Time-domain Moment Tensor Inversion Research Articles

Automatic Fault Plane Solution for the Provision of Rapid Earthquake Information in South Korea

The Korea Meteorological Administration (KMA) provides detailed hypocenter information after the earthquake early warning (EEW) service, due to increased public interest and for the study of fault movements. However, the rapid production of hypocenter information has limitations, including the necessity for the calculation of focal mechanisms, which requires expertise in seismology. Therefore, we developed automatic focal mechanisms (AFMs) based on the time domain moment tensor inversion method. A key feature of AFMs is the automatic collection and reforming of waveform data using information for EEW. Furthermore, we propose an additional module of the iterative inversion by reducing the low variance reduction data. This shows the increased variance reduction value rather than that of the first inversion. The variance reductions for the first inversion results were between 59 and 94%, whilst the results of the second inversion using the additional module were increased to 79–97%. The accuracy of the automatic results was similar to that of the manually determined results and was well adapted to the local earthquakes in and around the Korean Peninsula. The KMA provided the focal mechanisms of local earthquakes that could then be automatically determined using the EEW information within approximately 6–8 min and subsequently reported.

An active extensional deformation example: 19 May 2011 Simav earthquake (Mw= 5.8), Western Anatolia, Turkey

The Simav Earthquake that occurred on 19 May 2011 in western Turkey was investigated on the basis of seismological data and geological observations. Approximately WNW?ESE trending surface ruptures were observed on the Simav Fault. The focal mechanism parameters of the earthquake (Mw = 5.8) and its aftershocks (Mw??>??3.5) were estimated using time-domain moment tensor inversion. A total of 2245 events were located with Geiger?s conventional absolute location method then relocated using the double difference (DD) algorithm. The calculated locations at a depths between 2 and 16?km were found to be consistent with Coulomb stress variation in the area. Average variance reduction (VR) of the solutions was calculated as ~70%. The focal parameters of strike dip and slip of the main shock, occurring at a depth of 11?km dipping towards the NNE, were estimated at 277, 62 and ?92, respectively. The most striking indication of the study is that the area is dominated by normal faults with mainly WNW?ESE trends. It is also concluded that earthquakes in the region are caused by an active and regional NNE?SSW (N 12? E) trending (?3 axis) extension regime. The mean stress ratio is 0.80, indicating a triaxial stress state. This extension is probably associated with a slab?pull force and /or roll-back due to the complex subduction process of the African Plate beneath Anatolian block along both the Hellenic and Cyprus arcs in the eastern Mediterranean region.

Focal Mechanisms and Slip Models of Moderate Size Earthquakes in Albania and Adjacent Countries

The available data set of focal mechanisms in Albania and adjacent countries is enriched with the determination of source parameters for 27 earthquakes of moderate-size. The broad band waveforms recorded by the regional networks were used to do time-domain moment tensor inversion and also to invert for the slip distribution onto the fault plane for two events: one (11 February 2010) in the proximity of Lake Scutari (Albania-Montenegro borders) and the other (10 March 2010) in Serbia. Forward modelling is additionally applied, which incorporates synthetic Green's functions calculated for a 1D velocity model, to get a first-order evaluation of the distribution of strong ground shaking around the hypocenters. Forward modeling of strong ground shaking based on event-specific moment tensor and slip distribution indicates low levels of expected ground motion in accordance with observations. The new data confirm previous knowledge for the active tectonics of the region as for example the thrust/reverse faulting, along the low elevation areas of the foreland, and the sub-parallel normal faulting along the high elevation areas in the mountains.

Determination of full moment tensors of microseismic events in a very heterogeneous mining environment

The determination of source parameters and full moment tensors in mines is a difficult task, especially, when the velocity model of the mining environment is complex and strongly heterogeneous. The heterogeneities in the velocity model are usually caused by the presence of ore bodies, host rocks, tunnel systems and large excavations due to mining activity. The mined-out cavities introduce strong velocity contrasts in the model, cause multiple scattering of waves and result in a complex wave field with long coda waves. We have analysed five blasts and five induced microseismic events recorded at the Pyhäsalmi ore mine, Finland, and suggest a strategy of successfully inverting for the seismic moment tensors. We compute accurate locations using an eikonal solver and perform the time-domain moment tensor inversion from full waveforms using a generalized linear inversion. Green's functions are computed using a 3-D finite difference visco-elastic code capable of reproducing complex interactions of waves and structures. To suppress the sensitivity of the inversion to inaccuracies of locations and the velocity model, we analyse the data in the frequency range from 30 to 80Hz. The analysis of blasts and microseismic events proves that the moment tensor inversion is successful. The moment tensors of blasts display a high percentage of positive isotropic components. However, the presence of minor shear faulting triggered during blasting cannot be excluded. On the other hand, the moment tensors of microseismic events display significant negative isotropic and compensated linear vector dipole components. This indicates that the predominant mechanism of the events is probably related to the collapse of rock due to mining activity.

