Recent developments of the Middle East catalog

This article summarizes a recent study in the framework of the Global Earth model (GEM) and the Earthquake Model of the Middle East (EMME) project to establish the new catalog of seismicity for the Middle East, using all historical (pre-1900), early and modern instrumental events up to 2006. According to different seismicity, which depends on geophysical, geological, tectonic, and seismicity data, this region is subdivided to nine subregions, consisting of Alborz–Azerbaijan, Afghanistan–Pakistan, Saudi Arabia, Caucasus, Central Iran, Kopeh–Dagh, Makran, Zagros, and Turkey (Eastern Anatolia; after 30° E). After omitting the duplicate events, aftershocks, and foreshocks by using the Gruenthal method, and uniform all magnitude to Mw scale, 28,244 main events remain for the new catalog of Middle East from 1250 B.C. through 2006. The magnitude of completeness (Mc) was determined as 4.9 for five out of nine subregions, where the least values of Mc were found to be 4.2. The threshold of Mc is around 5.5, 5.0, 4.5, and 4.0, for the time after 1950, 1963, 1975, and 2000, respectively. The average of teleseismic depths in all regions is less than 15 km. Totally, majority of depth for Kopeh–Dagh and Central Iran, Zagros, and Alborz–Azerbaijan, approximately, is 15, 13, and 11 km and for Afghanistan–Pakistan, Caucasus, Makran, Turkey (after 30° E), and Saudi Arabia is about 9 km.     

Regional Variations of the <Emphasis Type="Italic">ω</Emphasis>-upper Bound Magnitude of GIII Distribution in the Iranian Plateau

The Iranian Plateau does not appear to be a single crustal block, but an assemblage of zones comprising the Alborz—Azerbaijan, Zagros, Kopeh—Dagh, Makran, and Central and East Iran. The Gumbel’s III asymptotic distribution method (GIII) and maximum magnitude expected by Kijko—Sellevoll method is applied in order to check the potentiality of the each seismogenic zone in the Iranian Plateau for the future occurrence of maximum magnitude (M_max). For this purpose, a homogeneous and complete seismicity database of the instrumental period during 1900–2012 is used in 29 seismogenic zones of the examined region. The spatial mapping of hazard parameters (upper bound magnitude (ω), most probable earthquake magnitude in next 100 years (M₁₀₀) and maximum magnitude expected by maximum magnitude estimated by Kijko—Sellevoll method (max M_{K ? S}^max) reveals that Central and East Iran, Alborz and Azerbaijan, Kopeh—Dagh and SE Zagros are a dangerous place for the next occurrence of a large earthquake.     

Imaging Faults and Shear Zones Using Receiver Functions

The geometry of faults at seismogenic depths and their continuation into the ductile zone is of interest for a number of applications ranging from earthquake hazard to modes of lithospheric deformation. Teleseismic passive source imaging of faults and shear zones can be useful particularly where faults are not outlined by local seismicity. Passive seismic signatures of faults may arise from abrupt changes in lithology or foliation orientation in the upper crust, and from mylonitic shear zones at greater depths. Faults and shear zones with less than near-vertical dip lend themselves to detection with teleseismic mode-converted waves (receiver functions) provided that they have either a contrast in isotropic shear velocity (V _s), or a contrast in orientation or strength of anisotropic compressional velocity (V _p). We introduce a detection method for faults and shear zones based on receiver functions. We use synthetic seismograms to demonstrate common features of dipping isotropic interfaces and contrasts in dipping foliation that allows determination of their strike and depth without making further assumptions about the model. We proceed with two applications. We first image a Laramide thrust fault in the western U.S. (the Wind River thrust fault) as a steeply dipping isotropic velocity contrast in the middle crust near the surface trace of the fault; further downdip and across the range, where basin geometry suggests the fault may sole into a subhorizontal shear zone, we identify a candidate shear zone signal from midcrustal depths. The second application is the use of microstructural data from exhumed ductile shear zones in Scotland and in the western Canadian Shield to predict the character of seismic signatures of present-day deep crustal shear zones. Realistic anisotropy in observed shear fabrics generates a signal in receiver functions that is comparable in amplitude to first-order features like the Moho. Observables that can be robustly constrained without significant tradeoffs are foliation strike and the depth of the foliation contrast. We find that an anisotropy of only a few percent in the shear zone is sufficient to generate a strong signal, but that the shear zone width is required to be >2 km for typical frequencies used in receiver function analysis to avoid destructive interference due to the signals from the boundaries of the shear zone.     

