Outer-rise Events Research Articles

Frequency-size distributions of Wadati-Benioff zone and near-boundary intraplate earthquakes: Implications for intermediate and deep seismicity

We conduct a systematic analysis of the frequency-moment distribution of earthquake populations in Wadati-Benioff zones, motivated by a three-fold increase in the available GlobalCMT database since the previous study by Okal and Kirby (1995), in which some datasets may have been undersampled. We use 100-km depths bins, processed with both least-squares and maximum-likelihood algorithms, and apply an F-test methodology to assess the robustness of our results, in particular regarding multi-segment behavior within individual depth bins. Our results generally support Okal and Kirby's (1995), and the new enlarged dataset suggests the resolution of the third segment predicted by these authors for the 500–600 km bin in Tonga, in the framework of their three-tier model for two-step source saturation of seismicity generated by transformational faulting within a wedge of metastable olivine. In the 200–300 and 300–400 km depth bins, we find comparable behaviors, revealing the artificial nature of the traditional 300-km boundary between “intermediate” and “deep” earthquakes. Our view is that it may be deeper than 300 km, and fundamentally variable between distinct subduction zones.We extend our study by considering frequency-moment distributions for intraplate earthquakes in oceanic lithosphere before it subducts at trenches. Those results support the interpretation of seismicity in the shallowest portions of slabs as involving the reactivation of existing faults under a process of dehydration embrittlement. However, outer rise events occurring immediately before subduction feature different population statistics, which are probably related to the release of bending stresses of a different origin.

The silent bending of the oceanic Nazca Plate at the Peruvian Trench

We predict the shape of the outer rise along the Peruvian Trench (6°-21°S) using flexural and gravity modelling in order to study the elastic properties of the subducting/oceanic Nazca Plate (NP). We include in our modelling the hotspot swell topography of the Nazca Ridge (NR) in the ridge-trench collision zone (~15°S). Results show an anomalous overthickening of the oceanic crust beneath the NR (from ~6 to 20 km), which is capable to produce most of the swell topography (500–700 km wide and up to 2.2 km high). The swell was formed likely under isostatic conditions (Te ~ 0 km) by the interaction of the NP with a hotspot-spreading center system. Despite the high buoyancy forces of the NR (0.5–4.0 × 1013 N/m) associated to the anomalous thick crust, the 45–50 Ma oceanic NP approaching the Peruvian Trench presents a well pronounced fore-bulge morphology with similar elastic thicknesses (Te = 35 ± 5 km) to those found for cold and old oceanic plate in the western Pacific. Consistently, our results do not show evidence for plate weakening (reduced Te values) in the NR-trench collision zone. We obtain similar results (Te = 40 ± 10 km) north of the NR implying that the oceanic NP is strong prior to subduction along most of the Peruvian convergent margin. This is consistent with the lack of great outer rise events (Mw ≥ 6) and the absence of reduced uppermost mantle velocities offshore Peru suggesting the presence of a poorly hydrated and rigid lithospheric mantle.

How Sediment Thickness Influences Subduction Dynamics and Seismicity

AbstractIt has long been recognized that sediments subducting along the megathrust influence the occurrence of giant (Mw ≥ 8.5) megathrust earthquakes. However, the limited observation span and the concurrent influence of multiple parameters on megathrust behavior prevent us from understanding how sediments affect earthquake size and frequency. Here, we address these limitations by using two‐dimensional, visco‐elasto‐plastic, seismo‐thermo‐mechanical numerical models to isolate how sediment thickness affects subduction geometry and seismicity. Our results show that increasing sediment thickness on the incoming plate results in a decrease of the slab dip, as the trench retreats due to the seaward growth of the sedimentary wedge that also unbends the slab. This decrease in megathrust dip results in a wider seismogenic zone, so that the maximum magnitude of megathrust earthquakes increases. Concurrently, the recurrence time of characteristic events increases and partial ruptures are introduced. The maximum magnitude estimated for subduction segments with the thickest sediment input (Makran, West‐Aegean, and Calabria) is distinctly higher than the instrumentally recorded magnitude. These segments may thus experience larger than as of yet observed earthquakes, albeit infrequently. Increasing sediment thickness also decreases megathrust normal stresses, as the seismogenic zone is more shallow and overlain by a lighter forearc structure. Thicker incoming plate sediments also favor more splay fault activity, whereas we observe more outer rise events for low sediment thickness. Finally, we demonstrate that modeling long‐term subduction dynamics and sediment subduction is crucial for understanding and quantifying megathrust seismicity and seismic potential of subduction zones.

