Normal-faulting Earthquakes Research Articles

Insights into the Lurking Structures and Related Intraplate Earthquakes in the Region of Bay of Bengal Using Gravity and Full Gravity Gradient Tensor

Comprehension of subsurface structures buried under thick sediments in the region of Bay of Bengal is vital as structural features are the key parameters that influence or are caused by the subsurface deformation and tectonic events like earthquakes. Here, we address this issue using the integrated analysis and interpretation of gravity and full gravity gradient tensor with few seismic profiles available in the poorly known region. A 2D model of the deep earth crust–mantle is constructed and interpreted with gravity gradients and seismic profiles, which made it possible to obtain a visual image of a deep seated fault below the basement associated with thick sediments strata. Gravity modelling along a NE–SW profile crossing the hypocentre of the earthquake of 21 May 2014 (M w 6.0) in the northern Bay of Bengal suggests that the location of intraplate normal dip fault earthquake in the upper mantle is at the boundary of density anomalies, which is probably connected to the crustal fault. We also report an enhanced structural trend of two major ridges, the 85°E and the 90°E ridges hidden under the sedimentary cover from the computed full gravity gradients tensor components.

Deep postseismic viscoelastic relaxation excited by an intraslab normal fault earthquake in the Chile subduction zone

The 2005 Mw 7.8 Tarapaca earthquake was the result of normal faulting on a west-dipping plane at a depth of ~90km within the subducting slab down-dip of the North Chilean gap that partially ruptured in the 2014 M 8.2 Iquique earthquake. We use Envisat observations of nearly four years of postseismic deformation following the earthquake, together with some survey GPS measurements, to investigate the viscoelastic relaxation response of the surrounding upper mantle to the coseismic stress. We constrain the rheological structure by testing various 3D models, taking into account the vertical and lateral heterogeneities in viscosity that one would expect in a subduction zone environment. A viscosity of 4–8×1018Pas for the continental mantle asthenosphere fits both InSAR line-of-sight (LOS) and GPS horizontal displacements reasonably well. In order to test whether the Tarapaca earthquake and associated postseismic relaxation could have triggered the 2014 Iquique sequence, we computed the Coulomb stress change induced by the co- and postseismic deformation following the Tarapaca earthquake on the megathrust interface and nodal planes of its M 6.7 foreshock. These static stress calculations show that the Tarapaca earthquake may have an indirect influence on the Iquique earthquake, via loading of the M 6.7 preshock positively. We demonstrate the feasibility of using deep intraslab earthquakes to constrain subduction zone rheology. Continuing geodetic observation following the 2014 Iquique earthquake may further validate the rheological parameters obtained here.

Fault Slip Distribution of the 2016 Fukushima Earthquake Estimated from Tsunami Waveforms

The 2016 Fukushima normal-faulting earthquake (Mjma 7.4) occurred 40 km off the coast of Fukushima within the upper crust. The earthquake generated a moderate tsunami which was recorded by coastal tide gauges and offshore pressure gauges. First, the sensitivity of tsunami waveforms to fault dimensions and depths was examined and the best size and depth were determined. Tsunami waveforms computed based on four available focal mechanisms showed that a simple fault striking northeast-southwest and dipping southeast (strike = 45°, dip = 41°, rake = −95°) yielded the best fit to the observed waveforms. This fault geometry was then used in a tsunami waveform inversion to estimate the fault slip distribution. A large slip of 3.5 m was located near the surface and the major slip region covered an area of 20 km × 20 km. The seismic moment, calculated assuming a rigidity of 2.7 × 1010 N/m2 was 3.70 × 1019 Nm, equivalent to Mw = 7.0. This is slightly larger than the moments from the moment tensor solutions (Mw 6.9). Large secondary tsunami peaks arrived approximately an hour after clear initial peaks were recorded by the offshore pressure gauges and the Sendai and Ofunato tide gauges. Our tsunami propagation model suggests that the large secondary tsunami signals were from tsunami waves reflected off the Fukushima coast. A rather large tsunami amplitude of ~75 cm at Kuji, about 300 km north of the source, was comparable to those recorded at stations located much closer to the epicenter, such as Soma and Onahama. Tsunami simulations and ray tracing for both real and artificial bathymetry indicate that a significant portion of the tsunami wave was refracted to the coast located around Kuji and Miyako due to bathymetry effects.

