Teleseismic Waveform Inversion Research Articles

New insight into the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey): An evidence for rupture-parallel pull-apart basin activation along the East Anatolian Fault Zone constrained by Geodetic and Seismological data

In this study, we investigated the main features of the causative fault of the 24 January 2020, Mw 6.8 Elazığ earthquake (Turkey) using seismological and geodetic data sets to provide new insight into the East Anatolian Fault Zone (EAFZ). We first constrained the co-seismic surface deformation and the rupture geometry of the causative fault segment using Interferometric Synthetic Aperture Radar (InSAR) interferograms (Sentinel-1A/B satellites) and teleseismic waveform inversion, respectively. Also, we determined the centroid moment tensor (CMT) solutions of focal mechanisms of the 27 aftershocks using the regional waveform inversion method. Finally, we evaluated the co-seismic slip distribution and the CMT solutions of the causative fault as well as of adjacent segments using the 27 focal solutions of the aftershocks, superimposed on the surface deformation pattern. The CMT solution of the 24 January 2020Elazığ earthquake reveals a pure strike-slip focal mechanism, consistent with the structural pattern and left-lateral motion of the EAFZ. The rupture process of the Elazığ event indicated that the rupture is started at 12 km around the hypocenter, and then propagated bilaterally along the NE-SW but mainly toward the southwest. The rupture slip has initially propagated toward the southwest (first 10 s) and northeast (4 s), and again toward the southwest (9 s). Maximum displacement is calculated as 1.3 m about 20 km southwest of the hypocenter at 6 km depth (centroid depth). The rupture stopped to down-dip around 20 km depth toward the southwest. The distribution of the slip vectors indicates that the rupture continued mostly through a normal oblique movement. Most of the moment release was released SW of the hypocenter and the rupture reached up to around 50 km. The focal mechanisms of analyzed 27 aftershocks show strike-slip, but mostly normal and normal oblique-slip faulting with an orientation of the tensional axes (NNE-SSW), indicating a normal oblique-slip, “transtensional” stress regime, parallel-subparallel to the strike of the EAFZ, consistent with SW-rupture directivity and co- seismic deformation pattern. Finally, based on the co-seismic surface deformation compatible with the distributional pattern of normal focal solutions, normal and normal oblique-slip focals of the aftershocks evidence the rupture-parallel pull-apart basin activation as a segment boundary of the left-lateral strike-slip movement of the EAFZ.

Back-propagating rupture evolution within a curved slab during the 2019 Mw 8.0 Peru intraslab earthquake

SUMMARY The 26 May 2019 MW 8.0 Peru intraslab earthquake ruptured the subducting Nazca Plate where the dip angle of the slab increases sharply and the strike angle rotates clockwise from the epicentre to north. To obtain a detailed seismic source model of the 2019 Peru earthquake, including not only the rupture evolution but also the spatiotemporal distribution of focal mechanisms, we performed comprehensive seismic waveform analyses using both a newly developed flexible finite-fault teleseismic waveform inversion method and a back-projection method. The source model revealed a complex rupture process involving a back-propagating rupture. The initial rupture propagated downdip from the hypocentre, then unilaterally northward along the strike of the slab. Following a large slip occurring ∼50–100 km north of the hypocentre, the rupture propagated bilaterally both further northward and back southward. The spatial distribution of focal mechanisms shows that the direction of T-axis azimuth gradually rotated clockwise from the epicentre northward, corresponding to the clockwise rotation of the strike of the subducting Nazca Plate, and the large-slip area corresponds to the high-curvature area of the slab iso-depth lines. Our results show that the complex rupture process, including the focal-mechanism transition, of the Peru earthquake was related to the slab geometry of the subducting Nazca Plate.

Revisiting the seismogenic characteristics of stable continental interiors: The case of three Indian events

Kinematic source process models using tele-seismic waveform inversion of three stable continental region (SCR) earthquakes from India (1993 Mw 6.2 Latur, 1997 Mw 5.8 Jabalpur and 2001 Mw 7.6 Bhuj) are presented in this paper to demonstrate the slip evolution and stress drop. These finite-fault models are methodologically unique to constrain the source dimensions, compared with assumptions and quasi-observations around blind faults. Our results show that these events do have a compact zone of singular asperity breakage within the Indian crust. Whereas the Bhuj and Jabalpur events have their rupture majorly restricted within the lower crustal regions. The Latur event ruptured a very shallow crustal asperity. The estimated rupture velocities are in the range of 2.6–3.2 km/s, Bhuj event the slowest and lengthiest. Our results do not favour an updip shallow component of asperity breakage for the Bhuj earthquake, as evident from lack of surface ruptures. Study also indicates that the 1993 Latur event occurred on a fault with high stress concentration which indicates stronger fault asperities or slip within a newer fault. Models on 1997 Jabalpur event imply higher frictional strength and very brittle nature at the lower crustal regions of the Indian slab, indicating a mechanically very strong lower crust. We conclude that an event like Latur can occur anywhere within continental interiors of Indian SCR, or elsewhere globally, and is an underestimated seismic hazard.

