Local Seismograph Networks Research Articles

Properties of the Petrinja (Croatia) earthquake sequence of 2020–2021 – Results of seismological research for the first six months of activity

We present locations of almost 14,000 events from the first six months of the Petrinja (Croatia) earthquakes sequence (mainshock 29 December 2020, Mw 6.4). The catalogue is estimated complete for ML ≥ 1.20. Initially sparse local seismograph network was densified a week after the mainshock, which reduced uncertainties of focal locations by about 75%. Source-specific station corrections were used in an iterative location scheme. The hypocentres were located with 30 different sets of velocity models and program control parameters in order to gain insight into the epistemic uncertainty of each earthquake location. The bulk of epicentres are located close to, and to the SE of the causative, dextral strike-slip Petrinja fault, but considerable activity has been triggered on smaller faults, up to 25 km away. About a half of the 78 first-motion polarity focal mechanism solutions (FMS) from the full first year of activity were found to have mechanisms similar to the foreshock, the mainshock and the largest of aftershocks, while the rest indicated almost pure reverse faulting. Strike-slip mechanisms occurred more often close to the main fault. The P-axes of FMS for aftershocks were found to be on the average rotated by 16° clockwise with respect to the P-axis of the mainshock, regardless of the style of faulting. The spatial distribution of aftershocks coincides very well with the areas of positive Coulomb stress change caused by the mainshock rupture. The inferred cut-off seismogenic depth in the area (7–10 km), estimated on the basis of published values of the geothermal gradient and the assumption of predominantly granitic upper crust, is considerably shallower than the depth of the deepest reliably located aftershocks (16–18 km). This fact may be explained by considering ophiolite-dominated crust, and by increased strain rate during the sequence, which raises the ability of crustal rocks for brittle failure.

CO2 Degassing relate to 2021 Nyiragongo Volcan éruption

The Nyiragongo volcano in the western part of the African Rift has the deep fractures that a facilite the upwelling of gases before and after the eruption, especially carbon dioxide (CO2 ). This paper presents the behaviour of carbon dioxide before and after the eruption on 22 May 2021. The Nyiragongo volcano eruption begun on 22 May 2021 at 18:15 LT (Local Time) in Goma. The lava flowed from the flanks of the volcano in two directions, one flow coming to a halt in the northeastern suburbs of Goma, The other flow cut off one kilometre of the national road 4 linking Goma to Butembo, a major regional road vital for suplying Goma town is frequently mentioned causing 32 victims, incorporation of volatile gases to the atmosphere, the destruction of 17 villages in the territory of Nyiragongo and crack in Goma and Gisenyi. The Nyiragongo Volcano is infamous for its extremely fluid lava that flows like water when the lava lake flows. This which is monitored by the scientific community of the Goma Volcanological Observatory (G .V. O), and the lava lake produces a huge amount of gas. CO2 is heavier than air, asphyxiating, and lethal at concentrations greater from 15%. The present paper shows the variation of CO2 near the volcano in monitoring sites to study the behavior one Month before the eruption, the CO2 showed an increasing trend in some sites and a few Months after the eruption we observed a slight decrease of 3% in all the CO2 monitoring sites. Nyiragongo is an important contributor of CO2 and SO2 to the troposphere. This 2021 eruption is similar to the one of 17 January 2002, which caused about 100 people’s death of and the evacuation of several people. The deadliest eruption of Nyiragongo was in 1977, when more than 600 people died. Post-eruption seismic activity was characterised by a large number of earthquakes of magnitude 3.5 or more. A few earthquakes of magnitude greater than 3.5 have been recorded by global seismic networks (USGS), the strongest of which had a magnitude of 5.2 in 2002. Compared to other eruptions that Nyiragongo volcano has experienced, the seismic activity before the eruption of 22 May 2021 was not too alarming. This is because there were no strong earthquakes (greater than or equal to 3.5) that preceded the eruption a few months before, as had been noted in previous eruptions. After the eruption, exceptional seismic activity was noticed. Many earthquakes of magnitude greater than or equal to 3.5 were recorded by the local seismographic network. These earthquakes were felt, the strongest having a magnitude of 5.2

Lithospheric heterogeneities of northeast India and Indo-Burma region: Geodynamic implications