Sources of uncertainties and artifacts in back-projection results

Summary Back-projecting high-frequency (HF) waves is a common procedure for imaging rupture processes of large earthquakes (i.e. Mw > 7.0). However, obtained back-projection (BP) results could suffer from large uncertainties since high-frequency seismic waveforms are strongly affected by factors like source depth, focal mechanisms, and the Earth's 3D velocity structures. So far, these uncertainties have not been thoroughly investigated. Here, we use synthetic tests to investigate the influencing factors for which scenarios with various source and/or velocity set-ups are designed, using either Tohoku-Oki (Japan), Kaikoura (New Zealand), Java/Wharton Basin (Indonesia) as test areas. For the scenarios, we generate either 1D or 3D teleseismic synthetic data, which are then back-projected using a representative BP method, MUltiple SIgnal Classification (MUSIC). We also analyze corresponding real cases to verify the synthetic test results. The Tohoku-Oki scenario shows that depth phases of a point source can be back-projected as artifacts at their bounce points on the earth's surface, with these artifacts located far away from the epicenter if earthquakes occur at large depths, which could significantly contaminate BP images of large intermediate-depth earthquakes. The Kaikoura scenario shows that for complicated earthquakes, composed of multiple sub-events with varying focal mechanisms, BP tends to image sub-events emanating large amplitude coherent waveforms, while missing sub-events whose P nodal directions point to the arrays, leading to discrepancies either between BP images from different arrays, or between BP images and other source models. Using the Java event, we investigate the impact of 3D source-side velocity structures. The 3D bathymetry together with a water layer can generate strong and long-lasting coda waves, which are mirrored as artifacts far from the true source location. Finally, we use a Wharton Basin outer-rise event to show that the wavefields generated by 3D near trench structures contain frequency-dependent coda waves, leading to frequency-dependent BP results. In summary, our analyses indicate that depth phases, focal mechanism variations, and 3D source-side structures can affect various aspects of BP results. Thus, we suggest that target-oriented synthetic tests, for example, synthetic tests for subduction earthquakes using more realistic 3D source-side velocity structures, should be conducted to understand the uncertainties and artifacts before we interpret detailed BP images to infer earthquake rupture kinematics and dynamics.

Shallow intraplate seismicity related to the Illapel 2015 Mw 8.4 earthquake: Implications from the seismic source

The September 16, 2015, Mw 8.4 Illapel, Chile earthquake is the first large event occurring in north-central Chile after the 1943 earthquake, filling a known seismic gap in the region. The earthquake took place in a complex tectonic region, nearby an area where transition from erosive to accretionary margin occurs due to the collision of Juan Fernandez Ridge (JFR) along the Chilean margin. We inverted the kinematic rupture process of the 2015 Mw 8.4 Illapel earthquake from the joint inversion of teleseismic body waves and near-field data. The relative weighting between datasets and the weighting of spatial/temporal constraints are objectively estimated by applying the Akaike's Bayesian Information Criterion. The coseismic slip model yields a total seismic moment of 4.92 × 1021 Nm occurred over ~120 s. The rupture shows both downdip and updip propagation with slip extending along the thrust interface from ~50 km depth to shallow near-trench depths (<15 km), with maximum slip of ~9 m located at shallow depths, where low average rupture speeds ~1.8–2 km/s are estimated. Outer-rise events, triggered within the oceanic Nazca plate after the mainshock, did not penetrate into the mantle and are related to preexisting faults due to both bending of Nazca plate and JFR uplift, which promotes a tensional stress regime in the surrounding area. This seismicity was triggered by static stress transfer from near-trench slip revealed by our source rupture modelling, suggesting outer-rise seismicity as a proxy for near-trench coseismic slip. Crustal seismicity within continental South American plate is observed prior to, and after, the mainshock, mainly related to extensional faulting within eroded and fractured wedge due to tectonic processes along erosive margins. We also show evidence of shallow seismicity after the mainshock associated with a long-lived crustal fault, which can represent a high seismic hazard for La Serena-Coquimbo conurbation.