The 6 September 2009 Mw5.4 Earthquake in Eastern Albania – FYROM Border: Focal Mechanisms, Slip Model, ShakeMap

On 6 September 2009 (GMT 21:49) a moderate Mw5.4 earthquake sequence burst at the eastern border of Albania with the Former Yugoslav Republic of Macedonia (FYROM). The main shock was located ~6 km north of the epicentre of the 30 November 1967 Mw6.2 Dibra (or Debar) earthquake, which caused loss of life and considerable damage to buildings. We use broad band waveforms recorded by the Hellenic Unified Seismic Network (HUSN), which receives real-time waveforms from the neighbouring networks, to compute focal mechanisms, obtain the slip model and derive the Shake Map of the mainshock. The focal mechanisms of 18 of the stronger events of the sequence, obtained through time-domain moment tensor inversion, indicate that deformation is taken up by NNE-SSW-trending normal faults, in agreement with the ~E-W extension previously identified within the Albanian orogen. Our results show that the 2009 main shock ruptured a roughly 9 km normal fault at a depth of 6 km, which strikes at 194° and dips west at ~45°. The slip of the main shock was confined to a single patch of ~9 km × 6 km, the average slip was 5 cm and the peak slip was 18 cm. The slip model was incorporated in a forward modelling scheme to simulate the ground motion distribution in the near field. The Shake Map thus obtained, based on the distribution of Peak Ground Velocity at phantom stations, outlines the mesoseismal area within the Dibra and Bulqiza districts in Albania, in accordance with macroseismic observations. The region affected by the 2009 sequence, together with the seismogenic region of the 1967 Dibra event, form a roughly NNE-SSW-trending structure which is an active seismotectonic zone in eastern Albania constituting a threat for nearby urban areas.

Seismic moment tensor inversion using a 3-D structural model: applications for the Australian region

SUMMARY There is significant seismic activity in the region around Australia, largely due to the plate boundaries to the north and to the east of the mainland. This activity results in serious seismic and tsunami hazard in the coastal areas of Australia. Hence seismicity is and will be monitored in real time by Geoscience Australia (GA), which uses a network of permanent broadband seismometers. Seismic moment tensor (MT) solutions are currently determined using 1-D, radially symmetric models of Earth and this requires augmentation by recording stations located outside of Australia. A 3-D model of the Australian continent developed recently using full waveform tomography now offers the opportunity to significantly improve the determination of MT solutions of earthquakes from tectonically active regions. A complete-waveform, time-domain MT inversion method has been developed using a point-source approximation. A series of synthetic tests using first a 1-D and then a 3-D structural model has been performed. The feasibility of deploying 3-D versus 1-D Earth structure for the inversion of seismic data has been studied and the advantages of using the 3-D structural model were illustrated with examples. The 3-D model is superior to the 1-D model, as a number of sensitivity tests show. The ultimate goal of this work is an automated MT inversion system in Australia relying on GA and other international stations, although more work remains to be done before the full implementation of such a scheme in real time.

The 24 May 2009 Mw5.2 earthquake sequence near Lake Doirani (FYROM-Greek borders): Focal mechanisms and slip model using empirical source time functions inversion

On 24 May 2009 (GMT 16:17) a moderate size (Mw5.2) earthquake sequence burst north of Lake Doirani along the Greek-FYROM borders and ruptured a N282°E striking normal fault of ∼ 6 km in length dipping at an angle of ∼ 60° to the north. The focal mechanisms for 25 of the stronger events, obtained through time-domain moment tensor inversion, reveal the prevalence of normal faulting along ∼ WNW–ESE striking planes, sometimes dip slip motions are combined with a considerable strike-slip component. Broad band waveforms from the Hellenic Unified Seismological Network (HUSN) and neighbouring networks were used to invert source time functions (STFs) and calculate the slip model for the strongest event (GMT 16:17). STFs were obtained through an empirical Green's function approach. No directivity could be clearly identified for this moderate magnitude event. Slip was released in a single patch of dimensions 6 km × 8 km, maximum slip is estimated at 15 cm, while average slip on the ruptured area (area of subfaults of non-zero slip) was 3 cm. One characteristic of the slip model was the large rise time, for a moderate size event, which was estimated to be 1.0 s, implying a slow slip-rate function. Brune's static stress drop is calculated ∼ 5 bars. It must be noted that the north dipping plane is identified as the fault plane, (slip vector trending N332°E) based on the model variance of the STFs inversion.