• Intercontinental subduction, Cenozoic volcanism and ductile sediments imaged. • Subducting Indian slab is imaged as high-Vp whilst volcanic front by low-Vp. • Dehydration of the slab, serpentinization of the forearc mantle imaged by low-Vp. • Very less seismicity due to ductility of the sediments. • Provides valuable information for intercontinental regions of analogous tectonics. In order to understand the geodynamical implications of intercontinental subduction, Cenozoic volcanism and ductile sediments in the northeast India and Indo-Burma region, a detailed 3-D P-wave tomography has been conducted using high-quality P-wave phase data recorded by local seismographic network ascribed to different agencies of India. This approach of using local earthquake data recorded by the portable seismographs installed for limited duration also provide an opportunity to adjudge the efficacy of local seismic network, which in turn enrich our understanding of intricate issues of Seismogenesis. P-wave tomographic imaging of lithospheric structure using arrival time data from local earthquakes recorded for the northeast India and Indo-Burma region revealed strong velocity heterogeneities beneath the region. The eastward subduction of the Indian slab beneath the Burmese arc with a dipping plane of intermediate-depth earthquakes has been imaged as high velocity (high-V) zones. Under the Cenozoic volcanic front in Burma, low velocity (low-V) anomalies are imaged. The low-V at shallow crustal layers may correspond to surface geological features, sedimentary materials or the faults associated with trapped fluids or loose pelgic sediments due to episodic tectonic upheavals, dehydration of the subducting Indian slab and serpentinization of the forearc mantle beneath the region, while high-V anomalies in the same depth range possibly indicate the presence of dense crystalline rocks under compressional stress that cause seismicity in the region. The distribution of seismicity is associated with major faults and lineaments associated with the distinct structural heterogeneities. The Bengal basin and the Central Burma Molasse basin exhibit low-V anomalies with very less seismicity due to ductility of the sediments. The low-V associated with the thick piles of sediments in the Bengal (>20 km) and Central Burma Molasse (≤12 km) Basin supported by the conspicuous distribution of low-Bouger gravity anomaly, which may have contributed significantly to major load along with an overthrust load of westward moving Indo-Burma ranges over the Indian lithosphere, which may amplify the stresses already acting on the Indian Plate due to load of the Himalaya, and making the region seismically more vulnerable. The present study is found to be in a good unison to the recent studies conducted in the region using different seismo-geophysical tools to different data set. The study provides valuable information on the lithospheric structures associated with seismogenesis and volcanic activity for the intercontinental regions, elsewhere in the world for our better understanding of geodynamical processes.

Seismic evidence of pop-up tectonics beneath the Shillong Plateau area of Northeast India

Our detailed 3-D seismic tomographic assimilation using high-quality phase arrival time data recorded by the local seismographic network demonstrated that heterogeneities in the crustal faults have contributed significantly to the pop-up tectonics beneath the Shillong Plateau, characterized by high-V and low-σ. The major seismogenic faults, namely, the north-dipping Dapsi thrust in association with Dauki fault in the south and south dipping Brahmaputra fault in the north, located either side of the Shillong Plateau that acted as the causative factors for the pop-up, which attributed to the lithostatic (high-V, low-σ) and sedimentary (low-V, high-σ) load, respectively. Seismicity is found confined to a depth ≤ 60 km. Uneven distribution of structural heterogeneities in the upper crust is responsible for earthquake genesis of varying strengths. It is intriguing to note that high-velocity anomalies and low-ϭ in the uppermost crust, interpreted as the Shillong Plateau that acted as a geometric asperity and the juxtaposition of high-V and low-V became the source zone of the 1897 Shillong earthquake (Ms 8.7) as a novel observation for the region. Structural heterogeneities are distinctly distributed between low-V, high-σ and high-V, low-σ in the lower crust plays a major role for future intense seismogenesis due to differential strain accumulation.

Eruption of Nyiragongo Volcano in May 2021 degassing of CO2 before and after eruption in the atmosphere

