Large Deep Earthquakes Research Articles

Multiple Observations of the Prompt Elastogravity Signals Heralding Direct Seismic Waves

AbstractThe recent first observations of the prompt elastogravity signals (PEGS) induced by the 2011 Mw = 9.1 Tohoku megathrust earthquake generated interest in how these signals might best be observed, especially for lower‐magnitude events. Simulations of these signals preceding the direct P wave, for different depths and focal mechanisms, first reveal that shallow strike‐slip earthquakes offer a better detection potential than subduction megathrust earthquakes. Consistently, clear PEGS are observed at several broadband seismometers during the 2012 Mw = 8.6 Wharton Basin earthquake. Due to their short source durations, large deep earthquakes are then shown to have an even larger detection potential, confirmed by the successful seismological observations for the 2018 Mw = 8.2 Fiji and 1994 Mw = 8.2 Bolivia earthquakes. Detection is even improved when an earthquake is recorded by a number of good‐quality stations, allowing for stacking techniques. Thanks to the deployment of the USArray network across Alaska, the recent 2018 Mw = 7.9 off‐Alaska earthquake (strike slip) is thus observed with an excellent signal‐to‐noise ratio. Array stacking is also shown to reveal the PEGS induced by the large 2010 Mw = 8.8 Maule megathrust earthquake, for which individual observations are impeded by the long‐lasting radiation generated by a distant large earthquake. As a whole, we show new observations and successful modeling of the PEGS for five earthquakes in the 7.9–8.8 magnitude range. These findings demonstrate that, even without considering promising future instruments, the PEGS detection is not restricted to exceptional events, confirming their potential for magnitude and focal mechanism determination within the few minutes following a large earthquake.

RevivingmB

We determine mB, the original body wave magnitude developed by Gutenberg and Richter over the period 1942–1956, for about 3300 Mw ≥ 6 earthquakes for the period 1988–present using modern broad-band seismograms. The main objective is to extend the database of energy-related parameters by combining mB databases for recent and old events. The radiated energy ER_B (in erg) computed from mB using the Gutenberg & Richter relation |$\log {E_{\mathrm{ R}\_\mathrm{ B}}} = 2.4{m_\mathrm{ B}} + 5.8$| agrees very well with ER estimated with modern techniques, especially for large deep earthquakes. Thus, ER_B is useful as a proxy for ER to investigate the global diversity of earthquake characteristics and physics over an extended period of time.

Big mantle wedge, anisotropy, slabs and earthquakes beneath the Japan Sea

The Japan Sea is a part of the western Pacific trench-arc-backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle. Local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the Japan Sea, which was determined using P and S wave arrival times of suboceanic earthquakes relocated precisely with sP depth phases. Ambient-noise tomography revealed a thin crust and a thin lithosphere beneath the Japan Sea and significant low-velocity (low-V) anomalies in the shallow mantle beneath the western and eastern margins of the Japan Sea. Observations with ocean-bottom seismometers and electromagnetometers revealed low-V and high-conductivity anomalies at depths of 200–300km in the big mantle wedge (BMW) above the subducting Pacific slab, and the anomalies are connected with the low-V zone in the normal mantle wedge beneath NE Japan, suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back-arc magmatism. The Pacific slab has a simple geometry beneath the Japan Sea, and earthquakes occur actively in the slab down to a depth of ∼600km beneath the NE Asian margin. Teleseismic P and S wave tomography has revealed that the Philippine Sea plate has subducted aseismically down to the mantle transition zone (MTZ, 410–660km) depths beneath the southern Japan Sea and the Tsushima Strait, and a slab window is revealed within the aseismic Philippine Sea slab. Seismic anisotropy tomography revealed a NW-SE fast-velocity direction in the BMW, which reflects corner flows induced by the fast deep subduction of the Pacific slab. Large deep earthquakes (M>7.0; depth>500km) occur frequently beneath the Japan Sea western margin, which may be related to the formation of the Changbai and Ulleung intraplate volcanoes. A metastable olivine wedge is revealed within the cold core of the Pacific slab at the MTZ depth, which may be related to the deep seismicity. However, many of these results are still preliminary, due to the lack of seismic stations in the Japan Sea. The key to resolving these critical geoscientific issues is seismic instrumentation in the Japan Sea, for which international cooperation of geoscience communities in the East Asian countries is necessary.

