Future Large Earthquake Analysis of Sulawesi Island Using Satellite Images and Moment Tensor Solution Approach

Sulawesi Island is the active tectonic region, where the tectonic architecture and potential earthquake sources until now remain largely unknown. The worst earthquake, an Mw 7.5 on September 28, 2018, in Palu, Indonesia, was caused catastrophic damage to life and property. The earthquake has highlighted the urgent need to raise knowledge of the cause of possible large future earthquakes and vulnerability. The main objective for this project is to create a thorough earthquake probabilistic hazard analysis map of the region, which is presently unavailable to better prepare for future earthquakes. The neotectonic and structural map was created using was supplemented with the 30-m resolution Shuttle Radar Topography Mission, Centroid Moment Tensor (CMT) solution, and seismologic data. The results demonstrate that faulting controls the geometry and the majority of these faults are active and capable of causing medium to large magnitude earthquakes with moment magnitudes ranging from 6.2 to 7.5 from 44 seismic sources. Our results show Sulawesi's northern deformation regimes have high seismicity risk and vulnerability. This study contributes a realistic seismic source for the Sulawesi neotectonic area particularly at the northwest, north, and east deformation regime, to understand the key large future earthquakes.

Estimation of the Tapered Gutenberg-Richter Distribution Parameters for Catalogs with Variable Completeness: An Application to the Atlantic Ridge Seismicity

The use of the tapered Gutenberg-Richter distribution in earthquake source models is rapidly increasing, allowing overcoming the definition of a hard threshold for the maximum magnitude. Here, we expand the classical maximum likelihood estimation method for estimating the parameters of the tapered Gutenberg-Richter distribution, allowing the use of a variable through-time magnitude of completeness. Adopting a well-established technique based on asymptotic theory, we also estimate the uncertainties relative to the parameters. Differently from other estimation methods for catalogs with a variable completeness, available for example for the classical truncated Gutenberg-Richter distribution, our approach does not need the assumption on the distribution of the number of events (usually the Poisson distribution). We test the methodology checking the consistency of parameter estimations with synthetic catalogs generated with multiple completeness levels. Then, we analyze the Atlantic ridge seismicity, using the global centroid moment tensor catalog, finding that our method allows better constraining distribution parameters, allowing the use more data than estimations based on a single completeness level. This leads to a sharp decrease in the uncertainties associated with the parameter estimation, when compared with existing methods based on a single time-independent magnitude of completeness. This also allows analyzing subsets of events, to deepen data analysis. For example, separating normal and strike-slip events, we found that they have significantly different but well-constrained corner magnitudes. Instead, without distinguishing for focal mechanism and considering all the events in the catalog, we obtain an intermediate value that is relatively less constrained from data, with an open confidence region.

Earthquake source characterization by machine learning algorithms applied to acoustic signals

Underwater seismic events generate acoustic radiation (such as acoustic-gravity waves), that carries information about the source and can travel long distances before dissipating. Effective early warning, emergency response, and information dissemination for earthquakes and tsunamis require a rapid characterisation of the fault properties: geometry and dynamics. In this work, we analysed hydrophone recordings of 201 earthquakes, located in the Pacific and the Indian Ocean, by employing acoustic signal processing and classification methods. The analysis allows identifying the type of earthquake (i.e. slip type, magnitude) and provides near real-time estimation of the effective properties of the fault dynamics and geometry. The results were compared against values reported by the Harvard Global Centroid Moment Tensor catalog (gCMT), revealing statistical significance between the extracted acoustic properties used to feed machine learning algorithms and the predicted slip and magnitude values.

