Regional Waveforms Research Articles

Regional waveform inversion of 2004 February 11 and 2007 February 09 Dead Sea earthquakes

SUMMARY Two felt moderate size earthquakes with local magnitudes 5.2 on 2004 February 11 and 4.4 on 2007 February 09 occurred to the east of the Dead Sea and along the northern part of the Arava/Araba fault (ARF), respectively. Being well recorded by the national seismic networks operating on the Dead Sea region, they offer a good opportunity to update the crustal model, revealing the tectonic process and present-day stress field on the Dead Sea region. The observed group velocity dispersion of Rayleigh and Love waves picked from the broad-band records for the 2004 and 2007 main shocks are used to update the Jordan Seismological Observatory model, which shows a good match between the observed and theoretical dispersion. Our updated model shows small traveltime residuals during our location process and yields a high variance reduction for our regional waveform inversion. The earthquake hypocentre, determined from the observed P- and S-wave traveltime data, and regional waveform inversion indicate a source depth of ∼14 and ∼12 km for the 2004 and 2007 main shocks, respectively. Focal mechanism obtained from both first motion polarities of local-regional observations and moment tensor inversion of regional observations of 2004 earthquake reveal a mainly normal faulting, with minor strike-slip component, along WNW–ESE fault, whose orientation is nearly consistent with the transverse Zarqa Ma'in fault. However, the focal mechanism of 2007 main shock shows left-lateral, strike-slip faulting along a near-vertical, near-NNE–SSW striking fault plane whose orientation is in good agreement with the surface expression of the observed northern ARF. The focal mechanism for a felt foreshock of 2007 earthquake indicates a similar left-lateral strike-slip fault with a slight normal component. Moment tensors estimated from regional waveforms indicate predominantly double-couple mechanisms for the three studied events. These results can be used to calibrate ground motions, evaluate 3-D velocity models for simulation of large scenario earthquakes and seismic risk assessment along the Dead Sea Fault.

Source parameters of March 10 and September 13, 2007, United Arab Emirates earthquakes

On March 10 and September 13, 2007 two earthquakes with moment magnitudes 3.66 and 3.94, respectively, occurred in the eastern part of the United Arab Emirates (UAE). The two events were widely felt in the northern Emirates and Oman and were accompanied by a few aftershocks. Ground motions from these events were well recorded by the broadband stations of Dubai (UAE) and Oman seismological networks and provide an excellent opportunity to study the tectonic process and present day stress field acting in this area. In this study, we report the focal mechanisms of the two main shocks by two methods: first motion polarities and regional waveform moment tensor inversion. Our results indicate nearly pure normal faulting mechanisms with a slight strike slip component. We associated the fault plane trending NNE–SSW with a suggested fault along the extension of the faults bounded Bani Hamid area. The seismicity distribution between two earthquake sequences reveals a noticeable gap that may be a site of a future event. The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated from displacement spectra. The moment magnitudes were very consistent with waveform inversion. The recent deployment of seismic networks in Dubai and Oman reveals tectonic activity in the northern Oman Mountains that was previously unknown. Continued observation and analysis will allow for characterization of seismicity and assessment of seismic hazard in the region.

Repeating earthquakes recorded by Liaoning Regional Seismograph Network

In the list of ‘repeating pairs’ or ‘doublets’ of earthquakes in China identified by Schaff and Richards using teleseismic waveform cross-correlation, there were 23 repeating pairs located in Liaoning Province. In this study the waveforms of these events were cross-correlated using records from Liaoning Regional Seismograph Network (LRSN), and the ‘repeating events’ in the sense of regional waveform cross-correlation were obtained. The result was compared with that of Schaff and Richards and was used for the assessment of the seismic phase picking and event location practice of LRSN. The result shows that ‘repeating events’ in the sense of teleseismic waveform cross-correlation and those in the sense of regional waveform cross-correlation have significant difference, although with some overlap. However, the overall assessment of the location accuracy and the phase pick errors of LRSN by using these two sets of ‘repeating events’, respectively, provides similar results, while ‘repeating events’ in the sense of regional waveform cross-correlation seem to be better performing in such an assessment. With the assumption that the separation between the ‘repeaters’ be less than 1 km, the uncertainty in routine earthquake location of LRSN is estimated to be below 5 km, with the average of 2 km. In the observational bulletins of LRSN the time error in phase picking is estimated to be within ±1s for 94% Pg readings and for 88% Sg readings.

