Structural Geology and the Seismotectonics of the 2004 Great Sumatran Earthquake

The paper sets out a method for structural analysis of seismotectonic data using centroid moment tensors and associated hypocenters from the Global Centroid Moment Tensor project, here illustrated for aftershocks from the 2004 great Sumatran earthquake. We show that the Sumatran segments of the megathrust were subject to compression in a direction near to orthogonal with the margin trend, consistent with the effect of relative movement of the adjacent tectonic plates. In contrast, the crust above the Andaman Sea segments was subject to margin-orthogonal extension, consistent with motion toward the gravitational potential well accumulated due to prior lateral (westward) rollback of the subducting edge of the northward moving Indian plate. Since this potential well is largely defined by topography, this episode of margin-orthogonal extension is at least in part “gravity driven.” It did not last long. Within 15 months, an earthquake cluster across an Andaman Sea spreading segment showed a return to kinematics driven by relative plate motion. The transition can be explained if fluid activity temporarily reduced basal friction (or effective stress) but then led to healing so that the megathrust once again began to develop friction-locked segments. The influence of slab rollback is in developing a gravitational potential well facing the megathrust, hence drawing the overriding crust toward it in the immediate postrupture phase while the megathrust is in a weakened state. Plate tectonics dominates during interseismic gaps, once the megathrust heals, and regains frictional resistance.y. The authors acknowledge funding support from the Australian Research Council: Discovery Project DP120103554 “A unified model for the closure dynamics of ancient Tethys constrained by geodesy, structural geology, argon geochronology and tectonic reconstruction” and Linkage Project LP130100134 “Where to find giant porphyry and epithermal gold and copper deposits.” The research was also supported by the “Satellites, Seismometers and Mass Spectrometers” initiative within the Research School of Earth Sciences at ANU. Bob Engdahl is thanked for his support during earlier versions and for providing data

Seismic moment tensors from synthetic rotational and translational ground motion: Green's functions in 1-D versus 3-D

Seismic moment tensors are an important tool and input variable for many studies in the geosciences. The theory behind the determination of moment tensors is well established. They are routinely and (semi-) automatically calculated on a global scale. However, on regional and local scales, there are still several difficulties hampering the reliable retrieval of the full seismic moment tensor. In an earlier study, we showed that the waveform inversion for seismic moment tensors can benefit significantly when incorporating rotational ground motion in addition to the commonly used translational ground motion. In this study, we test, what is the best processing strategy with respect to the resolvability of the seismic moment tensor components: inverting three-component data with Green’s functions (GFs) based on a 3-D structural model, six-component data with GFs based on a 1-D model, or unleashing the full force of six-component data and GFs based on a 3-D model? As a reference case, we use the inversion based on three-component data and 1-D structure, which has been the most common practice in waveform inversion for moment tensors so far. Building on the same Bayesian approach as in our previous study, we invert synthetic waveforms for two test cases from the Korean Peninsula: one is the 2013 nuclear test of the Democratic People’s Republic of Korea and the other is an Mw  5.4 tectonic event of 2016 in the Republic of Korea using waveform data recorded on stations in Korea, China and Japan. For the Korean Peninsula, a very detailed 3-D velocity model is available. We show that for the tectonic event both, the 3-D structural model and the rotational ground motion, contribute strongly to the improved resolution of the seismic moment tensor. The higher the frequencies used for inversion, the higher is the influence of rotational ground motions. This is an important effect to consider when inverting waveforms from smaller magnitude events. The explosive source benefits more from the 3-D structural model than from the rotational ground motion. Nevertheless, the rotational ground motion can help to better constraint the isotropic part of the source in the higher frequency range.SD thanks Bayrische Forschungsallianz (BayFOR) for funding her internship at the Australian National University (ANU, grant: BayIntAn-LMU-2017-66). Further funding comes from the European Research Council (advanced grant to HI: ROMY, number: 339991)

The 20 May 2016 Petermann Ranges earthquake: centroid location, magnitude and focal mechanism from full waveform modelling