Simulation of a Dispersive Tsunami due to the 2016 El Salvador\u2013Nicaragua Outer-Rise Earthquake (Mw 6.9)

The 2016 El Salvador–Nicaragua outer-rise earthquake (Mw 6.9) generated a small tsunami observed at the ocean bottom pressure sensor, DART 32411, in the Pacific Ocean off Central America. The dispersive observed tsunami is well simulated using the linear Boussinesq equations. From the dispersive character of tsunami waveform, the fault length and width of the outer-rise event is estimated to be 30 and 15 km, respectively. The estimated seismic moment of 3.16 × 1019 Nm is the same as the estimation in the Global CMT catalog. The dispersive character of the tsunami in the deep ocean caused by the 2016 outer-rise El Salvador–Nicaragua earthquake could constrain the fault size and the slip amount or the seismic moment of the event.

Outer rise seismicity boosted by the Maule 2010 Mw 8.8 megathrust earthquake

The Maule 2010 megathrust earthquake Mw 8.8 has been characterized by two coseismic high-slip patches (asperities) north and south of the epicenter, separated by a region of lower slip. Here, we invert full broadband waveforms to obtain regional moment tensors, yielding precise centroid depth and source parameters of outer rise events (Mw>4.5), including a large Mw 7.4 event that occurred just 1.5h after the Maule mainshock. Outer rise seismicity occurred mainly in two clusters: (1) a large number of outer rise events in the subducting plate located just seaward of the northern asperity of the Maule earthquake, and (2) a second cluster with fewer events seaward of the southern edge of the Maule rupture area. Thus, the outer rise seismicity is correlated with the coseismic rupture of the Maule earthquake, reflecting the stress state of the interplate coupled zone. The moment tensor results indicate similar extensional focal mechanisms for all outer rise events in the northern zone. In the southern region, most of the outer rise events are also extensional, except for one strike slip event located near the oceanic Mocha Fracture Zone. The centroid depths vary from 5 to 20km depth, and present similar magnitudes. Many of the outer rise events nucleated near the Mocha Fracture Zone, including the Mw 7.4 event and one strike–slip event. The calculated yield strength envelope for the oceanic Nazca lithosphere suggests that the centroid depths of intraplate tensional events span almost the entire upper-brittle part of the oceanic lithosphere.

Interrogation of the Megathrust Zone in the Tohoku-Oki Seismic Region by Waveform Complexity: Intraslab Earthquake Rupture and Reactivation of Subducted Normal Faults

Results from the 2011 Mw 9.1 Tohoku-Oki megathrust earthquake display a complex rupture pattern, with most of the high-frequency energy radiated from the downdip edge of the seismogenic zone and very little from the large shallow rupture. Current seismic results of smaller earthquakes in this region are confusing due to disagreements among event catalogs on both the event locations (>30 km horizontally) and mechanisms. Here we present an in-depth study of a series of intraslab earthquakes that occurred in a localized region near the downdip edge of the main shock. We explore the validity of 1D velocity model and refine earthquake source parameters for selected key events by performing broadband waveform modeling combining regional networks. These refined source parameters are then used to calibrate paths and further simulate secondary source properties, such as rupture directivity and fault dimension. Calculation of stress changes caused by the main event indicate that the region where these intraslab events occurred are prone to thrust events. This group of intraslab earthquakes suggest the reactivation of a subducted normal fault, and are potentially useful in enhancing our understanding on the downdip shear zone and large outer-rise events.

