Interplate Thrust Earthquakes Research Articles

Thermally-constrained fluid circulation and seismicity in the Lesser Antilles subduction zone

At subduction zones, fluid circulation and elevated pore pressure are key factors controlling the seismogenic behavior along the plate interface by reducing absolute fault strength, increasing the time return of high magnitude co-seismic rupture and favoring aseismic slip. The Lesser Antilles is an end-member subduction zone where the slow subduction of numerous trans-oceanic fracture zones and patches of pervasively fractured, hydrated and serpentinized exhumed mantle rocks increase the water input. Heat-flow variations measured in the trench and the forearc during the Antithesis 1 cruise reveal heat advection by fluid circulation and shed a new light onto the thermal control of seismicity location in the subduction zone.In the Northern Lesser Antilles, heat-flow anomalies, negative in the trench and positive in the forearc, reveal a ventilated fluid circulation with downward percolation of cold fluids at the sediment-starved, pervasively fractured trench and upward discharge of warm fluids through the Tintamarre Fault Zone in the forearc. In contrast, in the Central Lesser Antilles, a positive heat-flow anomaly at the trench and the accretionary wedge is typical of an insulated fluid circulation where warm fluids invade the plate interface flowing updip from the subduction depths up to the trench.The investigated margin segments correspond with a very low number of interplate thrust earthquakes, illustrating the frequent statement that fluids in subduction zones tend to reduce the interplate coupling, favor slow to aseismic slip behavior, and increase the time return of large seismic events. Moreover, the location of intraslab, and supraslab earthquakes at depth beneath the Central Lesser Antilles suggest a close relation to temperature-related dehydration reactions.

Rupture Characteristics Analysis of the 2020 Mw 7.4 Oaxaca, Mexico Earthquake Using Teleseismic, High-Rate GPS, and InSAR Data

The June 23 2020 Oaxaca Mw 7.4 interplate thrust earthquake struck the state of Oaxaca in Mexico, generating strong shaking and a long-lived tsunami. This earthquake is well recorded by the teleseismic, high-rate Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data, which provides an opportunity to understand the rupture characteristics of the Mexican subduction zone. Here, an integrated inversion strategy involving centroid moment tensor inversion and kinematic finite-fault inversion is used to study the rupture history of the 2020 Oaxaca earthquake. The fault geometry and source duration time derived from the centroid moment tensor solution are used as prior information in linear kinematic finite-fault joint inversion. The rupture initial point and relative weight of each dataset are determined to estimate a well-constrained rupture model. The finite-fault model shows the rupture expanded bilaterally around the hypocenter, the peak slip is 3.5 m, the main slip was located at a depth of 15–30 km, the whole rupture lasted about 20 s, and a 95% moment rate was released at 15 s. The half-duration of the finite-fault inversion is consistent with the centroid moment tensor inversion results (half-duration 9 s), which shows the good resolution of the temporal information. The total scalar moment was 1.5 × 1020 Nm, equivalent to a moment magnitude of Mw 7.4. The integrated inversion strategy used in this study is useful since the prior information can be derived and used to constrain the rupture process. Both the centroid moment tensor and finite-fault inversion mainly rely on identical temporal information provided by teleseismic P waveforms. The 2020 Oaxaca earthquake was mainly the interaction between Cocos and the North American plate, and the slow slip events may be the key factor affecting the seismogenic zone width in the Oaxaca region.

Optimizing a Model of Coseismic Rupture for the 22 July 2020MW7.8 Simeonof Earthquake by Exploiting Acute Sensitivity of Tsunami Excitation Across the Shelf Break