Contact of the Samoan Plume with the Tonga Subduction from Intermediate and Deep-Focus Earthquakes

The Tonga subduction zone in the south-west Pacific is the fastest convergent plate boundary in the world with the most active mantle seismicity. This zone shows unique tectonic features including Samoan volcanic lineament of plume-driven origin near the northern rim of the Tonga subducting slab. The proximity of the Samoa hotspot to the slab is enigmatic and invokes debates on interactions between the Samoa plume and the Tonga subduction. Based on long-term observations of intermediate and deep-focus Tonga earthquakes reported in the Global Centroid Moment Tensor (CMT) catalog, we provide novel detailed imaging of this region. Accurate traveltime residua of the P- and S-waves recorded at two nearby seismic stations of the Global Seismographic Network are inverted for the P- and S-wave velocities and their ratio and reveal their pronounced lateral variations. In particular, they differ for the southern and northern parts of the Tonga subduction region. While no distinct anomalies are detected in the southern Tonga segment, striking low-velocity anomalies associated with a high Vp/Vs ratio are observed in the northern Tonga segment close to the Samoa plume. These anomalies spread through the whole upper mantle down to depths of ~ 600 km. Together with the fast extension of the northern back-arc Lau Basin, slab deformation and geochemical enrichment in the northern Tonga region, they trace deep-seated magmatic processes and evidence an interaction of the Tonga subduction with the Samoa plume.

Rapid Earthquake Source Description Using Variometric-Derived GPS Displacements toward Application to the 2019 Mw 7.1 Ridgecrest Earthquake

Using near-field high-rate Global Positioning System (GPS) displacements to invert for earthquake fault slips in real time has the potential to improve the accuracy of earthquake early warning or tsunami early warning. For such applications, real-time retrieval of high-accuracy GPS displacements is essential. Here, we report on rapid modeling of the 2019 Mw 7.1 Ridgecrest earthquake with real-time GPS displacements derived from a variometric approach with readily available broadcast ephemeris. This method calculates station variations in real time by differencing continuous phase observations and does not rely on precise orbit and clock information. The phase ambiguity is also removed, and thus the method does not suffer from a relatively long convergence time. To improve the accuracy of variometric displacements, we use a local spatial filter to decrease the influence of residual errors that cannot be removed completely by the time difference. We invert for the centroid moment tensor, static fault slips, and fault rupture process from the derived displacements. Our results show that all inverted models are available within about 65 s after the origin time of the earthquake and are comparable with models inverted by real-time precise point positioning displacements. This study highlights the great value of variometric displacements for the rapid earthquake source description with only broadcast ephemeris.

Bayesian Inference of Centroid Moment Tensors of the 2019 Ambon (Mw 6.5) Aftershock Earthquake Sequence, Indonesia: A Preliminary Result

The Ambon Mw 6.5 earthquake on September 26th, 2019, had contributed to give severe damages and significantly increased seismicity around Ambon Island and surrounding areas. Mainshock was followed by aftershocks with spatial distribution added to the impact of destructions in this region. We investigated aftershocks sequences to reveal the effect of mainshock toward the change in the in-situ stress field, including the possibility of the existing faults reactivation and the generation of aftershocks. We inferred centroid moment tensor (CMT) for significant aftershock events with Mw more than 4.0 using waveform data recorded from October 18th to December 15th, 2019. The aftershock focal mechanism was determined using the Bayesian full-waveform inversion code ISOLA-Obspy. This approach provides the uncertainty of the CMT model parameters. From ten CMT solution we had inferred in three seismic clusters, we found that majority of events have a strike-slip mechanism. Four events located on the south of the N-S trendings have a dextral strike-slip fault type, reflected the rupture of the mainshocks fault plane. Three events in the cluster of Ambon Island are dextral strike-slip, confirming the presence of the fault reactivation. Meanwhile, three CMT solutions in the north show the dextral strike-slip faulting and may belong to the mainshock main fault, connected with the cluster in the south.