The October 11, 1999 and November 08, 2006 Beni Suef Earthquakes

Two felt moderate-sized earthquakes with local magnitudes of 4.9 on October 11, 1999 and 4.3 on November 08, 2006 occurred southeast of Beni Suef and Cairo cities. Being well recorded by the digital Egyptian National Seismic Network (ENSN) and some regional broadband stations, they provided us with a unique opportunity to study the tectonic process and present-day stress field acting on the northern part of the Eastern Desert of Egypt. In this study, we analyze the main shocks of these earthquakes and present 15 well recorded aftershocks (0.9 ≤ ML ≤ 3.3) which have small errors on both horizontal and vertical axes. The relocation analysis using the double difference algorithm clearly reveals a NW trending fault for the 1999 earthquake. The spatial distribution of its aftershocks indicates a propagation of rupture from the SW towards the NW along a fault length ~5 km dipping nearly ~40°SW. We also determined the focal mechanisms of the two main shocks by two methods (polarities and amplitudes ratios of P, SV and SH and regional waveform inversion). Our results indicate a normal faulting mechanism with a slight shear component for the first event, while pure normal faulting for the second one. The spatial distribution of the 1999 aftershocks sequence along with the retrieved focal mechanism confirmed the NW plane as the true fault plane. While for the 2006 event, the few aftershocks do not reveal any fault geometry; its focal mechanism indicated a pure normal fault nearly trending WNW-ESE that corresponds more likely to the extension of the 1999 earthquake fault. The seismicity distribution between the two earthquake sequences reveals a noticeable gap that may be a site of a future event. The NNE-SSW extensional stress indicated by the mechanisms of these events is in agreement with the regional stress field and the rifting of the northern Red Sea in its northern branches (Gulf of Suez and Gulf of Aqaba). The source parameters (seismic moment, moment magnitude, fault radius, stress drop and displacement across the fault) were also estimated and compared based on both the regional waveform inversion and the displacement spectra and interpreted in the context of the tectonic setting. The obtained results imply a reactivation of the pre-exiting NW-SE faults as a result of extensional deformation from the northern Red Sea-Gulf of Suez rifts.

Segmentation of the Blanco Transform Fault Zone from earthquake analysis: Complex tectonics of an oceanic transform fault

The Blanco Transform Fault Zone (BTFZ) forms the ∼350 km long Pacific–Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Nearby broadband seismic networks provide a unique framework for a detailed, long‐term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 129 earthquake source parameters; combined with 28 Harvard moment tensors, they represent the largest waveform derived OTF source parameter data set. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we determine along‐fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull‐apart basin. The longest transform (∼150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, seismically fully coupled, has a wider seismic zone (∼9 km) than other BTFZ transform segments and accommodates the largest (Mw 6.4–6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid postemplacement cooling. A short transform connects to the pull‐apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow width subparallel faults. Seismic coupling is low in extensional (≤15%) compared to transform areas (35–100%), implying different mechanical properties. Centroid depth variations are consistent with seismic slip cutoff near 600°C.

Three‐dimensional S velocity of the mantle in the Africa‐Eurasia plate boundary region from phase arrival times and regional waveforms

A new model of the three‐dimensional shear velocity structure of the Africa‐Eurasia plate boundary is presented. The new model is derived by jointly inverting different types of seismic data. The two main sources of information are regional waveforms and teleseismic S wave arrival times. We show that it is possible to find a model that fit the different data types nearly as well as when inverting solely one type of data. The main improvement in resolving power is achieved between depths of 300 and 700 km, though the improvements are not limited to this depth range. Our model reflects the complicated evolution of this plate boundary area. The transition zone is dominated by high‐velocity anomalies which we infer to represent a mix of lithosphere that subducted relatively recently or is not sufficiently cold and dense to traverse the 660‐km discontinuity. The only low‐velocity zone in the transition zone is beneath the Ionian Sea. The high‐velocity Hellenic slab is continuous throughout the upper mantle and into the lower mantle to about 1200 km, most likely representing subducted Neo‐Tethys lithosphere. The uppermost mantle is dominated by low velocities, consistent with the high level of tectonic activity. Low‐velocity regions are relatively strong beneath the Mid‐Atlantic Ridge, Turkey, and the Dead Sea region. The region's current lithosphere is relatively thin, except beneath the Adriatic and Ionian seas and the easternmost Atlantic Ocean.