Recurring large deep earthquakes in Hindu Kush driven by a sinking slab

AbstractHindu Kush subduction zone produces large intermediate‐depth earthquakes within a small volume every 10–15 years. Here we study the last three M ≥ 7 events within the cluster and find complex and diverse rupture processes. However, their main subevents appear to recur on the same fault patch, dipping 70° to the south. This recurrence requires an average of 9.6 cm/yr slip rate on the patch, much higher than the ~1 cm/yr surface convergence rate measured geodetically. The high slip rate is likely caused by significant slab internal deformation, such as localized slab stretching/necking. We infer that the Hindu Kush subducted slab below 210 km is sinking through the mantle at a vertical rate of 10 cm/yr.

The isolated ∼680 km deep 30 May 2015 MW 7.9 Ogasawara (Bonin) Islands earthquake

Deep-focus earthquakes, located in very high-pressure conditions 300 to 700 km below the Earth's surface within sinking slabs of relatively cold oceanic lithosphere, are mysterious phenomena. The largest recorded deep-focus earthquake (MW 7.9) in the Izu–Bonin slab struck on 30 May 2015 beneath the Ogasawara (Bonin) Islands, isolated from prior seismicity by over 100 km in depth, and followed by only a few small aftershocks. Globally, this is the deepest (680 km centroid depth) event with MW≥7.8 in the seismological record. Seismicity indicates along-strike contortion of the Izu–Bonin slab, with horizontal flattening near a depth of 550 km in the Izu region and rapid steepening to near-vertical toward the south above the location of the 2015 event. This event was exceptionally well-recorded by seismic stations around the world, allowing detailed constraints to be placed on the source process. Analyses of a large global data set of P, SH and pP seismic phases using short-period back-projection, subevent directivity, and broadband finite-fault inversion indicate that the mainshock ruptured a shallowly-dipping fault plane with patchy slip that spread over a distance of ∼40 km with a multi-stage expansion rate (∼5+ km/s down-dip initially, ∼3 km/s up-dip later). During the 17 s total rupture duration the radiated energy was ∼3.3×1016 J and the stress drop was ∼38 MPa. The radiation efficiency is moderate (0.34), intermediate to that of the 1994 Bolivia and 2013 Sea of Okhotsk MW 8.3 deep earthquakes, indicating that source processes of very large deep earthquakes sample a wide range of behavior from dissipative, more viscous failure to very brittle failure. The isolated occurrence of the event, much deeper than the apparently thermally-bounded distribution of Bonin-slab seismicity above 600 km depth, suggests that localized stress concentration associated with the pronounced deformation of the Izu–Bonin slab and proximity to the 660-km phase transition likely played a dominant role in generating this major earthquake.

Rupture complexity of the 1994 Bolivia and 2013 Sea of Okhotsk deep earthquakes

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.

Changbai intraplate volcanism and deep earthquakes in East Asia: a possible link?

The origin of intraplate volcanoes in Northeast Asia is considered to be associated with upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone. Among these intraplate volcanoes, Changbai is the largest and most active one, and very deep earthquakes (500–650 km depths) in the Pacific slab under East Asia occur ∼300 km to the east of the Changbai volcano. Integrating the findings of geophysical, geochemical and petrologic studies so far, we suggest a link between Changbai volcanism and the deep earthquakes in the Pacific slab. Many large shallow earthquakes occurred in the Pacific Plate in the outer-rise areas close to the oceanic trench, and sea water may enter down to a deep portion of the oceanic lithosphere through the active normal faults which generated the large outer-rise earthquakes. Sea water or fluids may be preserved in the active faults even after the Pacific Plate subducts into the mantle. Many large deep earthquakes are observed that took place in the subducting Pacific slab under the Japan Sea and the East Asian margin. At least some of the large deep earthquakes are caused by the reactivation of faults preserved in the subducting slab, and the fluids preserved in the faults within the slab may cause the observed non-double-couple components of the deep earthquake faulting. Fluids preserved in the slab may be released to the overlying mantle wedge through large deep earthquakes. Because large deep earthquakes occur frequently in the vicinity of the Changbai volcano, many more fluids could be supplied to this volcano than in other areas in Northeast Asia, making Changbai the largest and most active intraplate volcano in the region.