Earthquake Potential in the Zagros Region,Iran

Seismic strain and b value are used to quantify seismic potential in the Zagros region (Iran). Small b values (0.69 and 0.69) are related to large seismic moment rates (9.96×10¹⁷ and 4.12×10¹⁷) in southern zones of the Zagros, indicating more frequent large earthquakes. Medium to large b values (0.72 and 0.92) are related to small seismic moment rates (2.94×10¹⁶ and 6.80×10¹⁶) in middle zones of the Zagros, indicating less frequent large earthquakes. Small b value (0.64) is related to medium seismic moment rate (1.38×10¹⁷) in middle to northern zone of the Zagros, indicating frequent large earthquakes. Large b value (0.87) is related to large seismic moment rate (2.29×10¹⁷) in northwestern zone, indicating more frequent large earthquakes. Recurrence intervals of large earthquakes (M > 6) are short in southern (10 and 14 years) and northwestern (13 years) zones, while the recurrence intervals are long in the middle (46 and 114 years) and middle to northern (25 years) zones.     

The maximum expected earthquake magnitudes in different future time intervals of six seismotectonic zones of Iran and its surroundings

One of the crucial components in seismic hazard analysis is the estimation of the maximum earthquake magnitude and associated uncertainty. In the present study, the uncertainty related to the maximum expected magnitude μ is determined in terms of confidence intervals for an imposed level of confidence. Previous work by Salamat et al. (Pure Appl Geophys 174:763-777, 2017) shows the divergence of the confidence interval of the maximum possible magnitude m_max for high levels of confidence in six seismotectonic zones of Iran. In this work, the maximum expected earthquake magnitude μ is calculated in a predefined finite time interval and imposed level of confidence. For this, we use a conceptual model based on a doubly truncated Gutenberg-Richter law for magnitudes with constant b-value and calculate the posterior distribution of μ for the time interval T_f in future. We assume a stationary Poisson process in time and a Gutenberg-Richter relation for magnitudes. The upper bound of the magnitude confidence interval is calculated for different time intervals of 30, 50, and 100 years and imposed levels of confidence α?=?0.5, 0.1, 0.05, and 0.01. The posterior distribution of waiting times T_f to the next earthquake with a given magnitude equal to 6.5, 7.0, and 7.5 are calculated in each zone. In order to find the influence of declustering, we use the original and declustered version of the catalog. The earthquake catalog of the territory of Iran and surroundings are subdivided into six seismotectonic zones Alborz, Azerbaijan, Central Iran, Zagros, Kopet Dagh, and Makran. We assume the maximum possible magnitude m_max?=?8.5 and calculate the upper bound of the confidence interval of μ in each zone. The results indicate that for short time intervals equal to 30 and 50 years and imposed levels of confidence 1???α?=?0.95 and 0.90, the probability distribution of μ is around μ?=?7.16???8.23 in all seismic zones.     

The composition of basaltic lavas from Bayuda,Sudan and their place in the cainozoic volcanic history of north-east Africa

Six new analyses of young basaltic rocks from the Bayuda field show the predominant rock types to be strongly undersaturated basanites and nepheline trachybasalts. Both types are believed to represent magmas of deep-seated origin. Similar rocks are widely distributed in north-east Africa but mildly alkaline to tholeiitic basalts were erupted along the eastern margin of the continent in early and late Cainozoic times, whereas along the Tripoli-Tibesti zone to the west mildly alkaline basalts were probably confined to the early Tertiary. The Tripoli-Tibesti zone was one of uplift and strongly tensional tectonics in the late Mesozoic and early Cainozoic, and at this time may have been a line of potential lithospheric rifting, but a period of quiescence followed and resurgence of activity in the late Cainozoic produced weaker tensional structures and more strongly alkaline basic magmas. The region between these two main zones of activity was characterized throughout by intermittent alkaline volcanicity and weak tectonism. Neverthless, fracture zones which apparently controlled the volcanicity are beginning to be recognized in this area. It is argued that African volcanic activity is related to linear, rather than circumscribed, areas of mantle activity. Possible connections with epeirogenic movements within the Alpine orogenic belt appear to have been neglected in the debate on the causes of African igneous activity.     