Modeling the seismic cycle in subduction zones: The role and spatiotemporal occurrence of off‐megathrust earthquakes

Shallow off‐megathrust subduction events are important in terms of hazard assessment and coseismic energy budget. Their role and spatiotemporal occurrence, however, remain poorly understood. We simulate their spontaneous activation and propagation using a newly developed 2‐D, physically consistent, continuum, viscoelastoplastic seismo‐thermo‐mechanical modeling approach. The characteristics of simulated normal events within the outer rise and splay and normal antithetic events within the wedge resemble seismic and seismological observations in terms of location, geometry, and timing. Their occurrence agrees reasonably well with both long‐term analytical predictions based on dynamic Coulomb wedge theory and short‐term quasi‐static stress changes resulting from the typically triggering megathrust event. The impact of off‐megathrust faulting on the megathrust cycle is distinct, as more both shallower and slower megathrust events arise due to occasional off‐megathrust triggering and increased updip locking. This also enhances tsunami hazards, which are amplified due to the steeply dipping fault planes of especially outer rise events.

Aftershock seismicity of the 27 February 2010 Mw 8.8 Maule earthquake rupture zone

On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes Mw between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35km depth and 50–120km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~50km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50km depth and further inland at 150–160km trench perpendicular distance, with mostly shallow dipping (~28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the Mw 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~34.5°S). These crustal events occur in a ~30km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (Mw=6.9) on 11 March 2010.

Near-simultaneous great earthquakes at Tongan megathrust and outer rise in September 2009

The Earth's largest earthquakes and tsunamis are usually caused by thrust-faulting earthquakes on the shallow part of the subduction interface between two tectonic plates, where stored elastic energy due to convergence between the plates is rapidly released. The tsunami that devastated the Samoan and northern Tongan islands on 29 September 2009 was preceded by a globally recorded magnitude-8 normal-faulting earthquake in the outer-rise region, where the Pacific plate bends before entering the subduction zone. Preliminary interpretation suggested that this earthquake was the source of the tsunami. Here we show that the outer-rise earthquake was accompanied by a nearly simultaneous rupture of the shallow subduction interface, equivalent to a magnitude-8 earthquake, that also contributed significantly to the tsunami. The subduction interface event was probably a slow earthquake with a rise time of several minutes that triggered the outer-rise event several minutes later. However, we cannot rule out the possibility that the normal fault ruptured first and dynamically triggered the subduction interface event. Our evidence comes from displacements of Global Positioning System stations and modelling of tsunami waves recorded by ocean-bottom pressure sensors, with support from seismic data and tsunami field observations. Evidence of the subduction earthquake in global seismic data is largely hidden because of the earthquake's slow rise time or because its ground motion is disguised by that of the normal-faulting event. Earthquake doublets where subduction interface events trigger large outer-rise earthquakes have been recorded previously, but this is the first well-documented example where the two events occur so closely in time and the triggering event might be a slow earthquake. As well as providing information on strain release mechanisms at subduction zones, earthquakes such as this provide a possible mechanism for the occasional large tsunamis generated at the Tonga subduction zone, where slip between the plates is predominantly aseismic.

