabilistic seismic hazard analysis requires a seismicity rate model, or in other words, a forecast of earthquake rates. In the New Zealand National Seismic Hazard Model 2022, the seismicity rate model is constructed through independent forecasts of earthquakes on mapped faults and earthquakes distributed over cells in a spatial grid. Here, we explore the seismicity rate model for upper plate (hypocenter ≥ 40 km) events, to investigate the shape of magnitude–frequency distributions (MFDs) considering events nucleating (or for which the hypocenters are located) within individual fault zone. We find that more than 80% of the fault zones have MFDs that are better described by a Gutenberg–Richter (GR) distribution, instead of a characteristic distribution (i.e., rates of larger magnitudes much higher than the GR trend). Furthermore, the MFD classifications are neither influenced by time-dependent (and time-independent) considerations nor directly affected by the size (or area) of the fault zones. Fault zones with faster slip rates (>20 mm/yr) exhibit characteristic MFDs, whereas those with slower slip rates may or may not. Although multifault ruptures are prevalent in the characteristic distributions, large maximum magnitude (Mw >8.0) plays a pivotal role producing a characteristic MFD. On the other hand, physically unconnected multifault ruptures (i.e., involving rupture jumps ≥ 10 km) are mostly observed with GR distributions.

Comparison of Ground-Shaking Hazard for Segmented versus Multifault Earthquake-Rupture Models in Aotearoa New Zealand

Abstract Multifault ruptures are common for historical earthquakes, and here we consider their impact on seismic hazard. We compare ground-shaking hazard forecasts from the 2022 Aotearoa New Zealand National Seismic Hazard Model (NZ NSHM 2022), which incorporates many multifault ruptures (referred to as the multifault model) with modeled hazard from a simpler model of characteristic earthquakes on individual faults or fault segments (referred to as the segmented model). The multifault model includes very-low-probability rupture lengths of up to ∼1100 km and a mean of 221–234 km, whereas the segmented model primarily comprises rupture lengths of <200 km (mean, 43–51 km) and the maximum of 414 km. The annual rates of Mw 6.9–7.5 earthquakes are more than an order of magnitude higher for the segmented model (0.132–0.24/yr; recurrence times ∼4–7 yr) than the multifault model (0.027/yr; recurrence times 37 yr). Conversely, the rates of earthquakes are similar for segmented and multifault models at Mw>7.5 (0.018–0.031/yr; recurrence times 32–56 yr). Despite differences in rupture lengths and annual rates of earthquakes, the calculated ground-shaking hazard at 10% probability of exceedance (PoE) in 50 yr for the segmented model differs by <55% compared with the multifault model for 95% of sites across Aotearoa New Zealand. For 50% of sites, the modeled hazard differs by <20% between the two models. If a distributed seismicity model (DSM) is included in the hazard calculations, 95% of sites differ in modeled hazard by <18%, and 50% of sites differ by <2.2%. In most areas, seismic hazard at 10% PoE in 50 yr is greater for the segmented model than the multifault model, with notable exceptions along the central Alpine fault in the western South Island and the Taupō volcanic zone in the central North Island.

Deterministic seismic hazard analysis from physics-based earthquake simulations in the Eastern Betics (SE Iberia)