Ground velocity records of the 20 May 2016 Petermann Ranges earthquake are used to calculate its centroid-moment-tensor in the 3 D heterogeneous Earth model AuSREM. The global-centroid-moment-tensor reported a depth of 12 km, which is the shallowest allowed depth in the algorithm. Solutions from other global and local agencies indicate that the event occurred within the top 12 km of the crust, but the locations vary laterally by up to 100 km. We perform a centroid-moment-tensor inversion through a spatiotemporal grid search in 3 D allowing for time shifts around the origin time. Our 3 D grid encompasses the locations of all proposed global solutions. The inversion produces an ensemble of solutions that constrain the depth, lateral location of the centroid, and strike, dip and rake of the fault. The centroid location stands out with a clear peak in the correlation between real and synthetic data for a depth of 1 km at longitude 129.8° and latitude –25.6°. A collection of acceptable solutions at this centroid location, produced by different time shifts, constrain the fault strike to be 304 ± 4° or 138 ± 1°. The two nodal planes have dip angles of 64 ± 5° and 26 ± 4° and rake angles of 96 ± 2° and 77 ± 5°, respectively. The southwest-dipping nodal plane with the dip angle of 64° could be seen as part of a near vertical splay fault system at the end of the Woodroffe Thrust. The other nodal plane could be interpreted as a conjugate fault rupturing perpendicular to the splay structure. We speculate that the latter is more likely, since the hypocentres reported by several agencies, including the Geoscience Australia, as well as the majority of aftershocks are all located to the northeast of our preferred centroid location. Our best estimate for the moment magnitude of this event is 5.9. The optimum centroid is located on the 20 km surface rupture caused by the earthquake. Given the estimated magnitude, the long surface rupture requires only ∼4 km of rupture down dip, which is in agreement with the shallow centroid depth we obtained.This research was funded by the Australian Research Council Discovery Project DP140102533 and Linkage Project LP130100413. Calculations were performed on the ANU Terrawulf cluster, a computational facility developed with support from the AuScope initiative. AuScope Ltd is funded under the National Collaborative Research Infrastructure Strategy, an Australian Commonwealth Government Programme. This research/project was undertaken with the assistance of resources and services from the National Computational Infrastructure, which is supported by the Australian government

Resolvability of the centroid-moment-tensors for shallow seismic sources and improvements from modelling high-frequency waveforms

Shallow earthquakes in the depth range 0–30 km make up more than 60% of all world's earthquakes. However, resolving their seismic source parameters such as the depth and moment tensor components presents a challenge. Here, we investigate the effect of frequencies higher than 0.025 Hz on centroid‐moment‐tensor inversion for the earthquakes occurring in the top 10 km of the Earth's crust. For a synthetic source located at the depth of 1 km, the maximum amplitude of ground motion due to a vertical dip‐slip mechanism from the waveforms filtered at 0.01–0.15 Hz is about 1,400 times larger than that filtered at 0.01–0.025 Hz. We quantify the effect of this dramatic difference and other waveform differences by introducing the “balance of amplitudes” and “waveform similarity” functions for different depths and frequencies. They present a simple and fast way to estimate the resolvability of seismic sources at a given depth and frequency band. For the 20 May 2016, Mw = 5.9 Petermann Ranges earthquake in Central Australia analyzed at 0.01–0.025 Hz, a high uncertainty accompanies the estimated source parameters. When the frequency band is 0.01–0.15 Hz, the centroid depth is well constrained at 1 kmand the mechanism is a thrust fault striking ~314°N and dipping ~30°NE. These simulations require accurate Earth models. Our result, obtained at higher frequencies, is in a great agreement with various other studies that have been carried out for this earthquake and confirms a 20‐km long, shallow rupture.B. H. was supported by ARC Discovery Grant DP14010253

Centroid moment tensor catalogue using a 3-D continental scale Earth model: Application to earthquakes in Papua New Guinea and the Solomon Islands