Full three-dimensional tomography: a comparison between the scattering-integral and adjoint-wavefield methods

SUMMARY This paper analyses the computational issues of full 3-D tomography, in which the starting model as well as the model perturbation is 3-D and the sensitivity (Frechet) kernels are cal- culated using the full physics of 3-D wave propagation. We compare two formulations of the structural inverse problem: the adjoint-wavefield (AW) method, which back-propagates the data from the receivers to image structure, and the scattering-integral (SI) method, which sets up the inverse problem by calculating and storing the Frechet kernels for each data functional. The two inverse methods are closely related, but which one is more efficient depends on the overall problem geometry, particularly on the ratio of sources to receivers, as well as trade-offs in computational resources, such as the relative costs of compute cycles to data storage. We find that the SI method is computationally more efficient than the AW method in regional waveform tomography using large sets of natural sources, although it requires more storage.

Crustal structure of the south-central Andes Cordillera and backarc region from regional waveform modelling

SUMMARY We investigate the crustal structure in the Andes Cordillera and its backarc region using regional broadband waveforms from crustal earthquakes. We consider seismic waveforms recorded at regional distances by the CHile-ARgentina Geophysical Experiment (CHARGE) during 2000‐2002 and utilize previous seismic moment tensor inversion results. For each single station-earthquake pair, we fixed the source parameters and performed forward waveform modelling using ray paths that sample the crust of the highest elevation Cordillera and the accreted terranes in the backarc region. Our investigation indicates that synthetic seismograms for our earthquake-station geometry are most sensitive to crustal parameters and less sensitive to mantle parameters. We performed a grid search around crustal thickness, P-wave seismic velocity (Vp) and P -t oS-wave seismic velocity ratio (Vp/Vs), fixing mantle parameters. We evaluated this waveform analysis by estimating an average correlation coefficient between observed and synthetic data over the three broadband components. We identified all acceptable crustal models that correspond to high correlation coefficients that provide best overall seismogram fits for the data and synthetic waveforms filtered mainly between 10 and 80 s. Our results indicate along strike variations in the crustal structure for the north‐south high Cordillera with higher P-wave velocity and thickness in the northern segment (north of 33 ◦ S), and persistently high Vp/Vs ratio (>1.85) in both segments. This is consistent with a colder mafic composition for the northern segment and a region of crustal thickening above the flat slab region. In contrast, the results for the current volcanic arc (south of 33 ◦ S) agree with a warmer crust consistent with partial melt related to Quaternary volcanism presumably of an intermediate to mafic composition. A distinctive feature in the backarc region is the marked contrast between the seismic properties of the Cuyania and Pampia terranes that correlates with their heterogeneous crustal composition. The Cuyania terrane, composed of mafic-ultramafic rocks, exhibits high Vp, high Vp/Vs and a thicker layered crust versus the thinner more quartzrich crust of the eastern Sierras Pampeanas associated with low Vp and low Vp/Vs. These differences may have some effect on the mechanism that unevenly generates crustal seismicity in the upper ∼30 km in this active compressional region. In particular, the seismic properties of the Cuyania terrane, which shows evidence for a high Vp and high Vp/Vs crust, may be reflecting the complex tectonic evolution history of this terrane including accretion-rifting and re-accretion processes since the Palaeozoic that promote a high level of crustal seismicity in the upper ∼30 km, enhanced by the flat slab subduction in the segment between 31 ◦ S and 32 ◦ S. Another possible mechanism could be directly related to the presence of a strong lower crust above the flat slab that efficiently transfer stresses from the slab to the upper crust generating higher seismicity in the Cuyania terrane between 30 ◦ S and 34 ◦ S.

