Improved Joint Constraints on Moment Tensors, Source Time Functions, and Uncertainty Quantification for Moderately Large Earthquakes

We determined the centroid moment tensor (CMT) solutions of earthquakes that occurred along the North Anatolian fault (NAF) beneath the Sea of Marmara and the Aegean Sea, using data obtained from Turkey’s broad-band seismograph network. The CMT solution of the 2014 Aegean Sea earthquake (Mw 6.9) represents a strike-slip fault, consistent with the geometry of the NAF, and the source-time function indicates that this event comprised several distinct subevents. Each subevent is considered to have ruptured a different fault segment. This observation indicates the existence of a mechanical barrier, namely a NAF segment boundary, at the hypocenter. CMT solutions of background seismicity beneath the Aegean Sea represent strike-slip or normal faulting along the NAF or its branch faults. The tensional axes of these events are oriented northeast–southwest, indicating a transtensional tectonic regime. Beneath the Sea of Marmara, the CMT solutions represent mostly strike-slip faulting, consistent with the motion of the NAF, but we identified a normal fault event with a tensional axis parallel to the strike of the NAF. This mechanism indicates that a pull-apart basin, marking a segment boundary of the NAF, is developing there. Because ruptures of a fault system and large earthquake magnitudes are strongly controlled by the fault system geometry and fault length, mapping fault segments along NAF can help to improve the accuracy of scenarios developed for future disastrous earthquakes in the Marmara region.

The purpose of this research was to suggest an applicable procedure for computing the centroid moment tensor (CMT) automatically and in real time from earthquakes that occur in Indonesia and the surrounding areas. Gisola software was used to estimate the CMT solution by selecting the velocity model that best suited the local and regional geological conditions in Indonesia and the surrounding areas. The data used in this study were earthquakes with magnitudes of 5.4 to 8.0. High-quality, real-time broadband seismographic data were provided by the International Federation of Digital Seismograph Networks Web Services (FDSNWS) and the European Integrated Data Archive (EIDA) Federation in Indonesia and the surrounding areas. Furthermore, the inversion process and filter adjustment were carried out on the seismographic data to obtain good CMT solutions. The CMT solutions from Gisola provided good-quality solutions, in which all earthquake data had A-level quality (high quality, with good variant reduction). The Gisola CMT solution was justified with the Global CMT (GCMT) solution by using the Kagan angle value, with an average of approximately 11.2°. This result suggested that the CMT solution generated from Gisola was trustworthy and reliable. The Gisola CMT solution was typically available within approximately 15 minutes after an earthquake occurred. Once it met the quality requirement, it was automatically published on the internet. The catalog of local and regional earthquake records obtained through this technology holds great promise for improving the current understanding of regional seismic activity and ongoing tectonic processes. The accurate and real-time CMT solution generated by implementing the Gisola algorithm consisted of moment tensors and moment magnitudes, which provided invaluable insights into earthquakes occurring in Indonesia and the surrounding areas.

Recently, a widespread and densely continuous‐recording ocean‐bottom seismograph network has been deployed in the Japan Trench subduction zone. Utilizing the offshore network data improves azimuthal station coverage for offshore earthquakes in the Japan Trench subduction zone. It has a potential to obtain centroid moment tensor (CMT) solutions more accurately than conventional analyses using onshore networks and a simple one‐dimensional seismic velocity structure model. In this study, we conducted CMT inversion for subduction zone earthquakes that occurred between 1 April 2017, and 31 March 2024, with a moment magnitude range of 5.2–7.0. We used seismograms obtained from both the offshore and onshore networks. We calculated Green's functions using a three‐dimensional seismic velocity structure model. Our CMT solutions with thrust‐type mechanisms mostly indicated depths and dip angles consistent with the plate interface. For earthquakes in the outer‐rise region, our CMT solutions were characterized as normal‐fault mechanisms. The joint use of the offshore and onshore networks reduced the estimation errors of the CMT solutions compared with the only use of the onshore network, although the optimal solutions were consistent. The dip angles for the thrust earthquakes determined by our analysis were more consistent with the dip angle of the plate boundary than those determined by conventional CMT analyses. Additionally, we found that the conventional CMT analysis could introduce a systematic bias in depth and magnitude determinations. This finding highlights the importance of an offshore seismograph network and a reliable seismic velocity structure model for CMT inversions.