The low frequency of large destructive earthquakes in the Eastern Betics (SE Iberian Peninsula), not recorded in the instrumental seismic catalog, complicate the estimation of the maximum ground motion that could be reached. Knowing the characteristics and behavior of these large seismic ruptures is crucial for the study of seismic hazard. In this work, we have used a physics-based earthquake simulator to generate a 100 kyr catalog of seismic ruptures associated with seismogenic faults bordering the quaternary Guadalentin and Bajo Segura basins. From these ruptures, we have selected maximum earthquake scenarios, and those associated with certain probabilities of exceeding a severe magnitude from the synthetic catalog. The estimation of maximum peak accelerations (PGA) by means of ground motion prediction equations include a possible site amplification that we have calculated from a detailed geotechnical classification of soils. According to the simulations, the modeled faults have the capacity to generate maximum earthquakes with magnitudes ranging MW 6.7–7.4. Densely populated urban areas located along the Guadalentin valley, such as Lorca, could be subject to site-dependent PGA values close to 0.6 g on the Alhama de Murcia fault trace. Similar ground motion would affect the coastline of SE Alicante province when evaluating the maximum simulated event in the Bajo Segura offshore fault. But already from magnitudes around MW ∼ 6, PGA estimations exceed critical values of 0.5 g at the south side of the Bajo Segura basin and the city of Murcia, highly dependent of ground-motion amplification induced by thick deposits of very soft and saturated soils. Our scenarios reflect that other factors such as fault geometry, kinematics and the capability observed in the simulations to generate multi-fault ruptures also affect the spatial distribution of maximum ground motion in this region. We expect that this evaluation could contribute as an improvement guideline for future strong motion prediction studies prior to the settlement of new urban infrastructures in the Eastern Betics.

Complex multi-fault rupture and triggering during the 2023 earthquake doublet in southeastern Türkiye

Two major earthquakes (MW 7.8 and MW 7.7) ruptured left-lateral strike-slip faults of the East Anatolian Fault Zone (EAFZ) on February 6, 2023, causing >59,000 fatalities and ~$119B in damage in southeastern Türkiye and northwestern Syria. Here we derived kinematic rupture models for the two events by inverting extensive seismic and geodetic observations using complex 5-6 segment fault models constrained by satellite observations and relocated aftershocks. The larger event nucleated on a splay fault, and then propagated bilaterally ~350 km along the main EAFZ strand. The rupture speed varied from 2.5-4.5 km/s, and peak slip was ~8.1 m. 9-h later, the second event ruptured ~160 km along the curved northern EAFZ strand, with early bilateral supershear rupture velocity (>4 km/s) followed by a slower rupture speed (~3 km/s). Coulomb Failure stress increase imparted by the first event indicates plausible triggering of the doublet aftershock, along with loading of neighboring faults.

Evidence for a prehistoric multifault rupture along the southern Calico fault system, Eastern California Shear Zone, USA

Abstract Geomorphic mapping and paleoseismologic data reveal evidence for a late Holocene multifault surface rupture along the Calico-Hidalgo fault system of the southern Eastern California Shear Zone (ECSZ). We have identified ~18 km of continuous surface rupture along the combined Calico and Hidalgo faults in the vicinity of Hidalgo Mountain in the southern Mojave Desert. Based on the freshness of geomorphic fault features and continuity of surface expression, we interpret this feature to reflect a simultaneous paleorupture of both faults. Displacement along the paleorupture is defined by 39 field measurements to be generally pure right-slip with a mean offset of 2.3 m. Scaling relationships for this offset amount imply that the original surface rupture length may have been ~82 km (corresponding to a M7.4 earthquake) and that much of the rupture trace was erased by subsequent erosion of sandy and unconsolidated valley alluvium. Eight luminescence ages from a paleoseismic trench across the paleorupture on the Hidalgo fault bracket the timing of the most recent rupture to 0.9–1.7 ka and a possible penultimate event at 5.5–6.6 ka. This timing is generally consistent with the known earthquake clusters in the southern ECSZ based on previous paleoseismic investigations. The ages of these earthquakes also overlap with the age brackets of the most recent events on the Calico fault 42 km to the north and the Mesquite Lake fault 40 km to the south from earlier work. Based on these age constraints and the expected surface rupture length, we propose that the Calico fault system experienced a major, multifault rupture that spanned the entire length of the fault system between the historical Landers and Hector Mine ruptures but preceded these events by ~1–2 k.y. Coulomb stress change modeling shows that the Calico paleorupture may have delayed the occurrence of the Landers-Hector Mine cluster by placing their respective faults in stress shadows and may have also prevented a triggered event from occurring on the Calico fault following the historic events. This work implies that closely spaced ruptures in complex shear zones may repel each other and thereby stretch out the duration of major earthquake clusters. These results also suggest that complex multifault ruptures in the ECSZ may not follow simple, repeatable patterns.