An evaluation of earthquake hazard parameters in the Iranian Plateau based on the Gumbel III distribution

The Gumbel’s third asymptotic distribution (GIII) of the extreme value method is employed to evaluate the earthquake hazard parameters in the Iranian Plateau. This research quantifies spatial mapping of earthquake hazard parameters like annual and 100-year mode beside their 90 % probability of not being exceeded (NBE) in the Iranian Plateau. Therefore, we used a homogeneous and complete earthquake catalogue during the period 1900–2013 with magnitude M _w ? ≥?4.0, and the Iranian Plateau is separated into equal area mesh of 1° late?×?1° long. The estimated result of annual mode with 90 % probability of NBE is expected to exceed the values of M _w 6.0 in the Eastern part of Makran, most parts of Central and East Iran, Kopeh Dagh, Alborz, Azerbaijan, and SE Zagros. The 100-year mode with 90 % probability of NBE is expected to overpass the value of M _w 7.0 in the Eastern part of Makran, Central and East Iran, Alborz, Kopeh Dagh, and Azerbaijan. The spatial distribution of 100-year mode with 90 % probability of NBE uncovers the high values of earthquake hazard parameters which are frequently connected with the main tectonic regimes of the studied area. It appears that there is a close communication among the seismicity and the tectonics of the region.     

Volcanic rocks of the Witu Islands,Papua New Guinea: The origin of magmas above the deepest part of the New Britain Benioff zone

The Witu Islands are Quaternary volcanoes that overlie the deepest (about 300–580 km) part of the New Britain Benioff zone. The islands are about 100 km south of the transcurrent-divergent plate boundary that crosses the Bismarck Sea, and they surmount the southeastern end of the Willaumez-Manus Rise. The rocks are olivine- and quartz-normative tholeiitic basalts, low- and high-SiO₂ andesites, dacites, and rhyolites. Alkaline rocks that overlie the deep (greater than 300 km) parts of other Benioff zones have not been found in the Witu Islands. Compared to the Witu Islands rocks, those with similar SiO₂ contents from New Britain volcanoes that overlie progressively shallower parts of the Benioff zone to the south, are, for example, generally poorer in Na+K, Ti, and P, and higher in Ca and Al. There are similar progressive changes in trace-element abundances, but Zr and Nb contents are distinctly richer in Witu Islands rocks.⁸⁷Sr/⁸⁶Sr values range between 0.70311 and 0.7038, which are typical for rocks from New Britain as a whole and from other island arcs in the southwest Pacific. Two¹⁴³Nd/¹⁴⁴Nd values of 0.512211 and 0.512271, taken together with the Sr isotopic results, define a source region equivalent to those for oceanic-island basalts; there is no evidence for sea-water contamination of the sources. Perhaps the most striking feature of the Witu Islands rocks is their compositional diversity. Basalts range from olivine tholeiites similar to marginal-basin basalts from other areas, to quartz tholeiites similar in most respects to those typical of island arcs, and to incompatible-element-enriched tholeiites that are close to silica-undersaturation. Andesites on Unea Island have a strong island-arc signature, but the andesites, dacites, and rhyolite of Garove Island have some features that may be more in common with the silica-oversaturated rocks of oceanic areas. The mineralogy of Witu Islands basalts is characterised by phenocrysts of olivine (Fo_89-54), plagioclase (An_90-55) and Ca-augite. Cr-Al-rich spinels and aluminous magnetites are present as inclusions in some olivine phenocrysts. Groundmass fayalite, alkali feldspar, and dacitic to rhyolitic glasses high in K/Na are found in a few samples. In contrast to basalts from volcanoes above the shallower parts of the New Britain Benioff zone, those of the Witu Islands are characterised by rarity of low-Ca pyroxene, as phenocrysts or in the groundmass. Rocks richer in SiO₂ are characterised by the presence of orthopyroxene phenocrysts and lesser amounts of olivine. Hydrous minerals appear to be absent. Groundmass Fe-Ti oxides define crystallisation temperatures (about 800–1050°C) and oxygen fugacities (fO₂) corresponding to those of the Ni-NiO buffer, but up to two fO₂ log units above it. The suite as a whole is phenocryst-poor compared to most New Britain volcanic rocks. A significant degree of mantle heterogeneity is inferred by the chemical variability of the Witu Islands rocks. However, there are no compelling reasons in support of the interpretation that source heterogeneity is due to the effect of a slab-derived component. The cause of the heterogeneity is unclear, but may be due to mantle differentiation processes related to an anomalous tectonic setting.     