Harvard Centroid Moment Tensor (CMT) solutions for earthquakes from 1977 to 2004 showed that the stress fields are obviously different in northwestern Sichuan sub-block (NWSSB), western parts of Central Yunnan sub-block (CYSB) and eastern part of CYSB. The characteristics of the mean stress fields in these three regions are obtained by fitting to CMT solutions. The stress state in NWSSB is characterized by its sub-horizontal tensile principal axis of stress (T axis) in roughly N-S direction and west dipping compressive principal axis of stress (P axis); the one in western part of CYSB is characterized by its ENE dipping T axis and sub-horizontal medium principal axis of stress (B axis) in roughly N-S direction; the one in eastern part of CYSB is characterized by its sub-horizontal P axis in roughly NNW-SSE direction and sub-horizontal T axis in roughly WSW-ENE direction. Finite element method simulation clearly shows that the Indian Plate imposes great extrusion on Sichuan-Yunnan rhombic block (SYRB) near Assam massif. The value of the simulated compressive principal stress decreases with the distance from Assam massif. The simulated directions of the T axes in SYRB form annular distribution encircling Assam. For a homogeneous elastic medium with free boundary conditions on the top and bottom surfaces as well as the displacement boundary conditions derived from the GPS observations on the lateral boundaries, the computation results are consistent with the Harvard CMT solutions in NWSSB and western part of CYSB, while inconsistent with the Harvard CMT solutions in eastern part of CYSB. The inconsistency in eastern part of CYSB can be reduced when it includes inhomogeneous elastic media. The stress states in NWSSB and western part of CYSB revealed by the Harvard CMT solutions are not local, which are mainly controlled by the boundary force on the whole region. On the other hand, the stress state in eastern part of CYSB given by the Harvard CMT solutions is local, which may be affected by local topography, material inhomogeneity, and the drag force underneath.

Abstrak Penelitian ini dilakukan dengan tujuan untuk menganalisis Centroid Moment Tensor (CMT) gempa di sekitar Kepulauan Mentawai menggunakan metode Time Domain Moment Tensor (TDMT) dengan software MTTime. Dalam penelitian ini digunakan data sekunder gempa bumi 13 Maret 2022 dengan Mw 6,7 dan 10 September 2022 dengan Mw 6,2. Data yang didownload secara otomatis menggunakan menggunakan Obspy dari data center IRIS dengan jaringan GEOFON milik lembaga seismologi Jerman. Analisis CMT dengan metode TDMT menghasilkan parameter CMT yang meliputi strike, dip, rake, origin time, latitude, longitude, kedalaman centroid, diagram beachball, dan peta solusi CMT. Hasil analisis CMT juga diperoleh nilai komponen Double-Couple (DC), Compensated Linear Vector Dipole (CLVD) dan Isotropik (ISO) dimana dari komponen tersebut dapat diketahui sumber penyebab gempa bumi. Hasil dari penelitian untuk kedua event gempa bumi dihasilkan persentase DC yang lebih dominan dari komponen CLVD dan ISO. Hal tersebut menunjukkan bahwa sumber penyebab dari gempa bumi pada tanggal 13 Maret 2022 dan 10 September 2022 adalah aktivitas tektonik, serta persentase Variance Reduction (VR) dari setiap event diperoleh nilai lebih dari 50% sehingga hasil inversi waveform dari kedua event tersebut dikatakan reliable. Mekanisme sumber penyebab gempa bumi pada tanggal 13 Maret 2022 dan 10 September 2022 diakibatkan pergerakan patahan dengan tipe reverse, sehingga dapat diketahui bahwa pergerakan patahan di sekitar Kepulauan Mentawai memiliki mekanisme reverse. Hasil solusi CMT dari software MTTime cukup akurat ketika dibandingkan dengan Global CMT sehingga dapat dikembangkan untuk penentuan CMT secara real-time. Kata Kunci: Centroid Moment Tensor, Software MTTime, Kepulauan Mentawai, Inversi Waveform Abstract This research aims to analyze the Centroid Moment Tensor (CMT) of earthquakes around the Mentawai Islands using the Time Domain Moment Tensor (TDMT) method with MTTime software. The study utilizes secondary data from the 13 March 2022 earthquake with a magnitude of 6.7 and the 10 September 2022 earthquake with a magnitude of 6.2. Data were automatically downloaded using Obspy from the IRIS data center with the GEOFON network owned by the German seismological institute. CMT analysis with the TDMT method produces CMT parameters, including strike, dip, rake, origin time, latitude, longitude, centroid depth, beachball diagram, and CMT solution map. The results of the CMT analysis also obtained the values of the Double-Couple (DC), Compensated Linear Vector Dipole (CLVD), and Isotropic (ISO) components, which help determine the earthquake source. The analysis of both earthquake events revealed a higher percentage of DC compared to the CLVD and ISO components. This indicates that the sources of the earthquakes on 13 March 2022 and 10 September 2022 were tectonic activity. The Variance Reduction (VR) percentage for each event was over 50%, indicating that the waveform inversion results for both events are reliable. The source mechanism of the earthquakes on 13 March 2022 and 10 September 2022 is attributed to reverse fault movements, indicating that the fault movements around the Mentawai Islands have a reverse mechanism. The CMT solution results from the MTTime software are quite accurate when compared with the Global CMT, suggesting its potential for real-time CMT determination. Keywords: Centroid Moment Tensor, MTTime Software, Mentawai Islands, Waveform Inversion

