Reliable Fault Plane Solutions Research Articles

Focal mechanisms of recent seismicity at Campi Flegrei, Italy

The present study analysed volcano tectonic earthquakes that occurred in the caldera of Campi Flegrei, Italy from 2000 to 2019 to compute source focal mechanisms. We used data from a large number of seismic stations to estimate reliable fault plane solutions for 71 events of a maximum duration magnitude (MD) ranging between 0.5 ≤ MD ≤ 2.5. We found that most solutions were of a normal type and were rake-angle negative focal mechanisms in 86% of the cases. Only a few earthquakes occurred on normal dip-slip faults; the majority occurred on varying degrees of oblique slip. Only one event had a pure reverse-mechanism, and it was located far from the caldera centre. The spatial distribution of the computed mechanisms exhibited a remarkable mix of fault orientations without any relationship to the location area or source depth. The predominance of normal kinematics on high-angle and nearly vertical faults and the very shallow source depth exhibited good agreement with the fact that seismicity at Campi Flegrei occurred during periods of ground uplift. In fact, ground uplift elongated the shallowest crust, thus reducing the normal stress on existing locked faults; this facilitated earthquakes, particularly those characterised by normal, dip-slip, and strike-slip mechanisms. In contrast, ground subsidence produced a horizontal shortening of the upper crust, which increased the normal stress on high-angle faults and reduced earthquake occurrence. Our results indicate that the driving force of the recent volcano-tectonic seismicity in the study region is the ongoing ground uplift.

State of tectonic stress in Shillong Plateau of northeast India

Tectonic stress regime in the Shillong plateau, northeast region of India, is examined by stress tensor inversion. Some 97 reliable fault plane solutions are used for stress inversion by the Michael and Gauss methods. Although an overall NNW-SSE compressional stress is observed in the area, the stress regime varies from western part to eastern part of the plateau. The eastern part of the plateau is dominated by NNE-SSW compression and the western part by NNW-SSE compression. The NNW-SSE compression in the western part may be due to the tectonic loading induced by the Himalayan orogeny in the north, and the NNE-SSW compression in the eastern part may be attributed to the influence of oblique convergence of the Indian plate beneath the Indo-Burma ranges. Further, Gravitational Potential Energy (GPE) derived stress also indicates a variation from west to east.

Suitability of rapid energy magnitude determinations for emergency response purposes

SUMMARY It is common practice in the seismological community to use, especially for large earthquakes, the moment magnitude Mw as a unique magnitude parameter to evaluate the earthquake's damage potential. However, as a static measure of earthquake size, Mw does not provide direct information about the released seismic wave energy and its high frequency content, which is the more interesting information both for engineering purposes and for a rapid assessment of the earthquake's shaking potential. Therefore, we recommend to provide to disaster management organizations besides Mw also sufficiently accurate energy magnitude determinations as soon as possible after large earthquakes. We developed and extensively tested a rapid method for calculating the energy magnitude Me within about 10–15 min after an earthquake's occurrence. The method is based on pre-calculated spectral amplitude decay functions obtained from numerical simulations of Green's functions. After empirical validation, the procedure has been applied offline to a large data set of 767 shallow earthquakes that have been grouped according to their type of mechanism (strike-slip, normal faulting, thrust faulting, etc.). The suitability of the proposed approach is discussed by comparing our rapid Me estimates with Mw published by GCMT as well as with Mw and Me reported by the USGS. Mw is on average slightly larger than our Me for all types of mechanisms. No clear dependence on source mechanism is observed for our Me estimates. In contrast, Me from the USGS is generally larger than Mw for strike-slip earthquakes and generally smaller for the other source types. For ∼67 per cent of the event data set our Me differs ≤±0.3 magnitude units (m.u.) from the respective Me values published by the USGS. However, larger discrepancies (up to 0.8 m.u.) may occur for strike-slip events. A reason of that may be the overcorrection of the energy flux applied by the USGS for this type of earthquakes. We follow the original definition of magnitude scales, which does not apply a priori mechanism corrections to measured amplitudes, also since reliable fault-plane solutions are hardly available within 10–15 min after the earthquake origin time. Notable is that our uncorrected Me data show a better linear correlation and less scatter with respect to Mw than Me of the USGS. Finally, by analysing the recordings of representative recent pairs of strong and great earthquakes, we emphasize the importance of combining Mw and Me in the rapid characterization of the seismic source. They are related to different aspects of the source and may differ occasionally even more than 1 m.u. This highlights the usefulness and importance of providing these two magnitude estimates together for a better assessment of an earthquake's shaking potential and/or tsunamigenic potential.