Dense geophysical observations reveal a triggered, concurrent multi-fault rupture at the Mendocino Triple Junction

A central question of earthquake science is how far ruptures can jump from one fault to another, because cascading ruptures can increase the shaking of a seismic event. Earthquake science relies on earthquake catalogs and therefore how complex ruptures get documented and cataloged has important implications. Recent investments in geophysical instrumentation allow us to resolve increasingly complex, multi-fault ruptures for even moderate-sized earthquakes. We combine dense seismic and geodetic measurements to reveal an enigmatic rupture in late 2021 at the Mendocino Triple Junction in northern California. We show that rupture was dynamically triggered, yet concurrent, on two distinct faults roughly 30 km apart. Thus, this rupture combines features of complex ruptures usually considered to be single earthquakes, and triggered ruptures considered as multiple earthquakes. This event illustrates that moderate-sized earthquakes can exhibit similar complexity to that more commonly documented for large earthquakes.

The 2022 Mw 6.7 Menyuan Earthquake on the Northeastern Margin of the Tibetan Plateau, China: Complex Surface Ruptures and Large Slip

ABSTRACT On 8 January 2022, the Mw 6.7 Menyuan earthquake occurred near the stepover of the Lenglongling (LLLF) and Tuolaishan (TLSF) faults of the Qilian–Haiyuan fault zone in the middle of the northeastern Tibetan plateau. Field investigations and unmanned aerial vehicle-based photogrammetry revealed that the earthquake generated five surface rupture zones with different strikes and kinematic properties. Two large rupture zones, R1 (∼22.8 km long) and R2 (∼3.9 km long), occurred along the northern branch of the western LLLF and the eastern segment of TLSF, respectively, and featured left-lateral strike slips. Among the three small rupture zones, the left-lateral strike-slip-type R3 (0.6 km long) was located in the extension direction of R2, whereas the thrust-type R4 (∼3.3 km long) and R5 (∼1.1 km long) zones were located north of the central section of R1. These complex multifault ruptures were caused mainly by the rupture of strike-slip faults on both sides of the stepover structure. A small amount of compressive shortening strain was released during the earthquake due to regional oblique compression. The total length of the rupture zone was ∼31.7 km; the maximum left-lateral and vertical offsets were 3.5 ± 0.3 and 0.47 ± 0.04 m, respectively. Compared with the relationship observed between coseismic slips and magnitudes in historical and modern earthquakes in western China, the 2022 Menyuan earthquake produced a large coseismic slip in relation to its magnitude. The distribution characteristics of the aftershock belts and their relationship with rupture zones showed that the seismogenic fault of the earthquake was nearly east–west-striking TLSF, which may have triggered the rupture of the northern branch of the western LLLF. In addition, only a small segment of TLSF was ruptured, indicating that the accumulated strain could not be released completely during the earthquake and that this remains the most likely area for the occurrence of large earthquakes in the future.

Multi-fault rupture behavior of the 1786 M 73/4 Kangding earthquake on the eastern margin of the Tibetan Plateau

The 2022 LudingMs6.8 earthquake has drawn attention to the Xianshuihe fault zone. Historically, there was anM73/4earthquake in this region in 1786. Because the surface rupture of this historic earthquake was not obvious, there is still much debate over the extent of the surface rupture, which is critical for comprehending tectonic activity and assessing seismic risk for the Xianshuihe fault (XSHF). In particular, the seismogenic structure of this earthquake was connected by three active left-lateral strike-slip faults—the Anninghe fault (ANHF), Daliangshan fault (DLSF), and XSHF—where a large earthquake could cause multi-fault rupture.Giventhese criteria, we report the results of a series of trenches excavated in the vicinity of the epicenter at the northern section of the DLSF, the northern section of the ANHF, the Zheduotang section, and the Selaha section of the XSHF. We find that 1) three palaeoseismic events have been revealed on the northern section of the ANHF: BE1, ∼1003 AD; BE2, 1000–1182 AD; and BE3, 1536 AD asM7 earthquakes, and 2) two palaeoseismic events have occurred on the Selaha fault during the last 500 years. The first event corresponds to the AD 1725 KangdingM7 earthquake, and the latest event may be the AD 1786 M73/4earthquake. 3) Three palaeoseismic events occurred on Zheduotang: ZD1, 1215 BC∼315 AD; ZD2, 830 BC∼705 AD; and ZD3, the 1955 AD KangdingM71/2earthquake. 4) The 1786 KangdingM73/4earthquake was probably a multi-fault rupture event, as evidenced by the trenching profile evidence, nearby offset geomorphic features, and historical earthquake data. Not only the Moxi fault, a seismogenic structure, but also the southern end of the Selaha fault to the north and the Shimian fault (DLSF) to the south simultaneously broke during this earthquake.

