Tonga Earthquake Research Articles

Stress Drops of Intermediate‐Depth and Deep Earthquakes in the Tonga Slab

AbstractMultiple physical mechanisms have been proposed to explain the cause of intermediate‐depth and deep earthquakes, but they are still under debate. Source parameters such as stress drop, have the potential to provide insight into their physical mechanisms. We develop a modified spectral decomposition method to analyze 1‐year seismic data from temporary land‐based and ocean bottom seismographs in a complex subduction zone. By applying this method to investigate 1,083 intermediate‐depth and deep earthquakes in the Tonga slab, we successfully resolve the source spectra and stress drops of 743 MW 2.6–6.0 earthquakes. Although the absolute stress drops are subject to the choices of source model parameters, the relative stress drops are more reliably resolved. The median stress drop of Tonga earthquakes does not change with respect to magnitude but decreases with depth by 2–3 times in two separate depth ranges of 70–250 and 400–600 km, corresponding to intermediate‐depth and deep earthquakes, respectively. The median stress drops show spatial variations, with two high‐stress‐drop (five times higher than the ambient value) regions, coinciding with strong local deformation where the Tonga slab bends or tears. In the Tonga double seismic zone at 120–300 km depths, the median stress drop appears smaller in the lower plane than in the upper plane, suggesting a slower rupture velocity or a higher fluid content in the lower‐plane region. Our results suggest that intermediate‐depth and deep earthquakes in the Tonga slab generally follow the earthquake self‐similar model and favor the fluid‐related embrittlement hypothesis for both groups of earthquakes.

Antipodal seismic reflections upon shear wave velocity structures within Earth's inner core

Seismic evidence is presented for a high shear wave velocity, apparent discontinuity near ~100 km depth within a portion of Earth's inner core. Antipodally (>179°) focused data are stacked for five source–receiver diametric ray paths traversing the inner core. Two antipodal paths follow ray surfaces which are aligned with diameters between Tamanrasset (TAM), Algeria and Tonga earthquakes and Pitinga (PTGA), Brazil and Sulawesi earthquakes, providing clear examples of precursors to PKIIKP (an underside reflection at the inner core boundary). Waveform and stacked data (T > 4 s) were engaged in testing more than 16 inner core model series, varying compressional and shear wave velocities in upper inner core structures. The precursory seismic phases are successfully modeled as reflecting beneath a core liquid/solid interface at 100 km depth below the inner core boundary. This interface is highly reflective, and sensitive to a shear wave velocity contrast ≥5 km/s. An earlier precursory phase is observed at TAM and PTGA which may be modeled as an apparent discontinuity near ~250 km depth. This intermediate region has high shear wave velocities more akin to hcp-Fe mineralogy than Preliminary Reference Earth Model (PREM) values. Three other antipodal observations (China–Chile) nearly orthogonal to TAM paths exhibit seismic waves whose waveforms are more consistent with the PREM velocities above 100 km depth, while offering modest evidence for a solid/solid discontinuity at 100-km-depth. This research focused on seismology of the inner core potentially has mineral physics and geodynamic implications too broad to be simply encapsulated herein. Acknowledging some of these implications, we have focused upon measuring and mapping the seismic anomalies.

High Bulk and Shear Attenuation Due to Partial Melt in the Tonga‐Lau Back‐arc Mantle

AbstractSeismic attenuation measures energy loss during seismic wave propagation and quantifies the relaxation of rocks' elastic moduli. Abundant seismic studies have observed shear attenuation, but few have quantified bulk attenuation (bulk modulus relaxation), and these studies controversially locate finite bulk attenuation in different parts of the Earth, from the asthenosphere to the inner core. Here we present conclusive evidence localizing bulk attenuation to a specific region of the uppermost mantle. By analyzing amplitude spectral decay of P and S waves from Tonga earthquakes recorded at local seismic stations, we observe unusually low QP/QS ratios (<1.5) in the Tonga‐Lau mantle wedge. All seismic raypaths with significant path‐average bulk attenuation (Qκ−1 > 0.01 or Qκ < 100) are confined to the region immediately beneath the Lau back‐arc spreading centers west of the Tonga Arc. Tomography results show that the highest bulk attenuation (Qκ−1 = 0.037 ± 0.008 or Qκ = 27.0 ± 0.2) is about 75% of the in situ shear attenuation. The observed high bulk attenuation anomalies coincide with an inferred zone of partial melting, suggesting that the causative mechanism involves either infinitesimal movement of melt through pores or changes in melt fraction in response to seismic waves.