Source characteristics of Peruvian deep focus seismic doublet of November 24, 2015

The Peruvian deep focus seismic doublet of November 24, 2015 is the largest event recorded on the Peru-Brazil border in the last 50 years, with hypocenter depth at 655 km (Mw 7.4) for the first event and 675 km (Mw 7.5) for the second event, both events were separated by a distance of 50 km and by a time of 5 min. There was no damage on the surface, but the earthquake was felt in part of Peru (IV MM in Esperanza - Ucayali) and Brazil. The parameters that characterize the seismic source have been obtained from the inversion of teleseismic waveforms. For example, the duration of the rupture process was around 25 s for both events and the rupture velocity was between 2.5 and 2.8 km/s. The source time function is similar for both events: it starts with a small energy release before 10 s and then the greatest energy release between 10 s and 20 s. The focal mechanism is of normal fault type, corresponding to tensional tectonic regime of deep slab in the region.

Rupture Directivity Analysis of the 2018 Hokkaido Eastern Iburi Earthquake and Its Seismotectonic Implication

ABSTRACTThe Mw 6.6 Hokkaido Eastern Iburi earthquake striking southern Hokkaido Island on 5 September 2018 was a disastrous and peculiar event. In contrast to the usually shallow crustal earthquakes, this event occurred at a hypocentral depth about 37 km, close to the Moho discontinuity. To infer the rupture feature of the 2018 Hokkaido earthquake, we determine focal mechanism and centroid depth of the event with inversion of teleseismic waveforms. The result reveals that the centroid (at depth about 26 km) of this thrust earthquake is shallower than the hypocenter, which suggests the upward rupture propagation and dominant rupture in the lower crust. We also investigate the causative fault and rupture directivity based on waveform modeling. The steeply dipping fault (70°) with strike in the north–south direction is preferred to be the causative fault. The total dimension of rupture is estimated to be about 30 km, based on the aftershock distribution and rupture directivity. We propose that a seismogenic model with low temperature and complex stress field in the lower crust above the subduction‐zone interface may explain this event.

Complex Triggering Supershear Rupture of the 2018 Mw 7.5 Palu, Indonesia, Earthquake Determined from Teleseismic Source Inversion

ABSTRACTAn Mw 7.5 earthquake struck Palu in the northern coast of Sulawesi island, Indonesia, on 28 September 2018. Its focal mechanism was determined to be a left‐lateral strike‐slip fault, which is generally expected to not produce a tsunami. However, a large tsunami with runup heights of more than 6 m was observed along the coast of Palu city. Here, we show a complex triggering supershear source model as determined by teleseismic waveform inversion. Three asperities with different slip characteristics were found on the 120‐kilometer‐long rupture zone. Significant triggering rupture with a supershear speed was observed south of the epicenter, which was just beneath Palu city. This special rupture process can cause a strong directivity effect that produced anomalously large ground shaking with nonlinear effects in Palu area. The coseismic deformation determined from the inverted source model showed large horizontal displacements. These horizontal movements combined with complex bathymetry and topography could have pushed seawater to generate a tsunami even though the Palu earthquake was a strike‐slip event.

Simultaneous orthogonal shortening in the Afghan-Tajik Depression

Abstract The Afghan-Tajik Depression is a sedimentary basin in the Alpine-Himalayan mountain chain. It is traversed by series of north-south arcuate folds, suggesting that the basin is undergoing east-west compression. A second set of folds in the south of the depression runs east-west, crosscutting those trending north-south. We present results from teleseismic body waveform inversion and depth phase modeling for five recent earthquakes, and from detailed mapping of structures related to active faulting based on satellite imagery and topographic data. We argue that both sets of folds are active and that deformation is vertically partitioned, with north-south compression accommodated on east-west–trending thrust faults within the basement, and east-west compression accommodated on north-south–trending thrust faults above a detachment within the basin fill. The observation that orthogonal shortening can be accommodated simultaneously in this way has several important implications. Juxtaposed orthogonal fold systems identified in the geological record may not require temporally separate events, particularly in gravity-driven fold-and-thrust belts in foreland-basin settings. Pervasive detachment may limit the size of potential earthquakes by preventing single events from rupturing the entire seismogenic layer. However, it may also disguise geomorphic signatures of faulting and interseismic strain accumulation within the lower layer, hindering accurate seismic hazard assessment and regional tectonic interpretations.