Field- to nano-scale evidence for weakening mechanisms along the fault of the 2016 Amatrice and Norcia earthquakes, Italy

In August and October 2016, two normal fault earthquakes (Mw 6.0 and Mw 6.5, respectively) struck the Amatrice-Norcia area in the central Apennines, Italy. The mainshocks nucleated at depths of ~7–9km with the co-seismic slip propagating upward along the Mt. Gorzano Fault (MGF) and Mt. Vettore Fault System (MVFS). To recognize possible weakening mechanisms along the carbonate-hosted seismogenic faults that generated the Amatrice-Norcia earthquakes, the fresh co-seismic fault exposure (i.e., “nastrino”) exposed along the Mt. Vettoretto Fault was sampled and analyzed. This exposed fault belongs to the MVFS and was exhumed from ~2–3km depth. Over the fresh fault surface, phyllosilicates concentrated and localized along mm- to μm-thick layers, and truncated clasts and fluid-like structures were found. At the nano-scale, instead of their common platy-lamellar crystallographic texture, the analyzed phyllosilicates consist of welded nm-thick nanospherules and nanotubes similar to phyllosilicates deformed in rotary shear apparatus at seismic velocities or altered under high hydrothermal temperatures (>250°C). Moreover, the attitude of the Mt. Vettoretto Fault and its kinematics inferred from exposed slickenlines are consistent with the co-seismic fault and slip vectors obtained from the focal mechanisms computed for the 2016 mainshocks. All these pieces of evidence suggest that the Mt. Vettoretto Fault slipped seismically during past earthquakes and that co-seismic slip was assisted and facilitated at depths of <3km by phyllosilicate-rich layers and overpressured fluids. The same weakening processes may also have been decisive in facilitating the co-seismic slip propagation during the 2016 Mw 6.0 Amatrice and Mw 6.5 Norcia earthquakes. The microstructures found along the Mt. Vettoretto Fault, which is certainly a seismogenic fault, provide a realistic synoptic picture of co-seismic processes and weakening mechanisms that may occur in carbonate-hosted seismogenic faults.

Seismicity characterization of the Maravatío-Acambay and Actopan regions, central Mexico

We studied the seismic activity in the Maravatío-Acambay and Actopan regions in Central Mexico. These regions are of great importance due to the occurrence of shallow crustal normal-faulting earthquakes that caused widespread destruction near their epicenter and as far away as Mexico City. That was the case of the 19 November 1912 Acambay (Mw 6.9). We determined statistical seismicity characteristics such as the Båth's law (the size of largest aftershock with respect to that of the mainshock), the b-value and p-value. For the Maravatío aftershock sequence, we obtained a b-value of 0.88 and p-value of 0.68. Based on reported seismicity, we obtained a b-value of 1.12 for the Actopan region. We estimated the size of the largest aftershock of the Acambay event in the range of 4.7 <Mw < 5.9. By using the fragment-asperity model, we also performed the analysis of the magnitude distribution. The analysis showed similar values for the non-extensivity parameter (q-value): q = 1.6465 and q = 1.6555 for the Maravatío aftershock sequence and for the seismicity in the Actopan region, respectively. The similitude in the q-values could be associated with the similarities in the tectonic environment. In this model, the constant of proportionality (a) between released energy and the size of fragments showed that more energy is released for the Maravatío aftershock sequence than for the regular seismicity rate in the Acambay region. Finally, we analyzed the relation of the seismicity and the tectonic environment by quantifying the seismic coupling and the thickness of the seismogenic layer. The estimated seismogenic layer for the Maravatío-Acambay and Actopan regions are 14.6 km and 20.8 km, respectively. The seismic coupling coefficient at the Venta the Bravo fault in the Maravatío-Acambay region and Actopan region are 0.21 and 0.46, respectively. Our estimation of the seismic coupling coefficients shows that the regions can be classified as low-to-intermediate-coupling zones.