The Nyiragongo volcano is located in the western part of the African Rift. The deep fractures allow the upwelling of gases before and after the eruption, especially carbon dioxide (CO2). We want to understand the behaviour of carbon dioxide before and after the eruption of the Nyiragongo volcano on 22 May 2021. The Nyiragongo volcano erupted on the evening of Saturday 22 May 2021 at 18:15 local time in Goma, about 20 km north of the city of Goma. The lava flowed in two directions from the flanks of the volcano, one flow coming to a halt in the northeastern suburbs of Goma, The other flow cut off one kilometre of the national road 4 linking Goma to Butembo, a major regional road vital for the supply of the city of Goma, causing 32 victims, the production of several volatile gases in the atmosphere, the destruction of 17 villages in the territory of Nyiragongo and the cracking of the ground in the city of Goma and Gisenyi. The Nyiragongo Volcano is infamous for its extremely fluid lava that flows like water when the lava lake flows, which is monitored by the scientific community of the Goma Volcanological Observatory (O.V.G), the lava lake is the seat of an intense gas production. CO2 is a gas that is heavier than air, asphyxiating, and deadly at a concentration of 15%. The variation of CO2 in the different sites allows us to study the behaviour of the Nyiragongo Volcano, one Month before the eruption, the CO2 showed an increasing trend in some sites and a few Months after the eruption we observed a slight decrease in all the CO2 monitoring sites. Nyiragongo is an important contributor of CO2 and SO2 to the troposphere. This 2021 eruption is similar to the one of 17 January 2002, which caused the death of about 100 people and the evacuation of several people. The most deadly eruption of Nyiragongo was in 1977, when more than 600 people died.Post-eruption seismic activity was characterised by a large number of earthquakes of magnitude 3.5 or more. A few earthquakes of magnitudes greater than 3.5 have been recorded by global seismic networks (USGS), the strongest of which had a magnitude of 5.2 in 2002.Compared to other eruptions that Nyiragongo volcano has experienced, the seismic activity before the eruption of 22 May 2021 was not too alarming. This is because there were no strong earthquakes (greater than or equal to 3.5) that preceded the eruption a few months before, as had been noted in previous eruptions. After eruption, exceptional seismic activity was noticed. Many earthquakes of magnitudes greater than or equal to 3.5 were recorded by the local seismographic network. These earthquakes were felt, the strongest having a magnitude of 5.2

The Influence of a Transitional Stress Regime on the Source Characteristics of Induced Seismicity and Fault Activation: Evidence from the 30 November 2018 Fort St. John ML 4.5 Induced Earthquake Sequence

ABSTRACTOn 30 November 2018, a sequence of seismicity including a felt (ML∼4.5) induced earthquake occurred ∼16 km southwest of Fort St. John, British Columbia. Using a local seismograph network around the epicentral region, we identified > 560 seismic events over a two-week period, incorporating two mainshock events within a 45 min time interval, both with ML>4.3. This seismicity occurred close in location and depth to ongoing hydraulic fracturing operations. Using previously unpublished data, our analysis suggests that events, including the largest mainshock, occurred at the interval of fluid injection, which is shallower than previously reported. The events showed a mix of reverse, oblique normal, and strike-slip mechanisms within a well-defined structural corridor that forms the southern margin of the Fort St. John graben. The two mainshock events reveal opposing mechanisms: one as a reverse (re)activation of a normal fault (ML 4.5) and the other an oblique normal mechanism (ML 4.3). Stress inversion and bootstrap analysis of 72 well-constrained focal mechanisms indicate that the maximum principal stress direction is horizontal, oriented in a north-northeast direction (3°–36°). However, the intermediate and minimum stress axes fluctuate between horizontal and vertical and are nearly equal in magnitude, indicating that both reverse and strike-slip regimes can occur in response to relatively small stress perturbations. Stress inversions using event subsets before and after the largest mainshock reveals an approximately 30° counter-clockwise coseismic rotation of the principal stress axes in the hypocentral region. Furthermore, the observed seismicity suggests that the largest mainshock event exceeded the calculated Mmax using models based on injected volumes, suggesting that it may be an example of runaway rupture. This has important implications for risk analysis, because small changes in the stress field may be induced through ongoing operations in this area, destabilizing different faults within a complex structural environment.

Crustal velocity structure and seismotectonics of the Kinnaur region of northwest Himalaya: New constraints based on recent micro-earthquake data