Earthquake Triggering along the Xianshuihe Fault Zone of Western Sichuan, China

Western Sichuan is among the most seismically active regions in southwestern China and is characterized by frequent strong (M ≥ 6.5) earthquakes, mainly along the Xianshuihe fault zone. Historical and instrumental seismicity show a temporal pattern of active periods separated by inactive ones, while in space a remarkable epicenter migration has been observed. During the last active period starting in 1893, the sinistral strike–slip Xianshuihe fault of 350 km total length, was entirely broken with the epicenters of successive strong earthquakes migrating along its strike. This pattern is investigated by resolving changes of Coulomb failure function (ΔCFF) since 1893 and hence the evolution of the stress field in the area during the last 110 years. Coulomb stress changes were calculated assuming that earthquakes can be modeled as static dislocations in an elastic halfspace, and taking into account both the coseismic slip in strong (M ≥ 6.5) earthquakes and the slow tectonic stress buildup associated with major fault segments. The stress change calculations were performed for faults of strike, dip, and rake appropriate to the strong events. We evaluate whether these stress changes brought a given strong earthquake closer to, or sent it farther from, failure. It was found that all strong earthquakes, and moreover, the majority of smaller events for which reliable fault plane solutions are available, have occurred on stress–enhanced fault segments providing a convincing case in which Coulomb stress modeling gives insight into the temporal and spatial manifestation of seismic activity. We extend the stress calculations to the year 2025 and provide an assessment for future seismic hazard by identifying the fault segments that are possible sites of future strong earthquakes.

Evolution of the Stress Field in the Northern Aegean Sea (Greece)

Summary The evolution of the stress field in the area of the northern Aegean Sea during the 20thcentury has been studied. The area is dominated by dextral strike-slip faulting and is characterized by frequent strong earthquakes. Coulomb stress changes (ΔCFF) we recalculated assuming that earthquakes can be modeled as static dislocations in an elastic half-space, and taking into account both the coseismic slip in large (M≥7.0) earth quakes and the slow tectonic stress build-up along the major fault segments. The stress change calculations were performed for strike-slip faults of strike, dip, and rake appropriate to the large events. We evaluate whether these stress changes brought a given large earthquake closer to, or farther from, failure. It was found that each of the large events occurred in regions of increased calculated Coulomb stress. Moreover, the majority of smaller events for which reliable fault-plane solutions are available were also located in areas of positive ΔCFF. By extending the calculations to 2020, and assuming that no additional large (M≥7.0) earthquake occurs between 1999 and 2020, possible sites of future large earthquakes are identified.

A study of the active tectonics and deformation in the Mygdonia basin (N. Greece) using seismological and neotectonic data

In the present work we study the active tectonics setting and related deformation scheme in the Mygdonia basin, on the basis of the joint interpretation of seismological and neotectonic data. For this reason, the stress field derived from fault plane solutions of small-magnitude events from a local seismological experiment, as well as neotectonic observations are studied in order to determine its spatial distribution. The results show an almost identical spatial variation of the stress field determined from the two independent data sets, which also exhibits a very good correlation with the geomorphological changes in the Mygdonia basin. Moreover, the average stress field is almost identical to the corresponding stress pattern determined from the 3 largest events in the study area for which reliable fault plane solutions are available. Using the combined stress pattern and the corresponding moment-rate tensors derived from both data sets, the active crustal deformation is studied for the most active sections of the Mygdonia basin system. The results show a N-S extension at an average rate of 3mm/ yr for the central part of the Mygdonia system, in very good agreement with the available geodetic results.

The geometry of the Wadati–Benioff zone and lithospheric kinematics in the Hellenic arc

Accurate locations of 961 shallow and intermediate depth earthquakes, which occurred between 1956 and 1995 in the Hellenic arc (34–39°N, 19–28°E), are used to define the plate boundaries in this area. Reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used in order to define the interaction between the different plates in the arc. An ocean–continent type of interaction occurs on a curved surface, which is defined by the shallow branch (20–100 km) of the Wadati–Benioff zone. The intersection of this zone with the earth's surface is a curve which follows the convex (outer) side of the sedimentary arc (western Peloponnese–west of Cythera–south coast of Crete–east coast of Rhodes) and dips at low angle (∼30°) to the Aegean sea. Coupling between the subducted oceanic crust and the overriding of the Aegean lithospheric plate takes place on this surface. The deep branch (100–180 km) of the Wadati–Benioff zone is dipping freely (without coupling) at a high angle (∼45°) beneath the south Aegean trough and the volcanic arc. The high shallow seismicity ( h≤20 km) which is observed in the southwestern convex (outer) side of the arc (Ionian sea) is attributed to the fast southwestward motion of the Aegean plate. Location of strong deep earthquakes ( h>100 km) in the fore-arc area of the southwestern part of the Hellenic arc (west of Cythera) indicates that oceanic crust is ‘destroyed’ in this part of the Hellenic trench due to roll-back of the descending lithospheric slab.