AbstractThe Shumagin seismic gap along the Alaska Peninsula experienced a major,MW7.8, interplate thrust earthquake on 22 July 2020. Several available finite‐fault inversions indicate patchy slip of up to 4 m at 8–48 km depth. There are differences among the models in peak slip and absolute placement of slip on the plate boundary, resulting from differences in data distributions, model parameterizations, and inversion algorithms. Two representative slip models obtained from inversions of large seismic and geodetic data sets produce very different tsunami predictions at tide gauges and deep‐water pressure sensors (DART stations), despite having only secondary differences in slip distribution. This is found to be the result of the acute sensitivity of the tsunami excitation for rupture below the continental shelf in proximity to an abrupt shelf break. Iteratively perturbing seismic and geodetic inversions by constraining fault model extent along dip and strike, we obtain an optimal rupture model compatible with teleseismicPandSHwaves, regional three‐component broadband and strong‐motion seismic recordings, hr‐GNSS time series and static offsets, as well as tsunami recordings at DART stations and regional and remote tide gauges. Slip is tightly bounded between 25 and 40 km depth, the up‐dip limit of slip in the earthquake is resolved to be well‐inland of the shelf break, and the rupture extent along strike is well‐constrained. The coseismic slip increased Coulomb stress on the shallow plate boundary extending to the trench, but the frictional behavior of the megathrust below the continental slope remains uncertain.

Strong Motion Models for Duration and Arias Intensity for Strong Motion Records in Chile

Chile presents a high seismicity and very particular characteristics due to its subductive environment and high convergence rate of the interacting plates. In this paper, ground motion models (GMM) for Arias Intensity and strong ground motion duration are developed based on recently compiled Chilean earthquakes records. The Chilean derived GMM are later compared with global and subduction models. For the duration case, two definitions are considered: significant duration and bracketed duration. Care is taken to consider newly define rupture areas for the major earthquake in the catalog, which allows to redefine the distances to be used in the analysis. The strong motion records are processed using robust techniques and standardized procedures. The model parameters are selected using a Bayesian updating method. This allows to estimate the parameters as well as their corresponding uncertainties recognizing the variability of the coefficients and of the selected model. The derived ground motion predictive model associated is validated through a residual analysis and probability distributions, obtained from the adjustment method. Additionally, for the bracketed duration analysis, a classification method based on Support Vector Machine (SVM) is used, with the aim to incorporate the records with duration of zero seconds. The comparison of the resultant model with the existing ones allows to confirm the differences that the tectonic Chilean environment produces, characterized mainly by a high relative convergence rate between plates and interplate thrust earthquakes with high magnitudes.

Stress Interactions between an Interplate Thrust Earthquake and an Intraplate Strike-Slip Event: A Case Study of 2018 Mw 7.9 Gulf of Alaska Earthquake

ABSTRACT Most major earthquakes that have occurred in Alaska are related to rupture of the megathrust along the Alaska–Aleutian subduction zone. Large intraplate earthquakes in the oceanic lithosphere are rare. Recently, one large intraplate earthquake occurred near the Alaska subduction zone: the Mw 7.9 Gulf of Alaska earthquake (23 January 2018). The stress interaction between interplate megathrust and intraplate earthquake has been of interest because it helps to understand stress transfer and seismic hazard estimation. To explore the phenomenon of stress transfer, a viscoelastic spherical finite-element-method model accounting for topographic relief and lateral viscosity variations was used to calculate time-dependent stress transfer associated with the 1964 Mw 9.2 Alaska earthquake. We used postseismic Global Positioning System measurements to probe the differences in rheological properties between the continental mantle and oceanic mantle. The best-fitting model determined the viscosities of the continental mantle and oceanic mantle to be 3×1019 and 1×1020 Pa·s, respectively. Then, we calculated the viscoelastic stress transfers based on this viscosity structure. Because of viscoelastic stress loading associated with the 1964 Alaska earthquake, the ΔCFS values on the 2018 Alaska earthquake rupture fault exceeded the threshold (0.01 MPa) for triggering earthquakes.

