Characterization of fault plane and coseismic slip for the 2 May 2020, <i>M</i>_w 6.6 Cretan Passage earthquake from tide gauge tsunami data and moment tensor solutions

We present a source solution for the tsunami generated by the Mw 6.6 earthquake that occurred on 2 May 2020, about 80 km offshore south of Crete, in the Cretan Passage, on the shallow portion of the Hellenic Arc subduction zone (HASZ). The tide gauges recorded this local tsunami on the southern coast of Crete and Kasos island. We used Crete tsunami observations to constrain the geometry and orientation of the causative fault, the rupture mechanism, and the slip amount. We first modelled an ensemble of synthetic tsunami waveforms at the tide gauge locations, produced for a range of earthquake parameter values as constrained by some of the available moment tensor solutions. We allow for both a splay and a back-thrust fault, corresponding to the two nodal planes of the moment tensor solution. We then measured the misfit between the synthetic and the Ierapetra observed marigram for each source parameter set. Our results identify the shallow, steeply dipping back-thrust fault as the one producing the lowest misfit to the tsunami data. However, a rupture on a lower angle fault, possibly a splay fault, with a sinistral component due to the oblique convergence on this segment of the HASZ, cannot be completely ruled out. This earthquake reminds us that the uncertainty regarding potential earthquake mechanisms at a specific location remains quite significant. In this case, for example, it is not possible to anticipate if the next event will be one occurring on the subduction interface, on a splay fault, or on a back-thrust, which seems the most likely for the event under investigation. This circumstance bears important consequences because back-thrust and splay faults might enhance the tsunamigenic potential with respect to the subduction interface due to their steeper dip. Then, these results are relevant for tsunami forecasting in the framework of both the long-term hazard assessment and the early warning systems.

Deep Coseismic Slip in the Cascadia Megathrust can be Consistent with Coastal Subsidence

At subduction zones, the down-dip limit of slip represents how deep an earthquake can rupture. For hazards it is important - it controls the intensity of shaking and the pattern of coseismic uplift and subsidence. In the Cascadia Subduction Zone, because no large magnitude events have been observed in instrumental times, the limit is inferred from geological estimates of coastal subsidence during previous earthquakes; it is typically assumed to coincide approximately with the coastline. This is at odds with geodetic coupling models, it leaves residual slip deficits unaccommodated on a large swath of the megathrust. Here we will show that ruptures can penetrate deeper into the megathrust and still produce coastal subsidence provided slip decreases with depth. We will discuss the impacts of this on expected shaking intensities

Complex Slip Distribution of the 2021 Mw 7.4 Maduo, China, Earthquake: An Event Occurring on the Slowly Slipping Fault

The 21 May 2021 Maduo earthquake occurred on the Kunlun Mountain Pass–Jiangcuo fault (KMPJF), a seismogenic fault with no documented large earthquakes. To probe its kinematics, we first estimate the slip rates of the KMPJF and Tuosuo Lake segment (TLS, ∼75 km north of the KMPJF) of the East Kunlun fault (EKLF) based on the secular Global Positioning System (GPS) data using the Markov chain Monte Carlo method. Our model reveals that the slip rates of the KMPJF and TLS are 1.7 ± 0.8 and 7.1 ± 0.3 mm/yr, respectively. Then, we invert high-resolution GPS and Interferometric Synthetic Aperture Radar observations to decipher the fault geometry and detailed coseismic slip distribution associated with the Maduo earthquake. The geometry of the KMPFJ significantly varies along strike, composed of five fault subsegments. The most slip is accommodated by two steeply dipping fault segments, with the patch of large sinistral slip concentrated in the shallow depth on a simple straight structure. The released seismic moment is ∼1.5×1020 N·m, equivalent to an Mw 7.39 event, with a peak slip of ∼9.3 m. Combining the average coseismic slip and slip rate of the main fault, an earthquake recurrence period of ∼1250−400+1120 yr is estimated. The Maduo earthquake reminds us to reevaluate the potential of seismic gaps where slip rates are low. Based on our calculated Coulomb failure stress, the Maduo earthquake imposes positive stress on the Maqin–Maqu segment of the EKLF, a long-recognized seismic gap, implying that it may accelerate the occurrence of the next major event in this region.

