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A description of the initial excitation of spheroidal mode 0S2 initiated by the 2013 Sea of Okhotsk earthquake (Mw = 8.3) is presented. It is based on the newly revealed fact of the origination of a standing P-wave in the epicenter of the Sea of Okhotsk earthquake and excitation of the traveling surface P-waves on the Earth’s surface by this wave. A new interpretation of the nature of the Earth’s free spheroidal oscillations in the case of a strong earthquake (M ≥ 7.5) is proposed. The spheroidal mode 0S2 is interpreted as a combination of standing and traveling P-waves, transferring the energy of Earth’s deformations.

We used recent seismic data and advanced techniques to investigate 3D fault geometry over the transition from the partially coupled to the fully coupled plate interface inboard of the Nankai Trough off the Kii Peninsula, Japan. We found that a gently dipping plate boundary decollement with a thick underthrust layer extends beneath the entire Kumano forearc basin. The 1 April 2016 Off-Mie earthquake (Mw6.0) and its aftershocks occurred, where the plate boundary decollement steps down close to the oceanic crust surface. This location also lies beneath the trenchward edge of an older accretionary prism (∼14 Ma) developed along the coast of the Kii peninsula. The strike of the 2016 rupture plane was similar to that of a formerly active splay fault system in the accretionary prism. Thus, the fault planes of the 2016 earthquake and its aftershocks were influenced by the geometry of the plate interface as well as splay faulting. The 2016 earthquake occurred within the rupture area of large interplate earthquakes such as the 1944 Tonankai earthquake (Mw8.1), although the 2016 rupture area was much smaller than that of the 1944 event. Whereas the hypocenter of the 2016 earthquake was around the underplating sequence beneath the younger accretionary prism (∼6 Ma), the 1944 great earthquake hypocenter was close to oceanic crust surface beneath the older accretionary prism. The variation of fault geometry and lithology may influence the degree of coupling along the plate interface, and such coupling variation could hinder slip propagation toward the deeper plate interface in the 2016 event.

We analyzed macroseismic data and considered the effect of extremely long range propagation of sensible shocks during the deep-focus earthquake in the Sea of Okhotsk on May 24, 2013 (Mw = 8.3). In order to explain this effect, we formulated and qualitatively solved the problem of superposition of P-waves over the radial mode 0S0 of the natural oscillations of the Earth during this earthquake. Our results confirmed the possibility of such an interpretation of the observed macroseismic effect and also allowed us to explain the fact of anomalously low decay of seismic disturbances with distance.

The 2014 M w 7.9 Rat Islands intermediate-depth (∼107 km) earthquake is studied using seismic observations at teleseismic distances. The nodal plane with a steep-dip angle (87°) and a strike of 308° inferred from the Global CMT solution is selected as the causative fault plane because of the consistency with the distribution of the aftershocks relocated using the Joint Hypocenter Determination method. Its slip history is constrained using broadband P and SH waveforms and long-period (3–6 mHz) seismic waves in vertical and transverse components. The rupture process associated with this plane shows a rupture propagating at ∼2 km/s. The rupture essentially breaks one large slip patch (peak slip amplitude of 3.5 m) located 30 km west of the hypocenter. The size of this slip patch extends from 80 to 140 km depth, which would be the largest depth extent of earthquake slip ever documented. We show that this earthquake breaks the entire cold core of the subducting slab defined by the depth of the 600–650 °C isotherm. In addition, the lateral extent of the rupture appears to be bounded by the heterogeneities of subducting slab. Our results further indicate that, along this segment of Aleutian Arc, the intermediate-depth earthquakes might contribute to the westward transporting motion of Rat block. Lastly, the slip distribution of this earthquake as well as that of two other well studied M w > 7.5 intermediate-depth earthquakes suggests that the coldest portion of the subducting slab, outlined by the double Wadati–Benioff zone, is seismic during large intermediate-depth earthquakes. Such a result is important for estimating the seismic hazard of future large intermediate-depth earthquakes.

The existence of earthquakes within continental lithospheric mantle remains a highly controversial topic. Here, we present a detailed set of seismological analyses confirming the occurrence of a mantle earthquake beneath the Wind River Range of central Wyoming. Combining regional waveform inversion with the analysis of the delay and relative amplitudes of teleseismically-observed depth phases, we demonstrate that the 2013 Wind River earthquake – a M W 4.7 highly-oblique thrust-faulting event – occurred at 75 ± 8 km , well beneath the base of the crust. The magnitude, mechanism, and location of this earthquake suggest that it represents simple brittle failure at relatively high temperatures within the mantle lithosphere, as a result of tectonic, rather than magmatic, processes.

We apply a multiple source inversion method to systematically study the source processes of 25 large deep-focus (depth >400 km) earthquakes with Mw > 7.0 from 1994 to 2012, based on waveform modeling of P, pP, SH and sSH wave data. The earthquakes are classified into three categories based on spatial distributions and focal mechanisms of the inferred sub-events: 1) category one, with non-planar distribution and variable focal mechanisms of sub-events, represented by the 1994 Mw 8.2 Bolivia earthquake and the 2013 Mw 8.3 Okhotsk earthquake; 2) category two, with planar distribution but focal mechanisms inconsistent with the plane, including eighteen earthquakes; and 3) category three, with planar distribution and focal mechanisms consistent with the plane, including six earthquakes. We discuss possible physical mechanisms for earthquakes in each category in the context of plane rupture, transformational faulting and shear thermal instability. We suggest that the inferred source processes of large deep-focus earthquakes can be best interpreted by cascading failure of shear thermal instabilities in pre-existing weak zones, with the perturbation of stress generated by a shear instability triggering another and focal mechanisms of the sub-events controlled by orientations of the pre-existing weak zones. The proposed mechanism can also explain the observed great variability of focal mechanisms, the presence of large values of CLVD (Compensated Linear Vector Dipole) and the super-shear rupture of deep-focus earthquakes in the previous studies. In addition, our studies suggest existence of relationships of seismic moment ∼ (source duration) 3 and moment ∼ (source dimension) 3 in large deep-focus earthquakes.

It has been hypothesized that much of the crustal deformation attributed to the Laramide orogeny of the southwest North American Cordillera was caused by dynamic effects induced by the flat subduction of a large oceanic plateau that was embedded within the Farallon plate. Previous studies have identified within the North American mantle a seismic velocity anomaly that plausibly represents the remnants of the subducted plateau. Coupled plate kinematic and dynamic modeling of the anomaly, as well as surface geological findings identify this anomaly as the subducted conjugate to the Shatsky Rise. Here, we find clear evidence for a northeastward dipping (35° dip), thick (up to 400 km thick) slab-like seismic velocity anomaly within the top of the lower mantle below the central United States. Using a deep focus earthquake below Spain, we find that the observed seismic waveforms recorded with the dense USArray display multi-pathing indicative of sharp surfaces. Plate tectonic reconstructions in which the anomaly is migrated back in time suggest strong coupling of the plateau-thickened slab segment to the overriding North America Plate as it was subducted. In combination with the reconstructions, we interpret the structure as arising from eastward dipping Farallon subduction at the western margin of North America during the Cretaceous, in contrast with some recent interpretations. It appears that the plateau area of the slab has been further thickened, which might undergo a combination of pure shear bulk shortening during flat-slab subduction and/or by a folding instability during penetration into the lower mantle.