Dynamic rupture of subduction earthquakes located near the trench

We perform kinematic and dynamic source inversions of four interplate thrust earthquakes that occurred near the trench at the base of the continental wedge in Northern Chile and we compare them to deeper intraplate events. The magnitudes of these interplate events were between Mw 6.3 and Mw 6.5, with hypocentral depths varying between 17.8 km and 28 km. These earthquakes correspond to one foreshock and three aftershocks of the large 2014 Mw 8.2 Iquique earthquake. The geometry of the seismic rupture obtained from the kinematic and dynamic inversions were similar for the four earthquakes studied and their dynamic source parameters are in accordance with typical interplate earthquakes around Mw 6.5. We compare the dynamic rupture of these events with those of five intraplate, intermediate-depth earthquakes that occurred in Northern Chile, Japan, Argentina and Mexico with depths between 57 km and 250 km. We also compare these events with the Mw 6.9 Valparaiso earthquake that was also a shallow interplate thrust event. Results show that the main dynamic parameters of the interplate events (stress in the nucleation zone, overall stress drop and fracture energy rate) were smaller than those of the intraplate intermediate-depth earthquakes. Furthermore, we compare our results with those obtained with several methodologies commonly used to estimate values of stress drop, fault radius and corner frequency. We find some minor differences between them for most of the earthquakes analyzed. Finally, we infer from our results that the studied area, located in the subduction interface zone near the trench, is suitable for earthquake nucleation of small to moderate earthquakes and does not represent a barrier for a future large tsunamigenic rupture.

Narrow Rupture of the 2020 Mw 7.4 La Crucecita, Mexico, Earthquake

AbstractOn 23 June 2020, a large (Mw 7.4) interplate thrust earthquake struck near the town of La Crucecita in the state of Oaxaca in southern Mexico, following a 55-yr interseismic period. A seismic source model is well constrained by teleseismic waveforms, static Global Positioning System offsets, and tsunami data, suggesting that the earthquake occurred on the slab interface at a dip of ∼23°, with a narrow elliptical asperity concentrating around a shallow depth of ∼20 km. The rupture propagates bilaterally from the hypocenter, and the down-dip rupture is restricted to ∼25 km by slow slip events (SSEs). The down-dip shear stress is released by SSEs during the interseismic period, limiting the earthquake magnitude and possibly resulting in the characteristic earthquake. The 2020 La Crucecita event, thus, is a good reminder to assess the seismic and tsunami potential in this region. The stress changes caused by the coseismic slip of the 2017 Mw 8.2 Chiapas earthquake are too small to trigger the 2020 La Crucecita earthquake. However, combined with the postseismic afterslip effects that play a leading role, it greatly promotes the eventual occurrence of the La Crucecita event. The results demonstrate the importance of considering postseismic afterslip, when evaluating seismic hazard and its migratory pattern.

Evidence of an east-dipping slab beneath the southern end of the Philippine Trench (1°N–6°N) as revealed by ISC-EHB

We examine recently released global seismic datasets in 3D visualization to study slab configurations in the Mindanao-Molucca region, where the present-day arc-arc collision of divergent double subduction propagates northward and attains completion on Mindanao Island, Philippines. The activity of inter-plate thrust earthquakes in the Philippine Trench is not significant until Mindanao (6°N) is reached, while shallow earthquakes on the island exhibit predominantly strike-slip movements on the Cotabato and Philippine Faults. The spatial distributions of GCMT thrust-type earthquakes shallower than 60 km reveal that current collisions mostly occur along the Central and Talaud-Miangas Ridges. ISC-EHB events deeper than 80 km, as well as Slab2 contours, show the eastward-dipping trench of the Halmahera slab flipping to the westward-dipping Philippine Trench northward. We identify a zone of ISC-EHB earthquakes steeply dipping to the east beneath the southern end of the Philippine Trench (1°N–6°N) that is not modeled by Slab2. This feature of a steeply east-dipping slab is explained by collision of the subduction arc and is consistent with steeply plunging T-axes of earthquakes. The identification of the east-dipping slab and estimation of its extent are crucial for understanding geodynamic and plate-boundary evolution in the Mindanao-Molucca region.