Teleseismic Waveform Complexities Caused by Near Trench Structures and Their Impacts on Earthquake Source Study: Application to the 2015 Illapel Aftershocks (Central Chile)

AbstractThe complex near trench velocity structures, characterized by strongly varying bathymetry along with seawater, can produce substantial waveform complexities for near trench earthquakes, which makes it difficult to study earthquake source parameters through waveform modeling/inversion. Here we explore these wavefield complexities via modeling teleseismic records of a Mw6.6 near coast event and a Mw6.8 near trench event in the 2015 Illapel earthquake sequence. For the near coast event, the waveforms of direct P waves at teleseismic/diffracted distances are simple, and we obtain consistent source parameters between 1D regional and teleseismic waveform inversions. In contrast, the near trench event produces stronger and longer P coda waves (>100 s), resulting in many dramatic discrepancies between the regional and teleseismic inversions, in particular for the centroid depth. We adopt a spectral element method‐direct solution method hybrid approach to simulate synthetics with the complex source‐side 3‐D structures (bathymetry and water layer in this case) and investigate their roles in the genesis of strong P coda waves. Compared with the 1‐ and 2‐D synthetics, the 3‐D synthetics significantly improve the waveform fits up to 0.1 Hz when the source is placed at the preferred horizontal (~30 km from the trench axis) and vertical (~5 km beneath the ocean bottom) location. The refined location of the earthquake indicates that the plate interface is probably locked at very shallow depths and capable of nucleating strong earthquakes. We highlight the need for considering near trench 3‐D structures in seismic waveform analysis of near trench earthquakes, many of which are tsunamigenic.

Source characteristics of the 2017 Mw6.4 Moijabana, Botswana earthquake, a rare lower-crustal event within an ancient zone of weakness

The 2017 Moijabana earthquake in central Botswana (Mw6.4) was a large and deep event for a continental interior and occurred in a region with little historical seismicity. Based on InSAR measurements of surface deformation spanning the event and teleseismic observations, we determine the ruptured fault plane and finite rupture model of the earthquake. Although this oblique normal-faulting earthquake is too deep to uniquely determine the rupture plane geometry from InSAR alone, the best-fitting fault plane constrained by the joint inversion of teleseismic waveforms and InSAR data has a southwest dip and a strike of 126°, roughly consistent with the geologically mapped strike of the Kaapvaal craton's northern edge. Our results indicate that the earthquake had a total duration of ∼10 s, characterized by two major asperities. The first asperity nucleated in the lower crust and then the rupture propagated up-dip. The lower crustal asperity shows a much shorter rise time compared with the shallower asperity, indicating that contrasts in stress or material properties may have played an important role in the rupture process. The earthquake appears to have occurred in the Limpopo belt, a Proterozoic orogenic belt that represents an ancient zone of weakness between the Archean Zimbabwe and Kaapvaal cratons. In the present day, this zone of weakness may be responding to the stress field imposed by the East African Rift System.

Active backstop faults in the Mentawai region of Sumatra, Indonesia, revealed by teleseismic broadband waveform modeling

Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 “backthrust” sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as ‘unsticking’ of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults.

The 2005 and 2010 Earthquakes on the Sumatra–Andaman Trench: Evidence for Post‐2004 Megathrust Intraplate Rejuvenation

Great earthquakes on oblique subduction zones accommodate slip on strike-slip faults on the overriding and subducting plates, leading to increased intraplate seismicity. After the 2004 Sumatra earthquake, there was an increase in activity in the Wharton basin (WB), which is considered to be part of a diffuse plate boundary. Here, the intraplate deformation occurs in response to northwest-southeast directed compression as opposed to the north-northeast-south-southwest directed plate motion along the Sumatra-Andaman subduction zone. Earthquakes here mostly occur by left-lateral strike slip on north-south (N-S) trending fossil fractures, although ruptures on east-west planes are not uncommon. Here, we discuss the 24 July 2005 M-w 7.2 and the 12 June 2010 M-w 7.5 earthquakes, both of which were left-lateral strike-slip events sourced in the northern WB. Our source models for these events, based on teleseismic waveform inversion, favor slip on the N-S fossil fracture systems, consistent with previous studies in the WB. Although the former sourced at similar to 16 km depth was not tsunamigenic, the latter (similar to 34 km) triggered a small tsunami, possibly due to its oblique slip at similar to 5 km depth. We use conceptual models of plate deformation to suggest that both these events have resulted from post-2004 redistribution of stresses and rejuvenation of intraslab fracture systems within the WB, an actively deforming oceanic intraplate region.