From 2011 to 2012, we conducted marine seismic surveys using a multichannel seismic reflection (MCS) system and ocean bottom seismometers (OBSs) in and around the hypocentral region of the 1983 Nihonkai–Chubu earthquake. The survey covered the area from the Japan coast of the Japan Sea to the Yamato Basin and the Japan Basin. We investigated the relationship between the Nihonkai–Chubu earthquake and the crustal structure obtained from seismic reflection imaging and the P-wave velocity structure near the hypocentral region. The focus in the present paper is on seismic data that were acquired using both the MCS system and OBSs. Based on the results, the crustal structure of the Japan Sea around the hypocentral region of the 1983 Nihonkai–Chubu earthquake is divided into three types: island arc crust, thick oceanic crust, and oceanic crust. The Nihonkai–Chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust. There is a strong possibility that the east-dipping reflector detected in the crust was the source fault of the Nihonkai–Chubu earthquake. In addition, the location of anticlines formed by the east-dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the Nihonkai–Chubu earthquake was confirmed in the range of latitude from 39 ° 20 ′ to 41 ° 15 ′ N , although they were not found south of this segment. There is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the Nihonkai–Chubu earthquake. This area is located near the boundary between the thick oceanic crust and the oceanic crust.

The Okhotsk deep focus earthquake (M w = 8.3), the largest in the history of instrumental seismology, occurred on May 24, 2013, at 05:45 UTC in the Sea of Okhotsk near the western coast of the Kamchatka Peninsula. For the first time we have succeeded in catching the field of horizontal and vertical coseismic offsets generated by a strong deep seismic event, and investigating its characteristics using continuous GPS measurements. Based on these data and taking into account the seismological information, we have developed a dislocation model of the Okhotsk deep focus earthquake.

On 24 May 2013, the largest ever-recorded deep earthquake occurred beneath Sea of Okhotsk. A multiple point source inversion procedure is applied to constrain source process of this earthquake, based on waveform modeling of both direct P and SH waves and near-surface reflected pP and sSH waves. Our results indicate that the earthquake consists of six major subevents separated in space and time, encompassing a horizontal dimension of 64 ± 4 km along ~ N160°E and a downward depth extension of 35 ± 4 km. The geographic distribution and focal mechanisms of the inferred subevents and foreshock/aftershock locations do not fit into plane rupture. We suggest that the earthquake can be best explained by a cascading failure of shear instability within preexisting weak zones in the region, with the perturbation of stress generated by a shear instability triggering another.

The physical mechanism of deep earthquakes (depth >300 km) remains enigmatic, partly because their rupture dimensions are difficult to estimate due to their low aftershock productivity and absence of geodetic or surface rupture observations. The two largest deep earthquakes, the recent Great 2013 Sea of Okhotsk earthquake (M 8.3, depth 607 km) and the Great 1994 Bolivia earthquake (M 8.3, depth 637 km), together provide a unique opportunity to compare their rupture patterns in detail. Here we extend a travel-time sub-event location method to perform full teleseismic P-waveform inversion. This new method allows us to explain the observed broadband records with a set of sub-events whose model parameters are robustly constrained without smoothing. We find that while the Okhotsk event is mostly unilateral, rupturing 90 km along strike with a velocity over 4 km/s, the Bolivia earthquake ruptured about half this distance at a slow velocity (about 1.5 km/s) and displayed a major change in rupture direction. We explain the observed differences between the two earthquakes as resulting from two fundamentally different faulting mechanisms in slabs with different thermal states. Phase transformational faulting is inferred to occur inside the metastable olivine wedge within cold slab cores whereas shear melting occurs inside warm slabs once triggered.

We present results for joint kinematic inversion of high-rate GPS, strong motion and InSAR data for the 2016 M7.6 Melinka earthquake. We show that the source is a compact 35 s long rupture with 5.0 ± 0.15 m of peak slip. We find the Melinka earthquake occurs inside the slip region for the 1960 M9.5 Valdivia earthquake and within the highly locked portion of the megathrust inferred from inter-seismic velocity analysis. We show that there is very modest post-seismic deformation at a nearby GPS site QLLN and argue that this indicates the Melinka earthquake ruptures within the intermediate portion of the megathrust and is an isolated asperity surrounded by locked velocity weakening material. Further we find that the peak slip observed during this earthquake is larger than what has been accumulated in the intervening 57 years since the 1960 rupture and conclude that, at least in the area of the Melinka earthquake, this is indicates that the 1960 Valdivia event might not have used all the slip deficit available on the megathrust.

[1] Mass rearrangement within the crust and lithospheric mantle and ocean water redistribution caused by great earthquakes are made visible by their coseismic gravity signature, nowadays detectable by the Gravity Recovery And Climate Experiment (GRACE) space mission. Here we present a novel procedure for estimating the principal seismic source parameters (hypocenter and moment tensor) that relies solely on space gravity data from GRACE, applied for the first time to the 2011 Tohoku earthquake. It yields a seismic source model that is consistent with a thrust earthquake and geological information of the subduction zone. It closely resembles the Global Centroid-Moment-Tensor Project solution based on the teleseismic wave inversion, although the moment magnitude is higher (9.13 ± 0.11 compared to 9.08) and the hypocenter is further offshore by about 40 km, within the oceanic plate. This procedure will become an important tool in seismology as it complements Centroid-Moment-Tensor analysis by exploiting the new gravity data from GRACE.

The 2016 Ecuador M _W 7.8 earthquake ruptured the subduction zone boundary between the Nazca and South American plates. Joint modeling of seismic and tsunami observations indicates an ∼120 km long rupture area beneath the coastline north of the 1998 M_W 7.2 rupture. The slip distribution reveals two discrete asperities near the hypocenter and around the equator. Their locations and the patchy pattern are consistent with the prior interseismic geodetic strain, which showed highly locked patches also beneath the coastline. Aftershocks cluster along two streaks, one aligned nearly parallel to the plate convergence direction up-dip of the main slip patches, and the other on a trench-perpendicular lineation south of the 1958 rupture zone. Comparisons of seismic waveforms and magnitudes show that the 2016 event and 1942 earthquakes have similar surface wave magnitude M_S 7.5), overlapping rupture areas, and similar main pulses of moment rate. The same area ruptured as the southernmost portion of the larger earthquake of 1906 (M_W 8.6, M_S 8.6). The seismic behavior reflects persistent heterogeneous frictional properties of the Colombia–Ecuador megathrust.