Seismic detections of the 15 February 2013 Chelyabinsk meteor from the dense ChinArray

ChinArray is a dense portable broadband seismic network to cover the entire continental China, and the Phase I is deployed along the north-south seismic belt in southwest China. In this study, we analyze seismic data recorded on the ChinArray following the February 15, 2013 Chelyabinsk (Russia) meteor. This was the largest known object entering the Earth’s atmosphere since the 1908 Tunguska meteor. The seismic energy radiated from this event was recorded by seismic stations worldwide including the dense ChinArray that are more than 4000 km away. The weak signal from the meteor event was contaminated by a magnitude 5.8 Tonga earthquake occurred ~20 min earlier. To test the feasibility of detecting the weak seismic signals from the meteor event, we compute vespagram and perform F-K analysis to the surface-wave data. We identify a seismic phase with back azimuth (BAZ) of 329.7° and slowness of 34.73 s/deg, corresponding to the surface wave from the Russian meteor event (BAZ ~325.97°). The surface magnitude (M S) of the meteor event is 3.94 ± 0.18. We also perform similar analysis on the data from the broadband array F-net in Japan, and find the BAZ of the surface waves to be 316.61°. With the different BAZs of ChinArray and F-net, we locate the Russian meteor event at 58.80°N, 58.72°E. The relatively large mislocation (~438 km as compared with 55.15°N, 61.41°E by others) may be a result of the bending propagation path of surface waves, which deviates from the great circle path. Our results suggest that the dense ChinArray and its subarrays could be used to detect weak signals at teleseismic distances.

The 3 May 2006 (Mw 8.0) and 19 March 2009 (Mw 7.6) Tonga earthquakes: Intraslab compressional faulting below the megathrust

AbstractThe Tonga subduction zone is among the most seismically active regions and has the highest plate convergence rate in the world. However, recorded thrust events confidently located on the plate boundary have not exceeded Mw 8.0, and the historic record suggests low seismic coupling along the arc. We analyze two major thrust fault earthquakes that occurred in central Tonga in 2006 and 2009. The 3 May 2006 Mw 8.0 event has a focal mechanism consistent with interplate thrusting, was located west of the trench, and caused a moderate regional tsunami. However, long‐period seismic wave inversions and finite‐fault modeling by joint inversion of teleseismic body waves and local GPS static offsets indicate a slip distribution centered ~65 km deep, about 30 km deeper than the plate boundary revealed by locations of aftershocks, demonstrating that this was an intraslab event. The aftershock locations were obtained using data from seven temporary seismic stations deployed shortly after the main shock, and most lie on the plate boundary, not on either nodal plane of the deeper main shock. The fault plane is ambiguous, and investigation of compound rupture involving coseismic slip along the megathrust does not provide a better fit, although activation of megathrust faulting is responsible for the aftershocks. The 19 March 2009 Mw 7.6 compressional faulting event occurred below the trench; finite‐fault and W‐phase inversions indicate an intraslab, ~50 km deep centroid, with ambiguous fault plane. This event also triggered small megathrust faulting. There continues to be a paucity of large megathrust earthquakes in Tonga.