Three-dimensional full waveform inversion of short-period teleseismic wavefields based upon the SEM–DSM hybrid method

We present a method for high-resolution imaging of lithospheric structures based on full waveform inversion of teleseismic waveforms. We model the propagation of seismic waves using our recently developed direct solution method/spectral-element method hybrid technique, which allows us to simulate the propagation of short-period teleseismic waves through a regional 3-D model. We implement an iterative quasi-Newton method based upon the L-BFGS algorithm, where the gradient of the misfit function is computed using the adjoint-state method. Compared to gradient or conjugate-gradient methods, the L-BFGS algorithm has a much faster convergence rate. We illustrate the potential of this method on a synthetic test case that consists of a crustal model with a crustal discontinuity at 25 km depth and a sharp Moho jump. This model contains short- and long-wavelength heterogeneities along the lateral and vertical directions. The iterative inversion starts from a smooth 1-D model derived from the IASP91 reference Earth model. We invert both radial and vertical component waveforms, starting from long-period signals filtered at 10 s and gradually decreasing the cut-off period down to 1.25 s. This multiscale algorithm quickly converges towards a model that is very close to the true model, in contrast to inversions involving short-period waveforms only, which always get trapped into a local minimum of the cost function.

Automatic imaging of earthquake rupture processes by iterative deconvolution and stacking of high‐rate GPS and strong motion seismograms

AbstractBy combining the complementary advantages of conventional network inversion and backprojection methods, we have developed an iterative deconvolution and stacking (IDS) approach for imaging earthquake rupture processes with near‐field complete waveform data. This new approach does not need any manual adjustment of the physical (empirical) constraints, such as restricting the rupture time and duration, and smoothing the spatiotemporal slip distribution. Therefore, it has the ability to image complex multiple ruptures automatically. The advantages of the IDS method over traditional linear or nonlinear optimization algorithms are demonstrated by the case studies of the 2008 Wenchuan and 2011 Tohoku earthquakes. For such large earthquakes, the IDS method is considerably more stable and efficient than previous inversion methods. Additionally, the robustness of this method is demonstrated by comprehensive synthetic tests, indicating its potential contribution to tsunami and earthquake early warning and rapid response systems. It is also shown that the IDS method can be used for teleseismic waveform inversions. For the two major earthquakes discussed here, the IDS method can provide, without tuning any physical or empirical constraints, teleseismic rupture models consistent with those derived from the near‐field GPS and strong motion data.

Combination of double couple and non-double couple events during the Van, Turkey, 2011 earthquake sequence

A combination of double couple (DC) and non-double couple (non-DC) earthquakes hit Eastern Turkey, in the vicinity of Lake Van, in October–November 2011. Teleseismic waveform inversion was used to find the best fitting double couple and deviatoric moment tensors on four large and medium sized events of this sequence. The aftershocks of the Mw=7.1, 2011/10/23:10:41 earthquake built a NE–SW aftershock zone where the Mw=5.7, 2011/10/25 aftershock was located. The Mw=6.0, 2011/10/23:20:45 event was located around the terminal section of the Mw=7.1 aftershock zone which might be triggered by this event (aftershocks of this event propagated from W to E to build a W–E aftershock zone where the Mw=5.7, 2011/11/09 event was located). For these events the calculated best fitting grid search parameters are not very different from GCMT results, but DC components, after deviatoric moment tensor inversion, represent much more difference with GCMT and grid search. The important feature of deviatoric moment tensor inversion is the existence of a notable compensated linear vector dipole (CLVD) component on the Mw=5.7, 2011/10/25:14:55 and Mw=5.7, 2011/11/09:19:23 aftershocks. According to the regional seismotectonics, these CLVD components could be related to crustal rheology and volcanic activities. Based on the results, the existence of a cylindrical aftershock distribution could be taken as an indication of induced seismic activity on complex-ring structures resulted from magma or water–magma injection. However, the existence of Karst like structures suggests that the CLVD components may be under the influence of high-pressure water or gas injection rather than magma.