Outer rise stress changes related to the subduction of the Juan Fernandez Ridge, central Chile

Although outer rise seismicity is less common than interplate seismicity in subduction zones, a significant level of seismicity has occurred between the Nazca trench and Juan Fernandez Ridge, in central Chile, during the past 20 years. We first study the 9 April 2001 (Mw = 7.0) event and determine its focal mechanism, depth, and source time function by body‐waveform inversion from teleseismic broadband data. The results indicate tensional faulting in the upper part of the mechanical lithosphere. Its strike (41°) is similar to those observed in events down dip of the slab at about 100 km depth, which could indicate that these earthquakes occur in preexisting structures formed at the trench. Compressive outer rise events have also occurred during the 1980s in front of the rupture zone of the 1985 Mw 7.8 Valparaíso Earthquake. To understand their relation with the state of stress of the lithosphere, we construct yield stress envelopes of the oceanic lithosphere, including static and dynamic stresses. Dynamic stresses are due either to slab pull, ridge push, resistive, and drag forces. We explain the sequence of compressive and tensional events by the accumulation of stress prior to 1985 when the subduction is assumed to be locked and after by the unlockage of the subduction by the Valparaíso interplate event. The yield stress envelope analysis enables us to quantify the accumulation of compressive forces before 1985 and the tensional force after.

The April 9, 2001 Juan Fernández Ridge (M w 6.7) Tensional Outer-Rise Earthquake and its Aftershock Sequence

On April 9, 2001 a Mw 6.7 earthquake occurred offshore of the Chilean coast close to the intersection of the subducting Juan Fernandez Ridge (JFR) and the trench near 33°S. The mainshock as well as an unprecedented number of aftershocks were recorded on regional broad-band and short-period seismic networks. We obtained a regional moment tensor solution of the mainshock that indcates a tensional focal mechanism consistent with the Harvard CMT solution. Based on waveform modeling and relocation, the depth of the mainshock was found to be 10–12 km. We relocated 142 aftershocks, which are strongly clustered and restricted to 10–30 km in depth. The seismicity distribution indicates a conjugate normal fault system extending into the lithospheric mantle that correlates with ridge-parallel fractures observed by previous seismic and bathymetric surveys. In conjunction with the historic regional distribution of outer-rise and large interplate seismicity, our results indicate that, with the exception of anomalously large thrust events, preexisting fractures associated with large bathymetric features like ridges have to exist to allow the generation of outer-rise seismicity along the Chilean margin. Hence, flexural bending and time-dependent interplate earthquakes can locally affect the nucleation of outer-rise events. The occurrence of the outer-rise seismicity in the oceanic mantle suggests the existence of lithospheric scale faults which might act as conduits to hydrate the subducting slab.

Do intermediate‐ and deep‐focus earthquakes occur on preexisting weak zones? An examination of the Tonga subduction zone

It has been proposed that intermediate‐ and deep‐focus earthquakes occur on preexisting planes of weakness that were created at shallow depth. The purpose of this study is to test this hypothesis. We examined fault plane solutions for 360 events that occurred in the Tonga subduction zone from January 1976 to February 1999. For events in the slab, each fault plane solution is rotated by the slab dip angle so that the slab is in its original horizontal position. We then compare fault plane solutions for events in the slab with those for events at the outer rise. We find that an asymmetric fault system, corresponding to that found for outer rise events, persists down to about 450 km depth. This suggests that earthquakes down to this depth are caused by the reactivation of preexisting faults created in the oceanic plate before subduction. A similar pattern, though less well constrained, is found in the Kurile subduction zone down to 200 km depth. For earthquakes deeper than 450 km the fault plane solutions are much more scattered. In some places the fault plane pattern suggests the creation of new fault planes along orientations of maximum shear with zero internal friction. The pattern of fault plane solutions in particular localized volumes cannot be easily explained by the creation of new faults in the direction of maximum shear under a coherent regional tectonic stress field. Thus, either a highly heterogeneous stress field exists or preexisting planes of weakness also account for some deep earthquakes.