Assessing the role of selected constraints in Bayesian dynamic source inversion: Application to the 2017 Mw 6.3 Lesvos earthquake

Summary A dynamic finite-fault source inversion for stress and frictional parameters of the Mw 6.3 2017 Lesvos earthquake is carried out. The mainshock occurred on June 12, offshore the southeastern coast of the Greek island of Lesvos in the north Aegean Sea. It caused 1 fatality, 15 injuries, and extensive damage to the southern part of the island. Dynamic rupture evolution is modeled on an elliptic patch, using the linear slip-weakening friction law. The inversion is posed as a Bayesian problem and the Parallel Tempering Markov Chain Monte Carlo algorithm is used to obtain posterior probability distributions by updating the prior distribution with progressively more constraints. To calculate the first posterior distribution, only the constraint that the model should expand beyond the nucleation patch is used. Then, we add the constraint that the model should reach a moment magnitude similar to that obtained from our centroid moment tensor inversion. For the final posterior distribution, 15 acceleration records from Greek and Turkish strong motion networks at near regional distances ($\approx 30 - 150$ km) in the frequency range of 0.05–0.15 Hz are used. The three posterior distributions are compared to understand how much each constraint contributes to resolving different quantities. The most probable values and uncertainties of individual parameters are also calculated, along with their mutual trade-offs. The features best determined by seismograms in the final posterior distribution include the position of the nucleation region, the mean direction of rupture (towards WNW), the mean rupture speed (with 68 per cent of the distribution lying between 1.4–2.6 km/s), radiated energy (12–65 TJ), radiation efficiency (0.09–0.38), and the mean stress drop (2.2–6.5 MPa).

Análisis 3D de la deformación y cinemática de la Fosa Mesoamericana en la confluencia entre las placas de Rivera y Cocos con las placas Caribe y Norteamericana

Este trabajo propone un modelo cinemático 3D de la confluencia de las placas tectónicas Norteamericana y Pacífica con las placas de Rivera y Cocos, centrado en la zona de convergencia de la Fosa Mesoamericana en el área de México. A partir del análisis de más de 1300 mecanismos focales de terremotos (M ≥ 5.5) obtenidos de la base de datos en abierto del programa Global Centroid-Moment-Tensor (CMT), se han aplicado diversas técnicas de geología estructural para obtener la distribución espacial de los tensores de deformación (ey, ex, ez) y del factor de forma (k’) de dichos tensores. Estas técnicas consisten en el estudio de los Diedros Rectos de los mecanismos focales y de la aplicación del Modelo de Deslizamiento, basados en la cinemática del deslizamiento sobre el plano de falla teórico asociado a cada sismo. Este análisis permite realizar un estudio tridimensional de la deformación mediante el agrupamiento espacial 3D de los diferentes tensores (análisis cluster), agrupando zonas homogéneas en estilos de deformación, junto con los diagramas triangulares de clasificación de mecanismos focales (Kaverina) en relación con la geometría de la subducción y de la presencia de flujos mantélicos. También se han calculado perfiles profundos de la deformación y del factor de forma (k’) en relación a la geometría de la subducción y las principales estructuras tectónicas. Los resultados muestran una zonación 3D influenciada por el ángulo de subducción y el aumento de la velocidad de convergencia hacia el SE, en relación al acoplamiento mecánico de la subducción, apareciendo menos terremotos inversos en la convergencia de la Placa de Rivera con la Placa Norteamericana, frente a la Placa de Cocos con la Placa Norteamericana. La geometría 3D propuesta es congruente con la distribución espacial de grandes terremotos, el ángulo de subducción, la presencia de corrientes mantélicas a 100 km de profundidad y la presencia de terremotos en fallamiento normal. Este modelo también podría explicar la presencia de terremotos anómalos como terremotos lentos y silenciosos en el Gap de Guerrero.

A Testable Worldwide Earthquake Faulting Mechanism Model

In this article, we present a simple model to forecast global focal mechanisms. This model is based on a simple discrete counting distribution of the global centroid moment tensor catalog, and it also includes, using a Bayesian scheme, the a priori information from the Anderson theory of faulting. Our model is tested in hindcasting mode against independent data of global large earthquakes with Ms≥7. We obtained statistically significant good agreement between model and data using consistency test, demonstrating that this simple model can satisfactorily forecast focal mechanisms at the global scale. The defined testing procedure can be used to test the model in prospective mode against future events. These forecasts may inform short- to long-term hazard quantifications that require a finite source characterization, as well as real-time source inversion algorithms.