Deep earthquake rupture histories determined by global stacking of broadband P waveforms

We conduct a systematic investigation of time histories of moment release in deep earthquakes, whose physical mechanism remains unknown. We have constructed source time functions (STFs) of 111 large deep earthquakes (depth ≥100 km and Mw ≥ 6.4) by stacking P waveforms from broadband seismograms recorded by the Global Seismographic Network. Aligning and stacking waveforms cancels noise and allows us to identify the rupture initiation and termination times accurately. Each STF is scaled to a reference moment of 1019 N m, using the duration‐moment relation τ ∝ M01/3. In order to investigate depth dependence of the STFs, we bin them into three depth ranges, 100–350 km, 350–550 km, and deeper than 550 km, based on physical properties of the mantle and the distribution of the data. We infer a change in character near 550 km. Earthquakes deeper than about 550 km are shorter, have simpler time functions, and are more uniform in their characteristics. Further, we find longer than expected scaled durations and greater complexity in the 350–550 km depth range. The scaled durations of the 350–550 km earthquakes do not decrease with depth as 1/VS, instead possessing a slightly longer average scaled duration than those 100–350 km deep. We further categorize the earthquakes by subduction zone and find a similar depth dependence within individual zones. Finally, the events that have achieved highest moment rate in the first 1–2 s after initiation tend to be deeper and/or larger events, the latter suggesting that dynamic rupture propagation affects the final size of the earthquake. The depth dependence of duration and complexity suggests more complicated rupture histories between 350 and 550 km and a possible change in the physical mechanism of rupture at about 550 km.

Source characteristics of large deep earthquakes: Constraint on the faulting mechanism at great depths

Source parameters for eight large deep earthquakes are inferred from broadband data of the Global Seismograph Network. We completed an unprecedented data set composed of 14 largest deep earthquakes (Mw ranging from 7.0 to 8.3) from six different subduction zones studied with identical source parameter inversion methods. The study confirms earlier findings that deep earthquake source properties vary with the temperature of the subducting plate. Events in cold slabs show high aftershock activity, high rupture velocity, and high seismic efficiency. In contrast, earthquakes in warm slabs generally have low aftershock productivity, low rupture velocity, and low seismic efficiency. However, the 23 June 1991 Argentina event has source properties that are rather characteristic for earthquakes in cold slabs, and that differ from those of other large South American events. We suggest that two types of deep earthquakes occur in South America, and we interpret these results in the context of the shear instability model for the mechanism of deep earthquakes. In this model, shear failure occurs through a thermal runaway mechanism within a shear zone at great depth, and low seismic efficiency might indicate significant energy dissipation due to heating and possible melt production. The 1991 Argentina event, located in a pocket of deep seismicity within the slab core, may represent a shear instability with little melting as would be the case for large deep earthquakes in cold subduction zones, whereas the other known South American large deep events such as 1963 Peru, 1970 Colombia and 1994 Bolivia, are located in relatively aseismic regions and may involve significant melting within warmer regions of the slab.

Seismological constraints on the mechanism of deep earthquakes: temperature dependence of deep earthquake source properties

Seismological observations are of vital importance for understanding the mechanism of deep earthquakes. Most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes. Deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms. Deep earthquake aftershock sequences show temporal decay rates and magnitude–frequency relations ( b-values) that are similar to shallow earthquakes, with the aftershocks occurring preferentially along the mainshock fault planes. In addition, deep earthquake rupture velocities are similar to those of shallow earthquakes. However, there are also some observations that are clearly distinctive relative to shallow earthquakes. Stress drops of deep earthquakes show a large variation but are larger, on average, than shallow earthquakes. Different deep seismic zones show very different b-values, in contrast to shallow earthquakes, which show similar b-values worldwide. In addition, deep earthquakes show fewer aftershocks than shallow earthquakes. A variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab. Both deep earthquake b-values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs, suggesting that these factors are temperature-controlled to an extent much greater than with shallow earthquakes. Large deep earthquakes in warm slabs show slower rupture velocities, larger stress drops, and lower seismic efficiencies than similar earthquakes in cold slabs. The width of deep seismic zones is also probably temperature controlled, but deep earthquake rupture can propagate outside the normal limits of Benioff zone seismicity. Simple thermal models for the Tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200°C warmer than the temperature that limits smaller earthquakes in the slab. These observations can be used to evaluate physical models for deep earthquakes, including brittle slip along fluid-weakened faults, transformational faulting, and thermal (and perhaps melt lubricated) shear instabilities. It is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model, since hydrated materials are expected in only a narrow depth zone at the top of the slab, and the fault planes do not have the expected orientations for reactivated faults. The lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model, in which events should be confined within a narrow metastable wedge. Seismological studies have also failed to find evidence for the existence of metastable olivine in slabs. The temperature dependence of deep earthquakes argues in favor of a temperature-activated phenomenon, such as thermal shear instabilities and perhaps fault zone melting.