Coseismic Fault Model of Mw 8.3 2015 Illapel Earthquake (Chile) Retrieved from Multi-Orbit Sentinel1-A DInSAR Measurements

On 16 September 2015, a Mw 8.3 interplate thrust earthquake ruptured offshore the Illapel region (Chile). Here, we perform coseismic slip fault modeling based on multi-orbit Sentinel 1-A (S1A) data. To do this, we generate ascending and descending S1A interferograms, whose combination allows us to retrieve the EW and vertical components of deformation. In particular, the EW displacement map highlights a westward displacement of about 210 cm, while the vertical map shows an uplift of about 25 cm along the coast, surrounded by a subsidence of about 20 cm. Following this analysis, we jointly invert the multi-orbit S1A interferograms by using an analytical approach to search for the coseismic fault parameters and related slip values. Most of the slip occurs northwest of the epicenter, with a maximum located in the shallowest 20 km. Finally, we refine our modeling approach by exploiting the Finite Element method, which allows us to take geological and structural complexities into account to simulate the slip along the slab curvature, the von Mises stress distribution, and the principal stress axes orientation. The von Mises stress distribution shows a close similarity to the depth distribution of the aftershock hypocenters. Likewise, the maximum principal stress orientation highlights a compressive regime in correspondence of the deeper portion of the slab and an extensional regime at its shallower segment; these findings are supported by seismological data.

Rapidly Estimated Seismic Source Parameters for the 16 September 2015 Illapel, Chile M w 8.3 Earthquake

On 16 September 2015, a great (M w 8.3) interplate thrust earthquake ruptured offshore Illapel, Chile, producing a 4.7-m local tsunami. The last major rupture in the region was a 1943M S 7.9 event. Seismic methods for rapidly characterizing the source process, of value for tsunami warning, were applied. The source moment tensor could be obtained robustly by W-phase inversion both within minutes (Chilean researchers had a good solution using regional data within 5 min) and within an hour using broadband seismic data. Short-period teleseismic P wave back-projections indicate northward rupture expansion from the hypocenter at a modest rupture expansion velocity of 1.5–2.0 km/s. Finite-fault inversions of teleseismic P and SH waves using that range of rupture velocities and a range of dips from 16°, consistent with the local slab geometry and some moment tensor solutions, to 22°, consistent with long-period moment tensor inversions, indicate a 180- to 240-km bilateral along-strike rupture zone with larger slip northwest to north of the epicenter (with peak slip of 7–10 m). Using a shallower fault model dip shifts slip seaward toward the trench, while a steeper dip moves it closer to the coastline. Slip separates into two patches as assumed rupture velocity increases. In all cases, localized ~5 m slip extends down-dip below the coast north of the epicenter. The seismic moment estimates for the range of faulting parameters considered vary from 3.7 × 1021Nm(dip 16°) to 2.7 × 1021 Nm (dip 22°), the static stress drop estimates range from 2.6 to 3.5 MPa, and the radiated seismic energy, up to 1 Hz, is about 2.2–3.15 × 1016 J.

Great (≥Mw8.0) megathrust earthquakes and the subduction of excess sediment and bathymetrically smooth seafloor

Using older and in part flawed data, [Ruff (1989)][1] suggested that thick sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling. This circumstance was conjectured to favor rupture continuation and the generation of high-magnitude (≥Mw8.0) interplate thrust (IPT) earthquakes. Using larger and more accurate compilations of sediment thickness and instrumental (1899 to January 2013) and pre-instrumental era (1700–1898) IPTs (n = 176 and 12, respectively), we tested if a compelling relation existed between where IPT earthquakes ≥Mw7.5 occurred and where thick (≥1.0 km) versus thin (≤1.0 km) sedimentary sections entered the SZ. Based on the new compilations, a statistically supported statement (see Summary and Conclusions) can be made that high-magnitude earthquakes are most prone to nucleate at well-sedimented SZs. For example, despite the 7500 km shorter global length of thick-sediment trenches, they account for ∼53% of instrumental era IPTs ≥Mw8.0, ∼75% ≥Mw8.5, and 100% ≥Mw9.1. No megathrusts >Mw9.0 ruptured at thin-sediment trenches, whereas three occurred at thick-sediment trenches (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2). However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of high-magnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation. [1]: #ref-81