Using about 5200 handpicked P, S, pP, sP, PcP, and ScP phases from digitally recorded seismograms, together with International Seismological Centre reported phases, we obtain improved hypocentral locations for ∼1790 earthquakes, ∼983 of them having 90% confidence limits 50 km, in the period 1962 to September 1996, along the Indonesian subduction zone. We use an improved joint hypocenter determination method in which the solution is more stable than in the original one. We show that for deep earthquakes, use of the core-reflected phases is as good as use of depth phases, and since such core phases often have larger amplitudes and sharper onsets than the depth phases, their arrival times can be read very accurately. The positions of the relocated hypocenters show that (1) a portion of the Indonesian arc between ∼ 110° E and 123° E longitude, and deeper than ∼500 km, is dipping southward at an angle of ∼75°, that is, in a direction opposite to the upper part of the north dipping slab, and (2) east of about 108° E, the seismic zone is wider near 670 km than near the 500 km depth. The first suggests southward lateral flow in the mantle, relative to the plate motion vector here. From the contortion of the seismic zone along the eastern portion of the Indonesian arc, we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10-16S-1, over the last 10-20 Myr. Copyright 1999 by the American Geophysical Union.

Quantitative estimates of the crack density (ϵ), saturation rate (ξ) and porosity (ψ) parameters from seismic velocities (Vp, Vs) and Poisson’s ratio (σ) in the 2001 Bhuj earthquake area in western India indicate that the 2001 Bhuj earthquake hypocenter is associated with high-ϵ, high-ξ and high-ψ in the depth range of about 23–28 km, extending 15–30 km laterally. These anomalies may be due to a fluid-filled, fractured rock matrix, which might have contributed to trigger the 2001 Bhuj earthquake in the intraplate, stable continental region of the Indian Peninsula. This feature is similar to that of the 1995 Kobe earthquake [D. Zhao et al., Science 274 (1996) 1891–1894]. High-ψ areas are generally consistent with high-ϵ areas, but high-ξ areas have a wider distribution, indicating that micro-cracks exist in more localized areas within the crust, and that permeation of fluids in the hypocenter zone might have occurred extensively through the intergranular and fractured pores due to hidden intersecting fault geometry. Here we suggest the possibility that earthquake occurrence is closely related to in situ material heterogeneities, rather than stress conditions alone.

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.

We investigate the nature of laterally varying P wave speeds (Vp) and their relationship to a group of unusual deep earthquakes beneath the backarc of Tonga. This subhorizontal swath of “outboard” earthquakes occurs within the transition zone of the mantle but is several hundred kilometers away from the inclined Wadati-Benioff zone. Surprisingly, the seismogenic region is not associated with fast Vp. It follows that the effect of cold temperature, as implied by the presence of large earthquakes, must have been counteracted by other factors such as compositional or mineralogical effects. Our results are based on a sequence of seismic profiles in a fan-shot geometry. For a given azimuthal sector, we seek the simplest model of radially varying Vp that explains observed broadband waveforms and travel times. The lateral extent of anomalous Vp is then constrained by comparing the results from a wide range of azimuths. This approach fully utilizes information from triplicate P arrivals that are sensitive to seismic wave speeds at the depths of interest. Our results identify prominent anomalies of fast Vp (∼3% over average Earth models) both in the transition zone and in the lower mantle beneath the Tonga backarc. However, the region of fast Vp in the transition zone is offset by ∼300 km to the south of that in the lower mantle, and the region of offset coincides with the outboard earthquakes. A straightforward interpretation is that a large flux of subducted Pacific lithosphere lingers in the transition zone before descending into the lower mantle. Within this broad remnant of subducted lithosphere, the outboard earthquakes occur in a region where impounding of subducted material is most significant, a condition favorable for the accumulation of met astable olivine or volatiles. Such materials reduce Vp and are candidates for triggering earthquakes, thus accounting for the absence of fast Vp in the source zone of deep earthquakes.

We consider different formation models of deep focus earthquakes at the boundary between the Pamir and Hindu Kush mountains including the Hindu Kush megasource. Reconstruction of the Late Cretaceous‐ Paleogene basin between the Northern and Central Pamirs allows us to connect anomalously high seismicity with the deformation of the subduction plate (slab) of the oceanic lithosphere in this basin during its submersion into the mantle. In this sense the Vrancea zone in the Carpathians is an analog of the Hindu Kush megasource. The concentration of mantle earthquakes of the intermediate type with hypocenter depths up to 300 km and magnitudes up to 8 and greater [1] within a small megasource region at the boundary of tectonic zones in the Hindu Kush, Central, and Southwestern Pamirs has long attracted the interest of researchers (Fig. 1). Here, 90% of strong earthquakes in the Pamir‐Hindu Kush zone and 95% of released energy are concentrated on a territory of 200 × 120 km [2]. In the western region, mantle earthquakes are sharply interrupted at 69 ° 30 � E. The zone of mantle earthquakes approximately 350 km long with hypocenter depths up to 200 km is extended to the northeast of the megasource already on the territory of the Pamirs. Thus, the belt of mantle seismicity with a total length up to 500 km is clearly pronounced. The Hindu Kush source is confined to its southwestern part. L.I. Lobkovskii suggested a model based on the concept of two-level plate tectonics to explain the nature of the Hindu Kush earthquakes [3‐5]. Calculations demonstrated that dissipative heating of the upper parts of the mantle up to 700‐750 ° C occurs during the underthrust of the continental lithosphere in the collision zones. At such temperatures, the mechanism of brittle deformation (earthquake) becomes impossible. At the same time, the mantle part of the lithosphere separated by the asteno-layer from the crust can submerge into the upper mantle. This process is generally aseismic. Deep-focus mantle earthquakes appear only in the anomalous regions either as a result of insufficient heating, or due to the penetration of water or other fluids. In the latter case, the rocks become brittle and capable of seismogenic deformations. According to the other version [2], mantle seismicity in the Hindu Kush is related to the submersion of rocks of the ancient oceanic crust (Paleozoic or Early Mesozoic ophiolites) to the depths of 40‐70 km under the weight of the tectonic nappes. In this case, the basalt-eclogite phase transition occurs and their submersion to the upper mantle of lower density. This is an earthquake-related process. We eventually admit that the local source of mantle seismicity can be related to submersion, deformation, and separation of a slab of oceanic lithosphere in the basin that had closed recently. For example, seismicity in the Vrancea zone (East Carpathians) is related to the residual phenomena in the subduction zone of a deep basin that completely closed in the Middle‐Late Miocene (see Fig. 4.11 in [5] and references in this publication). Such an explanation was not applied to the Hindu Kush earthquakes because the existence of a young basin and related subduction were not assumed in this region.