Source mechanism and source parameters of May 28, 1998 earthquake, Egypt

On May 28, 1998, a moderate size earthquake of mb 5.5 occurred offshore the northwestern part of Egypt (latitude 31.45°N and longitude 27.64°E). It was widely felt in the northern part of Egypt. Being the largest well-recorded event in the area for which seismic data from the global digital network are available, it provides an excellent opportunity to study the tectonic process and present day stress field occurring along the offshore Egyptian coast. The source parameters of this event are determined using three different techniques: modeling of surface wave spectral amplitudes, regional waveform inversion, and teleseismic body waveform inversion. The results show a high-angle reverse fault mechanism generally trending NNW–SSE. The P-axis trends ENE–WSW consistently with the prevailed compression stress along the southeastern Hellenic arc and southwestern part of the Cyprean arc. This unexpected mechanism is most probably related to a positive inversion of the NW trending offshore normal faults and confirms an extension of the back thrusting effects towards the African margin. The estimated focal depth ranges from 22 to 25 km, indicating a lower crustal origin earthquake owing to deep-seated tectonics. The source time function indicates a single source with rise time and total rupture duration of 2 and 5 s, respectively. The seismic moment (Mo) and the moment magnitude (Mw) determined by the three techniques are 1.03 × 1017 Nm, 5.28; 1.24 × 1017 Nm, 5.33; and 1.68 × 1017 Nm, 5.42; respectively. The calculated fault radius, stress drop, and the average dislocation assuming a circular fault model are 7.2 km, 0.63 Mpa, and 0.11 m, respectively.

Non-linear crustal corrections in high-resolution regional waveform seismic tomography

SUMMARY We compare 3-D upper mantle anisotropic structures beneath the North American continent obtained using standard and improved crustal corrections in the framework of Non-linear Asymptotic Coupling Theory (NACT) applied to long period three component fundamental and higher mode surface waveform data. Our improved approach to correct for crustal structure in high-resolution regional waveform tomographic models goes beyond the linear perturbation approximation, and is therefore more accurate in accounting for large variations in Moho topography within short distances as observed, for instance, at ocean‐continent margins. This improved methodology decomposes the shallow-layer correction into a linear and non-linear part and makes use of 1-D sensitivity kernels defined according to local tectonic structure, both for the forward computation and for the computation of sensitivity kernels for inversion. The comparison of the 3-D upper mantle anisotropic structures derived using the standard and improved crustal correction approaches shows that the model norm is not strongly affected. However, significant variations are observed in the retrieved 3-D perturbations. The largest differences in the velocity models are present below 250 km depth and not in the uppermost mantle, as would be expected. We suggest that inaccurate crustal corrections preferentially map into the least constrained part of the model and therefore accurate corrections for shallow-layer structure are essential to improve our knowledge of parts of the upper mantle where our data have the smallest sensitivity.

Upper mantle structure of South America from joint inversion of waveforms and fundamental mode group velocities of Rayleigh waves

A new tomographic S wave velocity model for the upper mantle beneath South America is presented. We developed and applied a new method of simultaneously inverting regional S and Rayleigh waveforms and fundamental mode Rayleigh wave group velocities, to better constrain upper mantle S velocity structure and Moho depth. We used ∼5700 Rayleigh wave group velocity dispersion curves and 1537 regional wave trains with paths principally passing through the South American continent. The joint inversion of this data set provided a new three‐dimensional (3‐D) upper mantle S velocity model and a Moho depth model for South America, which fits both the group velocity and regional waveform data sets well. New features of the final three‐dimensional (3‐D) S velocity and Moho depth model correlate well with known geotectonic units on a regional scale. The Moho depth ranges from 30 km in the central Chaco basin to 42 km beneath the Amazonian craton and 45–70 km beneath the orogenic Andean belt. The imaged S velocity indicates an average lithosphere thickness of around 160 km for the Amazonian craton. High velocities are imaged beneath the Amazon and part of the Paraná and Parnaíba basins down to about 150 km. Low to very low velocities are imaged beneath the central Andes and the Chaco, Pantanal, and northwestern Paraná basins.