The 1 April 2014 Iquique, Chile,Mw8.1 earthquake rupture sequence

The 1 April 2014 Iquique, Chile,<i>M_w</i>8.1 earthquake rupture sequence

Mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and tsunami: Insight from seismic tomography

To clarify the generating mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0) and the induced tsunami, we determined high-resolution tomographic images of the Northeast Japan forearc. Significant lateral variations of seismic velocity are visible in the megathrust zone, and most large interplate thrust earthquakes are found to occur in high-velocity (high-V) areas. These high-V zones may represent high-strength asperities at the plate interface where the subducting Pacific plate and the overriding Okhotsk plate are coupled strongly. A shallow high-V zone with large coseismic slip near the Japan Trench may account for the mainshock asperity of the 2011 Tohoku-oki earthquake. Because it is an isolated asperity surrounded by low-velocity patches, most stress on it was released in a short time and the plate interface became decoupled after the Mw 9.0 earthquake. Thus the overriding Okhotsk plate there was shot out toward the Japan Trench and caused the huge tsunami.

Accelerated pacific plate subduction following interplate thrust earthquakes at the Japan trench

Interplate thrust earthquakes are usually followed by afterslips, and they let the fore-arc move slowly trenchward. However, we do not know if the subducting oceanic plate is accelerated landward after such earthquakes. The westward velocity of Global Positioning System (GPS) stations in NE Japan show gradient decreasing from east to west reflecting the E-W contractional strain built up by the inter-plate coupling. Here we show that such coupling significantly enhanced (∼1.5 times) after the 2003 Tokachi-Oki earthquake (Mw 8.0), Hokkaido, in the segments adjacent to the ruptured fault. The coupling seems to be further enhanced (∼3 times) after the 2011 Tohoku-Oki earthquake (Mw 9.0). It is unlikely that interplate friction suddenly increased over such a large region, and relatively strong pre-2003 coupling there would not allow such enhancements even if full coupling is attained. Hence they are attributable to the temporary acceleration of the Pacific Plate subduction. We propose a simple 2-dimensional model in which down-dip acceleration of the slab let the force balance rapidly recover promoted by a thin low-viscosity layer on the slab surface. The accelerated subduction would account for temporary activations of regional interplate seismicity after megathrust earthquakes.

Coseismic gravity changes of the 2010 earthquake in central Chile from satellite gravimetry

The first map of coseismic changes in gravity and geoid height has been drawn using the data from the Gravity Recovery and Climate Experiment (GRACE) satellites for the 2004 Sumatra‐Andaman earthquake. Here we present the second case of coseismic gravity change observation by satellite gravimetry, i.e., the change caused by an interplate thrust earthquake that occurred on 27 February, 2010 in Central Chile (Mw = 8.8). Gravity showed a negative jump with the largest drop of ∼5 μgal on the back‐arc side. The observed changes agree with those calculated assuming the realistic earth and fault parameters inferred from coseismic displacements of Global Positioning System (GPS) stations. Gravity in this area shows large seasonal and inter‐annual variability, and postseismic gravity changes could be isolated only by carefully removing hydrological signals.

Accelerographic measurements of the 27 February 2010 offshore Maule, Chile earthquake

AbstractThe mega earthquake of 27 February 2010, offshore Maule, Chile with moment magnitude of 8·8 is the fifth largest recorded earthquake in the world and, therefore, is considered to be a mega earthquake. This subduction interplate thrust earthquake occurred at the convergence of the Nazca (oceanic) plate with the South American continental plate. This type of earthquake affects the north‐western region of USA and Canada, as well as Alaska. Therefore, it is important to study the characteristics of the recorded accelerograms and their effects on the future behaviour of tall buildings in USA. Although the earthquake produced accelerographic records with very high ground motions in terms of peak ground accelerations and very long duration of ground shaking, the observed damage from this earthquake was more like the level of damage expected from a moment magnitude 8?0 rather than a moment magnitude of 8?8. This may be the result of asperities in the fault rupture surface. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Identification of High Frequency Pulses from Earthquake Asperities Along Chilean Subduction Zone Using Strong Motion