A comprehensive set of teleseismic waveforms from two South American deep‐focus earthquakes of the predigital era, the 1970 Colombia (Mw = 8.1) and 1963 Peru‐Bolivia (Mw = 7.7) events, are inverted for source mechanism, seismic moment, rupture history, and centroid depth. The P and SH wave inversion of the Colombia event confirms previous work, indicating that rupture occurred on a plane that dips steeply west. Rupture direction paralleled the trend of the Wadati‐Benioff zone. We decompose the source into subevents, based on a source time function which shows two major moment release pulses separated by ∼20 s. The first subevent is located near the initiation point at a depth of ∼630 km. The main moment release was located ∼70 km to the southeast and ∼20 km shallower. Rupture subsequently propagated farther southeast. The source time function has an initial subevent accounting for ∼30% of the moment release of the entire event, whereas the long‐period centroid moment tensor (CMT) analysis [Russakoff et al., 1997] has the initial subevent yielding ∼50%. The high‐angle nodal plane rotated ∼15° clockwise during the rupture, explaining the large compensated linear vector dipole (CLVD) component inferred from CMT solutions. Individual subevents have large CLVD and compressive isotropic components. A full moment tensor inversion of the Colombia and 1994 Bolivia events suggests that the initial subevents might contain a large non‐double‐couple (NDC) component. For the 1963 Peru‐Bolivia event, using P waves, rupture propagated NNW for a distance of ∼70 km, parallel to the high‐angle nodal plane and the trend of the Wadati‐Benioff zone. The focal mechanism changed dramatically after the second subevent, causing a very large NDC component. Both events, together with the 1994 Bolivia earthquake, have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km/s between subevents, with <2.0 km/s on average for the entire event. Low seismic efficiencies and rupture velocities support a highly dissipative, temperature‐dependent rupture mechanism for large deep‐focus South American earthquakes, compared with events in cold subducting slabs.

The 1977 Three Kings Ridge earthquake ( Ms = 6.7): broad-band aspect of the source rupture

Rapid characterization of the earthquake source and of its effects is a growing field of interest. Until recently, it still took several hours to determine the first-order attributes of a great earthquake (e.g. M_w ≥ 7.5), even in a well-instrumented region. The main limiting factors were data saturation, the interference of different phases and the time duration and spatial extent of the source rupture. To accelerate centroid moment tensor (CMT) determinations, we have developed a source inversion algorithm based on modelling of the W phase, a very long period phase (100–1000 s) arriving at the same time as the P wave. The purpose of this work is to finely tune and validate the algorithm for large-to-moderate-sized earthquakes using three components of W phase ground motion at teleseismic distances. To that end, the point source parameters of all M_w ≥ 6.5 earthquakes that occurred between 1990 and 2010 (815 events) are determined using Federation of Digital Seismograph Networks, Global Seismographic Network broad-band stations and STS1 global virtual networks of the Incorporated Research Institutions for Seismology Data Management Center. For each event, a preliminary magnitude obtained from W phase amplitudes is used to estimate the initial moment rate function half duration and to define the corner frequencies of the passband filter that will be applied to the waveforms. Starting from these initial parameters, the seismic moment tensor is calculated using a preliminary location as a first approximation of the centroid. A full CMT inversion is then conducted for centroid timing and location determination. Comparisons with Harvard and Global CMT solutions highlight the robustness of W phase CMT solutions at teleseismic distances. The differences in M_w rarely exceed 0.2 and the source mechanisms are very similar to one another. Difficulties arise when a target earthquake is shortly (e.g. within 10 hr) preceded by another large earthquake, which disturbs the waveforms of the target event. To deal with such difficult situations, we remove the perturbation caused by earlier disturbing events by subtracting the corresponding synthetics from the data. The CMT parameters for the disturbed event can then be retrieved using the residual seismograms. We also explore the feasibility of obtaining source parameters of smaller earthquakes in the range 6.0 ≤_Mw < 6.5. Results suggest that the W phase inversion can be implemented reliably for the majority of earthquakes of Mw= 6 or larger.