Strong-Motion and Teleseismic Waveform Inversions for the Source Process of the 2003 Bam, Iran, Earthquake

The Bam earthquake ( M w 6.5) occurred on 26 December 2003 in southeastern Iran, causing a major disaster in the city of Bam. A remarkable vertical peak ground acceleration value of 988 cm/s2 and two horizontal pulses were recorded inside the damaged city. Previous analyses showed that the earthquake was caused by a subsurface rupture on an unknown strike‐slip fault. In this study, we attempt to determine the precise fault location and source process of the 2003 Bam earthquake by performing a multiple‐time‐window linear waveform inversion of teleseismic and strong‐motion data, both individually and jointly. We examined the general features of the fault location and the source process by analyzing the teleseismic displacement waveforms and determined the precise features and fault geometry by inverting the three components of strong‐motion velocity records. The final estimate of the source process of the 2003 Bam earthquake was determined by joint inversion of the datasets. Our results show that a single fault model, characterized by the appropriate location of the hypocenter, rake angle variations, and the Rayleigh‐like speed of the rupture front can satisfactorily explain the three components of the strong‐motion records at BAM station.

Variations in rupture speed, slip amplitude and slip direction during the 2008 Mw 7.9 Wenchuan Earthquake

Theoretical and observational evidence implies that variable rupture velocity may be associated with high fault slip and high moment release rates during earthquakes. We investigate this relationship for the 2008 Mw 7.9 Wenchuan China, Earthquake, which appears to have experienced a highly variable moment release rate. We used an empirical Green's function (EGF) deconvolution analysis of teleseismic waveforms to retrieve the primary rupture characters for this event. Based on field observations and the deconvolved source—time function from thrust and strike–slip EGFs, we divided the ruptured fault into three segments, allowing us to determine the spatial slip distribution with different average rupture velocities on each segment. We deployed a grid search analysis, in which we integrated the teleseismic waveform inversion and forward modelling of the regional surface wave to determine the optimum rupture speed in each fault segment. Our result shows that the 2008 Wenchuan Earthquake had slip amplitude, direction and rupture velocity that were highly spatially variable. The earthquake initially ruptured with nearly pure thrust motion and a slow rupture velocity of 1.7 km s—1. Then, the rupture speed increased up to 3.1—3.3 km s—1 and produced the largest slip on the second segment, where two parallel faults ruptured simultaneously. Rupture velocity then slowed down to 2.5—2.9 km s—1 on the final segment, which underwent primarily strike–slip motion. In total, the fault extended 300 km with an average rupture speed in the range of 2.6—2.9 km s—1. The high rupture speed (close to the shear wave velocity) in the second segment may be related to the simultaneous rupture of the two parallel faults on well–established pre–existing structures. The spatial distribution of rupture velocity and slip are likely related to the strongly rotational and 3–D deformation in the eastern Tibetan Plateau margin, which leads to the heterogeneous stress field of the Longmen Shan region.

Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implications

We use teleseismic waveform inversion, along with depth phase analysis, to constrain the centroid depths and source parameters of large African earthquakes. The majority of seismic activity is concentrated along the East African Rift System, with additional active regions along stretches of the continental margins in north and east Africa, and in the Congo Basin. We examine variations in the seismogenic thickness across Africa, based on a total of 227 well-determined earthquake depths, 112 of which are new to this study. Seismogenic thickness varies in correspondence with lithospheric thickness, as determined from surface wave tomography, with regions of thick lithosphere being associated with seismogenic thicknesses of up to 40 km. In regions of thin lithosphere, the seismogenic thickness is typically limited to ≤20 km. Larger seismogenic thicknesses also correlate with regions that have dominant tectonothermal ages of ≥1500 Ma, where the East African Rift passes around the Archean cratons of Africa, through the older Proterozoic mobile belts. These correlations are likely to be related to the production, affected by method and age of basement formation, and preservation, affected by lithospheric thickness, of a strong, anhydrous lower crust. The Congo Basin contains the only compressional earthquakes in the continental interior. Simple modelling of the forces induced by convective support of the African plate, based on long-wavelength free-air gravity anomalies, indicates that epeirogenic effects are sufficient to account for the localization and occurrence of both extensional and compressional deformation in Africa. Seismicity along the margins of Africa reflects a mixture between oceanic and continental seismogenic characteristics, with earthquakes in places extending to 40 km depth.