Liparitic volcanism of some regional volcanic structures in Kazakhstan,Middle Asia and Caucasus

Liparitic volcanism is a typical feature of the orogenic phase giving rise to the Kazakhstan, Middle Asia and Caucasus folded systems. The main characteristics of the liparitic volcanism common to these three regions are the following:

1. Geo-structural zonation of the volcanic structures.
2. Dismembered Moho surface within the volcanic structures.
3. Synchronous, yet independent evolution of liparitic and andesitic volcanisms.
4. Ignimbritic character of the liparitic volcanism.
5. Lateral petrochemical zonation with some features common to the liparitic and andesitic rock series.

Geo-structural and petrochemical zonations are likely governed in the regions studied by a deep-seated plutonic body.     

1974 activity of chokai volcano,Japan

Shallow intrusion of magma caused phreatic explosions and mud flows at the snow-covered summit of Chokai volcano, northeast Honshu, Japan, after 153 years of dormancy. Total heat emission by the eruption is estimated at more than 3.0 × 10²¹ erg. Equivalent amount of magma is about 2.2 × 10⁸ ton. Focal mechanisms of the associated volcanic earthquakes, which had been variable during the period of eruption. became stable after the cessation of the surface activity with pressure axis in a NW direction which is also the strike of the epicenter distribution. This temporal change of focal mechanisms may be interpreted as the result of propagation of increased pore pressure in the direction of the maximum pressure in the post eruptive period. The magmatic pressure which certainly predominated during the eruption period and caused carthquakes with variable mechanisms, decreased through surface activity.     

Recent geodynamics of dangerous faults

The analysis of the existing information concerning the present-day deformation activity of the fault zones in seismically active and aseismic regions suggests that the notions of an active fault and a dangerous fault should be distinguished. It is shown that a fault which is active for an expert in geotectonics will not be considered dangerous by an expert in geotechnical monitoring of buildings. The definition is given according to which a dangerous fault is understood as a zone of linear destruction which accommodates the contemporary short-period (a few months and years) pulsed and/or alternating motions with strain rates above 5 × 10^–5 per annum and earthquakes with M ≥ 5. A technique is developed for identifying the dangerous faults based on monitoring the recent ground surface displacements in accordance with a special protocol which ensures an increased degree of detail in time and space. Based on the idea of the probable accumulation of dangerous strains during the operating cycle of the objects, the criteria for assessing their geodynamical risks are formulated.     

Eruption of Piip Crater (Kamchatka)

In autumn of 1966 on the northern slope of Kliuchevskoy volcano a chain of new adventive craters broke out at the height of about 2200 m. Eighty-four hours before the beginning of the eruption a swarm of preliminary volcanic earthquakes had appeared. The number of preliminary shocks was 457 with total energy of 4 × 10¹⁷ erg. With the beginning of the lava flow the earthquakes stopped and a continuous volcanic tremor appeared. The total energy of volcanic tremor amounts to 10¹⁶ erg. During the eruption numerous explosive earthquakes with the energy of 10¹⁵–10¹⁶ erg were recorded and besides the microbarograph of the Volcanostation recorded 393 explosions with an energy more than 10¹³ erg and their total energy was equal to 10¹⁷ erg. All together it has been formed 8 explosive craters and the lowest 9th crater was effusive. The slag cone was formed round this effusive crater, the lava effusion of basaltic-andesite composition (52,5% SiO₂) tooke place from the lava boccas at the cone base and from the crater. The lava flow covered a distance of 10 km along the valley of the Sopochnoy river and descended to a height of about 800 m. The lava flow velocity at the outflow reached 800 m/hr, the lava temperature was 1050°C. The effused lava volume amounts to 0.1 km³. The eruption stopped on December 25–26, 1966.     