Crustal deformation associated with the 2011 eruption of the Nabro volcano, Eritrea

We investigate the crustal deformation associated with the 2011 eruption of the Nabro volcano, Afar, Eritrea. The Nabro volcano erupted on the night of 12 June 2011. A seismic sequence started 5h before the onset of the volcanic eruption. It included 25M>3 earthquakes, of which one Mw 5.6 normal fault earthquake occurred on 12 June at about the same time as the onset of the eruption, and one Mw 5.6 strike-slip earthquake occurred at the end of the main sequence on 17 June. The deformation associated with the eruption and the seismic activity was resolved by Interferometric Synthetic Aperture Radar (InSAR) measurements of the TerraSAR-X and ENVISAT satellites. Interferograms were generated using ascending and descending track pairs. The Nabro caldera and the associated channel of magma flow are characterized by significant loss of coherence which limited our InSAR observations at the near field of the volcano. Therefore, detailed assessment of co- and post-eruptive seismicity and monitoring of post-eruptive deformation using continued InSAR observations were added to the co-eruptive analysis in order to better constrain the different magmatic and tectonic components and determine the final source model. We carried out tens of different inversion models. Our best-fit model includes a dike, a normal fault and a strike-slip fault, consistent with the mechanisms of the major earthquakes. Coulomb stress calculations based on our model are found to be in agreement with post-eruptive seismicity. Finally, the source mechanism and geometry of our model are found to be in accord with the major tectonic structures in this area.

Deep electrical resistivity tomography along the tectonically active Middle Aterno Valley (2009 L'Aquila earthquake area, central Italy)

Three 2-D Deep Electrical Resistivity Tomography (ERT) transects, up to 6.36 km long, were obtained across the Paganica-San Demetrio Basin, bounded by the 2009 L'Aquila Mw 6.1 normal-faulting earthquake causative fault (central Italy). The investigations allowed defining for the first time the shallow subsurface basin structure. The resistivity images, and their geological interpretation, show a dissected Mesozoic-Tertiary substratum buried under continental infill of mainly Quaternary age due to the long-term activity of the Paganica-San Demetrio normal faults system (PSDFS), ruling the most recent deformational phase. Our results indicate that the basin bottom deepens up to 600 m moving to the south, with the continental infill largely exceeding the known thickness of the Quaternary sequence. The causes of this increasing thickness can be: (1) the onset of the continental deposition in the southern sector took place before the Quaternary, (2) there was an early stage of the basin development driven by different fault systems that produced a depocentre in the southern sector not related to the present-day basin shape, or (3) the fault system slip rate in the southern sector was faster than in the northern sector. We were able to gain sights into the long-term PSDFS behaviour and evolution, by comparing throw rates at different timescales and discriminating the splays that lead deformation. Some fault splays exhibit large cumulative throws (>300 m) in coincidence with large displacement of the continental deposits sequence (>100 m), thus testifying a general persistence in time of their activity as leading splays of the fault system. We evaluate the long-term (3–2.5 Myr) cumulative and Quaternary throw rates of most of the leading splays to be 0.08–0.17 mm yr−1, indicating a substantial stability of the faults activity. Among them, an individual leading fault splay extends from Paganica to San Demetrio ne’ Vestini as a result of a post-Early Pleistocene linkage of two smaller splays. This 15 km long fault splay can explain the Holocene surface ruptures observed to be larger than those occurred during the 2009 L'Aquila earthquake, such as revealed by palaeoseismological investigations. Finally, the architecture of the basin at depth suggests that the PSDFS can also rupture a longer structure at the surface, allowing earthquakes larger than M 6.5, besides rupturing only small sections, as it occurred in 2009.