Complex and Segmented Rupture Associated with the 14 August 2003 Mw 6.2 Lefkada, Ionian Islands, Earthquake

The 14 August 2003 (M w 6.2) Lefkada earthquake ruptured the Lefkada Segment of the Cephalonia transform fault zone (CTFZ), a major structure along the Ionian Islands of Greece. We invert 30 P and 9 S waves recorded by the Global Seismographic Network to recover the slip distribution on the fault. Teleseismic (Benetatos et al., 2005) and regional (Zahradnik et al., 2005) waveform modeling has revealed the multiple-event (mainly double) character of the mainshock. Our slip model indicates that the earthquake did occur as two distinct subevents, separated in space by approximately 40 km and in time by 14 sec. The two subevents ruptured the Lefkada segment of CTFZ. Rupture initiated in the north and propagated to the south with an average speed of 2.4 km/sec. The moment was released in three distinct patches. The first patch of moment release, located beneath the western coast of Lefkada Island, is confined in a small area (� 25 10 km 2 ), extends in depth from 10 to 25 km, and here the maximum slip of � 34cm is observed. The second patch of moment release is located offshore further to the south, but close to the northern coast of Cephalonia Island, and is confined in an even smaller area (� 15 10 km 2 ). The third patch, for which the uncertainty is the highest, occurred at the intersection of the two fault segments, which is actually the intersection of the Lefkada and Cephalonia segments of the CTFZ. To obtain a satisfactory fit to the regional and teleseismic body waves the contribution from subevents on both segments has to be considered. An extensive sensitivity analysis is performed to quantify the uncertainty of the slip model. ShakeMaps of peak ground velocities, simulated from the slip model, are found to be in good agreement with the distribution of reported damage and intensity level of VII, at the regions near the epicenter and especially on the western coast of Lefkada Island.

W17 Single and paired pulse TMS: Comparisons between natural and magnetic activation and between monophasic and biphasic TMS

ized linear inverse source estimates of cortical current density. Determination of the priors in the resolution of the linear inverse problem was performed with the use of information from the hemodynamic responses of the cortical areas as revealed by block-designed (strength of activated voxels) fMRI. We estimate functional cortical connectivity by computing the directed transfer function (DTF) and the Partial Directed Coherence (PDC) on the estimated cortical current density waveforms in regions of interest (ROIs) on the modeled cortical mantle. Furthermore, this method allows to detect changes in the time course of information flow between cortical regions in different frequency bands. Results: The reliability of these techniques was further demonstrated by simulation studies and the elaboration of high-resolution EEG and fMRI signals collected during different cognitive and motor tasks in healthy subjects. Discussion: Multimodal integration of high resolution EEG and fMRI data offers a significant aid for the study of the ‘‘when’’ , ‘‘where’’, and ‘‘how’’ in the cortical brain regions.

Seismotectonics of western Turkey from high resolution earthquake relocations and moment tensor determinations

We have relocated 364 earthquakes in western Turkey between 2002 and 2003 using more than 45,000 accurate P and S differential travel‐times. We also have determined moment tensors for 71 of these earthquakes using their broadband regional waveforms. After relocation the events are highly concentrated in three clusters and delineate two nearly vertical faults. Their strikes agree with the strike of one of the fault planes of the averaged fault‐plane solution of the cluster, which allowed us to pick the true fault plane and the slip direction. We identified a NNW‐striking left‐lateral strike‐slip fault near Buldan that connects the Gediz graben in the north and the eastern end of the Buyuk Menderes graben in the south. The other right‐lateral strike‐slip fault near Izmir strikes ENE–WSW direction. Our results indicate that part of the N–S extension occurring in western Turkey is accommodated by strike‐slip faults and is accompanied by E–W shortening.

The March 11, 2002 Masafi, United Arab Emirates earthquake: Insights into the seismotectonics of the northern Oman Mountains

A moderate ( M ∼5) earthquake struck the northeastern United Arab Emirates (UAE) and northern Oman on March 11, 2002. The event was felt over a wide area of the northern Emirates and was accompanied by smaller (felt) events before and after the March 11 main shock. The event was large enough to be detected and located by global networks at teleseismic distances. We estimated focal mechanism and depth from broadband complete regional waveform modeling. We report a normal mechanism with a slight right-lateral strike-slip component consistent with the large-scale tectonics. The normal component suggests relaxation of obducted crust of the Semail ophiolite (specifically, the Khor Fakkan Block) while the right-lateral strike-slip component of the mechanism is consistent with shear across the Oman Line. Felt earthquakes are rare in the region, however no regional seismic network exists in the UAE to determine local seismicity. This event offers a unique opportunity to study the active tectonics of the region as well as inform future studies of seismic hazard in the UAE and northern Oman.