3‐D visualization of Tonga Earthquake

Scientists at the Scripps Institution of Oceanography (SIO; La Jolla, Calif.) have created interactive three‐dimensional (3‐D) visualizations of the 3 May magnitude 8.0 earthquake that occurred near Neiafu,Tonga. The earthquake occurred at 1526 UTC at a depth of 16 kilometers (as recorded by the USArray seismic network operated at SIO, http://anf.ucsd.edu). A tsunami warning was initially issued, but it was subsequently canceled, and to date, no fatalities have been reported.These 3‐D visualizations are available for free download at the Scripps Visualization Center Web site (http://www.siovizcenter.ucsd.edu).The first visualization shows the hypocenter of the 3 May Tonga main shock and locations of historical earthquakes. Global topography and bathymetry data are also included for reference. The second visualization shows a more localized view of the Tonga region, along with vertical cross sections of the velocity structure in the region. An obvious correlation can be seen between changes in the velocity structure and the historical earthquake locations, which map out the geometry of the Tonga subduction zone.

Detailed structure and sharpness of upper mantle discontinuities in the Tonga subduction zone from regional broadband arrays

Recordings of deep Tonga earthquakes from two arrays of 12 broadband seismographs each in the Fiji and Tonga islands are stacked and searched for reflections and conversions from upper mantle discontinuities in the Tonga subduction zone. The arrays operated as part of the Seismic Arrays in Fiji and Tonga (SAFT) experiment from July 2001 to August 2002. In comparison with the commonly used teleseismic approaches, the short path lengths for the local data provide smaller Fresnel zones and high‐frequency content for precise mapping of discontinuity topography and sharpness. To enhance the low‐amplitude discontinuity phases s410p, P660p and S660p, deconvolved seismograms from each event/array pair are aligned on the maximum amplitude of the direct P wave and subsequently slant stacked. For the 410‐km discontinuity, the results show no systematic variations in depth with distance to the cold slab. The 660‐km discontinuity varies between 656 and 714 km in depth. For the southern and central parts of the subduction zone, the largest depths occur in the core of the Tonga slab. For the northern part, two separate depressions of the 660‐km discontinuity are observed. These anomalies are interpreted as being induced by the active, steeply subducting Tonga deep zone and a subhorizontally lying remnant of subducted lithosphere from the fossil Vityaz trench, respectively. Interpreting the deflections of the 660‐km discontinuity in terms of local temperatures implies a thermal anomaly of −800°K to −1200°K at 660 km depth. Except for the southern region where it may thicken, the width of the depressed 660‐km discontinuity region implies that the Tonga slab seems to penetrate the 660‐km discontinuity with little deformation. Waveform modeling suggests that both the 410‐ and 660‐km discontinuities are sharp. The 660‐km discontinuity is at most 2 km thick in many parts of the region, and a first‐order discontinuity cannot be precluded. The 410‐km discontinuity thickness shows somewhat more variability and ranges from 2 to 10 km outside the slab and is at most 10 km thick within the slab. This suggests that the subduction process does not produce dramatic effects on the sharpness of the discontinuities.

Stress field in the subducting lithosphere and comparison with deep earthquakes in Tonga

We present a numerical model of the subducting lithosphere that provides an alternative explanation for stresses causing deep earthquakes. Our model lithosphere is composed of α olivine, β spinel, γ spinel, and perovskite + magnesiowüstite. The heat conduction equation is solved to determine temperature conditions in the slab and to locate the equilibrium phase transformations in pressure‐temperature space. Volumetric strains in the subducting lithosphere are calculated from the density of individual phases and from the heat released or consumed in the phase changes. These strains are used as sources of stress in the subducting lithosphere. Dislocation creep and Peierls stress creep laws are included in the viscoelastic rheology. Volumetric reductions due to equilibrium phase transformations cause high shear stress in the transition zone because of the variable viscosity inside the subducting slab. Aspects of the model shear stresses are in agreement with observations of high seismic activity in the Tonga‐Wadati Benioff zone. Compression is oriented along the dip of the slab, and extension is oriented in the plane perpendicular to the compression axis. Since our model stresses agree with the seismic observations, and because the model stresses are larger than those caused by buoyancy forces, our model provides a possible explanation of stresses causing deep earthquakes. Also, our model does not need metastability of olivine to explain the occurrence of high shear stress in the transition zone.