Seismic Source Parameters of Normal-Faulting Inslab Earthquakes in Central Mexico

We studied 62 normal-faulting inslab earthquakes in the Mexican subduction zone with magnitudes in the range of 3.6 ≤ M w ≤ 7.3 and hypocentral depths of 30 ≤ Z ≤ 108 km. We used different methods to estimate source parameters to observe differences in stress drop, corner frequencies, source dimensions, source duration, energy-to-moment ratio, radiated efficiency, and radiated seismic energy. The behavior of these parameters is derived. We found that normal-faulting inslab events have higher radiated seismic energy, energy-to-moment ratio, and stress drop than interplate earthquakes as expected. This may be explained by the mechanism dependence of radiated seismic energy and apparent stress reported in previous source parameter studies. The energy-to-moment ratio data showed large scatter and no trend with seismic moment. The stress drop showed no trend with seismic moment, but an increment with depth. The radiated seismic efficiencies showed similar values to those obtained from interplate events, but higher than near-trench events. We found that the source duration is independent of the depth. We also derived source scaling relationships for the mentioned parameters. The low level of uncertainties for the seismic source parameters and scaling relationships showed that the obtained parameters are robust. Therefore, reliable source parameter estimation can be carried out using the obtained scaling relationships. We also studied regional stress field of normal-faulting inslab events. Heterogeneity exists in the regional stress field, as indicated by individual stress tensor inversions conducted for two different depth intervals (Z 40 km, respectively). While the maximum stress axis (σ 1) appears to be consistent and stable, the orientations of the intermediate and minimum stresses (σ 2 and σ 3) vary over the depth intervals. The stress inversion results showed that the tensional axes are parallel to the dip direction of the subducted plate. At depths greater than 40 km, the maximum horizontal stresses are oblique to the dip direction following the general trend of the slab geometry.

Postseismic gravity change after the 2006-2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands.

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 Mw8.3 thrust and 2007 Mw8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and ~1018 Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.

New Attenuation Relationship for Peak Ground and Pseudo-Spectral Acceleration of Normal-Faulting Earthquakes in Offshore Northeast Taiwan

Ground motions from normal-faulting earthquakes are generally considered to be smaller than those of strike-slip and thrust events. On 11 April 2011 a crustal normal-faulting earthquake [the Fukushima earthquake (M_w 6.6)] occurred in Eastern Japan. The peak ground acceleration (PGA) observed was considerably higher than the predictions of several ground-motion prediction equations (GMPEs), which were derived mainly from thrust or strike-slip earthquakes. In northeast Taiwan, the tectonic structure of the Ryukyu Arc and the Okinawa Trough typically entail normal-faulting earthquakes. Because of the normal-faulting earthquakes relevance to ground motions and nuclear power plant sites in northeast Taiwan, we evaluated the impact of the ground motion of normal-faulting earthquakes in offshore northeast Taiwan using a newly constructed attenuation relationship for PGA and pseudo-spectral acceleration (Sa). We collected 832 records from 13 normal-faulting earthquakes with focal depths of less than 50 km. The moment magnitude (M_w) of the 13 events was between 4 - 6. The Sa and PGA of normal-faulting earthquakes offshore northeast Taiwan determined with the newly constructed attenuation relationship were higher and lower, respectively, than those obtained using attenuation equations commonly used in the Taiwan subduction zone.

Ground-Motion Prediction Model From Intermediate-Depth Intraslab Earthquakes at the Hill and Lake-Bed Zones of Mexico City

We present ground-motion prediction equations, based on Bayesian regressions, to predict response spectra at sites located within the hill and lake-bed zones of Mexico City for intermediate-depth, normal-faulting intraslab earthquakes with magnitude ranges 5.2 ≤ Mw ≤ 7.4; distances from sites to the fault surface 103 ≤ R ≤ 464 km; and focal depths of the events 40 ≤ H ≤ 128 km, all at sites located in hill and lake-bed zones in Mexico City. The equations are built as functions of magnitude and distance to the fault surface, using more than 20 intraslab earthquakes. We compare our results with previous attenuation models for intraslab (hill zone) and interplate earthquakes (hill and lake-bed zones) in Mexico City. The results also show that the uncertainty for intraslab earthquakes at sites located within the hill zone is larger than that for sites located in lake-bed zone.