Geological structure informs rupture propagation and surface rupture complexity during the 2016 Kaik?ura earthquake, New Zealand: insights for future large earthquake hazard

We summarise the geological setting of complex surface rupture of the 2016 Mw 7.8 Kaik?ura earthquake in the Marlborough Tectonic Domain of New Zealand. The event was complex both seismologically and geologically but not totally dissimilar to other large historical events globally. The earthquake occurred in the comprehensively imbricated, steeply-dipping Pahau Terrane crust that exhibits numerous tectonic overprints with diverse faulting styles. The current strike slip faults of the Marlborough Fault System are immature in their structural development and occupy, at least in part, inherited faults of earlier deformation phases. Several of the faults that ruptured in 2016 may connect at seismogenic depths. A listric fault geometry is likely for many of the faults that ruptured in 2016. This interpretation is supported by crustal seismic mapping identifying listric geometries for other large faults within the region. Examination of other historic surface rupturing earthquakes in the Marlborough Tectonic Domain and globally show some complexity but not to the same level of multifault rupture as in 2016. We conclude that multifault ruptures may be enhanced in the Kaik?ura region where the Australian plate crust is thinner than farther west and the plate boundary deformation, at rates of >20 mm year?1, transfers between closely-spaced faults with acute changes in surface geometry and with diverse rupture characteristics. The trend in seismic hazard assessment since 2016 is to include multifault ruptures universally, but this would be inconsistent with historic events in the Marlborough Tectonic Domain. Consideration of geological structure and history may usefully be incorporated into seismic hazard methodology to evaluate when and where multifault source models are indeed appropriate.

Late Holocene earthquakes on the Papatea Fault and its role in past earthquake cycles, Marlborough, New Zealand

ABSTRACT The north-striking sinistral reverse Papatea Fault ruptured with a very large (up to 12 m) oblique slip as part of the 2016 M W 7.8 Kaikōura earthquake in the northeastern South Island. Paleoseismic studies were undertaken at three sites along the Papatea Fault, named Murray’s roadcut, Jacqui’s Gully (both on the main strand), and Wharekiri trench (western strand). These sites provide evidence for up to three Late Holocene paleoearthquakes prior to 2016 (=E0) on this previously unmapped active fault, with preferred OxCal-modelled timings of 98–149 (E1), 546–645 cal yr BP (E2), and >738 cal yr BP (E3). Event correlations between the sites are generally consistent across these past events, implying that the two strands of the Papatea Fault link at depth and rupture together co-seismically as in 2016. Comparisons of its paleoseismic record with the Kekerengu Fault and uplift data from Waipapa Bay and Kaikōura, suggest that the Papatea Fault may have three distinct rupture modes: (i) Kaikōura-type multi-fault ruptures with multi-metre, anelastic block displacements and associated major landscape change; (ii) multi-fault earthquake ruptures with other regional fault combinations; and (iii) single-fault Papatea ruptures with metre-scale displacement. OxCal models offer the possibility that the E1 fault rupture occurred in 1855 CE.