Spatio‐temporal variation of the tidal triggering effect on earthquake occurrence associated with the 1982 South Tonga earthquake of Mw 7.5

We observed a clear tidal triggering effect on earthquakes closely related to the occurrence of the 1982 South Tonga earthquake (Mw 7.5) by precisely measuring the correlation between the earth tide and earthquake occurrence using 385 shallow earthquakes of reverse‐fault type in the Tonga‐Kermadec subduction zone. The result of statistical analysis indicates that the tidal effect, having been predominant for the normal stress on the fault plane, concentrated in and near the future focal area for several years preceding the South Tonga earthquake. The high correlation disappeared after the earthquake. The frequency distribution of phase angles for the pre‐event period exhibited a peak at the phase angle where the tidal normal stress is at its maximum to accelerate the fault slip, which indicates that the observed high correlation is not a statistical chance but physically reasonable.

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.

Defocusing of teleseismic P‐waves by the Tonga‐Kermadec Slab

We analyze P‐wave amplitude reduction for Tonga earthquakes observed at the SKIPPY broadband seismograph network in Australia. The negative amplitude anomaly diminishes with a focal depth, suggesting that the anomaly is due to defocusing caused by the fast Tonga slab. The SKIPPY network covers the distance range where the effect of the Tonga slab is most pronounced. We calculate theoretical amplitudes using simplified slab models based on previous delay time tomography, and show that the theoretical amplitude anomaly reproduces the observations reasonably well. A 100‐km thick slab provides a better fit to the data than a 50‐km thick slab. A thin slab penetrating deep into the lower mantle is inconsistent with the pattern of amplitude variation with distance.

Depression of the 660 km discontinuity beneath the Tonga Slab determined from near‐vertical ScS reverberations

The occurrence of the March 9, 1994 (Mw 7.6) deep Tonga earthquake during a temporary deployment of broadband seismographs provides an excellent opportunity to study the seismic discontinuity structure in the vicinity of a subducting slab. Near vertical ScS reverberations from the 410, 520, and 660 discontinuities are visible at 10 stations in the South Pacific at frequencies between 0.003–0.033 Hz without stacking. We use a regional velocity model and ray theory to model the interval from ScS to ScS3. A grid search is then performed on the depth and impedance contrast at each of the seismic discontinuities to optimize the fit to both the underside and topside reflections. Stacked records indicate prominent discontinuities at 400 km and 665 km depth beneath and to the west of the slab, and 410 km, 500 km, and 640 km to the east of the slab. Analysis of individual records suggests a local depression of 30 km in the 660 and a broadening of the transition zone directly beneath and to the west of the Tonga slab, in a region of possible near horizontal slab. These results are consistent with a large cool region just to the west of the intersection of the Tonga slab with the 660.

Quasi‐Love phases between Tonga and Hawaii: Observations, simulations, and explanations

Seismograms of some shallow Tonga earthquakes observed at Hawaii contain SV‐polarized phases in the Love wave time window, most prominently on the vertical component. Given the geometry of the observations (Δ ≈ 40–45°), such phases may be explained either as body waves or as mode‐converted surface waves. Detailed synthetic seismogram modeling of representative events reveals several instances where the body wave explanation is inadequate, even when plausible uncertainties in the source mechanism are taken into account. The observed, SV‐polarized phase can instead be generated through Love‐Rayleigh scattering, which requires laterally varying seismic anisotropy along the Tonga‐Hawaii path. Trial‐and‐error forward modeling with simple structures based on the transversely isotropic mid‐Pacific velocity model PA5 of Gaherty et al [1996] obtains velocity structure that yields synthetic seismograms matching the observations. This model, while non unique, suggests first‐order constraints on the lateral variation in anisotropic properties, and associated mantle flow, along the Tonga‐Hawaii path. By examining trade‐offs in model parameters, we conclude that robust features of the model are: (1) a transition from radial to mixed radial and azimuthal anisotropy 3°–5° from Hawaii; (2) the NW‐SE alignment of the axis of azimuthal anisotropy; (3) higher degree of P anisotropy relative to S anisotropy; and (4) the presence of azimuthal anisotropy within upper 200–250 km of the mantle. Taken together, these features imply a disruption of mantle fabric by the processes forming Hawaii‐Emperor volcanic system. A model with anisotropic gradients in both the lithospheric lid and shallow asthenosphere is the simplest extension of our starting model. However, an equivalent data fit can be obtained if the azimuthal‐anisotropy gradients are restricted to line beneath the high‐velocity “lid” of model PA5, so that mantle hot spot flow need not penetrate the lithospheric lid.