Unusual low-angle normal fault earthquakes after the 2011 Tohoku-oki megathrust earthquake

A few low-angle normal fault earthquakes at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, were detected immediately after the 2011 Tohoku-oki earthquake. After that, however, no such normal fault events have been observed until the occurrence of the 2014 M W 6.6 Fukushima-oki earthquake. Here we analyze the teleseismic body waveforms of the 2014 Fukushima-oki earthquake. We first compare the observed teleseismic body waves of the 2014 Fukushima-oki earthquake with those of the largest previous low-angle normal fault aftershock (M W 6.6), which occurred on 12 March 2011, and then estimate the centroid depth and moment tensor solution of the 2014 Fukushima-oki earthquake. The teleseismic body waves and moment tensor solution of the 2014 Fukushima-oki earthquake are similar to those of the 2011 normal fault aftershock, which suggests that the 2014 Fukushima-oki earthquake occurred at a similar depth and had a similar mechanism to that of the 2011 aftershock. We detected five low-angle normal fault aftershocks at approximately the depth of the plate interface, with a strike nearly parallel to the trench axis, and confirmed that all of them except for the 2014 Fukushima-oki earthquake occurred within 17 days after the mainshock. The occurrence of these low-angle normal fault events is likely to reflect the reversal of shear stress due to overshooting of slip during the 2011 Tohoku-oki earthquake. We speculate that a fast but heterogeneous recovery of stress state at the plate interface may explain why these events preferentially occurred immediately after the megathrust event, while one of them occurred with a significant delay. In order to better understand the characteristics of stress state in the crust, we have to carefully observe the ongoing seismic activity around this region.

Small size very low frequency earthquakes in the Nankai accretionary prism, following the 2011 Tohoku-Oki earthquake

A large number of shallow low frequency events were recorded after the 2011 Mw 9.0 Tohoku-oki earthquake by the cabled network of broadband ocean bottom seismometers (DONET) deployed in the eastern part of the Nankai trough. This low frequency event activity was intense for the first few days after the great earthquake and gradually decreased. Signals of the events are most clearly visible at the frequency range around 2–8Hz. Some of the events are accompanied by a very long frequency (VLF) signal, which is clearly observed at around 0.02–0.05Hz. The magnitude and source duration estimated by waveform analysis for one of the largest very low frequency earthquakes (VLFEs) was 3.0–3.5 and 17s. This source duration is extremely long compared to ordinary earthquakes of comparable magnitude. These newly detected VLFEs are likely to be normal fault earthquakes located at shallow depths within the accretionary prism, in contrast to the previously reported VLFEs that were explained by a low angle thrusting along the decollement zone. On the other hand, the low frequency events with no clear VLF signal were previously regarded as being low frequency tremors (LFTs). We show that events with and without the VLF signal likely represent the same phenomenon, and the VLF signal is only observed when a large magnitude event occurs near the station. The waveforms of VLFEs are characterized by the coexistence of long source duration and high-frequency radiation of signals, and such features were previously explained by the co-occurrence of shear failure and hydrofractures under the influence of fluid brought into the decollement zone. Our result indicates that the stress state and the mechanical environment, which promote the occurrence of VLFEs, exist not only along the decollement zone but also in the shallower part of the accretionary prism.

Source study of three moderate size recent earthquakes in the Guerrero seismic gap

We study three recent earthquakes of different types in the Guerrero seismic gap zone with M ≥ 6.5: the 15 July 1996 Guerrero near-coast interplate earthquake (Mw = 6.6), the 18 April 2002 Guerrero near-trench interplate earthquake (Mw = 6.7), and the 11 December 2011 Guerrero normal-faulting inslab earthquake (Mw = 6.5). We compute the slip distributions, estimate source parameters, and model strong ground motions with the finite-fault stochastic method. We use different methods to estimate source parameters in order to observe differences in stress drop and radiated seismic energy among these events. The similarity in seismic magnitude gives us the opportunity to compare our results for the three different types of earthquake and interpret them in terms of the tectonic environments and seismic hazard. We analyze the peak ground accelerations and their relation with the stress drop. We simulated ground motions with the stochastic method. The model parameters are validated against recordings and a stress drop of 3, 15, and 70 MPa is estimated for the near-trench interplate, near-coast interplate, and normal-faulting inslab events, respectively. The near-trench interplate event has the lowest radiated seismic energy and the lowest rupture velocity. This is reflected in the energy to moment ratio of 2.17 × 10−6, 4.52 × 10−6, and 3.96 × 10−6 for the near-trench interplate, near-coast interplate, and normal-faulting inslab events, respectively. We define asperities using two different criteria: (1) based on average displacement and (2) maximum displacement. The asperity area for the near-coast and near-trench interplate events represents about 23 and 24–25 % of the total rupture area, which in the case of the normal-faulting inslab event, it is only 19 and 23 % based on average and maximum displacement, respectively.