• New ‘minimum’ 1-D crustal velocity model of Kinnaur Himalaya through VELEST algorithm. • High micro-earthquake activity to the north of HHSB in the Kinnaur Himalaya. • Crustal seismicity is concentrated between 5 and 25 km depth above the Main Himalayan Thrust. • Two distinct zones with thrust fault mechanism in the HHSB and normal fault mechanism to the northern part. • Major improvement in the crustal structure and the seismotectonics of the region. The occurrence of micro-earthquakes is monitored in the Kinnaur region of NW Himalaya using a local seismograph network of 12 broadband stations. About 800 micro-earthquakes recorded from 2008 to 2012 within the magnitude range of 1.0 to 4.5, are relocated using the newly developed 1-D ‘minimum’ crustal velocity model. The velocity model developed using the VELEST program based on P- and S-phase data divides the upper 35 km thick crust into 8 layers in which Vp/Vs appears to be quite variable. The seismogenic zones in this part are mainly located within a depth range between 5 and 25 km. On the basis of integration of different studies, this region around the epicentre zone of the M S 6.8 earthquake of 1975 can be divided into several seismogenic zones. The southern zone falls in the Higher Himalayan Seismic Belt, where the occurrence of M L > 5.0 earthquakes is rare. However, in comparison with other regions of the NW Himalaya, the northern zone in the South Tibetan Detachment shows a unique feature of high seismicity. Seismicity is bounded to the east by the Kaurik-Chango fault zone. The north–south oriented Kaurik Chango normal fault is transverse to the strike of major tectonic features of the Himalaya and follows the trend of present-day seismicity along with the fault plane solution of M S 6.8 Kinnaur earthquake. Results from microseismicity and fault plane solutions provide valuable information to understand the seismotectonics of the region. The stress inversion shows variable stress patterns from north to south.

Relocated Hypocenters and Structural Analysis from Waveform Modeling of Aftershocks from the 2011 Prague, Oklahoma, Earthquake Sequence

Abstract We present an analysis of aftershocks with M L >3 from the 2011 Prague, Oklahoma, earthquake sequence, an intraplate sequence of moderate‐size earthquakes within the North American craton. We apply waveform analysis to seismograms from temporary local seismograph networks to relocate the aftershocks’ hypocenters. The relocations show that the hypocenters are confined to the Precambrian basement. The character of the recorded seismograms waveforms are reconstructed by finite‐difference forward modeling of seismic sources buried at different depths in 2D crustal models. The numerical modeling indicates that the observed complex waveforms, and particularly the coda, are created by waves scattering off numerous shallow crustal velocity anomalies. We infer that these anomalies correspond to paleoriver channels that are embedded in the Pennsylvanian limestone formation. The modeling indicates that the amount of energy within the coda depends on the depth of the source. Our waveform modeling of the observed seismograms is consistent with our relocations of the aftershocks to depths of 5–10 km, within the Precambrian basement. Electronic Supplement:Empirical and synthetic data, model parameters, and geological figures.

A Historical Review of Induced Earthquakes in Texas

In Texas, earthquakes have occurred in close association with activities accompanying petroleum production since 1925. Here we develop a five‐question test to categorize individual events as “tectonic,” “possibly induced,” “probably induced,” or “almost certainly induced.” In Texas, the probably induced and almost certainly induced earthquakes are broadly distributed geographically—in the Fort Worth basin of north Texas, the Haynesville Shale play area of east Texas, along the Gulf Coast in south Texas, and the Permian basin of west Texas. As the technologies applied to manage petroleum fields have evolved, induced earthquakes have been associated with different practices. In fields being driven by primary recovery prior to 1940, earthquakes occurred in fields extracting high volumes of petroleum from shallow strata. Subsequently, as field pressures decreased and secondary recovery technologies became common, earthquakes also occurred in association with waterflooding operations. Since 2008, the rate of earthquakes with magnitudes greater than 3 has increased from about 2 events/yr to 12 events/yr; much of this change is attributable to earthquakes occurring within a few kilometers of wastewater disposal wells injecting at high monthly rates. For three sequences monitored by temporary local seismograph networks, most hypocenters had focal depths at and deeper than the depth of injection and occurred along mapped faults situated within 2 km of injection sites. The record clearly demonstrates that induced earthquakes have been broadly distributed in several different geographic parts of Texas over the last 90 years.

Seismological evidence of an active footwall shortcut thrust in the Northern Itoigawa–Shizuoka Tectonic Line derived by the aftershock sequence of the 2014 M 6.7 Northern Nagano earthquake