Cyclic stressing and seismicity at strongly coupled subduction zones

We use the finite element method to analyze stress variations in and near a strongly coupled subduction zone during an earthquake cycle. Deformation is assumed to be uniform along strike (plane strain on a cross section normal to the trench axis), and periodic earthquake slip is imposed consistent with the long‐term rate of plate convergence and degree of coupling. Simulations of stress and displacement rate fields represent periodic fluctuations in time superimposed on an average field. The oceanic plate, descending slab, and continental lithosphere are assumed here to respond elastically to these fluctuations, and the remaining mantle under and between plates is assumed to respond as Maxwell viscoelastic. In the first part of the analysis we find that computed stress fluctuations in space and time are generally consistent with observed earthquake mechanism variations with time since a great thrust event. In particular, trench‐normal extensional earthquakes tend to occur early in the earthquake cycle toward the outer rise but occur more abundantly late in the cycle in the subducting slab downdip of the main thrust zone. Compressional earthquakes, when they occur at all, have the opposite pattern. Our results suggest also that the actual timing of extensional outer rise events is controlled by the rheology of the shallow aseismic portion of the thrust interface. The second part of the analysis shows the effects of mantle relaxation on the rate of ground surface deformation during the earthquake cycle. Models without relaxation predict a strong overall compressional strain rate in the continental plate above the main thrust zone, with the strain rate constant between mainshocks. However with significant relaxation present, a localized region of unusually low compressional, or even slightly extensional, strain rate develops along the surface of the continental plate above and somewhat inland from the downdip edge of the locked main thrust zone. The low strain rate starts in the middle or late part of the cycle, depending on position. This result suggests that the negligible or small contraction measured on the Shumagin Islands, Alaska, during 1980 to 1991, may not invalidate an interpretation of that region as being a moderately coupled subduction zone. In contrast, mantle relaxation causes only modest temporal nonuniformity of uplift rates in the overriding plate and of extensional stress rates in the subducting plate, even when the Maxwell time is an order of magnitude less than the recurrence interval.

Spatio-temporal variations of seismicity in the southern Peru and northern Chile seismic gaps

The spatio-temporal variation of seismicity in the southern Peru and northern Chile seismic gaps is analyzed with teleseismic data (m b ≥ 5.5) between 1965 and 1991, to investigate whether these gaps present the precursory combination of compressional outer-rise and tensional downdip events observed in other subduction zones. In the outer-rise and the inner-trench (0 to 100 km distance from the trench) region, lower magnitude (5.0 ≤ m b < 5.5) events were also studied. The results obtained show that the gaps in southern Peru and northern Chile do not present compressional outer-rise events. However, both gaps show a continuous, tensional downdip seismicity. For both regions, the change from compressional to tensional regime along the slab occurs at a distance of about 160 km from the trench, apparently associated with the coupled-uncoupled transition of the interplate contact zone. In southern Peru, an increase of compressional seismicity near the interplate zone and of tensional events (5.0 ≤ m b ≤ 6.3) in the outer-rise and inner-trench regions is observed between 1987 and 1991. A similar distribution of seismicity in the outer-rise and inner-trench regions is observed with earthquakes (m b < 5.5). In northern Chile there is a relative absence of compressional activity (m b ≥ 5.5) near the interplate contact since the sequence of December 21, 1967. After that, only a cluster of low-magnitude compressional events has been located in the area 50 to 100 km from the trench. The compressional activity occurring near the interplate zone in both seismic gaps represents that a seismic preslip is occurring in and near the plate contact. Therefore, if this seismic preslip is associated with the maturity of the gap, the fact that it is larger in southern Peru than in northern Chile may reflect that the former gap is more mature than the latter. However, the more intense downdip tensional activity and the absence of compressional seismicity near the contact zone observed in northern Chile, may also be interpreted as evidence that northern Chile is seismically more mature than southern Peru. Therefore, the observed differences in the distribution of stresses and seismicity analyzed under simple models of stress accumulation and transfer in coupled subduction zones are not sufficient to assess the degree of maturity of a seismic gap.