Although both earthquake mechanism and 3-D Earth structure contribute to the seismic wavefield, the latter is usually assumed to be layered in source studies, which may limit the quality of the source estimate. To overcome this limitation, we implement a method that takes advantage of a 3-D heterogeneous Earth model, recently developed for the Australasian region. We calculate centroid moment tensors (CMTs) for earthquakes in Papua New Guinea (PNG) and the Solomon Islands. Our method is based on a library of Green's functions for each source-station pair for selected Geoscience Australia and Global Seismic Network stations in the region, and distributed on a 3-D grid covering the seismicity down to 50 km depth. For the calculation of Green's functions, we utilize a spectral-element method for the solution of the seismic wave equation. Seismic moment tensors were calculated using least squares inversion, and the 3-D location of the centroid is found by grid search. Through several synthetic tests, we confirm a trade-off between the location and the correct input moment tensor components when using a 1-D Earth model to invert synthetics produced in a 3-D heterogeneous Earth. Our CMT catalogue for PNG in comparison to the global CMT shows a meaningful increase in the double-couple percentage (up to 70%). Another significant difference that we observe is in the mechanism of events with depth shallower then 15 km and Mw < 6, which contributes to accurate tectonic interpretation of the region

Teleseismic Tomography When Stations Follow Profiles: Pitfalls and Remedies

Online Material: Discussion, figures of resolution matrix analy-sis of alternative cell geometries; alternative station distributions

Seismic tomography reveals a mid-crustal intrusive body, fluid pathways and their relation to the earthquake swarms inWest Bohemia/Vogtland

The region of West Bohemia/Vogtland in the Czech–German border area is well known for the repeated occurrence of earthquake swarms, CO2 emanations and mofette fields. We present a local earthquake tomography study undertaken to image the Vp and Vp/Vs structure in the broader area of earthquake swarm activity. In comparison with previous investigations, more details of the near-surface geology, potential fluid pathways and features around and below the swarm focal zone could be revealed. In the uppermost crust, for the first time the Cheb basin and the Bublák/Hartoušov mofette fields were imaged as distinct anomalies of Vp and Vp/Vs. The well-pronounced low-Vp anomaly of the Cheb basin is not continuing into the Eger rift indicating a particular role of the basin within the rift system. A steep channel of increased Vp/Vs is interpreted as the pathway for fluids ascending from the earthquake swarm focal zone up to the Bublák/Hartoušov mofette fields. As a new feature, a mid-crustal body of high Vp and increased Vp/Vs is revealed just below and north of the earthquake swarm focal zone. It may represent a solidified intrusive body which emplaced prior or during the formation of the rift system. We speculate that enhanced fluid flow into the focal zone and triggering of earthquakes could be driven by the presence of the intrusive body if cooling is not fully completed. We consider the assumed intrusive structure as a heterogeneity leading to higher stress particularly at the junction of the rift system with the basin and prominent fault structures. This may additionally contribute to the triggering of earthquakes

Upper-mantle P- and S-wave velocities across the Northern Tornquist Zone from traveltime tomography

This study presents P- and S-wave velocity variations for the upper mantle in southern Scandinavia and northern Germany based on teleseismic traveltime tomography. Tectonically, this region includes the entire northern part of the prominent Tornquist Zone which follows along the transition from old Precambrian shield units to the east to younger Phanerozoic deep sedimentary basins to the southwest. We combine data from several separate temporary arrays/profiles (276 stations) deployed over a period of about 15 yr and permanent networks (31 stations) covering the areas of Denmark, northern Germany, southern Sweden and southern Norway. By performing an integrated P- and S-traveltime analysis, we obtain the first high-resolution combined 3-D VP and VS models, including variations in the VP/VS ratio, for the whole of this region of study. Relative station mean traveltime residuals vary within ±1 s for P wave and ±2 s for S wave, with early arrivals in shield areas of southern Sweden and later arrivals in the Danish and North German Basins, as well as in most of southern Norway. In good accordance with previous, mainly P-velocity models, a marked upper-mantle velocity boundary (UMVB) is accurately delineated between shield areas (with high seismic mantle velocity) and basins (with lower velocity). It continues northwards into southern Norway near the Oslo Graben area and further north across the Southern Scandes Mountains. This main boundary, extending to a depth of at least 300 km, is even more pronounced in our new S-velocity model, with velocity contrasts of up to ±2–3 per cent. It is also clearly reflected in the VP/VS ratio. Differences in this ratio of up to about ±2 per cent are observed across the boundary, with generally low values in shield areas to the east and relatively higher values in basin areas to the southwest and in most of southern Norway. Differences in the VP/VS ratio are believed to be a rather robust indicator of upper-mantle compositional differences. For the depth interval of about 100–300 km, thick, depleted, relatively cold shield lithosphere is indicated in southern Sweden, contrasting with more fertile, warm mantle asthenosphere beneath most of the basins in Denmark and northern Germany. Both compositional and temperature differences seem to play a significant role in explaining the UMVB between southern Norway and southern Sweden. In addition to the main regional upper-mantle velocity contrasts, a number of more local anomaly features are also outlined and discussed