Time-Domain Moment Tensors for shallow (h≤40km) earthquakes in the broader Aegean Sea for the years 2006 and 2007: The database of the Aristotle University of Thessaloniki

Abstract We present a catalog of moment tensor (MT) solutions and moment magnitudes, Mw, for 119 shallow (h ≤ 40 km) earthquakes in Greece and its surrounding lands (34°N–42°N, 19°E–30°E) for the years 2006 and 2007, computed with the 1D Time-Domain Moment Tensor inversion method (TDMT_INV code of Dreger, 2003 ). Magnitudes range from 3.2 ≤ Mw ≤ 5.7. Green's functions (GF) have been pre-computed to build a library, for a number of velocity profiles applicable to the broader Aegean Sea region, to be used in the inversion of observed broad band waveforms (10–50 s). All MT solutions are the outcome of a long series of tests of different reported source locations and hypocenter depths. Quality factors have been assigned to each MT solution based on the number of stations used in the inversion and the goodness of fit between observed and synthetic waveforms. In general, the focal mechanisms are compatible with previous knowledge on the seismotectonics of the Aegean area. The new data provide evidence for strike-slip faulting along NW–SE trending structures at the lower part of Axios basin, close to the heavily industrialized, and presently subsiding, region of the city of Thessaloniki. Normal faulting along E–W trending planes is observed at the Strimon basin, and in Orfanou Gulf in northern Greece. A sequence of events in the east Aegean Sea close to the coastline with western Anatolia sheds light on an active structure bounding the north coastline of Psara–Chios Islands about 20–25 km in length exhibiting right lateral strike-slip faulting.

Source parameters of the M 6.5 Skyros Island (North Aegean Sea) earthquake of July 26, 2001

Teleseismic body wave modelling, time domain moment tensor inversion of regional waveforms and spectral analysis of the far-field P-wave pulses are used to derive the source parameters of the July 26, 2001 Skyros earthquake (M 6.5). Its epicentre is located south of the Sporades Islands in the North Aegean Sea (Greece). Previous focal mechanism solutions indicate motion on strike-slip faults. The time domain moment tensor inversion is applied for the first time to the regional waveforms of the recently established broadband network in Greece. Its application gave results which are highly consistent with teleseismic waveform modelling. The results of this study, in combination with the distribution of aftershocks, indicate left-lateral strike slip motion on a NW-SE striking fault with parameters: fault plane (strike = 151°, dip = 83°, rake = 7°) and auxiliary plane (strike = 60°, dip = 84°, rake = 173°), depth 12 km and M 0 = 5.98e18 Num. Moreover, the time domain moment tensor inversion technique yielded a pure double couple source with negligible CLVD. The spectral analysis of the far-field P-wave pulses resulted in a fault length L ~ 32 km, stress drop 6m ~ 9 bars and average displacement u ~ 30 cm. These values are in very good agreement with those estimated from empirical scaling relations applicable to the Aegean area.

Spatial segmentation characteristic of focal mechanism of aftershock sequence of Wenchuan Earthquake

Moment tensor solutions of 88 earthquakes were determined by using the broadband waveform data recorded in six stations within 450 km around the Wenchuan Earthquake sequence by means of the time domain moment tensor inversion method. It was found that the type of the focal mechanism solution is characteristic of obvious spatial segmentation. There are six segments along the main rupture zone from southwest to northeast, where initially the focal mechanism is of main thrust type, finally of main right-lateral strike-slip type and between these two areas there is a transition zone characterized in multiple types of focal mechanisms appearing in turn. Earthquakes of left-lateral strike-slip type perpendicular to the main rupture zone occurred near Xiaoyudong Town. The stress field of each segment is inversed by means of the FMSI program, and it was found that, along the main rupture zone from southwest to northeast, the direction of the maximum principal stress is gradually changing from near EW to NW-SE, and finally changing back to near EW.