Tsunami from the Mariana Earthquake of April 5, 1990: Its abnormal propagation and implications for tsunami potential from outer‐rise earthquakes

The tsunami generated by the Mariana earthquake of April 5, 1990 was observed on the Japanese and Pacific islands as far as Hawaii. The observed tsunami amplitudes are not a simple function of distance from the source but vary with large‐scale bathymetry. Numerical computations of tsunami propagation are made for actual bathymetry and the computed amplitudes are compared with the observations. From the comparisons, the seismic moment is estimated to be 1.4 × 1020Nm, very similar to that from seismic waves, and indicates that the seismic waves and tsunami are equally excited. The numerical computations also show that the tsunami propagated toward the Japanese coast through two different paths: one is through the trench system with high velocity and small amplitude, the other along the ridge system with low velocity but large amplitude. The two kinds of tsunami are identified on recently‐installed ocean bottom pressure gauges. Since the Mariana earthquake was an outer‐rise earthquake in a weakly coupled subduction zone, where the size of outer‐rise event is larger than underthrusting events, it is possible that even larger earthquakes occur in this region. The tsunami potential from such events must be considered, including the unusual tsunami propagation.

LARGE EARTHQUAKES IN THE TONGA REGION ASSOCIATED WITH SUBDUCTION OF THE LOUISVILLE RIDGE

Subduction of the Louisville Ridge influences both interplate and intraplate seismicity in the southern Tonga region (22°S–26°S). Five earthquakes with diverse mechanisms and seismic moments greater than 1 × 1027 dyn cm have occurred in this region since 1975. These include two outer rise, one underthrusting, and two intermediate depth events, all of which occurred in close proximity to the subducting ridge. In order to understand better the nature of these events and to determine the role played by the Louisville Ridge in the subduction process, detailed rupture models have been determined for each earthquake by body and surface wave analysis. Three events occurred at shallow depths near the oblique intersection of the Louisville Ridge and the Tonga are. These are the compressional (thrust) outer rise event of October 11, 1975 (Mw = 7.4); the tensional (normal) outer rise event of October 10, 1977 (Mw = 7.4); and the subsequent underthrusting event on December 19, 1982 (Mw = 7.5), the latter being the largest documented interplate event in Tonga. The intermediate depth events of June 22, 1977 (d = 70 km, Mw = 8.1) and April 13, 1980 (d = 160 km, Mw = 7.6) both have normal fault mechanisms; however; the 1977 event has a downdip tension axis, whereas the 1980 event has a downdip compression axis. The 1977 event is unusual due to its large size and the fact that most intermediate depth activity in Tonga involves downdip compression. The body wave radiation for the 1977 event indicates concentrated rupture in the depth range 70–90 km, near the downdip projection of the Louisville Ridge. It appears that the Louisville Ridge perturbs the regional stress regime at both intermediate and shallow depths in the slab. The ridge appears to be a buoyant feature that produces locally strong interplate coupling to the north of its junction with the Tonga trench, which accounts for the 1982 thrust event and the preceding 1975 compressional outer rise event located seaward of the thrust rupture zone. South of the ridge intersection the plate interface appears to be less strongly coupled: tensional stresses due to slab pull are communicated to shallow depths, resulting in the October 10, 1977 (Mw = 7.4), tensional outer rise event. The locally enhanced interplate coupling, along with a downdip stress barrier to the intraplate compressional stresses that arise from deep resistance to the slab subduction, are two effects of the ridge that appear to explain the accumulation of in‐plate tension in the region of the June 22, 1977, event, the April 13, 1980, downdip compressional event reflects a return to in‐plate compression following the great 1977 rupture which relaxed the tensional stress. The unusual stress regime of the subducting ridge and its oblique trajectory down the plate are thus responsible for the diversity of large earthquake occurrence in the southern Tonga arc.