A destructive M 6.7 earthquake struck Northern Nagano prefecture on November 22, 2014. The main shock occurred on the Kamishiro fault segment of the northern Itoigawa–Shizuoka Tectonic Line (ISTL). We used data recorded at 41 stations of the local seismographic network in order to locate 2118 earthquakes that occurred between November 18 and November 30, 2014. To estimate hypocenters, we assigned low Vp models to stations within the Northern Fossa Magna (NFM) basin thus accounting for large lateral crustal heterogeneities across the Kamishiro fault. In order to further improve accuracy, the final hypocenter locations were recalculated inside a 3D velocity model using the double-difference method. We used the aftershock activity distribution and focal mechanism solutions of major events in order to estimate the source fault area of the main shock. Our analysis suggests that the shallow part of the source fault corresponds to the surface trace of the Kamishiro fault and dips 30°–45° SE, while the deeper part of the source fault corresponds to the downdip portion of the Otari–Nakayama fault, a high angle fault dipping 50°–65° SE that formed during the opening of the NFM basin in the Miocene. Along its surface trace the Otari–Nakayama fault has been inactive during the late Quaternary. We verified the validity of our model by calculating surface deformation using a simple homogeneous elastic half-space model and comparing it to observed surface deformation from satellite interferometry, assuming large coseismic slip in the areas of low seismicity and small coseismic slip in the areas of high seismicity. Shallowing of the source fault from 50°–65° to 30°–45° in the upper 4km, in the areas where both surface fault traces are visible, is a result of footwall shortcut thrusting by the Kamishiro fault off the Otari–Nakayama fault.

Seismicity and Seismotectonics of Jeddah-Makkah Region, West-Central Saudi Arabia

Jeddah-Makkah regionis have been suffering from earthquake crisis where some moderate to destructive earthquakes have been recorded. These earthquake activities are oriented along major faults or clustered in certain spots. Moreover, these earthquake events have annual recurrence periods, so the identification of these seismogenic source zones is of utmost importance for mapping the most hazardous localities which should be avoided in the future urban planning. Historical and instrumental earthquakes have been collected from national and international data centers and unified in catalogue. The existence of microearthquakes inland suggests that there is a significant level of tectonic activity at away from the axial trough of the Red Sea. Then, seismogenic source zones have been defined depending on the major tectonic trends; distribution of earthquake epicenters, seismicity rate (a & b-values) and fault plane solution of major earthquakes. It is concluded that Jeddah-Makkah region is affected by the outlined five seismogenic source zones; three of these zones aligned of the main Red Sea axial trough (southwestern Jeddah, western Jeddah, and northwestern Jeddah zones), while the other two zones are located in the land area of the region (Thewal-Rabegh and Jeddah-Makkah zones). These inland zones correlated well with the main trends of major tectonics which refleet the reactivation of tectonic movements along these fault trends. The Red Sea zones are in agreement with the main path of the axial trough. The range of b-value in these identified zones is 0.65 to 1.03 through these identified zones. The area characterized by higher b-values could be indicative of a relative low stress regime which was a result of resulting from the stress release by the earthquakes. Whereas, the areas of lower b-values can be considered as an evidence of a relatively higher stress regime associated with a dominantly extensional stresses. Based on aforementioned, the region is suffering from different stress level accumulations which, in turn, cause earthquakes with different magnitudes. Accordingly, deployment of local seismograph network through Jeddah-Makkah region is highly recommended. These results will support, to a great extent, seismic hazard assessment and risk mitigation of the region.

Earthquake sequences in the southern block of the Pernambuco Lineament, NE Brazil: Stress field and seismotectonic implications

The Pernambuco Lineament (PeL) is an E–W-striking, 700km long shear belt located in the central part of the Borborema Province, Brazil. Seismic sequences have been monitored in the region since 1991, and they are clearly correlated with the main shear belt and its NE-trending branches. A new sequence of small earthquakes occurred in April 2010 away from the main belt in the southern block of the lineament. We monitored this seismicity using a ten station network for 154days and used the observations to determine the nature of the seismicity and the stress patterns. In addition, we used data from five previous local seismograph networks to determine the stress tensor. The results indicate that the seismicity is concentrated in three clusters, two of which are associated with normal faults: the first strikes 96°, dips 51°, and has a rake of −65°, and the second strikes 253°, dips 64°, and has a rake of −120°. These faults are 2.2km and 1.5km long and 2.0–3.0km and 3.0–3.5km deep, respectively, and are not correlated with the Pernambuco Lineament or its NE-trending branches. The inversion of focal mechanisms from our investigation and previous studies indicates E–W-trending compression and N–S-trending extension that affects both the northern and southern blocks of the PeL. The new data indicate that the fault behavior in the shear zone area is more complex than has previously been observed along reactivated mylonitic belts.