Mantle discontinuities under southern Africa from precursors to P′ P′df

We investigate the reflection properties of upper‐mantle discontinuities beneath southern Africa using precursors to the df branch of PKPPKP (P′ P′). The P′ P′df branch is weaker than the ab and bc branches, but it does not have the complication of a caustic and appears across a wider distance range. Stacks from hundreds of short‐period seismograms recorded in California from the March 9, 1994 Tonga earthquake (Mw = 7.6) show an ∼5% reflection (at 3.5 s dominate period) from 660‐km depth indicating a sharp “660” under southern Africa. A 3.5 s period reflection from 410‐km depth is also visible in these stacks, but only ∼2% the strength of P′ P′df. This result contrasts with the observation of the “410” and the “660” reflecting comparable amounts of high‐frequency energy under the Indian Ocean [Benz and Vidale, 1993a], indicating either a diffuse “410” boundary under southern Africa or global variations in the impedance change across the “410”. A 1.5 s period reflection may indicate the existence of fine‐scale heterogeneity near 320‐km depth. Reflectivity synthetic seismograms also show that a previously claimed reflection from 785‐km depth has the more likely explanation as PcPPKP.

Aftershock sequences of moderate‐sized intermediate and deep earthquakes in the Tonga Subduction Zone

Intermediate and deep focus earthquakes are generally thought to produce few aftershocks. The two year operation of a network of broadband seismographs in the Tonga‐Fiji region provides an opportunity to monitor the aftershock activity of moderate‐sized earthquakes. In addition to the March 9, 1994 (Mw 7.6) event that shows 144 aftershocks, we found that all nine intermediate and deep earthquakes with Mw > 6.0 showed some aftershock activity, suggesting that aftershock sequences are ubiquitous. On average, deep Tonga earthquakes show an order of magnitude fewer aftershocks above a given magnitude than shallow California earthquakes of similar moment. The largest aftershocks of each sequence have an mb that is 2.0 units smaller on average than the Mw of the mainshock, whereas the California earthquakes show a mean difference of 1.1. Since b‐values of the deeper sequences appear to be similar to shallow sequences, the difference in aftershock productivity results from the larger size difference between the mainshock and the larger aftershocks. The most prolific sequences show an activity level not far below shallow earthquakes of the same moment. This is in contrast with observations of little aftershock activity for deep earthquakes in South America, Japan, and Kuriles, and suggests that the Tonga subduction zone is unusually prolific in aftershock production, perhaps due to cold slab temperatures.

Complex scattering within D″ observed on the very dense Los Angeles Region Seismic Experiment Passive Array

Several seismic phases that scattered within a few hundred kilometers of the base of the mantle are observed in a very dense seismic section. The Los Angeles Region Seismic Experiment passive phase array was composed of 88 seismometers placed along a 175 km profile. Records from two deep earthquakes in Tonga and one earthquake near Honshu, Japan show a secondary arrival between clear P and PcP arrivals. Modeling with layered structures shows that the Tonga and Honshu seismic sections are consistent with an increase in seismic velocity 140 and 240 km above the core‐mantle boundary, respectively, and a ≃10‐km thick low‐velocity zone at the base of the mantle beneath a region in the mid Pacific. Several of these arrivals are not coherent enough to appear in higher resolution stacks from the much larger Southern California Seismic Network. This experiment illustrates that fine‐scale passive array data can reveal small‐scale deep Earth structure invisible to larger‐scale seismic networks.

The March 9, 1994 (Mw 7.6), deep Tonga earthquake: Rupture outside the seismically active slab

We investigate the rupture process of the March 9, 1994, Mw 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.