Rupture of the Large (Mw 7.8), Deep Earthquake of 1973 Beneath the Japan Sea with Implications for Seismogenesis

The earthquake of 29 September 1973 beneath the western edge of Japan Sea is the largest deep earthquake along the western Pacific ( M w 7.8, focal depth 575 km). The source region is near the tip of the Japan Wadati-Benioff zone, where a continuous belt of seismicity clearly defines a unique, shallow-dipping (∼30°) slab down to depths about 600 km. This unique geometry of slab predicts a limited vertical extent available for earthquake rupture, in direct conflict with previous reports of a large, subvertical fault for the 1973 earthquake. Using a comprehensive data set of nearly 20 P and SH waveforms recorded at teleseismic distances, we investigate rupture associated with this event in some detail by inversion of waveforms. In particular, we use wave trains with all relevant depth phases ( pP, sP, and sS ) that are crucial for resolving the vertical extent of rupture. In addition to a northward component of rupture as in previous reports, we find clear evidence for a significant component of eastward rupture propagation that cannot fit within a subvertical, north–south striking nodal plane. Meanwhile, all regions of major slip are at essentially the same depth. Thus the true fault plane appears subhorizontal, with the entire region of slip extending only about 15 ± 6 km across the slab9s thickness. The spatio-temporal distribution of slip is well explained by a simple, circular rupture front, propagating at a speed of 3–4 km/sec. A comparison among the largest deep earthquakes shows that most rupture planes are more or less dimensionally equal, with high aspect ratios ranging approximately from 0.5 to 1. While some events have fairly restricted source regions, others seem to extend more than 30 km into the thickness of the slabs.

PKIKP...and Those Mysterious Precursors

Research Article| January 01, 2001 PKIKP...and Those Mysterious Precursors Larry J. Ruff Larry J. Ruff Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2001) 72 (1): 80–81. https://doi.org/10.1785/gssrl.72.1.80 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Larry J. Ruff; PKIKP...and Those Mysterious Precursors. Seismological Research Letters 2001;; 72 (1): 80–81. doi: https://doi.org/10.1785/gssrl.72.1.80 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search A simple question about one seismogram can open the door into the wonderful world of wave propagation. Seismographs in North America are far away from earthquakes that occur in the subduction zone along Indonesia, and thus the first arrivals from these events will be core phases—P waves that have traveled through the Earth's core. On 7 August 2000, there was a fairly large (Mw = 6.5) deep earthquake beneath the Banda Sea region of the Indonesia arc. This deep event is notable in producing sharp single-pulse P-wave seismograms (for example, see the source time function at... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Aftershocks of the March 9, 1994, Tonga earthquake: The strongest known deep aftershock sequence

The March 9, 1994 (Mw 7.6, depth 564 km), deep Tonga earthquake produced an exceptionally strong aftershock sequence for a deep earthquake. Using eight temporary broadband stations deployed on the Fiji, Tonga, and Niue islands, we identify 144 aftershocks with mb ranging from 3.2 to 6.0 during the first 41 days following the mainshock. The number and seismic moment of the March 9 aftershocks lie within the range of aftershock productivities of typical shallow earthquakes of similar moment. The aftershocks show a power law decay with time following the mainshock, with an exponent (p value) of −1.0, similar to shallow aftershock sequences. Fifty aftershocks can be accurately located using a hypocentroidal decomposition method; these aftershocks align along a near‐vertical ENE striking plane similar to one of the mainshock nodal planes. Sixteen of the 18 well‐located aftershocks during the first 2.5 hours are located to the NNE of the mainshock epicenter, consistent with NNE rupture propagation derived from waveform analysis. Later aftershocks are located south of the epicenter, suggesting aftershock expansion beyond the region of substantial moment release in the mainshock. Several of the aftershocks are located outside of the normal bounds of the seismically active slab, consistent with evidence that the mainshock rupture terminated outside the active slab. Five possible foreshocks occurred along the preferred fault plane within the 22 days prior to the mainshock. At least three “triggered” aftershocks occurred at distances of 50 to 110 km within the first six hours following the mainshock; triggered aftershocks in adjacent slab material may be common for large deep earthquakes. While there is some variation in the focal mechanisms of the aftershocks, most resemble the mainshock focal mechanism. All the mechanisms show compressional axes dipping to the west, consistent with background seismicity in this region. Overall, most of the aftershock characteristics of the 1994 Tonga event are similar to those found for shallow earthquakes, suggesting similarities between shallow and deep earthquakes in the mechanical processes that produce aftershocks.