SEISMIC COUPLING AND OUTER RISE EARTHQUAKES

Variation of interplate seismic coupling at subduction zones is a major factor controlling the size of the largest underthrusting events. This variation also has a profound effect on the regional intraplate stresses in the vicinity of the subduction zone. Outer rise seismicity is strongly correlated with variations in interplate coupling, reflecting the stress state of the interplate coupled zone. Over 200 outer rise earthquakes with known focal mechanisms are used to investigate the relationship between stresses in the outer rise and interplate seismic coupling. These events occur within the downgoing (i.e., oceanic) plate near the bathymetric trench axes and generally fall into the categories of tensional (normal) or compressional (thrust) with their tensional or compressional stress axes oriented approximately horizontal and perpendicular to the trench. In uncoupled subduction zones, only tensional outer rise earthquakes occur, which indicates that the outer rise is dominated by tensional stresses associated with plate bending and/or slab pull forces. In strongly coupled subduction zones, both tensional and compressional outer rise events are found. These events are related both spatially and temporally to the distribution of large underthrusting earthquakes and are thus an integral part of the earthquake cycle. In the strongly coupled regions, tensional outer rise events follow large underthrusting events as the outer rise is temporarily in tension due to the underthrusting motion. Compressional outer rise events take place as compressional stress slowly accumulates oceanward of locked sections of the interplate zone. In four instances, compressional outer rise earthquakes have been followed by large underthrusting events which have occurred 2, 4, 7, and 19 years after the associated outer rise event. The remaining compressional outer rise events are located in regions that are either known seismic gaps or in regions where the seismic potential is unknown. The occurrence of compressional outer rise earthquakes suggests that compressional stress is accumulating in the adjacent interplate region and that there is the potential for a future large underthrusting event in the region. Thirty compressional outer rise events have been located in trench segments of Middle and South America, the Kurile Islands, the Tonga and Kermadec islands, the New Hebrides Arc, and the Solomon Islands regions. In both the southern Kamchatka and northern New Hebrides regions the outer rise seismicity indicates that the stress regimes in the outer rise have changed with time from tensional, following a previous large underthrusting event, to compressional at present. Thus three stages of the cycle from underthrusting to tensional outer rise regime to compressional outer rise regime are present, requiring only the occurrence of the next underthrusting event to complete the cycle. The occurrence of compressional outer rise events is useful for assessing the seismic potential of a region on an intermediate time scale.

Interplate coupling and temporal variation of mechanisms of intermediate-depth earthquakes in Chile

Abstract We investigated the temporal variation of the mechanism of large intraplate earthquakes at intermediate depths in relation to the occurrence of large under-thrusting earthquakes in Chile. Focal mechanisms were determined for three large events (1 March 1934: M = 7.1, d = 120 km; 20 April 1949: M = 7.3, d = 70 km; and 8 May 1971: MW = 7.2, d = 150 km) which occurred down-dip of the great 1960 Chilean earthquake (MW = 9.5) rupture zone. The 1971 event is down-dip compressional: θ (strike) = 12°, δ (dip) = 80°, and λ (rake) = 100°. The 1949 earthquake focal mechanisms is θ = 350°, δ = 70°, and λ = −130°. The data available for the 1934 event are consistent with a down-dip tensional mechanism. Thus, the two events which occurred prior to the great 1960 Chilean earthquake are down-dip tensional. Published fault plane solutions of large intermediate-depth earthquakes (28 March 1965 and 7 November 1981) which occurred down-dip of the Valparaiso earthquakes of 1971 (MW = 7.8) and 1985 (MW = 8.0) are also down-dip tensional. These results suggest that before a major thrust earthquake, the interplate boundary is strongly coupled, and the subducted slab is under tension at intermediate depths; after the occurrence of an interplate thrust event, the displacement on the thrust boundary induces transient compressional stress at intermediate depth in the down-going slab. This interpretation is consistent with the hypothesis that temporal variations of focal mechanisms of outer-rise events are due to changes of interplate coupling.