Rupture zones in the Aegean region

Rupture zones of strong shallow earthquakes can be investigated by field observations of surface fault traces (SF), accurate location of spatial clusters of aftershocks or other small earthquakes (CL), reliable fault plane solutions (FP) and information on the spatial distribution of sites with high ( I ≥ VIII) macroseismic intensities (MA). The validity of these four techniques was successfully tested by comparing the results of their application to several cases of strong shallow earthquakes. These techniques were then applied to spatially define the rupture zones of 150 strong ( M≥6.0) shallow earthquakes in the Aegean and surrounding area. The type of faulting was determined for all these zones by the use of reliable fault plane solutions, which are available for this area. The orientation of most of these rupture (fault) zones is in good agreement with the presently acting stress field with some exceptions which are attributed to the rupture of pre-existing faults. The tectonic implications of these zones are discussed.

Lithospheric boundaries and plate motions in the Cyprus area

Accurate location of earthquakes recorded by seismographs and reliable fault plane solutions of strong recent earthquakes are used to define the plate boundaries and the pattern of plate motion in Cyprus and surrounding area. Spatial distribution of shallow ( h<60 km) and intermediate depth (60 km ≤ h ≤130 km) earthquakes define a continuous boundary between the Eurasian and the African lithospheric plates in this area. This boundary is formed of two arcuate structures, the eastern and western, which have their concave side to the north and are connected by a NNE striking transform dextral fault, the Paphos Transform Fault (PTF), just west of Cyprus. The eastern structure consists of the Cyprean arc and its continuation to the Gulf of Adana, which then joins the Eastern Anatolian Fault. The western one is an arc-like structure which strikes in a northwest direction following the Florence rise and joins the Rhodes thrust fault. The African plate is slowly subducted under the Eurasian plate from south to north but in the Cyprean arc the Cyprean microplate overrides also the Levantine lithosphere in a SW direction. Some interesting tectonic similarities between the Cyprean and the Hellenic arcs are pointed out.

Evidence from a cluster of small earthquakes for a fault at 18 km depth beneath Oak Ridge, southern California

Abstract A swarm of about 50 small earthquakes (M ∼ 1.5) occurred for a month during 1989 beneath Oak Ridge, southern California. Location accuracy using conventional analysis of arrival-time picks is limited for these events by the weak, emergent nature of arrivals on the available seismograms. However, waveform cross-correlation techniques are found to provide precise relative event locations due to the similarity of the waveforms recorded at individual stations. The relocated events form a small cluster about 1 km across at a depth of ∼ 18 km and are aligned along a plane that dips 35° to the northwest. Estimated standard errors for the locations are generally less than 50 m. The time evolution of the sequence shows a gradual migration of activity away from its initiation point. Three additional events occurred several months later; these align along the same plane but are displaced about 500 m to the southeast from the main swarm. Reliable fault-plane solutions are difficult to obtain for these events due to the small number of station records available, the limited range of takeoff angles, and the weak initial arrivals on many of the seismograms. Stacking the records at each station over the different events greatly reduces prearrival noise levels and assists in resolving the average P first motions. Analysis of these first-motion data indicates that the slip planes of probable focal mechanisms are not in agreement with the plane defined by the seismicity. The seismicity alignment may represent the extension of the Simi fault, in which case some shallowing of the fault dip would be required to match the observed 35° dip at 18 km.

Large seismic faults in the Hellenic arc

Using information concerning reliable fault plane solutions, spatial distribution of strong earthquakes (Ms³ 6.0) as well as sea bottom and coastal topography, properties of the seismic faults (orientation, dimension, type of faulting) were determined in seven shallow (h &lt; 40 km) seismogenic regions along the convex part of thc Hellenic arc (Hellenic trench) and in four seismogenic regions of intermediate depth earthquakes (h = 40-100 km) along the concave part of this arc. Except for the northwesternmost part of the Hellenic trench, where the strike-slip Cephalonia transform fault dominates, all other faults along this trench are low angle thrust faults. III thc western part of the trench (Zante-west Crete) faults strike NW-SE and dip NE, while in its eastern part (east Crete-Rhodos) faults strike WNW-ESE and dip NNE. Such system of faulting can be attributed to an overthrust of the Aegean lithosphere on the eastern Mediterranean lithosphere. The longest of these faults (L = 300 km) is that which produced the largest known shallow earthquake in the Mediterranean area (21 July 365, Ms = 8.3) which is located near the southwestern coast of Crete. The second longest such fault (L = l 70 km) is that which produced a large earthquake (December 1303, Ms = 8.0) in the easternmost part of the trench (east of Rhodos island). Both earthquakes were associated with gigantic tsunamis which caused extensive damage in the coast of many Eastern Mediterranean countries. Seismic faults of the intermediate depth earthquakes in the shallow part of the Benioff zone (h = 40- 100 km) are of strike-slip type, with a thrust component. The orientations of these faults vary along the concave part of the arc in accordance with a subduction of remnants of all old lithospheric slab from the convex side (Mediterranean) to the concave side (Aegean) of thc Hellenic arc. The longest of these faults (L = 220 km) is that which produced the largest known intermediate depth earthquake in the whole Mediterranean area (12 October 1856, M = 8.2) north of Crete. The second longest such fault (L = 160 km) produced a large earthquake (26 June 1926, M = 8.0) in the easternmost part of the concave part of the arc (near Rhodos). Both earthquakes caused very serious damage in several Eastern Mediterranean countries but were not associated with tsunamis.