A study of the 2 December 2002 ( M5.5) Vartholomio (western Peloponnese, Greece) earthquake and of its largest aftershocks

Broadband data from the Greek National Seismological Network are used to study the moderate size ( M5.5) earthquake, which occurred on 2 December 2002 near the town of Vartholomio, in western Peloponnese (Greece). Time domain moment tensor inversion applied to retrieve the focal mechanism of the mainshock and of three of the larger aftershocks of the sequence, revealed almost pure strike-slip faulting along NW–SE or NE–SW trending nodal planes. The relative source time functions for the mainshock, obtained from an empirical Green's function analysis, do not reveal any clear directivity to any of the stations. A careful observer might suggest directivity towards NW, if any. Optimum values are 0.4 s for the rise time and 2.7 km/s for the rupture velocity. The spatial and temporal distribution of fault slip showed that the major part of the resolved slip occurred beneath the mainshock's epicenter, ~20 km underneath the western coast of Peloponnese. This probably accounts for the considerable damage observed to the nearby towns. The resolution between the two nodal planes does not permit an identification of the fault plane; however the statistics on the slip distribution model, the preliminary analysis of aftershock locations and macroseismic data favour the NW–SE trending plane as the fault plane, which is connected with sinistral strike-slip motions. These are the first implications for sinistral strike-slip motions in this area and more data are needed in the future to get more reliable resolution of the motions.

Moment tensor solutions for small and moderate earthquakes in the Ibero‐Maghreb region

We applied time domain moment tensor inversion of local and regional waveforms to small and moderate (Mw = 3.5–5.7) shallow earthquakes from the Iberian Peninsula, northern Morocco, and northern Algeria. For the 6+ years period from November 1995 to March 2002 and the previous Network of Autonomously Recording Seismograms (NARS) experiment, moment tensor solutions were obtained for 58 events, considerably increasing the total number of available solutions in the study area. For each event we performed a moment tensor inversion and a double‐couple grid search. For simple faulting events the grid search is valuable as a quality test for its ability to reveal potential ambiguities of the solutions and to assess confidence limits of fault plane parameters or principal axes orientation. The computed mechanisms show regional consistency: A large part of the Iberian Peninsula is characterized by normal faulting mechanisms with SW‐NE oriented T axes. Thrusting and SE‐NW compression is dominant in Algeria. In the Alboran Sea, the westernmost part of the Mediterranean, and the transition between both regimes, strike‐slip mechanisms dominate with approximately N‐S oriented P axes. This pattern suggests a regional anomaly characterized by clockwise rotation of the principal horizontal stress orientations.

Moment tensor solutions of the 1994 to 1996 Double Spring Flat, Nevada, earthquake sequence and implications for local tectonic models

AbstractThe 12 September 1994 Mw 5.8 Double Spring Flat, Nevada, earthquake initiated at the intersection of a northeast- and northwest-striking set of conjugate faults within an overlapping zone between the Genoa and Antelope Valley fault zones, of the eastern Sierra Nevadan range frontal fault system. The mainshock ruptured on the northeast-striking fault plane. Eight days after the mainshock, the aftershock activity migrated from the mainshock fault plane to the northwest-striking conjugate fault. Over the next 2 years, aftershocks migrated southward onto another set of conjugate faults and then onto the Antelope Valley fault zone. The focal mechanisms of 17 M > 4 aftershocks were estimated from a time-domain moment tensor inversion using regional broadband data. The T axis (minimum stress direction) is oriented east-west (N80°E to N100°E) for the (M > 4) events as is, commonly observed along the eastern Sierra Nevadan range front in northwestern Nevada. From these results, we make some general points that can be considered in seismic hazard assessment. The maximum magnitude in overlapping normal fault zone is limited to the size of the overlapping zone. This makes small- to moderate-size (M < 6) strike-slip earthquakes more likely than large range-front (M > 7) earthquakes. The seismicity within this overlapping zone may indicate interseismic strain accumulation from east-west extension mainly through strike-slip deformation. The apparent scarcity of modern normal-faulting earthquakes along the Sierran range-front faults suggests a characteristic model, while a Gutenberg and Richter model for the recurrence behavior of earthquakes applies to the overlap zones between the normal faults. The pattern of seismicity and principle stress directions from the aftershock fault-plane solutions suggest a tectonic model of changing fault geometry for the overlapping zone between the Genoa and Antelope Valley fault zones. Two plausible long-term tectonic outcomes may develop with this model: a normal fault growth model where the overlapping segments of the Genoa and Antelope Valley faults eventually become “hard linked” (form a throughgoing fault) or a normal fault growth model where the overlapping segment of the Genoa fault system grows southward while the Antelope Valley fault is isolated in the formation of new basins and ranges.