We present a procedure for rapid determination of an earthquake moment magnitude, M wp, at regional distances from the P ‐wave first arrivals based on Tsuboi et al. (1995, 1999) and Kanjo et al. (2006). We apply an automated window‐length selection and use a variable P ‐wave velocity instead of a constant value to make a better estimation for M wp magnitude. This procedure is applied to 46 earthquakes with magnitude ( M w) greater than 4.8, which have been recorded and analyzed for Global Centroid Moment Tensor (Global CMT; the Global Centroid Moment Tensor Project database was searched using www.globalcmt.org/CMTsearch.html [last accessed July 2012]) solutions between 2008 and 2011 by the Prime Ministry Disaster and Emergency, Presidency National Earthquake Department (AFAD). Our results show good agreement between the M wp and M w magnitudes (about ±0.27 mu), and we expect this procedure can be used for the rapid determination of moment magnitudes of regional earthquakes. Effective emergency response for large regional destructive earthquakes requires accurate information about the size and location of event within a few minutes. The 23 October 2011 M w 7.1 Van, Turkey earthquake was a destructive regional earthquake that needed rapid determination of the moment magnitude because the local ( M L) and duration ( M D) magnitudes determined by national network operators were underestimated. The widely used M wp moment magnitude algorithm (Tsuboi et al. , 1995, 1999) considers very broadband, P ‐wave displacement seismogram to approximate far‐field source time functions. These displacement seismograms are integrated and corrected for geometrical spreading and an average radiation pattern to obtain scalar moments at each station. Application of the standard moment magnitude formula, averaging over stations, and applying a magnitude correction (Whitmore et al. , 2002), gives a moment magnitude, M wp, for an event. We introduce a procedure to …

The development of real-time tsunami forecast and rapid tsunami warning systems is crucial in order to mitigate tsunami disasters. The present study shows that tsunami prediction from a seismic centroid moment tensor (CMT) solution would work satisfactorily for the 2013 Santa Cruz Islands earthquake (Mw 8.0) tsunami even though the earthquake source had been modeled as a complicated source characterized by two patches of slip in a past study. We numerically solved the equations for a linear dispersive wave on a spherical coordinate system from the initial tsunami height distribution derived from the CMT solution and a classical scaling law for earthquake faults. The tsunami simulations well explain the observed tsunami arrival times, polarities of initial wave, and maximum amplitudes obtained by deep-ocean pressure measurements. The comparison of the simulation results from dispersive and non-dispersive modeling indicates that the dispersive modeling reproduced the observed waveforms better than the conventional non-dispersive approach. Also, the area affected by a maximum height greater than 0.4 m is decreased by approximately 34% by using dispersion modeling. Those results indicate that the tsunami prediction based on CMT solutions is useful for early warning, and the modeling of dispersion can significantly improve performance.

We performed nonlinear waveform inversion for source depth, time function, and mechanism, by modeling direct P and S waves and corresponding surface reflections at teleseismic distances. This technique was applied to moderate size events, and so we make use of short period or broadband records, and utilize SV waveforms in addition to P and SH . For the inversion we used a direct search method called the neighborhood algorithm (NA), which requires just two control parameters to guide the search in a conceptually simple manner, and is based on the rank of a user-defined misfit measure. We use a simple generalized ray scheme to calculate synthetic seismograms for comparison with observations, and show that the use of a derivative-free method such as the NA allows us to easily substitute more complex synthetics if necessary. The source mechanism is represented in two different ways; the superposition of a double-couple component with an isotropic component, and a general moment tensor with six independent components. Good results are obtained with both synthetic input data and real data. We achieve good depth resolution and obtain useful constraints on the source-time function and source mechanism, including an isotropic component estimate. Such estimates provide important discriminants between man-made events and earthquakes. We illustrate inversion with real data using two earthquakes, and in both cases the source parameter estimates compare well with the corresponding centroid moment tensor solutions. We also apply our technique to a known nuclear explosion and obtain a very shallow depth estimate and a large isotropic component.