We analyze the coda of teleseismic P waves to deterministically map seismic scatterers in a 1000 km on-a-side cube beneath the Izu-Bonin trench by migrating and stacking waveforms. This method was applied to several hundred short-period vertical-component western United States seismometer recordings of 17 deep-focus Izu-Bonin earthquakes. Except for isolated arrivals, the midmantle appears devoid of sharp discontinuities. S-to-P conversions at the 660-km discontinuity generated the largest signals; the 410-km discontinuity occasionally generated signals. Horizontal discontinuities poorly explain other signals; however, S-to-P conversions generated at a nearly north-south trending, steeply dipping discontinuity at 30°N, 145°E, and 1000 km beneath and parallel to the Izu-Bonin trench explain most additional signals. Observed coherent S-to-P conversions of 2-s period waves limit the width of a gradient zone to

We analyze residual spheres for teleseismic S wave arrival times from 10 intermediate and deep focus earthquakes in the Kurile subduction zone. Long-period WWSSN and CSN recordings of transverse component S and ScS arrivals provide good azimuthal coverage for each event, with arrival time estimates that compare well with short-period observations. In order to isolate the near-source structure, substantial lower mantle and receiver path effects are suppressed both by using empirical station/path statics derived from S, sS, ScS, and sScS arrivals for a total of 28 northwestern Pacific events and by constructing differential residual spheres. Ray tracing through three-dimensional aspherical shear velocity models supports the inferred large magnitude of the deep mantle and near receiver effects, although existing models do not appear to provide accurate corrections. The events are relocated in the PREM structure, and a modest spatial smoothing of the residuals is applied. The resulting residual spheres are modeled using a finite difference travel time method that accounts for three-dimensional ray path perturbations. Our starting models are derived from slab structures determined by previous P wave residual sphere analyses. We explore the compatibility of the P and S wave observations to constrain the Vs/Vp velocity ratio of the slab and to test whether the S wave arrival times place additional constraints on the slab geometry. The modeling supports the previous results favoring penetration of the Kurile slab to at least 800 km depth for most of the length of the arc, with some evidence that the northern portion of the slab broadens by as much as a factor of 3 upon entering the lower mantle. The preferred slab shear velocity models have relatively low (2–3%) shear velocity contrasts below 670 km depth, which can be accounted for by slab temperature reductions of 300°–400°.

Both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine (`MO'). Dynamically consistent models of subducting slabs, in which the kinetics of the olivine to spinel (wadsleyite or ringwoodite) transition is treated in a simplified way, are carried out to evaluate the buoyancy effect of MO on the subduction velocity. Assuming slab thicknesses according to the cooling half-space model, we find that MO forms in slabs with ages greater than approximately 70 Myr, and that a significant slowing down of subduction velocity occurs for ages greater than 100 Myr. Because a decrease in the subduction velocity for old lithospheric ages is not observed, our models suggest that the amount of MO in slabs is less than approximately 5000 km2 in cross section, and that for old lithosphere the cooling half-space model is not applicable.

To constrain dynamic source properties of deep earthquakes, we have systematically constructed broadband time functions of deep earthquakes by stacking and scaling teleseismic P waves from U.S. National Seismic Network, TERRAscope, and Berkeley Digital Seismic Network broadband stations. We examined 42 earthquakes with depths from 100 to 660 km that occurred between July 1, 1992 and July 31, 1995. To directly compare time functions, or to group them by size, depth, or region, it is essential to scale them to remove the effect of moment, which varies by more than 3 orders of magnitude for these events. For each event we also computed short-period stacks of P waves recorded by west coast regional arrays. The comparison of broadband with short-period stacks yields a considerable advantage, enabling more reliable measurement of event duration. A more accurate estimate of the duration better constrains the scaling procedure to remove the effect of moment, producing scaled time functions with both correct timing and amplitude. We find only subtle differences in the broadband time-function shape with moment, indicating successful scaling and minimal effects of attenuation at the periods considered here. The average shape of the envelopes of the short-period stacks is very similar to the average broadband time function. The main variations seen with depth are (1) a mild decrease in duration with increasing depth, (2) greater asymmetry in the time functions of intermediate events compared to deep ones, and (3) unexpected complexity and late moment release for events between 350 and 550 km, with seven of the eight events in that depth interval displaying markedly more complicated time functions with more moment release late in the rupture than most events above or below. The first two results are broadly consistent with our previous studies, while the third is reported here for the first time. The greater complexity between 350 and 550 km suggests greater heterogeneity in the failure process in that depth range.

We stack the teleseismic depth phases sS, sP, and pP produced by deep focus earthquakes to image precursory arrivals that result from near-source, underside reflections off the 410-km seismic velocity discontinuity (hereinafter referred to as the 410) and use differential time measurements between these phases and their precursors to compute discontinuity depths near seven subduction zones around the Pacific Ocean margin. We begin by selecting seismograms with high-quality depth phase arrivals recorded by several long-period networks between the years 1976 and 1996. Filtering the waveforms and stacking them along theoretical travel-time curves reveals clear precursors which vary in shape and timing. We compute confidence levels to evaluate the reliability of the observed precursory features using a bootstrap method that randomly resamples the seismograms prior to stacking. We measure the differential travel time between the reference pulse and the precursor using a cross-correlation technique and convert this time to an apparent discontinuity depth using the isotropic Preliminary Reference Earth Model (PREM) at 25-s period, corrected to an oceanic crustal thickness. The lateral resolution of our long-period stacks for 410 topography is limited compared to that sometimes achieved in short-period analyses but is much higher than that obtained from global SS precursor studies. For most subduction zones the results indicate little change in the average depth to the 410-km discontinuity in the local areas sampled by the precursor bounce points compared to broad regional depths inferred from SS precursor results. This implies that any large variations in depth to the 410-km discontinuity near subduction zones are limited to a narrow zone within the slab itself where they may be difficult to resolve with long-period data. Coverage for the Tonga and Peru-Chile subduction zones is sufficiently dense that we can observe lateral variations in 410 depths. In Tonga the results suggest depth variations perpendicular to the slab of up to 33 km, after correcting for probable lateral heterogeneity in velocity above 400 km depth, and variations parallel to the slab orientation as large as 13 km. The cross-slab variation is consistent with the elevation of olivine phase transformations in cold regions; the variation along strike suggests a more complex thermal heterogeneity that may be related to the subduction history of this region. We see evidence for additional reflectors above the 410 in some of the waveform stacks, but the inconsistency and weak amplitude of these features preclude definitive interpretations.

The duration of each subevent of 48 earthquakes with magnitude larger than 5.5 and depth greater than 100 km was determined from stacked traces of broadband records of Global Seismograph Network stations. We fitted the source time function by one or more triangles convolved with attenuation. We found that global stacks of displacement seismograms yield reliable estimates of the rupture duration. The durations, scaled to a moment of 1019 N m, of both the subevents and the entire earthquake show a slight decrease with depth from 9 s for events at 100 km depth to about 7 s for events at 600 km depth. Assuming that the rupture velocity is a constant fraction of the shear wave speed, this decrease can be completely explained by the increase in shear velocity of 20%. In this sense, deep earthquakes are comparable to intermediate ones. For some intermediate-depth events, Vidale and Houston [1993] found durations up to twice as long. We find that almost all of their slow events have been recorded at large epicentral distances. At these distances, we conjecture that the end of the P wave train may be extended by the arrival of reflections from the D″ layer.