The global aftershock zone

Abstract The aftershock zone of each large (M ≥ 7) earthquake extends throughout the shallows of planet Earth. Most aftershocks cluster near the mainshock rupture, but earthquakes send out shivers in the form of seismic waves, and these temporary distortions are large enough to trigger other earthquakes at global range. The aftershocks that happen at great distance from their mainshock are often superposed onto already seismically active regions, making them difficult to detect and understand. From a hazard perspective we are concerned that this dynamic process might encourage other high magnitude earthquakes, and wonder if a global alarm state is warranted after every large mainshock. From an earthquake process perspective we are curious about the physics of earthquake triggering across the magnitude spectrum. In this review we build upon past studies that examined the combined global response to mainshocks. Such compilations demonstrate significant rate increases during, and immediately after (~ 45 min) M > 7.0 mainshocks in all tectonic settings and ranges. However, it is difficult to find strong evidence for M > 5 rate increases during the passage of surface waves in combined global catalogs. On the other hand, recently published studies of individual large mainshocks associate M > 5 triggering at global range that is delayed by hours to days after surface wave arrivals. The longer the delay between mainshock and global aftershock, the more difficult it is to establish causation. To address these questions, we review the response to 260 M ≥ 7.0 shallow (Z ≤ 50 km) mainshocks in 21 global regions with local seismograph networks. In this way we can examine the detailed temporal and spatial response, or lack thereof, during passing seismic waves, and over the 24 h period after their passing. We see an array of responses that can involve immediate and widespread seismicity outbreaks, delayed and localized earthquake clusters, to no response at all. About 50% of the catalogs that we studied showed possible (localized delayed) remote triggering, and ~ 20% showed probable (instantaneous broadly distributed) remote triggering. However, in any given region, at most only about 2–3% of global mainshocks caused significant local earthquake rate increases. These rate increases are mostly composed of small magnitude events, and we do not find significant evidence of dynamically triggered M > 5 earthquakes. If we assume that the few observed M > 5 events are triggered, we find that they are not directly associated with surface wave passage, with first incidences being 9–10 h later. We note that mainshock magnitude, relative proximity, amplitude spectra, peak ground motion, and mainshock focal mechanisms are not reliable determining factors as to whether a mainshock will cause remote triggering. By elimination, azimuth, and polarization of surface waves with respect to receiver faults may be more important factors.

Comment on "A Unified Seismic Catalog for the Iranian Plateau (1900-2011)" by Mohammad P. Shahvar, Mehdi Zare, and Silvia Castellaro

A recent paper in the March/April 2013 issue of Seismological Research Letters , “A Unified Seismic Catalog for the Iranian Plateau (1900–2011)” by Mohammad P. Shahvar, Mehdi Zare, and Silvia Castellaro (henceforth referred to as Shahvar et al. , 2013) provides a unified homogeneous earthquake catalog, for the period 1900–2011, particularly for seismic‐hazard assessment studies in Iran. They subdivide the territory of the Iranian plateau into two domains only according to seismicity to derive empirical relationships to convert the original magnitudes ( M s, m b, M L, and M N) to a uniform scale ( M w), applying standard least squares (SR) and inverted least squares (ISR) regression methods and orthogonal regression method (OR). Some contradictions in Shahvar et al. (2013) encouraged us to declare a few points. The distinction between seismotectonic zones is based on all available geologic, tectonic, geophysical, and seismological data. In other words, a seismotectonic zone is considered an area that under present‐day geodynamic regimes has an internally consistent tectonic setting and unified seismicity pattern (Ye et al. , 1993, 1995). Shahvar et al. (2013) claim that the Iranian plateau can de facto be divided into two domains only because seismicity in the Alborz and Central Iran zones show statistically undistinguishable features. In contrast, Karimiparidari et al. (2013) distinguish six seismotectonic zones based on Mirzaei et al. (1998) and Tavakoli (1996). It is noteworthy that the second author of the two concurrent inconsistent papers is in common. Based on Nowroozi (1971) and Bird et al. (1975), it is stated by Shahvar et al. (2013) that the continental Arabian shield subducts beneath the Zagros belt. However, local seismograph networks have found no reliable depths in the Zagros deeper than ∼20 km, even in places where deeper earthquakes were reported to have occurred (e.g., Niazi et al. , 1978; Tatar et al. , …

A hybrid multiple event location technique to obtain ground truth event locations

SUMMARY We present a multiple event location technique that is able to produce minimally biased (location accuracy of 5 km or better) events from a cluster of earthquakes, without relying on prior ground truth information or dense local seismograph networks. In the hybrid hypocentroidal decomposition and reciprocal cluster analysis (HDC-RCA) method, HDC determines accurate relative locations in an event cluster (but a biased hypocentroid) using regional and/or teleseismic data. RCA uses local phases and invokes the reciprocity principle by relocating the local station network centroid with the events fixed (by HDC) as fictitious stations. Since the location of the local station network centroid is exactly known, the mislocation vector between the true and relocated centroids represents the offset with which the entire event cluster must be shifted to obtain unbiased absolute locations. We have developed applicability criteria to help determine under what conditions HDC-RCA is able to determine minimally-biased locations in an event cluster. The performance of the HDC-RCA method is validated on several event clusters containing previously determined ground truth events and applied to clusters for which ground truth events were not previously determined. Events promoted to GT5 level are provided in an electronic supplement.