Agrigan: An introduction to the geology of an active volcano in the Northern Mariana Island arc

Agrigan is the tallest (965 m a.s.l.) and largest (44 km²) of the volcanoes of the northern Mariana Islands. Its slopes are asymmetric to the east; a small caldera (4 km²) dominates the interior. The volcanic edifice has been disrupted along three sets of faults: 1) exterior slump faults, 2) radial faults, and 3) interior faults related to caldera-collapse. The rocks of the volcano are characterized by porphyritic clinopyroxene-olivine-plagioclase basalts and subordinate andesites. Cumulate xenoliths composed of Fo₈₁, An₉₅ and diopside are common in the basalts. Development of the volcano began with 3–4 km of submarine growth. The earliest recognizable flows are the result of fissural Hawaiian- and Strombolian-type eruptions. These were followed by the eruption of more viscous lavas from above the present summit. Flank eruptions of basalt and andesite preceded voluminous outpourings of andesitic pyroclastics contemporaneous with caldera-collapse. Subsequent magmatic resurgence is localized along a N10E rift zone. Violent ejection of lapilli and ash occurred in 1917.     

Rayleigh wave phase velocity tomography and strong earthquake activity on the southeastern front of the Tibetan Plateau

To investigate the relationship between velocity structure and earthquake activity on the southeastern front of the Tibetan Plateau, we make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from the regional Yunnan Seismic Network. These data are used to compute Rayleigh wave Green's Functions by cross-correlating between two stations, extracting phase velocity dispersion curves, and finally inverting to image Rayleigh wave phase velocity with periods between 5 and 34 s by ambient noise tomography. The results show significant lateral variations in crustal and uppermost mantle structures in the studied region. Phase velocity anomalies at short periods(5–12 s) are closely related to regional tectonic features such as sediment thickness and the depth of the crystalline basement. The Sichuan-Yunnan rhombic block, enclosed by the Honghe, Xiaojiang and Jianchuan faults, emerges as a large range of low-velocity anomalies at periods of 16–26 s, that inverts to high-velocity anomalies at periods of 30–34 s. The phase velocity variation in the vicinity of the Sichuan-Yunnan rhombic block suggests that the low-velocity anomaly area in the middle-lower crust may correspond to lower crustal channelized flow of the Tibetan Plateau. The spatial distribution of strong earthquakes since 1970 reveals that the Yunnan region is inhomogeneous and shows prominent characteristics of block motion. However, earthquakes mostly occur in the upper crust, with the exception of the middle-Yunnan block where earthquakes occur at the interface zone between high and low velocity as well as in the low-velocity zones, with magnitudes being generally less than 7. There are few earthquakes of magnitude 5 at the depths of 15–30 km, where gather earthquakes of magnitude 7 or higher ones which mainly occur in the interface zone between high and low velocities with others extending to the high-velocity abnormal zone.     

A Statistical Framework for Calculating and Assessing Compositional Linear Trends Within Fault Zones: A Case Study of the NE Block of the Clark Segment,San Jacinto Fault,California, USA