The Chilean subduction zone is one of the most active of the world with M = 8 or larger interplate thrust earthquakes occurring every 10 years or so on the average. The identification and characterization of pulses propagated from dominant asperities that control the rupture of these earthquakes is an important problem for seismology and especially for seismic hazard assessment since it can reduce the earthquake destructiveness potential. A number of studies of large Chilean earthquakes have revealed that the source time functions of these events are composed of a number of distinct energy arrivals. In this paper, we identify and characterize the high frequency pulses of dominant asperities using near source strong motion records. Two very well recorded interplate earthquakes, the 1985 Central Chile (Ms = 7.8) and the 2007 Tocopilla (Mw = 7.7), are considered. In particular, the 2007 Tocopilla earthquake was recorded by a network with absolute time and continuos recording. From the study of these strong motion data it is possible to identify the arrival of large pulses coming from different dominant asperities. The recognition of the key role of dominant asperities in seismic hazard assessment can reduce overestimations due to scattering of attenuation formulas that consider epicentral distance or shortest distance to the fault rather than the asperity distance. The location and number of dominant asperities, their shape, the amplitude and arrival time of pulses can be one of the principal factors influencing Chilean seismic hazard assessment and seismic design. The high frequency pulses identified in this paper have permitted us to extend the range of frequency in which the 1985 Central Chile and 2007 Tocopilla earthquakes were studied. This should allow in the future the introduction of this seismological result in the seismic design of earthquake engineering.

Seismicity and state of stress in Guerrero segment of the Mexican subduction zone

We take advantage of a relatively dense network of seismic stations in the Guerrero segment of the Mexican subduction zone to study seismicity and state of stress in the region. We combine our results with recent observations on the geometry of the subducted Cocos plate imaged from receiver function (RF) analysis, an ultraslow velocity layer mapped in the upper crust of the subducted slab, and episodic slow slip events (SSEs) and nonvolcanic tremors (NVTs) reported in the region to obtain a comprehensive view of the subduction process. Seismicity and focal mechanisms confirm subduction of the Cocos plate below Mexico at a shallow angle, reaching a depth of 25 km at a distance of 65 km from the trench. The plate begins to unbend at this distance and becomes horizontal at a distance of ∼120 km at a depth of 40 km. Some of the highlights of the inslab seismicity are as follows: (1) A cluster of earthquakes in the depth range of 25–45 km, immediately downdip from the strongly coupled part of the plate interface, revealing both downdip compressional and extensional events. This seismicity extends from ∼80 to 105 km from the trench and may be attributed to the unbending of the slab. (2) The slab devoid of seismicity in the distance range of ∼105–160 km. (3) Sparse inslab seismicity revealing downdip extension in the distance range of 160–240 km. The NVTs are also confined to this distance range. The episodic SSEs occur on the horizontal segment, in the distance range of ∼105–240 km, where an ultraslow velocity layer in the upper crust of the slab has been mapped from waveform modeling of converted SP phases. Thus, in the distance range of ∼105–160 km, SSEs occur but NVTs and inslab earthquakes are absent. This suggests that metamorphic dehydration reactions in the subducting oceanic crust and upper mantle begin at a distance of ∼160 km, giving rise to both the inslab earthquakes and NVTs. No inslab earthquake occurs beyond 240 km. The receiver function images and P wave tomography suggest that the slab begins a steep plunge at a distance of ∼310 km, reaching a depth of 500 km around 340 km from the trench. The negative buoyancy of such a slab should give rise to large extensional stress in the slab. Yet inslab seismicity is remarkably low, which may be explained by a slab that is not continuous up to a depth of 500 km, but is broken at a shallower depth. The resulting slab window may permit subslab material to flow through the gap. This may provide an explanation for the recent rift‐related basalts found near Mexico City. The fore‐arc, upper plate seismicity, which during the period of study (1995–2007) consisted of a moderate earthquake (Mw5.8) near the coast (H = 12 km), its numerous aftershocks, and two shallow events farther inland, demonstrates a trench‐normal extension in the upper plate near this convergent margin, a state of stress that may be explained by tectonic erosion and/or seaward retreat of the trench. Seismicity, location of the mantle wedge, and rupture areas of Mexican earthquakes suggest that the downdip limit of rupture during large/great earthquakes in Guerrero may be 105 ± 15 km. Shallow‐dipping, interplate thrust earthquakes are not the only type of events that affect the seismic hazard in the region. The magnitudes of inslab downdip compressional and extensional earthquakes that occur within ∼20 km inland from the coast, in the depth range of 25–45 km, may reach 6.5 and 7.5, respectively. In addition, we now identify normal‐faulting earthquakes in the upper plate. These sources need to be taken into account in the hazard estimation.