Identification of Paleoearthquakes and Coseismic Slips on a Normal Fault Using High-Precision Quantitative Morphology: Application to the Jiaocheng Fault in the Shanxi Rift, China

The quantitative morphology of bedrock fault surfaces combined with aerial surveys and field identification is a useful approach to identify paleoearthquakes, obtain coseismic slips, and evaluate the seismogenic capacity of active faults in bedrock areas where traditional trenching methods are not applicable. Here, we report a case study of the Jiaocheng Fault (JCF) in the Shanxi Rift, China. Although several studies have been conducted on the JCF, its coseismic slip history and seismogenic capacity are still unclear. To address these problems, we investigated two bedrock fault surfaces, Sixicun (SXC) and Shanglanzhen (SLZ), on the JCF’s northern segment using quantitative morphological analysis together with aerial and field surveys. Quantitative fractal analysis based on the isotropic empirical variogram and moving window shows that both bedrock fault surfaces have the characteristics of vertical segmentation, which is likely due to periodic earthquakes, the coseismic slip of which can be determined by the height of the segments. Three seismic events at SXC, with a coseismic vertical slip of 1.74, 1.65, and 1.99 m, and three seismic events at SLZ, with a coseismic vertical slip of 1.32, 2.35, and 1.88 m, are identified. Compared with the previous studies, these three seismic events may occur in the Holocene, but it requires absolute dating ages to support, which is also the focus of our future work. Considering the seismologic capability (M&gt;7.5) and the relationship between the recurrence interval of ~2.6 kyr and elapsed time of more than 3 kyr, the seismic hazard of the northern and middle segments of the JCF requires immediate attention.

Improving Coseismic Slip Measurements

A physics-based method estimates the duration of earthquakes’ coseismic phase and can help improve the precision of coseismic slip models and magnitude estimates.

Tsunami hazard in Lombok & Bali, Indonesia, due to the Flores backarc thrust

The tsunami hazard posed by the Flores backarc thrust, which runs along the northern coast of the islands of Bali and Lombok, Indonesia, is poorly studied compared to the Sunda megathrust, situated ~250 km to the south of the islands. However, the 2018 Lombok earthquake sequence demonstrated the seismic potential of the western Flores Thrust when a fault ramp beneath the island of Lombok ruptured in two Mw 6.9 earthquakes. Although the uplift in these events mostly occurred below land, the sequence still generated 1–2.5 m-high local tsunamis along the northern coast of Lombok (Wibowo et al., 2021). Historical records show that the Flores fault system in the Lombok and Bali region has generated at least six ≥ Ms 6.5 tsunamigenic earthquakes since 1800 CE. Hence, it is important to assess the possible tsunami hazard represented by this fault system. Here, we focus on the submarine fault segment located between the islands of Lombok and Bali (below the Lombok Strait). We assess modeled tsunami patterns generated by fault slip in six earthquake scenarios (slip of 1–5 m, representing Mw 7.2–7.9+), with a focus on impacts on the capital cities of Mataram, Lombok and Denpasar, Bali, which lie on the coasts facing the strait. We use a geologically constrained earthquake model informed by the Lombok earthquake sequence (Lythgoe et al., 2021), together with a high-resolution bathymetry dataset developed by combining direct measurements from GEBCO with sounding measurements from the official nautical charts for Indonesia. Our results show that fault rupture in this region could trigger a tsunami reaching Mataram in < 8 minutes and Denpasar in ~10–15 minutes, with multiple waves. For an earthquake with 3–5 m of coseismic slip, Mataram and Denpasar experience maximum wave heights of ~1.3–3.3 m and ~0.7 to 1.5 m, respectively. Furthermore, our earthquake models indicate that both cities would experience coseismic subsidence of 20–40 cm, exacerbating their exposure to both the tsunami and other coastal hazards. Overall, Mataram city is more exposed than Denpasar to high tsunami waves arriving quickly from the fault source. To understand how a tsunami would affect Mataram, we model the associated inundation using the 5 m slip model and show that Mataram is inundated ~55–140 m inland along the northern coast and ~230 m along the southern coast, with maximum flow depths of ~2–3 m. Our study highlights that the early tsunami arrival in Mataram, Lombok gives little time for residents to evacuate. Raising their awareness about the potential for locally generated tsunamis and the need for evacuation plans is important to help them respond immediately after experiencing strong ground shaking.