Utilizing chemical data derived from the various fault zone architectural components of the Clark strand of the San Jacinto fault, southern California, USA, we apply for the first time non-central principal component analysis to calculate a compositional linear trend within molar A–CN–K space. In this procedure A–CN–K are calculated as the molar proportions of Al₂O₃ (A), CaO* + Na₂O (CN), and K₂O (K) in the sum of molar Al₂O₃, Na₂O, CaO*, and K₂O. CaO* is the molar CaO after correction for apatite. We then derive translational invariant chemical alteration intensity factors, t, for each architectural component through orthogonal projection of analyzed samples onto the compositional linear trend. The chemical alteration intensity factor t determines the relative change in composition compared to the original state (i.e., the composition of the altered wall rocks). It is dependent on the degree of intensity to which the process or processes responsible for the change in composition of each architectural component has been active. These processes include shearing, fragmentation, fluid flow, and generation of frictional heat. Non-central principal component analysis indicates that principal component 1 explains 99.7 % of the spread of A–CN–K data about the calculated compositional linear trend (i.e., the variance). The significance level for the overall one-way analysis of variance (ANOVA) is 0.0001. Such a result indicates that at least one significant difference across the group of means of t values is different at the 95 % confidence level. Following completion of the overall one-way ANOVA, the difference in means t test indicated that the mean of the t values for the fault core are different than the means obtained from the transition and damage zones. In contrast, at the 95 % confidence level, the means of the t values for the transition and damage zones are not statistically distinguishable. The results of XRD work completed during this study revealed that the <2 µm fraction is composed primarily of illite/smectite with ~15 % illite in the damage zone, of illite/smectite with ~30 % illite in the transition zone, and of discreet illite with very minor smectite in the fault core. These changes parallel the increasing values of the chemical alteration intensity factors (i.e., t). Based on the above results, it is speculated that when fault zones are derived from tonalitic wall rocks at depths of ~400 ± 100 m, the onset of the illite/smectite to illite conversion will occur when t values exceed 0.20 ± 0.12, the average chemical alteration intensity factor calculated for the transition zone. Under such conditions during repeated rupturing events, frictional heat is produced and acidic fluids with elevated temperatures (≥ ~125 °C) are flushed through the fault core. Over time, the combination of shearing, fragmentation, and frictionally elevated temperatures eventually overcomes the kinetic barrier for the illite/smectite to illite transition. Such settings and processes are unique to fault zones, and as a result, they represent an underappreciated setting for the development of illite from illite/smectite. The success of non-central principal component analysis in this environment offers the first statistically rigorous methodology for establishing the existence of compositional linear trends in fault zones. This method also derives quantifiable alteration intensity factors that could potentially be used to compare the intensity of alteration at different segments of a fault, as well as offer a foundation to interpret the potential driving forces for said alteration and differences therein.     

Soufrière of guadeloupe 1976–1977 eruption — mass and energy transfer and volcanic health hazards

The Soufrière volcano in Guadeloupe island delivered a phreatic eruption that commenced on July 8th, 1976 and lasted until March 1st, 1977. This eruption was similar to the 1797, 1798, 1809 and 1956 outbreaks. Phreatic activity ejected blocks derived from the fissure walls and fine pyroclasts produced by hydrothermal alteration of the old dome’s rocks. Field observations and measurements allowed the present authors to calculate the mass and energy transfer of steam and ashes: 10⁷ tons of water (very low considering that on the mountain summit the annual precipitation is 10 tons m)²,10⁶ m³ of ashes. The most important energy transfers was thermal: about 5 × 10²⁰ ergs for each phreatic eruption. The total kinetic energy output was 2 × 10¹⁹ ergs for a total thermal energy output of 64 × 10²⁰ ergs. The gases and fine pyroclasts did pollute the atmosphere, waters and soils and consequently affected the population living on the slopes of the volcano.     

Geology and tectonic setting of the Mule Creek caldera,New Mexico,U.S.A.

The 10-km diameter Mule Creek caldera is the youngest felsic eruptive center in the Mogollon-Datil volcanic field of southwestern New Mexico. The caldera forms a topographic basin surrounded by a raised rim. The caldera wall is well displayed on the south and west sides of the structure where it dips 20–30 degrees toward the center of the basin. Mudflow breccia fills the caldera and is banked up against the caldera wall. Post-caldera porphyritic quartz latite domes and flows crop out along the ring-fracture zone. The caldera is superimposed upon an older volcanic complex of flow-banded rhyolite and porphyritic andesite lava. The Mule Creek caldera probably originated by explosive eruption of about 10 km³ of pumice and ash, in part preserved in the matrix of the mudflow breccia. Periods of explosive volcanism during the deposition of mudflow breccia are documented by tuffaceous beds interbedded with the breccia. A thin rhyolite ash-flow sheet originated in the caldera and overlies the mudflow breccia. The youngest felsic rocks around the caldera are (1) domes and flows of crystal-rich porphyritic quartz latite of variable mineralogy, interpreted as a defluidized magma, and (2) widespread crystal-poor, flow-banded rhyolite, dated at 18.6 m.y., which is not directly related to the caldera sequence. The Mule Creek caldera and other volcanic features farther south represent the only documented overlap of felsic volcanism with early stages of Basin-Range tectonism in the Mogollon-Datil field.     