A hybrid method for calculating the radiated wave energy of deep earthquakes

The energy radiated as waves is a parameter vital to the understanding of earthquake rupture physics. Calculations of this energy, however, are difficult and wrought with uncertainty. We developed a new hybrid method for the calculation of radiated wave energy and other key seismic parameters. The method calculates these parameters in an internally consistent and objective manner, allowing for robust comparison between parameters and across earthquakes. In addition, the method does not require deconvolution or filtering of data, thus results contain all recorded high frequencies. We use the method to calculate source–time functions, seismic moment, duration, and radiated wave energy for 19 recent, deep (depth>292 km) earthquakes ranging in size from M w=6.5 to 8.2. We compute effective seismic efficiency for the three largest events. The principle result is that the effective seismic efficiencies are less than 0.3. If earthquake rupture adhered to the Orowan model, then the average frictional stress during rupture is equal to the final shear stress on the fault plane, and the seismic efficiency is 1. But since the measured seismic efficiency is less than 1, the rupture process of these large deep earthquakes partitions more energy into friction and the average frictional stress during rupture is higher than the final fault plane stress.

Source and aftershock properties of the 1996 Flores Sea Deep Earthquake

The June 17, 1996 (Mw 7.8) Flores Sea event is one of the largest deep earthquakes, and also produced an energetic aftershock sequence, allowing study of the spatial relationships between moment release, aftershock occurrence, and slab geometry. The locations of the mainshock, rupture sub‐events, aftershocks, and background slab seismicity are determined using a relative location algorithm. The event occurred at the edge of a 150 km wide aseismic region that represents a tear in the lowermost part of the Indonesian slab. The aftershocks define a 90 km long region parallel to slab strike. Two other aftershocks locate 10–25 km outside the previously active slab. The mainshock propagated about 75 km eastward along an E‐W striking, southward dipping plane, with a rupture velocity of 3.5 km/s. There was a substantial change in focal mechanism during the rupture, with the fault plane strike changing by about 25°. Overall, the Flores Sea deep earthquake shows numerous large aftershocks, and a moderate rupture velocity (3.5 km/s) and stress drop (∼ 7‐20 MPa.) These characteristics are similar to the 1994 Tonga deep earthquake, but different from the 1994 Bolivia event, suggesting they may be typical of large deep earthquakes in colder slabs. Systematic differences in rupture parameters for deep earthquakes in warm and cold slabs suggest that the rupture mechanism is highly sensitive to temperature.

Determination of the isotropic component of deep focus earthquakes by inversion of normal-mode data

SUMMARY We determine the isotropic component of large deep earthquakes by inversion for their full six-component moment tensors using the normal-mode data. We show that it is possible to reduce the correlation between the isotropic component and other components and to determine the isotropic component independently by analyses of normal-mode data at periods longer than 500 s. We find no significant isotropic component for the earthquakes we studied; the magnitudes of the estimated isotropic components are comparable to the uncertainty due to the mislocation of the centroid. The magnitude of the isotropic component is at most 5 per cent of the deviatoric seismic moment if it exists. This suggests that, if a rapid phase change of mantle minerals occurs during the rupture of deep earthquakes, the mantle material that transformed would be confined within an extremely thin layer. We also conduct moment tensor inversion using body waves and surface waves in the period band between 20 and 500 s. We find that there is a relatively consistent non-double-couple component of the deviatoric moment tensor for the Japan Sea earthquake (1994 July 21). This is likely to be caused by source complexity

Transformation-induced faulting requires an exothermic reaction and explains the cessation of earthquakes at the base of the mantle transition zone