Insights from the geological record of deformation along the subduction interface at depths of seismogenesis

Subduction interfaces are loci of interdependent seismic slip behavior, fluid flow, and mineral redistribution. Mineral redistribution leads to coupling between fluid flow and slip behavior through decreases in porosity/permeability and increases in cohesion during the interseismic period. We investigate this system from the perspective of ancient accretionary complexes with regional zones of mélange that record noncoaxial strain during underthrusting adjacent to the subduction interface. Deformation of weak mudstones is accompanied by low-grade metamorphic reactions, dissolution along scaly microfaults, and the removal of fluid-mobile chemical components, whereas stronger sandstone blocks preserve veins that contain chemical components depleted in mudstones. These observations support local diffusive mass transport from scaly fabrics to veins during interseismic viscous coupling. Underthrusting sediments record a crack porosity that fluctuates due to the interplay of cracking and precipitation. Permanent interseismic deformation involves pressure solution slip, strain hardening, and the development of new shears in undeformed material. In contrast, coseismic slip may be accommodated within observed narrow zones of cataclastic deformation at the top of many mélange terranes. A kinetic model implies interseismic changes in physical properties in less than hundreds of years, and a numerical model that couples an earthquake simulator with a fluid flow system depicts a subduction zone interface governed by feedbacks between fluid production, permeability, hydrofracturing, and aging via mineral precipitation. During an earthquake, interseismic permeability reduction is followed by coseismic rupture of low permeability seals and fluid pressure drop in the seismogenic zone. Updip of the seismogenic zone, there is a post-seismic wave of higher fluid pressure that propagates trenchward.

The July 2020 Mw 6.3 Nima Earthquake, Central Tibet: A Shallow Normal-Faulting Event Rupturing in a Stepover Zone

On 22 July 2020, an Mw 6.3 earthquake with a predominantly normal-faulting mechanism struck the Yibug Caka fault zone, central Tibet, where the overall tectonic environment is characterized by left-lateral strike-slip motion. This event offers a chance to gain insight into the tectonic deformation and the cause of shallow normal-faulting earthquakes in this little studied region. Here, we use Sentinel-1A/B Interferometric Synthetic Aperture Radar data to investigate the coseismic and postseismic deformation related to this earthquake. The earthquake ruptured a previously mapped West Yibug Caka fault and is dominated by normal slip with a peak value of 1.9 m at depth of 6.9 km. Postseismic deformation analysis indicates that the observed subsidence signals of up to ∼4.7 cm are a consequence of afterslip. Most of the afterslip is confined at depths between 0.8 and 8.4 km, peaking at 0.27 m at depth of 6.1 km. The significant coseismic slip and afterslip involved in the earthquake highlights a complex interaction between the major normal fault and the secondary synthetic fault. By an integrated analysis of the inversions, regional geology geomorphology, fault kinematics, and seismicity background, we propose a tectonic model that attributes the occurrence of this normal-faulting event to the release of extensional stress in a stepover zone controlled by the northeast-striking sinistral strike-slip Riganpei Co fault and Bu Zang Ai fault. Compared with that the structural stepover often acts as a barrier to affect the propagation of earthquake rupture, our study demonstrates that the failure of a stepover may potentially induce the occurrence of earthquake along the bounding strike-slip faults.