Attenuation tomography in West Bohemia/Vogtland

We present a three-dimensional (3-D) P-wave attenuation (Qp) model for the geodynamically active swarm earthquake area of West Bohemia/Vogtland in the Czech/German border region. Path-averaged attenuation t* is calculated from amplitude spectra of time windows around the P-wave arrivals of local earthquakes. Average t/t* value or Qp for stations close to Nový Kostel are very low (< 150) compared to that of stations located further away from the focal zone (increases up to 500 within 80 km distance). The SIMUL2000 tomography scheme is used to invert the t* for P-wave attenuation perturbation. Analysis of resolution shows that our model is well-resolved in the vicinity of earthquake swarm hypocenters. The prominent features of the model are located around Nový Kostel focal zone and its northern vicinity. Beneath Nový Kostel a vertically stretched (down to depth of 11 km) and a highly attenuating body is observed. We believe that this is due to fracturing and high density of cracks inside the weak earthquake swarm zone in conjunction with presence of free gas/fluid. Further north of Nový Kostel two highly attenuating bodies are imaged which could represent fluid channels toward the surface. The eastern anomaly shows a good correlation with the fluid accumulation area which was suggested in 9HR seismic profil

Upper-mantle velocities below the Scandinavian Mountains from P- and S- wave traveltime tomography

The relative traveltime residuals of more than 20 000 arrival times of teleseismic P and S waves measured over a period of more than 10 yr in five separate temporary and two permanent seismic networks covering the Scandinavian (Scandes) Mountains and adjacent areas of the Baltic Shield are inverted to 3-D tomograms of P and S velocities and the VP/VS ratio. Resolution analysis documents that good 3-D resolution is available under the dense network south of 64° latitude (Southern Scandes Mountains), and patchier, but highly useful resolution is available further north, where station coverage is more uneven. A pronounced upper-mantle velocity boundary (UMVB) that transects the study region is defined. It runs from SE Norway (east of the Oslo Graben) across the mountains to the Norwegian coast near Trondheim (around the Møre−Trøndelag Fault Complex), after which it follows closely along the coast further north. Seismic velocities in the depth interval 100−300 km change significantly across the UMVB from low relative VP and even lower relative VS on the western side, to high relative VP and even higher relative VS to the east. This main velocity boundary therefore also separates relatively high VP/VS ratio to the west and relatively low VP/VS to the east. Under the Southern Scandes Mountains (most of southern Norway), we find low relative VP, even lower relative VS and hence high VP/VS ratios. These velocities are indicative of thinner lithosphere, higher temperature and less depletion and/or fluid content in a relatively shallow asthenosphere. At first sight, this might support the idea of a mantle buoyancy source for the high topography. Under the Northern Scandes Mountains, we find the opposite situation: high relative VP, even higher relative VS and hence low VP/VS ratios, consistent with thick, dry, depleted lithosphere, similar to that in most of the Baltic Shield area. This demonstrates significant differences in upper-mantle conditions between the Southern and Northern Scandes Mountains, and it shows that upper-mantle velocity anomalies are very poor predictors of topography in this region. An important deviation from this principal pattern is found near the topographic saddle between the Southern and Northern Scandes Mountains. Centred around 64°N, 14°E, a zone of lower S velocity and hence higher VP/VS ratio is detected in the depth interval between 100 and 300 km. This ‘Trøndelag−Jämtland mantle anomaly’ (TJMA) is still interpreted as part of relatively undisturbed lithosphere of shield affinity because of high relative P velocity, but the relatively low VP/VS ratios indicate lower depletion, possibly higher fluid content, and most likely lower viscosity relative to the adjacent shield units. We suggest that this mantle anomaly may have influenced the collapse of the Caledonian Mountains, and in particular guided the location and development of the Møre−Trøndelag Fault Complex. The TJMA is therefore likely to have played an important role in the development of the ‘two-dome architecture’ of the Scandes Mountains