In many subduction zones, earthquakes occur continuously to the bottom of the mantle transition zone and stop abruptly at the endothermic phase change marking the top of the lower mantle, suggesting cause and effect. New experiments on polycrystals undergoing the ilmenite ↔ perovskite transformation show that transformation-induced faulting occurs only in the exothermic direction of the reaction. The close structural and thermodynamic analogies between the transformation studied here in CdTiO3 and those expected in the silicates of the earth's mantle argue strongly that these results are relevant to transformations occurring in subducting lithosphere. Moreover, consideration of the thermodynamics of this asymmetry show that this result should be a general phenomenon. If so, it follows that faulting by this mechanism cannot occur in the lower mantle, providing a natural explanation for the termination of earthquakes just before 700 km depth. If a ‘wedge’ containing metastable olivine exists at depth in subduction zones and is necessary for deep earthquakes, our results remove the constraint that this ‘wedge’ must taper to zero where the deepest earthquakes occur. The characteristics of the large deep earthquakes of 1994 and their aftershocks show that deep earthquakes can propagate beyond the zone of previous hypocenters and trigger aftershocks there, providing important new information about the mechanism of deep earthquakes and pointing to the need for additional experiments.

Metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere

Earth's deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km. This depth interval closely brackets the mantle transition zone, characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher‐pressure phases. Deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal, thermodynamic, and mechanical ferment inside slabs. Numerical simulations of reaction rates show that the olivine → spinel transformation should be kinetically hindered in old, cold slabs descending into the transition zone. Thus wedge‐shaped zones of metastable peridotite probably persist to depths of more than 600 km. Laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting. This instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser, finer‐grained phase. Hence in cold slabs, such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite. It is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs. This consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features. It explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone, where minerals in downgoing slabs should transform to spinel and ilmenite structures. The onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab. Even if a slab penetrates into the lower mantle, earthquakes should cease at depths near 700 km, because the seismogenic phase transformations in the slab are completed or can no longer occur. Substantial metastability is expected only in old, cold slabs, consistent with the observed restriction of deep earthquakes to those settings. Earthquakes should be restricted to the cold cores of slabs, as in any model in which the seismicity is temperature controlled, via the distribution of metastability. However, the geometries of recent large deep earthquakes pose a challenge for any such models. Transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics, including focal mechanisms indicating localized shear failure rather than implosive deformation, are so similar to those of shallow earthquakes. Finally, metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab. Such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and/or are detached from subducting slabs. Metastability in downgoing slabs could have considerable geodynamic significance. Metastable wedges would reduce the negative buoyancy of slabs, decrease the driving force for subduction, and influence the state of stress in slabs. Heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage, causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity. Because wedge formation should occur only for fast subducting slabs, it may act as a “parachute” and contribute to regulating plate speeds. Wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel, favoring the subsequent sinking of the slab into the lower mantle.

P′P′ Precursors Under Africa: Evidence for Mid-Mantle Reflectors

Observations of precursors to P′P′ from a recent exceptionally large deep earthquake in the Fiji Islands (moment magnitude = 7.6) at an array of broadband stations in California revealed mid-mantle reflectors near depths of 785 kilometers and 1200 kilometers under the southern African rift. Such observations, previously reported primarily in subduction zones, suggest that these reflectors may have a global character. Our analysis, which also indicated a sharp, uplifted 670-kilometer discontinuity, demonstrates the power of sparse regional broadband arrays for the study of weak, frequency-dependent features in deep-Earth structure.

The 1994 Bolivia and Tonga events: Fundamentally different types of deep earthquakes?

The 1994 Bolivia and Tonga events show large differences in source and aftershock properties. The Bolivia earthquake, characterized by high stress drop, slow rupture propagation, and a weak aftershock sequence, appears similar to other large deep earthquakes that are isolated from active seismic zones, including the 1970 Colombia earthquake. In contrast, the 1994 Tonga event showed stress drop, rupture propagation velocities, and an aftershock sequence similar to that found for shallow earthquakes. The aftershock sequence of the 1994 Tonga event is many times stronger than has been observed for any other deep earthquake. A survey of deep earthquake aftershocks suggests that aftershock occurrence is correlated with the overall subduction zone magnitude‐frequency relation (b‐value). The largest deep events occur in isolated regions lacking smaller earthquakes, or in subduction zones characterized by anomalously low b‐values. The 1994 Tonga event is an exception and is the largest deep earthquake to occur within an active subduction zone showing a large b‐value, perhaps explaining the strong aftershock sequence. The Bolivia and Tonga events may represent end members of a population of deep earthquakes showing exceptional diversity in source properties, or they may represent two fundamentally different types of deep earthquakes, perhaps resulting from different physical mechanisms.