THE SOUTH SANDWICH ISLANDS EARTHQUAKE OF 27 JUNE 1929: SEISMOLOGICAL STUDY AND INFERENCE ON TSUNAMI RISK FOR THE SOUTH ATLANTIC

While no large interplate thrust earthquakes are known at the South Sandwich subduction zone, historical catalogues include a number of earthquakes with reported magnitudes of 7 or more. We present a detailed seismological study of the largest one (27 June 1929; M PAS = 8.3). The earthquake relocates 80 km north of the north-western corner of the arc. Its mechanism, inverted through the PDFM method, features normal faulting on a steeply dipping fault plane ( φ = 71°, δ = 70°, λ = 272°). The seismic moment, 1.7 x 10 28 dyn.cm, supports Gutenberg and Richter’s estimate, and is 28 times the largest shallow CMT in the region. The 1929 event is interpreted as representing a lateral tear in the South Atlantic plate, comparable to similar earthquakes in Samoa and Loyalty, deemed “STEP faults” by Govers and Wortel (2005). Hydrodynamic simulations using the MOST method (Titov and Synolakis, 1997) suggest deep-water tsunami amplitudes reaching 30 cm off the coast of Brazil, where run-up should have been observable, and 20 cm along the Gulf of Guinea (Ivory Coast, Ghana). We also simulate a number of potential sources obtained by assigning the 1929 moment to the geometries of other known earthquakes in the region, namely outer-rise normal faulting events at the center of the arc and its southern extremity, and an interplate thrust fault at the southern corner, where the youngest lithosphere is subducted. A common feature of these models is the strong focusing of tsunami waves by the South Atlantic Ridge, the southwest Indian Ocean Ridge, and the Agulhas Rise, resulting in amplitudes always enhanced in Ghana, southern Mozambique and certain parts of the coast of South Africa. This study documents the potential tsunami hazard to South Atlantic shorelines from earthquakes in this region, principally normal faulting events.

Low-frequency tremors associated with reverse faults in a shallow accretionary prism

The shallow part of a seismogenic subduction zone is thought to play an important role in tsunami genesis during large interplate thrust earthquakes. Near the updip portion of the seismogenic zone along the Nankai trough, splay faults, which are major active reverse faults in the accretionary prism, likely rupture during large interplate earthquakes such as the 1944 M w 8.2 Tonankai earthquake off Kii Peninsula. We observed low-frequency tremors associated with reverse faults in a shallow accretionary prism off Kii Peninsula at close range by using ocean bottom seismographs. The tremors were characterized by a dominant frequency range of 2–8 Hz and a lack of energy in the frequency range above 10 Hz. Their duration ranged from tens of seconds to a few minutes. Their source locations lay in three clusters on the landward slope of the Nankai trough. Activity in each cluster continued for from one day to two weeks. Many of the low-frequency tremors were located near the shallowest part of a major splay fault. The episodic activity of low-frequency tremors and of previously reported very-low-frequency earthquakes, which indicate reverse faulting in the shallow accretionary prism, suggests that the reverse faults in the accretionary prism are conditionally stable faults that can become unstable under sufficiently strong dynamic loading, such as that caused by a large earthquake. Such reverse faults, as typified by a splay fault, can rupture during large interplate earthquakes and generate large tsunamis.