A catastrophicMw7 earthquake ruptured on 12 January 2010 on a complex fault system near Port‐au‐Prince, Haiti. Offshore rupture is suggested by aftershock locations and marine geophysics studies, but its extent remains difficult to define using geodetic and teleseismic observations. Here we perform the multitaper multiple signal classification (MUSIC) analysis, a high‐resolution array technique, at regional distance with recordings from the Venezuela National Seismic Network to resolve high‐frequency (about 0.4 Hz) aspects of the earthquake process. Our results indicate westward rupture with two subevents, roughly 35 km apart. In comparison, a lower‐frequency finite source inversion with fault geometry based on new geologic and aftershock data shows two slip patches with centroids 21 km apart. Apparent source time functions from USArray further constrain the intersubevent time delay, implying a rupture speed of 3.3 km/s. The tips of the slip zones coincide with subevents imaged by backprojections. The different subevent locations found by backprojection and source inversion suggest spatial complementarity between high‐ and low‐frequency source radiation consistent with high‐frequency radiation originating from rupture arrest phases at the edges of main slip areas. The centroid moment tensor (CMT) solution and a geodetic‐only inversion have similar moment, indicating most of the moment released is captured by geodetic observations and no additional rupture is required beyond where it is imaged in our preferred model. Our results demonstrate the contribution of backprojections of regional seismic array data for earthquakes down toM≈ 7, especially when incomplete coverage of seismic and geodetic data implies large uncertainties in source inversions.

A large, Ms = 7.5, shallow earthquake occurred beneath the Mariana trench on April 5, 1990. From the relocated aftershock distribution, the fault area is estimated to be 70 × 40 km2. A tsunami observed on the Japanese islands verifies that the depth of the main shock is shallow. For waveform analysis, we use long‐period surface waves and body waves recorded at global networks of GDSN, IRIS, GEOSCOPE and ERIOS. The centroid moment tensor (CMT) solution from surface waves indicates normal faulting on a fault whose strike is parallel to the local axis of the Mariana trench, with the tension axis perpendicular to it. The seismic moment is 1.4 × 1020 Nm (× 1027 dyn.cm) which gives Mw = 7.3. Far‐field P and SH waves from 13 stations are used to determine the source time function. Since the sea around the epicentral region is about 5 km deep, body waveforms are contaminated with water reverberations. The inversion results in a source time function with a predominantly single event with a duration of 10 sec, a seismic moment of 2.1 × 1020 Nm, and a focal mechanism given by strike = 198°, dip = 48°, slip = 90°. The short duration indicates a small area of the rupture. The location of the main shock with respect to the aftershock area suggests that the nodal plane dipping to the west is preferred for the fault plane. The local stress drop of the single subevent is estimated to be 150 MPa (1.5 Kbars). The Mariana earthquake is considered to have occurred in an uncoupled region, in response to the gravitational pull caused by the downgoing Pacific plate.

We determine centroid-moment tensor (CMT) solutions by minimizing waveform differences between observed and simulated seismograms based on an adjoint method. Synthetic seismograms and Frechet derivatives are calculated based on a spectral-element method. The non-linear adjoint CMT inversion algorithm requires three simulations for each iteration: one ‘forward’ simulation to obtain synthetics for the current source parameters, one ‘adjoint’ simulation which involves injecting time-reversed differences between observed and simulated seismograms as simultaneous virtual sources at each of the receivers, and an extra forward simulation to compute the step length in the conjugate-gradient direction. Whereas the vertical component of the adjoint wavefield reflects the radiation pattern near the centroid location, the components of the adjoint strain tensor capture the elements of the moment tensor. We use the method to determine adjoint CMT solutions for two representative southern California earthquakes using recent 3-D crustal model CVM-6.2. The adjoint CMT solutions are in good agreement with classical Hessian-based CMT solutions involving 3-D Green's functions. In general, adjoint CMT inversions require fewer numerical simulations than traditional Hessian-based inversions. This faster convergence holds promise for multiple moment-tensor and kinematic rupture inversions in 3-D earth models.

The largest oceanic intraplate earthquake ever detected and the largest event worldwide since 1994 occurred in a remote oceanic region near the Balleny Islands on March 25, 1998 (03:12:26 UT; Mw 8.2). An intraplate earthquake of this magnitude is extremely rare, especially for the Antarctic plate, which shows low seismicity.The earthquake occurred about 300 km west of the Balleny Transform between the Antarctic passive margin and the Australian‐Antarctic spreading center (Figure 1). The best double‐couple mechanism from the Harvard Centroid Moment Tensor (CMT) solution shows strike‐slip faulting with nodal planes trending north‐south and east‐west (Figure 1). The CMT solution also shows a large, compensated linear vector dipole (CLVD) component that can be interpreted as indicating northwest‐southeast extension.