Neotectonics and Paleoseismicity of a Major Junction Between Two Strands of the Awatere Fault, South Island, New Zealand

<p><b>In northeastern South Island, New Zealand, obliquely-convergent relativemotion between the Pacific and Australian plates is accommodated by slip acrossactive dextral-oblique faults in the Marlborough fault system. The Awatere Fault isone of four principal active strike-slip faults within this plate boundary zone, andincludes two sections (the eastern and Molesworth sections) that have differentstrikes and that join across a complex fault junction in the upper Awatere Valley.</b></p> <p>Detailed mapping of the fault traces and measurement of 97 geomorphicdisplacements along the Awatere Fault in the vicinity of the fault junction show thatthe eastern and Molesworth sections of the fault intersect one another at a low angle(10-15º), at the eastern end of an internally faulted, elongate, ~15 km long and up to3 km wide fault wedge or sliver. The region between the fault sections is split by aseries of discontinuous, en-echelon scarps that are oriented from ~10º to 20-30ºclockwise from the principal fault sections. Based on other observations ofdiscontinuities in strike-slip earthquake ruptures around the globe, this low-angleintersection geometry suggests that the junction between these fault sections may notact as a significant barrier to earthquake rupture propagation. This interpretation ofthe mechanical significance of the fault junction to earthquake ruptures is counter toprevious suggestions, but is supported by new paleoseismic data from fourpaleoseismic trenches excavated on each side of the junction. In a new paleoseismictrench on the Molesworth section at Saxton River, 18 km to the west of the junction,up to ten surface-rupturing events in the past ~15 ka are recognised from 12radiocarbon ages and 1 optically stimulated luminescence age. In two new trencheson the eastern section near to Upcot Saddle, 12 km northeast of the fault junction,five events took place in the past 5.5 ka, based on 21 radiocarbon ages. Thischronology from Upcot Saddle is combined with data from two previous trencheslocated ~55 km to the northeast at Lake Jasper, to infer nine events on the easternsection since 8330-8610 cal. years B.P. These well-dated events on the easternsection are compared to those on the Molesworth section to the west of the faultjunction. At 95% confidence, five events on both sections have occurred withstatistical contemporaneity since ~6 ka B.P. These five events may have rupturedboth the eastern and Molesworth sections simultaneously, in accordance with the interpretation that the fault section junction does not arrest rupture propagation.</p> <p>Alternatively, these events may have been separate earthquakes that occurred withinthe statistical resolution provided by radiocarbon dating.</p> <p>The most recent event to rupture the eastern section was the Mw ~7.5 1848Marlborough earthquake. The coseismic slip distribution and maximum traceablelength of this surface rupture are calculated from the magnitude and distribution ofsmall, metre-scale geomorphic displacements attributable to this earthquake. Thesedata suggest this event ruptured >100-110 km of the eastern section, with meansurface displacement of 5.3 ±1.6 m. Based on these parameters, the momentmagnitude of this earthquake would be Mw 7.4-7.7. This magnitude estimate isindistinguishable from previous calculations that were based on attenuation ofshaking intensity isoseismals that were assigned from contemporary historicalaccounts of that earthquake. On the basis of similar rupture lengths and coseismicdisplacements, it is inferred that the penultimate event had a similar momentmagnitude to the 1848 earthquake.</p> <p>Horizontal displacement of a flight of 6 fluvial terraces at Saxton River by theMolesworth section of the Awatere Fault is constrained to have occurred at a nearconstantrate of 5.5 ±1.5 mm/a since ~15 ka B.P. These rates are based on two newoptically stimulated luminescence ages for the highest terrace treads of 14.5 ±1.5 and6.69 ±0.74 ka B.P. These rates are indistinguishable from recent strike-slip rateestimates for the eastern section of 5.6 ±1.1 and 6 ±2 mm/a. Comparing themagnitudes and ages of the terrace riser displacements at Saxton River to the timingof paleoearthquakes on the Molesworth section implies a mean per-eventdisplacement of 4.4 ±0.2 m since ~15 ka. The new terrace ages also record twoperiods of aggradation that post-date the Last Glacial Maximum.</p>