Aftershock Investigation in the Andaman-Nicobar Islands: An Antidote to Public Panic?

Public panic prevailed in every part of the Andaman and Nicobar Iislands of India following the megathrust Sumatra earthquake ( Mw 9.3) on 26 December 2004. In this article, we present a very brief analysis of our continuous three-month (January-March 2005) monitoring and recording of aftershock data following the main earthquake to show how this endeavor reduced public panic and constituted an important ingredient to a disaster management program for the Andaman-Nicobar region. Monitoring was conducted using six short-period three-component temporary digital seismograph stations set up in different parts of the Andaman and Nicobar islands. Our findings demonstrate that 1) there was no aftershock gap zone as recorded by the far-distant seismographic network, hence negating the possibility of immediate strong quakes in the Andaman and Nicobar islands; 2) there was no strong shaking at full moon (26 January 2005) due to tidal stresses, although the rate of aftershock activity increased by about 31% from the events of the preceding day; 3) there is a north-south trending 850 × 350-km2 rupture area beneath the Andaman and Nicobar islands; and 4) eruptions of mud at the Baratang volcanic zone and lavas at the Barren volcanic zone occurred because of strong shaking due to the mainshock and a series of aftershock clusters within 100 km of the individual volcanic zones. These eruptions may continue for a couple of years, until the aftershock sequence ceases. We show that this new dataset from the local seismographic network constituted an important factor in reassuring the coastal people and islanders about imminent dangers from future earthquakes and tsunamis, and we propose geochemical tests of the erupted mud samples from the Baratang volcanic zone, with geophysical exploration followed by drilling, to ascertain the presence of oil and gas reserves and the phenomenon of gas seepage under the …

Scenarios of Seismic Risk in the United Arab Emirates, an Approximate Estimate

We estimate the losses due to 10 scenario earthquakes in 150 settlements of the United Arab Emirates (UAE). For southern Iran, we use four source zones and the maximum magnitudes in them as determined by GSHAP (7.2 ≤ M ≤ 8.1). For six local scenario earthquakes, we use the range 5.5 ≤ M ≤ 6.5, place the sources mainly on mapped faults and vary the distance to major cities from 10 to 60 km. In the test case of the Masafi earthquake (M5, 11 March 2002), the method and data bank we use yield the correct results, suggesting that our approach to the problem is valid for the UAE. The sources in Iran are expected to cause only minor damage, except for an M8.1 earthquake in the Makran region. For such an event we expect some deaths, several hundred injured and a loss of 3–6% of the value to the building stock in the northeastern UAE, including Oman. The losses for local scenarios with epicenters in the unpopulated areas of the UAE and for scenarios with M < 5.8 are estimated to be minor. Because the two major mapped faults run through several of the large cities, scenarios with short epicentral distances from cities have to be considered. Scenarios with M6 near cities lead to estimates of about 1000 ± 500 deaths, and several thousand injured. Most buildings are expected to be damaged to a moderate degree and the loss to buildings is estimated around 1/4of their value. If the magnitude should reach 6.5, the losses to humans and to building value could be staggering. These estimates are approximate because: (1) there exists no local seismograph network that could map active faults by locating microseismicity; (2) there exist no historically old buildings that could serve as tests for effects due to strong ground motion in the past; (3) there exist no microzonation of the subsurface properties in this region of unconsolidated building ground; (4) there exist no detailed inventory of building fragility. Nevertheless, our conclusion that there exists a substantial seismic risk in the UAE is reliable, because our method yields accurate results in the cases of earthquakes with known losses during the last several decades in the Middle East.