Enumerating Plausible Multifault Ruptures in Complex Fault Systems with Physical Constraints

ABSTRACT We propose a new model for determining the set of plausible multifault ruptures in an interconnected fault system. We improve upon the rules used in the Third Uniform California Earthquake Rupture Forecast (UCERF3) to increase connectivity and the physical consistency of ruptures. We replace UCERF3’s simple azimuth change rules with new Coulomb favorability metrics and increase the maximum jump distance to 15 km. Although the UCERF3 rules were appropriate for faults with similar rakes, the Coulomb calculations used here inherently encode preferred orientations between faults with different rakes. Our new rules are designed to be insensitive to discretization details and are generally more permissive than their UCERF3 counterparts; they allow more than twice the connectivity compared with UCERF3, yet heavily penalize long ruptures that take multiple improbable jumps. The set of all possible multifault ruptures in the California fault system is nearly infinite, but our model produces a tractable set of 326,707 ruptures (a modest 29% increase over UCERF3, despite the greatly increased connectivity). Inclusion in the rupture set does not dictate that a rupture receives a significant rate in the final model; rupture rates are subsequently determined by data constraints used in an inversion. We describe the rupture building algorithm and its components in detail and provide comparisons with ruptures generated by a physics-based multicycle earthquake simulator. We find that greater than twice as many ruptures generated by the simulator violate the UCERF3 rules than violate our proposed model.

Spatial and temporal stress field changes in the focal area of the 2016 Kaikōura earthquake, New Zealand: A multi-fault process interpretation

To understand the stress controls on the occurrence of a multi-fault rupture, we estimated the crustal stress between April 2013 to December 2018, i.e., before and after the Mw7.8 Kaikōura earthquake that occurred in New Zealand on 13 November 2016. We used both the focal mechanism solutions from the temporary seismic networks and the GeoNet moment tensor solutions and selected the solutions that differed significantly from the mainshock fault planes and rakes. Then, we performed stress tensor inversions for the selected focal mechanism solutions. Using the stress tensor inversion results, we also calculated the slip tendency. Prior to the Kaikōura earthquake, the stress regime was the strike-slip type, and the maximum eigenvalue of the stress tensor (σ1) was oriented WNW–ESE. The stress field orientation did not change significantly after the earthquake. This suggests that the stress change during the Kaikōura earthquake was too small to alter the stress orientations, implying that there may have been large differential stress prior to the Kaikōura earthquake. However, the average stress ratio in different clusters changed in two different patterns after the earthquake, suggesting possible changes in the magnitude of different components of the stress tensor, or of pore pressure in different regions. A high slip tendency was observed at the hypocentre, while a low slip tendency was observed at the northern end of the Kaikōura earthquake faults. This may suggest that the stress orientation and the stress ratio controlled the initiation and the end of the multi-fault rupture. These results corroborate previous fault propagation models.

Late Holocene coseismic uplift of the Kaikōura coast, New Zealand

Abstract The complex 2016 MW 7.8 Kaikōura earthquake ruptured >20 faults and caused highly variable uplift and subsidence of an ~110 km stretch of coastline. The earthquake raised questions about fault interactions in regions of oblique convergence and especially subduction to strike-slip transition zones like the Kaikōura region. We integrate 2016 coastal vertical deformation observations with new mapping and dating of Holocene marine terraces to: (1) compare spatial patterns of 2016 coseismic and longer-term vertical motions, (2) investigate possible past multi-fault ruptures or temporal clusters of earthquakes around Kaikōura, and (3) assess the relative contributions of crustal faults and the Hikurangi subduction interface to late Holocene coastal uplift. We identify possible multi-fault ruptures or loose clusters of earthquakes at ca. 850–550 yr B.P. and ca. 350–100 yr B.P. Most (and possibly all) of the Kaikōura coast has been uplifted over the late Holocene; the 25-km-long Parikawa section of coast subsided coseismically in 2016 but appears to be uplifted through reverse slip on an offshore fault. Late Holocene uplift everywhere along the coastline of interest can be attributed to slip on known upper-plate faults; slip on a shallow-dipping (<20°) subduction interface cannot be ruled out but is not required to explain uplift.