We evaluate the methodologies that aim at resolving the space-time slip distribution during large earthquakes. In one common approach, the source process of an event is assumed to be composed of a series of subevents. Critical points in teleseismic body waves, such as peaks and coherent arrivals, are viewed as discrete points in the spatiotemporal history of an earthquake. We show that these features can be interpreted as the instantaneous centroid locations of the moment release history. We demonstrate this result with an analysis of the Mw=8.2 Bolivia 1994 deep event. We reconstruct the slip distribution in two contrasting ways. First, broadband P waves are interpreted as being composed of subevents. Five peaks in the moment release rate are located relative to the onset of the rupture using the azimuthal variations of onset-to-peak times. Second, we invert broadband P waves for the slip distribution on a subhorizontal fault. We use a nonlinear tomographic imaging technique called simulated annealing and an L1 misfit norm. Rupture velocities appear to be

One of the largest (Mw = 7.8) deep earthquakes (595 km) in recorded history occurred beneath the Flores Sea on June 17, 1996. We analyze broadband body waveforms to determine the seismic source characteristics of this important earthquake. The source time function has a small initial subevent (5 s duration) followed by three major subevents with a total source duration of 29 s. Directivity analysis indicates an unilateral rupture to the east-southeast with a subhorizontal rupture length of 75 km. A time-independent moment tensor (TIMT) inversion gives a total seismic moment of 5.4×1020 N m ( Mw = 7.8). A time-dependent moment tensor (TDMT) inversion indicates rupture on a near-vertical fault (strike of 130°, dip of 80°SW, rake of −166°) during the first 5 s, releasing 8% of the seismic moment. The major part of the moment was released along a shallower dipping plane (strike of ∼100°, dip of ∼55°, rake of ∼−45°). This change of faulting geometry may imply that the rupture initiated at a structural, thermal, or phase boundary of high stress concentration and then triggered the major moment release along a regional weak zone. The mainshock produced at least 23 (mb>2.9) aftershocks within the first 3 days. Seventeen of the largest aftershocks were located using the Joint Hypocenter Determination algorithm. The aftershocks define a plane dipping antithetically to the dip of the subducting slab with lateral and depth dimensions of 145 km and 76 km, respectively. The aftershock locations correlate with the southeast striking nodal plane of the moment tensor mechanism.

We report the observation of T phases recorded at a number of Pacific island sites, following the great Bolivian deep earthquake of June 9, 1994. By studying their arrival times at the various stations, we unravel the process of conversion from seismic to acoustic energy at the South American coastline. We find that the maxima of amplitude of the phase are incompatible with propagation along the great circle from source to receiver but rather require the generation of the acoustic wave at a common point at the Arica Bight for all stations in the South Pacific, and even farther south for stations in Hawaii and the Bonin Islands. The timing of these conversions corresponds to the arrival of regional S waves at the conversion points. The high-frequency nature of the T phase requires the channeling of the S wave from the seismic source to conversion point through low-attenuation material, which in turns adds supportive evidence for the mechanical continuity of the downgoing slab under that section of South America, despite the observed gap in seismicity between 300 and 600 km.

We investigate the rupture process of the March 9, 1994, M w 7.6 deep Tonga earthquake and its relationship to the background seismicity of the subducted Tonga slab. Variations in observed P and S wave pulse duration indicate that the rupture propagated to the NNE and extended well beyond the background seismicity. We inverted 47 P and SH waveforms, including regional broadband waveforms from the Southwest Pacific Seismic Experiment, using a method that solves for the focal mechanism change during the rupture and the distribution of moment release along the fault plane. The results indicate that significant moment release occurred in previously aseismic regions outside the active seismic zone and that the rupture terminated 10-20 km beyond the bounds of the previous seismic activity. A significant change in focal mechanism occurred when the rupture propagated into the previously aseismic region. Rupture along the near-vertical NNE striking nodal plane provides a somewhat better fit to the body waveforms than rupture along the near-horizontal nodal plane. This result, combined with the planar alignment of aftershocks and the general NNE directivity of the waveforms, provides strong evidence that the rupture occurred on the near-vertical plane. Thermal modeling of the Tonga slab indicates that the rupture terminated in material about 200°C warmer than the temperature that normally limits the occurrence of smaller earthquakes. Additionally, aftershocks seem to be suppressed in the outer regions of the rupture, which contain about half of the moment release but only 1 of the 15 well-located aftershocks. We suggest that slabs may be composed of an inner cold core, where seismic rupture initiates and small earthquakes occur, and a thermal halo of warmer material, which can sustain rupture and only a few aftershocks. The mechanism by which rupture propagates through the warmer material need not be similar to the process governing rupture nucleation in the cold slab core; nucleation may occur through a process limited to the cold core such as transformational faulting, and propagation through the warmer material may occur through ductile faulting or plastic instabilities. Isolated deep earthquakes in other subduction zones, such as the 1994 Bolivia event, may occur almost completely within the warmer zone, accounting for the lack of background seismicity and the dearth of aftershocks.

We investigate the role of grain-size reduction during the olivine-spinal transformation on rheological properties of subducting slabs on the basis of a scaling model for micro structural development during nucleation and growth. In this model, the size of spinal grains nucleating at olivine grain-boundaries is controlled by the relative rates of nucleation and growth, taking into account the impingement through the collision of grains due to growth. When the volume fraction of spinal reaches a certain threshold value (critical volume fraction ~ 1 to 10 %, depending on the P,T conditions in the slab), the new phase will form a continuous film and will significantly reduce the strength of the two-phase aggregate if spinal grain-size is small. The size of spinal grains delta 0 at this stage is calculated and is shown to be highly sensitive to temperature. At relatively high temperatures (T > 1000 K), delta_0 shows an Arrhenius-type dependence on temperature, that is delta_0 ~ exp(-E* /RT) with E* ~ 400 kJ/mol, whereas a more complicated temperature dependence is found at low temperatures (T < 900 K), where a grain-size reduction of up to 4 orders in magnitude is possible. Strength profiles of slabs due to combined effects of temperatures and of grain-size reduction are calculated. It is shown that: (1) the strength of slabs will have an unusual temperature dependence through the temperature dependence of grain-size; and (2) a subducting slab has a complicated rheological structure containing a weak region below the tip of a metastable olivine-bearing wedge in a cold slab. Possible implications of these anomalous rheological structures of slabs on the dynamics of subduction are discussed including the mechanisms of deep earthquakes.