Protolith ages and deformation mechanism of metamorphic rocks in the Zhangbaling uplift segment of the Tan-Lu Fault Zone

Protolith ages and Indosinian deformation mechanism of metamorphic rocks in the Zhangbaling uplift segment of the Tan-Lu Fault Zone are important, unsolved problems. Our LA-ICP-MS zircon dating work indicates that protolith ages of the greenschist-facies Zhangbaling Group are 754–753 Ma, and those of the amphibolite-facies Feidong Complex are 800–745 Ma. These rocks belong to the earliest cover of the Yangtze Plate. Their ages and metamorphic features suggest that the rocks did not come from the Dabie Orogen. The Indosinian structures in the Zhangbaling Group and lower Sinian strata formed in a flatlying ductile detachment zone with a shear sense of top-to-the-SSW whereas those in the underlying Feidong Complex are characterized by ENE-WSW inclined folds developed under a ductile regime. It is suggested therefore that the sinistral Tan-Lu Fault Zone of the Indosinian period is buried under the Hefei Basin west of the Zhangbaling uplift segment and the uplift segment is a displaced block neighboring the fault zone. Detachment deformation between the upper rigid and lower ductile crust during displacement of the Zhangbaling uplift segment resulted in the formation of the flat-lying ductile detachment zone and its underlying drag fold zone of a ductile regime. The protolith ages and deformation mechanism in the Zhangbaling uplift segment further prove sinistral origination of the Tan-Lu Fault Zone during the continent-continent collision of the North China and Yangtze plates and support the indentation model for the two-plate collision that considers the Tan-Lu Fault Zone as an oblique convergence boundary.     

b-value and fractal dimension variations in Iran

Iran sits on a region with a high intrinsic level of seismic activity due to its tectonic setting. Through statistical examination of the earthquakes listed in the catalogue from International Institute of Earthquake Engineering and Seismology (IIEES), this research attempted to calculate some seismicity factors and find correlation between them. A preliminary analysis indicated changes in the b-value of the Gutenberg-Richter relationship over the study region. Thus, the study area was divided into five zones (Alborz, Zagros, Azerbaijan, Central and East) and b-value was computed for each zone. Considering faulting mechanism styles and the b-values in the region, it was found that the lowest b-values belong to the thrust events and strike-slip faulting earthquakes have intermediate values. These findings support previous studies. Furthermore, results of b-value calculation were used for the estimation of accumulated differential stresses (σ₁–σ₂) over each zone. Overall, the b-value for Iran is averagely low which signifies the high stress tectonic regime in this region. Also, by having calculated fractal dimension (D) in each zone, a correlation obtained showing that in Iran region, the b-value correlates to fractal dimension by D = 4.2b–2 relation which does not support Aki's (1981) speculation of D = 3b/c.     

The linear relation between temperatures based on the silica content of groundwater and regional heat flow: A new heat flow map of the United States

Application of the silica geothermometer to over 70,000 non-thermal groundwaters from the United States has shown that there is a correlation between the average silica geotemperatures for a region (T SiO₂ in °C) and the known regional heat flow (q in mW m^?2) of the form: 1 $$TSiO_2 = mq + b$$ wherem andb are constants determined to be 0.67°C m² mW^?1 and 13.2°C respectively. The physical significance of ‘b’ is the mean annual air temperature. The slope ‘m’ is related to the minimum average depth to which the groundwaters may circulate. This minimum depth is estimated to be between 1.4 and 2.0 km depending on the rock type. A preliminary heat flow map based on equation (1) is presented using theT SiO₂ for new estimates of regional heat flow where conventional data are lacking. Anomalously high localT SiO₂ values indicate potential geothermal areas.