Routine Regional Moment Tensor Inversion for Earthquakes in the Greek Region: The National Observatory of Athens (NOA) Database (2001-2006)

The routine use of regional broadband data for the determination of moment tensors of even very small events ( Mw ∼ 3.5) has considerably enhanced our understanding of tectonic processes in many active regions worldwide ( e.g. , Kao and Jian 2001; Braunmiller et al. 2002; Kubo et al. 2002; Pondrelli et al. 2002; Clinton et al. 2006; Risteau 2008). This development can be attributed to a number of reasons, such as the ability to generate accurate synthetic seismograms for a given velocity model and an increase in the amount and quality of data at regional distances. An additional reason may also be the variety of available methods for regional moment tensor inversion that utilize different parts of the recorded waveform, like surface waves (Thio and Kanamori 1995), body and surface waves (Zhao and Helmberger 1993), or just the long-period part of the signal (Ritsema and Lay 1995). Greece has high seismicity, and large events in the past have caused significant damage and casualties (for an overview see Papazachos and Papazachou 1997). Thus, a systematic study of source properties can help seismic hazard mitigation efforts and offer insights into the seismogenic patterns of the Greek region. Most available source mechanisms of previous events in Greece were derived either from P -wave polarities (Papazachos et al. 1983, 1988; Papadopoulos et al. 1986) or from the inversion of teleseismic waveforms recorded by the Global Seismic Network (Lyon-Caen et al. 1988; Hatzfeld et al. 1996; Bernard et al. 1997; Kiratzi and Louvari 2003; Benetatos et al. 2004). However, the former approach depends heavily on station coverage and the reliability of the first-motion readings, while the latter one can be used only for moderate to large events ( Mw > 5.0). Since 1997 the National …

Source processes of the 2009 Irian Jaya, Indonesia, earthquake doublet

On January 3, 2009 UT, an earthquake doublet with moment magnitudes of 7.6 and 7.4 occurred along the New Guinea trench near the northern coast of Irian Jaya, Indonesia. The events were separated by three hours in time and 60 km in space, having similar thrust faulting mechanisms. The Solomon and New Britain Islands regions located east of the source area are well known for the occurrence of earthquake doublets due to the complex tectonic settings. To investigate the source processes of the earthquake doublet, we have performed teleseismic waveform inversions using P-wave displacement records from the IRIS-DMC database. The results of the inversions are consistent with the direction of the subduction along the New Guinea trench for both of the earthquakes. Significant slips exceeding 5.0 m appear near the hypocenters, and the slip distributions look to be complementary to each other. The rupture starting point of the second event is close to the edge of the slip distribution of the first event. Our source models support the triggering mechanism of doublet events suggested by Lay and Kanamori (1980). The results also suggest the state of stress in the Irian Jaya region and the mechanism of earthquakes in this subduction zone.

Validation of Teleseismic Inversion of the 2004 Mw 6.3 Les Saintes, Lesser Antilles, Earthquake by 3D Finite-Difference Forward Modeling

We study the 21 November 2004 Mw 6.3 Les Saintes earthquake that occurred south of the island of Guadeloupe at a shallow depth, damaging some buildings on the island. The objective of this work is to assess the potential of a teleseismic source study to reproduce local ground motion. The velocity model of this area is not currently well known, and the near-field seismograms are affected by paths and site effects. We first analyze this earthquake as a point source and then as an extended fault using teleseismic broadband waveform inversion approach from the P and SH waveforms at stations well distributed in azimuth. We obtain two stable and reliable solutions of the spatiotemporal history of the earthquake rupture on each nodal plane. The models show a rupture duration of 6 sec and a maximum slip of about 1 m but have different locations of the main asperity. We then carry out the forward modeling of the seismic wave propagation at the regional scale, coupled with the obtained kinematic source models taking into account the topography, the bathymetry, and the sea layer. We investigate if one of the fault models could produce synthetics that match the local observed ground motions. The simulation results provide a better understanding of the uncertainty of source, path, and site effects even if our knowledge of the underground geological structure is limited to the 1D stratified model. It is inferred that the 1D model does not seem to be appropriate in the northern direction (middle of Guadeloupe Island) due to the unknown basin structure, while it is relatively sufficient in the eastern and the northwestern directions from the source. By careful comparison of the synthetic and observed displacement seismograms in terms of main phase forms and arrival times at low frequency, especially at the closest station GBGA, the fault model with northeast faulting is estimated to be the more relevant to the measured local ground motions.