A constitutive model is developed to describe the creep behavior of olivine-spinel aggregates, and conditions for transformation shear instability and deep earthquake nucleation are predicted. Using the Eshelby-Mori-Tanaka theory [Eshelby, 1957; Mori and Tanaka, 1973], a piecewise incremental method is employed to calculate the stress-strain relation for the olivine-spinel aggregate. The creep strain and strain rate depend on differential stress, the aspect ratio of spinel inclusions, latent heat of the phase transformation, the relaxation of the volumetric phase transformation strain e ph kk , and the creep law parameters of spinel. A constant strain rate calculation suggests that above a critical percentage transformation ξ c an earthquake may nucleate owing to the system's potential for a shear stress drop, provided e kk ph of -8% is not relaxed and spinel grain size d xp remains small. A normalized critical stiffness, calculated from the potential shear stress drop Δτ and the critical shear strain γ c needed to change from one steady state creep condition to another, suggests that a plate-like, partially transformed region is required for deep earthquake nucleation. Such a region may be formed by localized deformation or preexisting heterogeneities in a subducting rock.

A prominent and spatially localized phase arriving approximately 4 s after the initial compressional arrival, repeatedly observed on seismograms recorded in the central Rio Grande Rift from intermediate- and deep-focus earthquakes at distances near 90°, is identified as a probable compressional to shear wave conversion from a lower crustal root of the Socorro midcrustal magma body (SMB). This phase is inconsistent with previously determined crustal models containing a simple sill-like midcrustal magma body with a negligible conduit system. Finite element synthetic seismogram modeling suggests that a partially molten root with a western boundary dipping steeply to the east extends several kilometers from the 19-km-deep upper surface of the magma body down into the lower crust of the southwestern portion of the Albuquerque-Belen basin. The location and geometry of this feature suggest that the intracrustal intrusion of mantle magmas responsible for the ongoing inflation of the SMB and associated seismicity is occurring via an off-axis conduit system located beneath the active margin of the Albuquerque-Belen basin.

The effects of a strong non-linear thermo-kinetic coupling in the olivine-spinel transition are investigated with a 2D model for fast-subducting slabs. The thermo-kinetic coupling problem is solved in the Lagrangian framework by the differential algebraic method and by using experimental input parameters. Narrow regions with metastable olivine are found in fast and cold slabs down to the 660 km discontinuity, while the thermo-kinetic coupling reduces the depth and width of the zone where olivine and spinel coexist. In contrast to recent kinetic models, both latent heat release and heat diffusion place important constraints on the structure of the metastable wedge. Both processes trigger the ongoing phase transformation in the cold slab interior. A thermal instability over a short timescale (10 4 –10 5 yr) can occur due to a positive thermo-kinetic feedback. This effect is enhanced at greater depths. The depth of the metastable wedge is found to be extremely sensitive to certain geophysical and thermodynamic parameters such as the lithospheric basal temperature, slab width and the activation energy for diffusion. Variations in these parameters within the experimental uncertainties can cause significant changes in the depth of the metastable wedge. Hence predictions about the exact position of the metastable olivine wedge are rather difficult. We propose that deep focus earthquakes may occur due to a thermal runaway caused by shear heating, while the thermal instability is enhanced by the latent heat release associated with the olivine-spinel phase transition.

We have located and estimated source parameters of 109 earthquakes (0 to 5 M) using seismograms recorded at 2.5 km depth in the Cajon Pass borehole, southern California. The borehole is about 4 km from the San Andreas fault, at the boundary between the locked, almost aseismic Mojave and San Bernardino segments, where the San Jacinto fault approaches the San Andreas. This area is of interest both on account of its tectonic complexity and its high potential seismic hazard. The clear, relatively unattenuated downhole recordings are rotated to determine the incoming azimuth of the P wave, and the delay time between the P and S arrivals is used to estimate the hypocentral distance. The difference between the hypocenters located this way and the Southern California Seismic Network (SCSN) locations of the same earthquakes increases with hypocentral distance from the borehole. Of the earthquakes within 20 km of the borehole, 95% are within 1–2 km of the SCSN epicenters and all are within 5 km of the SCSN depths. All but three of the 58 earthquakes located here which the SCSN did not record were within 20 km of the borehole and so the location errors are similar to those of SCSN A and B quality events. Most of the small earthquakes occurred within the relatively active San Jacinto fault zone, but at least eight are located close to or within the San Andreas fault zone. The stress drops for these events range from 0.1 to 18 MPa. These earthquakes appear similar to the other events in this study suggesting that the San Andreas fault is similar to other faults at the scale of these small earthquakes. The variation in stress drop and slip orientation of these small earthquakes suggests that larger scale heterogeneity continues to these small scales. Also, the location of these small earthquakes in major established fault zones implies that earthquake source dimensions are not geometrically controlled by fault zone width.

Broadening of seismogram envelopes around direct S waves of deep earthquakes that occurred in a lithospheric slab reveals random inhomogeneities in the upper mantle and crust. Regional differences in the character of envelope broadening were studied in relation to the volcanic front in the central part of Japan. A total of 58 earthquakes that occurred along the subducting Pacific plate ranging from 80 to 500 km in depth were observed at 73 stations of the Kanto-Tokai seismic observation network and used in this analysis. Time lags of the maximum peak arrival and the half-maximum arrival were measured from the onset of the direct S wave on root-mean-squared traces of horizontal component seismograms in frequency bands of 1, 2, 4, and 8 Hz. The envelope broadening is weak at stations located along the coastline of the Pacific Ocean, however, the dependence of time lags on the hypocentral distance and frequency becomes strong at stations near the volcanic front. Moreover, the time lags become very long with increasing hypocentral distances and frequencies west of the volcanic front. This broadening of the seismogram envelope can be interpreted as the effect of the diffraction and multiple forward scattering on the seismic wave propagation through random inhomogeneities. The parabolic approximation is applied to simulate the envelope broadening when the correlation distance of velocity inhomogeneities is longer than the seismic wavelength. The numerical simulation indicates that the dependence of the envelope broadening on the travel distance and frequency depends greatly on the type of the autocorrelation function for the randomness. By comparing the observed frequency dependence of the envelope broadening with the result of the numerical simulation, the type of the autocorrelation function was estimated to be close to Gaussian at stations east of the volcanic front and close to exponential west of the volcanic front. The regional difference in the randomness on both sides of the volcanic front might reflect the difference in the short-wavelength component inhomogeneities.

[1] We use a three-dimensional strike-slip fault model in the framework of rate and state-dependent friction to investigate earthquake behavior and scaling relations on oceanic transform faults (OTFs). Gabbro friction data under hydrothermal conditions are mapped onto OTFs using temperatures from (1) a half-space cooling model, and (2) a thermal model that incorporates a visco-plastic rheology, non-Newtonian viscous flow and the effects of shear heating and hydrothermal circulation. Without introducing small-scale frictional heterogeneities on the fault, our model predicts that an OTF segment can transition between seismic and aseismic slip over many earthquake cycles, consistent with the multimode hypothesis for OTF ruptures. The average seismic coupling coefficient c is strongly dependent on the ratio of seismogenic zone width W to earthquake nucleation size h*; c increases by four orders of magnitude as W/h* increases from � 1 to 2. Specifically, the average c = 0.15 � 0.05 derived from global OTF earthquake catalogs can be reached at W/h* ≈ 1.2–1.7. Further, in all simulations the area of the largest earthquake rupture is less than the total seismogenic area and we predict a deficiency of large earthquakes on long transforms, which is also consistent with observations. To match these observations over this narrow range of W/h* requires an increase in the characteristic slip distance dc as the seismogenic zone becomes wider and normal stress is higher on long transforms. Earthquake magnitude and distribution on the Gofar and Romanche transforms are better predicted by simulations using the visco-plastic model than the half-space cooling model.