Up-dip directivity in near-source during the 2009 L'Aquila main shock

S U M M A R Y In this study we have investigated the directivity associated with the initial up-dip rupture propagation during the 2009 April 6 (Mw 6.1) L’Aquila normal-faulting earthquake. The objective is the understanding of how the peculiar initial behaviour of rupture history during the main shock has affected the near-source recorded ground motions in the L’Aquila town and surrounding areas. We have modelled the observed ground velocities at the closest near-source recording sites by computing synthetic seismograms using a discrete wavenumbers and finite difference approach in the low frequency bandwidth (0.02–0.4 Hz) to avoid site effects contaminations. We use both the rupture model retrieved by inverting ground motion waveforms and continuous high sampling-rate GPS time-series as well as uniform-slip constant-rupture speed models. Our results demonstrate that the initial up-dip rupture propagation, characterizing the first 3 s of the rupture history during the L’Aquila main shock and releasing only ∼25 per cent of total seismic moment, controls the observed ground motions in the near-source. This initial stage of the rupture is characterized by the generation of ground velocity pulses, which we interpret as a forward directivity effect. Our modelling results confirm a heterogeneous distribution of rupture velocity during the initial up-dip rupture propagation, since uniform rupture speed models overestimate up-dip directivity effects in the footwall of the causative fault. The up-dip directivity observed in the near field during the 2009 L’Aquila main shock is that expected for a normal faulting earthquake, but it differs from that inferred from far-field observations that conversely provide evidence of along-strike directivity. This calls for a careful analysis as well as for the realistic inclusion of rupture directivity to predict ground motions in the near source.

Dynamic lithospheric response to megathrust and precursory seismicity features of megathrust

Temporal variations of seismic properties in inner trench regions before and after three megathrusts with magnitudes greater than or equal to 8.8 since 2004 are investigated to understand the nature of megathrust earthquakes. The seismicity was increased significantly, and the fault-type compositions changed after megathrusts. The seismicity displays characteristic fault-type-dependent distribution on rupture planes. The postseismic thrustal events were populated around the down-dip rupture margins due to the concentration of shear stress after coseismic ruptures. Normal-faulting earthquakes increased after the megathrusts particularly in shallow-depth regions with large slips, which may be associated with lithospheric rebound and development of splay faults. The earthquake occurrence rate (b value) displays a characteristic slip-dependent feature. The earthquake occurrence rates decrease with the slip amount of forthcoming megathrust, which may be caused by continuous accumulation of plate-driven stress and tectonic loading around the future rupture planes on the slab surface. The slip dependency of earthquake occurrence rates is enhanced with time until the occurrence of megathrust. The level of seismicity after megathrust is inversely proportional to that before megathrust, yielding comparable average seismicity over the rupture zone. It was also observed that the dynamic lithospheric response is highly correlated with slip distribution on the rupture plane. The tension axes of the normal-faulting earthquakes for 100days after the Tohoku-Oki earthquake focus to an apparent pole, suggesting a radial viscoelastic deformation of lithosphere. The temporal changes of slip-dependent b values fit well with an exponential function, suggesting an exponential increase of normal stress on the locked region with time until the occurrence of the megathrust. This observation suggests that the temporal variation of slip-dependent b values may be useful for prediction of forthcoming megathrusts at least several tens years beforehand.