A deep earthquake aftershock sequence and implications for the rupture mechanism of deep earthquakes

A distinguishing characteristic of deep earthquakes has been the absence of observable aftershock sequences1,2. Here we report the first extensive deep-earthquake aftershock sequence to be observed; it was recorded by an array of eight broadband seismographs following the 9 March 1994 deep Tonga earthquake. The aftershocks show a power-law decay with time following the main shock, as is typical of shallow events. Most of the well located aftershocks are concentrated along a steeply dipping plane consistent with one of the nodal planes of the main-shock mechanism and the mechanisms of three large aftershocks. Assuming these aftershocks denote the main-shock rupture area, they define a 50 × 65 km fault plane extending across the entire width of the active seismic zone and into the surrounding aseismic region. The width of the aftershock zone is wider than the expected width of the metastable olivine wedge, demonstrating that either the width of the metastable olivine material exceeds previous estimates, or the aftershocks are not confined in such a wedge.

S-Pconversion from the transition zone beneath Tonga and the nature of the 670 km discontinuity

SUMMARY Using more than 300 recordings of deep Tonga earthquakes from the Warramunga Seismic Array (central Australia), we have searched for short-period S-P conversions from the 670 km discontinuity arriving in the P-wave codas. The array seismograms were stacked and plotted in sections to optimize observation of S670P yielding the following results: For the northern part of Tonga (17.3°–18.4°S), S670P-phases are observed with apparent conversion depths between about 660 and 680 km. For the central part (20.2°–22.3°S), S670P is not clearly observed. For the southern part (23.1°–25.7°S), S670P is observed for conversion depths between about 660 and 700 km. Intermittent occurrence of S-P conversion may result from different characteristic source radiation patterns among the three sections. Overall uncertainties in conversion depths (resulting from unmodelled lateral velocity variations and hypocentral location errors) are perhaps a few tens of kilometres. The S-P rays must travel either within or very close to the subducted slab because of the source/receiver/slab-dip geometry, so we conclude that large (>50 km) deformations of the 670 km discontinuity do not occur beneath Tonga. Our observations appear to be inconsistent with a chemical discontinuity at 670 km, but are easily explained if the 670 km discontinuity is a strongly pressure-dependent and, perhaps, moderately endothermic phase change in both the slab and surrounding mantle.

Duration and depth of faulting of the 22 June 1977 Tonga earthquake

AbstractThe 22 June 1977 (Mw = 8.2) Tonga earthquake has the longest rupture duration ever reported for a normal fault event. The 150-km depth range spanned by aftershocks of the earthquake is also unusually large. There has been substantial controversy over both the depth and duration of faulting for this great event, obscuring its tectonic significance. We study the source process of the Tonga event using long-period Rayleigh waves recorded by the Global Digital Seismograph Network (GDSN) and International Deployment of Accelerometers (IDA) networks. For a standard assumption of a Haskell source, a total duration of 84 ± 4 sec is obtained using a least-squares inversion method. We introduce the use of the spectral amplitude as a weighting factor in measuring the misfit between the data and a given source finiteness model, which reduces the scatter and improves the resolution of source duration determined from data ranging in period from 150 to 300 sec. Using a more realistic shape for the source-time function in the inversion (drawing upon results from body-wave analysis) reveals a much longer (165-sec process time) component of the source process of the Tonga earthquake. The fundamental mode Rayleigh waves do not resolve any horizontal source directivity. However, the centroid depth of the earthquake is well resolved as 96 km with 90 per cent confidence range (93, 104 km). The estimated error in the depth determination due to the uncertainties in the source finiteness and earth models is only a few kilometers. The results indicate that the rupture of the earthquake excited long-period seismic waves at depths somewhat greater than the 70 to 80 km depth range where the primary body-wave radiation occurred, favoring rupture on the steeply dipping plane of the focal mechanism. The fundamental mode Rayleigh waves with periods longer than 150 sec cannot resolve vertical extent of the faulting; however, additional information from body-wave and free oscillation analyses indicates a vertical fault extent of about 50 km with a frequency-dependent variation in seismic radiation with depth.