The Mw=6.3 deep-focus earthquake beneath Granada, Spain, in 2010 consisted of three resolvable sub-events occurring within a time span of 5 s. Estimated sub-event seismic moment partitioning is 12%, 7% and 81%, respectively. All sub-events had similar focal mechanisms with a vertical and a near-horizontal nodal plane, and all occurred within 5 km of each other at a similar depth, suggesting rupture on the near-horizontal nodal plane. However, directivity analyses indicate that the first sub-event ruptured unilaterally on the vertical plane. Its modeled rupture length of ∼9 km and stress drop of ∼2 MPa are typical of crustal earthquakes. In contrast, the following sub-events show no clear directivity. The third, best resolved, sub-event had a hypocenter ∼2 km from the first and a focal mechanism indistinguishable from the first, but it had a rupture dimension of 40 MPa. This requires an ambient stress field significantly greater than the stress drop of the first sub-event, implying that the first sub-event ruptured as a slip pulse with a transient weakening mechanism. The large stress drops of the second and third sub-events suggest a crack-like rupture without fault healing and with nearly total stress drop. Fault-zone melting and metastable olivine are viable mechanisms for these ruptures. In contrast, the rupture characteristics of the first sub-event seem incompatible with most mechanisms currently under consideration for deep-focus earthquakes.

Mantle reflectivity profiles obtained from long-period recordings of multiple-ScS reverberations from intermediate and deep-focus earthquakes surrounding China reveal remarkably sharp images of China deep structure. The Tibetan Plateau and fold belts to the north are underlain by a midlithospheric, low-velocity zone (LVZ) evidenced by coincident G (lid-LVZ boundary) and L' (base of the LVZ) reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north-south deformation. The mantle beneath northeastern China, Mongolia and southeastern Russia has a typically cratonic reflectivity signature with H (Hales) and deep L (Lehmann) discontinuities. The Sino-Korean Paraplatform of eastern China is a region of active extensional tectonics, as expressed by an “oceanic” reflectivity pattern at shallow depths (a G discontinuity at ∼80 km), but a more continental structure below. The southern Sea of Japan, Korean Peninsula, Yellow Sea and bordering regions of the China mainland are underlain by a ∼5.4% impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410-km discontinuity produced at the subduction zones to the east. Regional inversion of the reflection coefficient of the 410-km discontinuity, R(410), reveals a pronounced high beneath Mongolia and north to northwestern China which is best explained by increased upper mantle olivine content, signifying extensive basalt depletion in a tectosphere that extends below the 410-km discontinuity. A low value of R(660) associated with topographic depression of the 660-km discontinuity beneath eastern Asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity. Reflection defocusing is an alternative but requires ≥38 km of intermediate-scale topography within the depressed region, exceeding the amplitude of the depression by a factor of 1.5–2. Either mechanism requires strong heterogeneity far west of seismically active slabs. Two lower mantle reflectors (one beneath southern Honshu, Kyushu, and the Sea of Japan; the other below Shantung province (China) and the Yellow Sea) correlate with horizontally elongated velocity anomalies in western Pacific seismic tomography models. We speculate that they may represent slabs stagnating below the 660-km discontinuity.

A large (M w =7.1) deep earthquake (606 km) occurred beneath Sakhalin Island, about 200 km away from the nearest well-defined Wadati-Benioff zone in southwestern Kurile. Moment tensors obtained in various frequency bands using the teleseismic data show that the Sakhalin event produced double-couple radiation over a very broadband frequency range, and the P waveforms appear simple at most teleseismic stations (Δ=30-90 o ). On the other hand, many broadband seismometers successfully recorded the seismic waves at regional distances (Δ=3-25 o ) in Japan, showing two significant arrivals in the P wave seismograms whose time interval is about 3 s

Subduction along the Kuril-Kamchatka arc has been proposed as a primary example in which subducted lithosphere plunges steeply into the lower mantle with little deformation. The occurrence of several unusual earthquakes along this arc during the last few years places new constraints on the configuration and the state of strain of the down-going slab. We systematically determined the source parameters of 27 large- to moderate-sized earthquakes (mb≥5.5) that occurred between 1963 and 1990 at depths greater than 200 km along this subduction zone. To our surprise, our results show that the configuration and strain due to earthquakes in subducted lithosphere are highly variable along both the strike and dip of the arc. Approximately 75 km farther west of the deepest earthquakes in the northern portion of the arc, the earthquake of August 14, 1988, occurred at a comparable depth of 636±17 km. Thus at least part of the slab has been displaced in a direction perpendicular to the overall strike of the Wadati-Benioff zone. Earthquakes in the central part of the arc delineate shoaling of the subducted slab at a depth of approximately 570 km, reminiscent of the configuration of subducted lithosphere along the Tonga arc. Both observations indicate that the subducted slab is deforming, probably in response to resistance encountered near the 670-km discontinuity. For all earthquakes north of 48°N, focal mechanisms of all events deeper than 200 km show a remarkably consistent pattern of downdip compression (strain). In the Southern Kuril, the Sakhalin Island earthquake (M0∼8×1019N m)of May 12, 1990, is one of three large deep-focus earthquakes that have occurred over 150 km farther west of the well-defined Wadati-Benioff zone. The relationship between the source region of these events and the rest of the subducted slab is equivocal due to the lack of background seismicity. Nonetheless, for earthquakes south of 48°N, net downdip extension dominates between depths of 200–450 km before switching to downdip compression below 540 km. Thus the strain field within the slab is segmented both along strike and dip. If the slab in southern Kuril-Kamchatka can serve as a stress guide down to depths of 615 km, the changeover from extension to compression occurs at an unusually large depth within the transition zone of the upper mantle. Alternatively, the deep-focus events west of the island of Sakhalin could have occurred in a detached piece of lithosphere. As a whole, the configuration and state of strain of the subducted slab along the Kuril-Kamchatka arc are highly variable, close to a microcosm of variations among different deep subduction systems observed on a global scale. In particular, the subducted lithosphere is apparently undergoing considerable deformation as it approaches the base of the upper mantle. Our results do not preclude the possibility of subducted lithosphere ever reaching the lower mantle. Nevertheless, it seems unlikely that the down-going slab along the Kuril-Kamchatka arc penetrates into the lower mantle unhindered.