Stress Drops of the 1997–1998 Colfiorito, Central Italy Earthquakes: Hints for a Common Behaviour of Normal Faults in the Apennines

Stress drop estimates of moderate-magnitude earthquakes in the Umbria–Marche region, in the northern Apennines, exhibit a large scatter. For the two Mw 5.7 and 6.0 main shocks of 26 September 1997 near Colfiorito, several papers resulted in stress drop estimates of 20 MPa, but values as low as 2–3 MPa were proposed as well. Also for the largest aftershocks (Mw > 4), estimates spread from < 1 MPa up to values ten times larger. We have critically revisited methods and data used in the literature. We have specifically faced the trade-off between source and propagation effects, as we believe that it is responsible for a part of the large scatter. To keep this trade-off under control, we have applied a methodology that combines the best fit of both source spectra after Empirical Green’s Function (EGF) deconvolution and observed ground motion spectra, finding that the results of the two different data sets converge independently at the same solution. We have used ground motions observed in the Colfiorito basin, where an accelerograph and a co-located seismological broad-band station recorded three clusters of earthquakes in a broad magnitude interval (1.7 ≤ Mw ≤ 6.0). We have found that the mainshock–aftershock sequences result in stress drops of 2–5 MPa at Mw ≥ 5.6, with an average tendency to decrease at smaller magnitudes where stress drop variability increases. These findings confirm the source scaling recently assessed through Empirical Green’s Function deconvolution for another well-monitored seismic sequence of normal-faulting earthquakes, which struck the city of L’Aquila in the central Apennines in April 2009. The similar scaling law of the two areas suggests common mechanisms of stress release for the shallow normal faults in the Apennines. The propensity of smaller earthquakes to increase in variability, with a tendency toward smaller stress drops, may reflect an effect of fault strength heterogeneities for smaller size ruptures.

Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal-faulting earthquake related to a fracture zone in the Pacific plate

We describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

Skewed orientation groups in scatter plots of earthquake fault plane solutions: Implications for extensional geometry at oceanic spreading centers

AbstractSystematic analysis of earthquake focal solutions derived from centroid moment tensors shows well‐defined orientation groups in scatterplots of fault plane normals and associated slip line vectors. Consideration of the geometry implied by these orientation groups can allow resolution of the ambiguity inherent in the choice as to which of the two conjugate fault plane solutions should apply, and in many cases, the same classification can be applied to the entire orientation group. Examining scatter plots of data from normal fault earthquakes on spreading ridges typically shows orthogonal relations but there are also many cases where there is a skew with respect to the great circles defined by faults on adjacent transform faults. This can be explained by finite rock strength in the adjacent transforms, requiring resolved shear stress to allow movement, thus requiring rotation of the trajectories of the deviatoric stress axes: anticlockwise for right‐lateral transforms and clockwise for left‐lateral transforms. This asymmetry also requires formation of tilt block geometries reminiscent of Basin and Range style continental extension.

A reassessment of outer-rise seismicity and its implications for the mechanics of oceanic lithosphere

We use body-waveform modelling to constrain the source parameters of earthquakes occurring globally in oceanic lithosphere beneath the subduction zone outer rise and outer trench slope. These data are then used to map the stress state in the lithosphere of the downgoing plate as it bends into the subduction zone. Our results provide new constraints on the faulting of oceanic lithosphere at the outer rise, which is important for understanding the transmission of plate-driving forces through the subduction system. In all cases, shallow normal-faulting earthquakes are observed at the top of the plate, and are separated in depth from any deeper thrust-faulting earthquakes. No temporal variation associated with large thrust-faulting earthquakes on the subduction interface is seen in the depth extent of each type of faulting at the outer rise. The transition depth from trench-normal extension to compression is found to vary in agreement with models in which deformation is driven by the combination of in-plane stresses and bending stresses, resulting principally from slab pull. Combining the seismologically derived constraints on the thickness of the elastic core of the plate with estimates of the plate curvature, we place upper bounds on the strength of the lithosphere at the outer rise, which is required to be ≲300 MPa for a constant yield stress model, or governed by an effective coefficient of friction of ≲0.3.