Newly imaged shape of the deep seismic zone within the subducting Pacific plate beneath the Hokkaido corner, Japan‐Kurile arc‐arc junction

The geometry of the deep seismic zone associated with the subducting Pacific plate beneath the North American plate in the Hokkaido corner, Japan‐Kurile arc‐arc junction has been investigated based on the hypocenters accurately relocated by a dense local seismographic network with three‐dimensional P and S wave velocity structures. The model suggests that the lateral changes in the dip of the subduction zone are more rapid than previously believed. We found that the deep seismic zone in the Pacific plate dips at 20° to 30° on the side of the Japan arc, the dip angle increased in a narrow region around 143°–144°E by 10° to 20°, and the dip was 40° to 50° on the side of the Kurile arc. In the transitional zone around 143°–144°E an unusual distribution of microearthquakes has been discovered within the Pacific plate, which was located on a near‐vertical plane with an area of 50 × 100 km2 with strike that was perpendicular to the Kurile Trench axis. This feature was named the Tokachi‐Oki slab‐cracking zone. We suggest that this is a slab‐cracking zone torn by the extensional stress due to the rapid lateral changes in the dip of the subduction zone. The slab‐cracking zone may possibly grow into the large disruption or tear of the slab in the future.

コーダ波エンベロープの解析による散乱強度非一様分布の検出

Deterministic analysis of coda waves has become one of the effective approaches to estimate inhomogeneous structure in the crust and uppermost mantle. Seismic array observation gives coherent wave trains, which are suitable for estimating scattering properties beneath the array. Local or regional seismographic network offers distribution of coda wave energy in space and time, which has an advantage of estimating scattering properties in a wide area below the station network. In this paper, several studies of the latter analysis are introduced. One of those analyses, proposed by Nishigami (1991), is to invert coda envelopes observed by a local seismographic network into spatial distribution of scattering coefficients. An observational equation is derived under the assumption of S to S single isotropic scattering and spherical source radiation, where observed data are the deviations of coda envelope from a master curve decaying smoothly. The unknown parameters are the relative scattering coefficients assigned to small blocks that divide the three-dimensional space. The observational equation can be solved by a recursive stochastic inversion. This analysis method applied to the central California data delineates deep structure of the San Andreas fault system as a structure with strong scattering. Several regions in central Japan were also analyzed by this method and detailed distribution of S wave reflectors was estimated below active volcanoes, Mt. Ontake and Mt. Nikko-Shirane. Weak scattering correlated with a seismic gap area was also recognized in the Hokuriku region. Revenaugh (1995a) proposed another method, callled Kirchhoff coda migration, which stacks the energy of single-scattered waves within the coda of teleseismic P waves observed by a local seismographic network. This method was applied to southern California and the following images were obtained, strong P to P scattering from the slab subducting beneath the Transverse Ranges at depths of 50-200km; P to Rg scattering correlated with topographic roughness; and P to S scattering from the aftershock area of the 1992 Landers earthquake. These two methods analyzing the seismic network data seem to be effective to estimate the heterogeneous structure in the crust and uppermost mantle. From the viewpoint of imaging the deep structure of active faults, the inversion analysis of coda envelopes from local earthquakes may be more effective. The variance reduction of the observed data, however, is low, for example, about 14% for the central California analysis. In the future, the simple models of scattering and source radiation assumed in the inversion analysis should be improved into more realistic ones to increase the model fitting. Forward modeling such as proposed by Obara (1997) will also be important to understand the whole shape of coda envelopes related to inhomogeneous distribution of scattering and attenuation intensity.

A seismologically motivated survey of blasting activity in the northern Rhine area

AbstractDiscrimination between quarry blasts and earthquakes has gained importance due to signature of the Comprehensive Test Ban Treaty. In this context, large chemical explosions are significant. In the routine analysis of data from local seismograph networks, discrimination between smaller blasts and micro-earthquakes is not always clear. Many quarries are in operation and blasts far outnumber natural earthquakes in the highly industrialized northern Rhine area.We compiled a list of active quarries in the Northern Rhine Area and mapped their locations. We then created a database from a questionnaire sent out to all quarries on the list. From the 33% of questionnaires that were returned, we discerned some representative values for the main blasting parameters and explosive consumption. In the study area of 72,000 km2, approx. 21,000 blasts are fired per year (80 per working day). Most of the blasts (72%) have total explosive charges between 400 and 4500 kg. Shots with charges above 10 tons are rare (20-30 per year). Some 80% of the blasts are ripple-fired with a nominal firing time interval of 20 ms.Based on empirical amplitude vs. distance curves from vibration control measurements, a relation between maximum charge weight per delay time, L (kg), and a ‘quarry blast’ magnitude, MQB, is derived: MQB= 0.6·log10(L) + 0.131. Using this relation and extrapolating the database from the questionnaire shows that for magnitudes between 1 and 2, blasts occur 200–250 times more frequently than micro-earthquakes in the Northern Rhine area.