The 4 August 2004, Lake Ontario, Earthquake

Other| January 01, 2006 The 4 August 2004, Lake Ontario, Earthquake Won-Young Kim; Won-Young Kim Lamont-Doherty Earth Observatory of Columbia University Palisades, NY 10964 wykim@ldeo.columbia.edu (W.-Y. K.) Search for other works by this author on: GSW Google Scholar Savka Dineva; Savka Dineva Department of Earth Sciences, University of Western Ontario 1151 Richmond Street London, Ontario N6A 5B7, Canada (S. D., S. M., D. E.) Search for other works by this author on: GSW Google Scholar Shutian Ma; Shutian Ma Department of Earth Sciences, University of Western Ontario 1151 Richmond Street London, Ontario N6A 5B7, Canada (S. D., S. M., D. E.) Search for other works by this author on: GSW Google Scholar David Eaton David Eaton Department of Earth Sciences, University of Western Ontario 1151 Richmond Street London, Ontario N6A 5B7, Canada (S. D., S. M., D. E.) Search for other works by this author on: GSW Google Scholar Author and Article Information Won-Young Kim Lamont-Doherty Earth Observatory of Columbia University Palisades, NY 10964 wykim@ldeo.columbia.edu (W.-Y. K.) Savka Dineva Department of Earth Sciences, University of Western Ontario 1151 Richmond Street London, Ontario N6A 5B7, Canada (S. D., S. M., D. E.) Shutian Ma Department of Earth Sciences, University of Western Ontario 1151 Richmond Street London, Ontario N6A 5B7, Canada (S. D., S. M., D. E.) David Eaton Department of Earth Sciences, University of Western Ontario 1151 Richmond Street London, Ontario N6A 5B7, Canada (S. D., S. M., D. E.) Publisher: Seismological Society of America First Online: 09 Mar 2017 Online ISSN: 1938-2057 Print ISSN: 0895-0695 © 2006 by the Seismological Society of America Seismological Research Letters (2006) 77 (1): 65–73. https://doi.org/10.1785/gssrl.77.1.65 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Won-Young Kim, Savka Dineva, Shutian Ma, David Eaton; The 4 August 2004, Lake Ontario, Earthquake. Seismological Research Letters 2006;; 77 (1): 65–73. doi: https://doi.org/10.1785/gssrl.77.1.65 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search Abstract A Mw 3.1 earthquake occurred in Lake Ontario along the United States–Canada border, about 30 km south from Port Hope, Ontario, Canada, on 4 August 2004. Despite its small size, the shock was very well recorded by broadband seismographic stations deployed in recent years in Ontario, Canada, and in New York State. More than 40 broadband stations at local and regional ranges provided high-quality digital data. Waveform data analysis constrained the source at a depth of 4 (±2) km, which places the shock in the shallow Precambrian basement beneath Paleozoic platform deposits. Th e source mechanism from the regional waveform inversion for the double-couple moment tensor is predominantly strike-slip faulting. A NS striking (8°) nodal plane dipping to the east (dip = 59°) is the likely fault plane which represents right-lateral strike-slip motion. Th e subhorizontal P-axis orientation (trend = 234° and plunge = 12°) is consistent with the maximum horizontal compressional stress (SHmax) direction in eastern North America. Although the 4 August 2004 event is a small shock and has the seismic moment of M0 = 4.45 (±2.30) × 1013 Nm, it is the largest instrumentally recorded earthquake that has occurred in Lake Ontario. This and other significant earthquakes in the region suggest a broad-scale strike-slip faulting stress regime with a shallow seismogenic layer in the Erie–Ontario Lowlands region. The shallow focal depths of earthquakes in the region increase the risk of higher ground shaking compared to other seismic zones in northeastern North America with a deeper seismogenic layer. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The 9 December 2003 Central Virginia Earthquake Sequence: A Compound Earthquake in the Central Virginia Seismic Zone