[1] The directivity method of Warren and Silver (2006) has been used to distinguish the fault planes of deep earthquakes in the Tonga and Middle America subduction zones. These studies identified exclusively subhorizontal fault planes between 100 and 300 km depth, raising the question of whether the observations represent new constraints on the physical mechanism of the earthquakes or a bias in the methodology. Here, the strengths, weaknesses, and biases of the method are investigated through the comprehensive analysis of 120 synthetic earthquakes with varying depth, rupture vector orientation, rupture complexity, signal-to-noise ratio, and station distribution. These synthetic tests, which allow the evaluation of the effect of each of the varied parameters on the rupture vector determination, show that fault planes can be identified for a wide variety of conditions. The method underestimates rupture velocities by 76%–94%, but this does not affect the determination of the orientation of the rupture vector. The most important parameter for determining the orientation of the rupture vector, and thereby allowing identification of the fault plane, is the distribution of stations recording the earthquake; better coverage around the earthquakes results in tighter constraints on the rupture vector. The broader distribution of recording stations for deeper earthquakes results in tighter constraints on the rupture vectors for deeper earthquakes and, therefore, more ease in identifying the fault plane. However, the difference in resolution does not systematically prohibit the identification of certain orientations of fault planes at shallower depths and suggests that previous observations of exclusively subhorizontal fault planes between 100 and 300 km depth are not the result of systematic bias in the methodology. Instead, the observed dominance of subhorizontal fault planes provides important constraints on the physical mechanism of intermediate-depth earthquakes. The observed subhorizontal fault orientation is inconsistent with the reactivation of the dominant trenchward-dipping outer rise normal faults.

We investigate the fault parameters of the 2005 Miyagi-Oki (Japan) earthquake using duration variations of teleseismic P-waves. The results show that the earthquake has a thrust-type mechanism and a seismic moment of 4.46 ¡_ 1019 Nm. Rupture directivity analysis suggests that the earthquake occurred as a result of a bilateral faulting on the fault plane with a strike of 247¢X, a dip of 17¢X and a slip of 125¢X. The optimal rupture azimuth, measured counterclockwise from the strike on the fault plane, is 170¢X (or 350¢X). The rupture length and average source duration are estimated to be 73.4 km and 14.5 sec, respectively. Thus the rupture velocity is 2.53 km sec-1 (~0.57 times the value of S-wave velocity), which is lower than the value for other similarly sized earthquakes. This implies that the 2005 Miyagi-Oki earthquake was probably a slow event. Consequently, there may have been less release of high-frequency seismic energy, leading to lower radiated seismic energy and radiation efficiency (~0.32 - 0.48). In other words, relatively larger fracture energy occurred during earthquake faulting in addition to the heat due to friction. The ratio of the static stress drop to the apparent stress (> 2.0) also suggests that the earthquake can be modeled as a frictional overshoot in a stress model, which implies the transformation of a lower percentage of strain energy into seismic-wave energy during the process of earthquake rupturing.

This paper describes a comprehensive study of the attenuation of Modified Mercalli intensity in New Zealand earthquakes, which has resulted in significantly different and new findings when compared with those of earlier work. The current study used recently revised magnitudes for the 30 events in the carefully selected data set. Magnitudes ranged from ML = 5.0 to Ms = 7.8. Special effort was also put into establishing the depths of the events, which ranged from very shallow to 65 km. An expression of a form also used in peak ground acceleration attenuation studies was adopted, I = a + bM + cr + dlog10r with r being taken as the mean distance from the centre of the fault rupture surface to each isoseismal. A two-step stratified regression analysis was used because it results in more realistic estimates of standard errors, although its mean curves were virtually the same as those derived from the standard one-step method. It was found possible to model accurately the attenuation of the deepest events from regressions of the data of the shallowest events, but the inverse was not true. In a study of the influence of source mechanisms it was found that the attenuation was the same for events with normal and strike-slip faulting. In contrast, events with reverse fault mechanisms were found to have higher intensities (for the same M and r) than normal and strike-slip events, by a factor corresponding to that found by Campbell in a study of peak ground accelerations, although the statistical test was narrowly short of the conventional significance level.

Publisher's version (útgefin grein), The May 25, 1987, Vatnafjöll earthquake occurred on the Mid‐Atlantic plate boundary, at a junction between a transform (the South Iceland Seismic Zone) and a spreading segment (the Eastern Volcanic Zone). The South Iceland Seismic Zone has had the most destructive earthquakes in the history of Iceland with average intervals of 80–100 years and estimated maximum magnitudes around 7. The 1987 Vatnafjöll earthquake was the largest earthquake (mb = 5.8; MS = 5.8; Mw = 5.9) in south Iceland since the 1912 magnitude 7 earthquake and is the first event sufficiently large to allow a meaningful study of source process in this geologically interesting region. Foreshocks and aftershocks of the Vatnafjöll earthquake were located with relatively good accuracy. They define an elongated N‐S epicentral area, 12 km long and 4 km wide with hypocentral depths mostly between 6 and 13 km. We determined centroid source parameters of the Vatnafjöll earthquake by inverting long‐period teleseismic P and SH waves. The centroid source mechanism and foreshock and aftershock distribution indicate N‐S right‐lateral strike‐slip faulting on a near‐vertical plane. The rupture initiated near the base of the crust and propagated upward along an approximately 10‐km‐long fault but did not reach the surface. The centroid depth is well constrained and is between 5 and 8 km depth at the 95% confidence level. The seismic moment is 9.1×1024dyn cm. The source has relatively short duration of 3 s, which may indicate higher stress drop than global average of earthquakes of this size. The South Iceland Seismic Zone is a 70‐km‐long E‐W striking transform zone between two overlapping rift zones, the Eastern and Western Volcanic Zones. It is characterized by “bookshelf” type of tectonics; that is, there is no surface faulting in the direction of the transform. Instead, N‐S right‐lateral strike‐slip faults are distributed along and perpendicular to the transform zone. The Vatnafjöll earthquake has characteristics of earthquakes in the South Iceland Seismic Zone and is probably not related directly with magmatic activity in the Eastern Volcanic Zone. The transform zone thus extends farther into the rift zone than was previously recognized., This research was supported by the NSF under grant EAR87-96171. Partial support also came from a NATO Science fellowship. LDGO contribution 4735

[1] Rupture velocity is an important source parameter, which is often difficult to determine, especially for deep-focus earthquakes where there is usually limited near-source information. To help overcome this problem, we developed a new method to estimate rupture velocities of deep-focus earthquakes with better resolution. We first carry out teleseismic P waveform inversions to determine slip distributions for a range of rupture velocities on the two nodal planes. Then forward modeling of regional data is performed using the slip distributions determined in the teleseismic inversions to estimate the rupture velocity. Using this method, we attempted to determine the rupture velocities of large deep-focus earthquakes surrounding Japan, which are well recorded on teleseismic and regional networks. Empirical Green functions are used for both the teleseismic and regional analyses. Although it is difficult to determine the rupture velocity from only the teleseismic data, the analyses including regional data show clear difference which can resolve the rupture velocity and fault geometry. For three deep earthquakes, we obtained rupture velocities of about 1∼2 km/s, which correspond to 20∼40% of the shear wave velocity and are much slower than typical values for shallow earthquakes.