The 9 December 2003 central Virginia earthquake sequence was a compound earthquake consisting of two nearly identical events occurring about 12 sec apart. The second event is separated by about 300 m from the first event along the azimuth of approximately 195 (±10)°. The source mechanism determined from regional waveform inversion indicates predominantly thrust faulting at a depth of approximately 10 (±2) km. The sequence with two events attained the combined seismic moment of M 0 2.64 (±1.01) × 10 15 N m ( M w 4.3). The focal mechanism indicates a subhorizontal P axis trending 301° and plunging 19°. A regional stress model for the central Virginia seismic zone (cvsz) derived from the 9 December 2003 events and 11 previous earthquakes indicates a thrust-faulting stress regime with σ 1 trending 133 (±12)° and plunging 14 (±3)°. The least principal stress axis ( σ 3 ) trends 25 (±10)° and plunges 52 (±3)°. The σ 1 axis is rotated approximately 68° clockwise relative to the average maximum horizontal compressional stress (S Hmax ) direction for eastern North America (approximately 65°). The 9 December 2003 earthquake sequence occurred among the systems of Paleozoic and Mesozoic faults above the southern Appalachian decollement, which is at depths from 12 to 19 km in the Piedmont geologic province of central Virginia.

The 2002 Molise earthquake sequence: What can we learn about the tectonics of southern Italy?

On October 31, 2002 a M L = 5.4 earthquake occurred in southern Italy, at the margin between the Apenninic thrust belt (to the west) and the Adriatic plate (to the east). In this area, neither historical event nor seismogenic fault is reported in the literature. In spite of its moderate magnitude, the earthquake caused severe damage in cities close to the epicenter and 27 people, out of a total of 29 casualties, were killed by the collapse of a primary school in S. Giuliano di Puglia. By inverting broadband regional waveforms, we computed moment tensor solutions for 15 events, as small as M L = 3.5 ( M w = 3.7). The obtained focal mechanisms show pure strike-slip geometry, mainly with focal planes oriented to NS (sinistral) and EW (dextral). In several solutions focal planes are rotated counterclockwise, in particular for later events, occurring west of the mainshock. From the relocated aftershock distribution, we found that the mainshock ruptured along an EW plane, and the fault mechanisms of some aftershocks were not consistent with the mainshock fault plane. The observed stress field, resulting from the stress tensor inversion, shows a maximum principal stress axis with an east–west trend (N83°W), whereas the minimum stress direction is almost N–S. Considering both the aftershock distribution and moment tensor solutions, it appears that several pre-existing faults were activated rather than a single planar fault associated with the mainshock. The finite fault analysis shows a very simple slip distribution with a slow rupture velocity of 1.1 km/s, that could explain the occurrence of a second mainshock about 30 h after. Finally, we attempt to interpret how the Molise sequence is related to the normal faulting system to the west (along the Apennines) and the dextral strike-slip Mattinata fault to the east.

Stator Iron Loss of Tubular Permanent-Magnet Machines

Whilst methods of determining the iron loss in rotating permanent magnet machines have been investigated extensively, the study of iron loss in linear machines is relatively poorly documented. This paper describes a simple analytical method to predict flux density waveforms in discrete regions of the laminated stator of a tubular permanent magnet machine, and employs an established iron loss model to determine the iron loss components, on both no-load and on-load. Analytical predictions are compared with the iron loss deduced from finite element analyses for two tubular permanent magnet machine designs, and it is shown that if a machine has a relatively high electrical loading, the on-load iron loss can be significantly higher than the no-load value.

Crust‐mantle coupling in the Gulf of Cadiz (SW‐Iberia)

In the Gulf of Cadiz and Cape St. Vincent region at the SW‐Iberian margin, seismic catalogues report seismicity down to ∼100 km depth. According to structural models for the region, this includes earthquakes that occur in the uppermost continental mantle. We invert regional waveforms to estimate moment tensors and centroid depths for moderate earthquakes, obtaining eight solutions (Mw 3.8 to 5.3) between 6 and 60 km depth. Moment tensor solutions and previous source estimates show predominately reverse and strike‐slip faulting style with ∼NNW‐SSE oriented P‐axes, close to plate convergence between Africa and Eurasia. We obtain similar stress tensor estimates from crustal and upper mantle earthquakes, both with sub‐horizontal, ∼NNW σ1‐axes. This indicates analogous deformation in both domains, and that earthquakes are unrelated to any subduction process.