Active seismic deformation in the Italian peninsula and Sicily

Recent and historical seismicity as well as reliable fault plane solutions are used in order to perform a moment tensor analysis and estimate the active crustal and sub-crustal deformation of the Italian peninsula and Sicily. The results show that in Northern Italy, along the Alps, the deformation is taken up by compression at N162°E and a rate of 1 mm/yr. The thickening of the seismogenic layer is taking place at a rate of 0.1 mm/yr. In Central Italy, along the Apennines, extension is prevailing at N28°E and a rate of 3 mm/yr which causes thinning of the seismogenic layer at a rate 0.5 mm/yr. In Southern Italy, at Calabria, the deformation is taken up as extension at N40°E and a rate of 11 mm/yr. At the island of Sicily, compression is occurring at N25°E and a rate of 1 mm/yr. These results are in agreement with plate motion models for the area. The analysis of the deep seismicity of the Tyrrhenian Sea showed that the descending slab is in a state of down dip compression at N146°E and a rate of 2 mm/yr.

Determination of Fault Plane Solutions for Small Earthquakes in Japan.

Fault plane solutions for small earthquakes in Japan are determined by using the P wave first motion data given in the Japan University Network Earthquake Catalog and the Seismological Bulletin of the Japan Meteorological Agency. By carefully selecting from the catalog the events that occurred inland in Japan, we can determine reliable fault plane solutions for the earthquakes with a magnitude as small as three. We have determined fault plane solutions for 174 earthquakes which occurred in 1987, compared with 107 fault plane solutions reported by the Seismological Bulletin for the same year but for a much wider area. The fault plane solutions obtained in this study reflect the same pattern of the regional and tectonic stress distribution as previous studies. This result indicates that first motion data reported in the Japan University Network Earthquake Catalog are reliable enough to determine fault plane solutions. The construction of a database for fault plane solutions of microearthquakes in Japan is essential for the detailed study of the stress accumulation process in the Japanese islands.

Active deformation in the Vrancea region, Rumania

Recent and historical seismicity as well as reliable fault plane solutions are used to study the active deformation caused by the occurrence of intermediate depth (60–170 km) earthquakes of the Vrancea region, Rumania. In this area, located in the southeastern part of the Carpathian arc, the westward subduction of the Carpathian trench has terminated, leaving continental lithosphere, at present, at the arc. The principalT axis of the intermediate depth events trends N159°E and has a plunge of 74°, which is the same as the dip of the subducted plate. TheP axis has a trend of 314° and a shallow plunge of 15°. The analysis of the moment tensor of six focal mechanisms showed that the dominant mode of deformation of the subducted lithosphere is a down-dip extension at a rate of about 2 cm/yr, based on seismicity data.

A formulation for reliable estimation of active crustal deformation and its application to central Greece

SUMMARY A procedure for the analysis of data, concerning seismic moment release and fault-plane solutions, for the estimation of active crustal deformation is proposed. The formulation takes advantage of all the available historical and instrumental data for the calculation of the 'size' of the deformation in a seismic zone and of all the reliable fault-plane solutions which are available for a broader seismic belt for the determination of the 'shape' of the deformation. A detailed analysis of the errors involved in each parameter, based on a Monte Carlo numerical method, is suggested. The proposed procedure is applied to a region of extensional tectonics in central Greece. The resulting seismic strain rates show that the dominant mode of deformation in the area is extension at a rate of the order of 6 mm yr-' and a mean azimuth of N25W. A vertical contraction rate of about 1 mm yr-' has also been calculated. The results from geodetic measurements performed by Billiris et al. (1991) declare that the calculated seismic deformation represents about 60 per cent of the total strain of this area.

Rates of active crustal deformation in the Aegean and the surrounding area

Active crustal deformation is calculated for 26 zones of shallow seismicity in the Aegean and the surrounding regions. The data analysis is based on a procedure developed in a previous paper (Papazachos and Kiratzi, 1993). This procedure takes advantage of all the available historical and instrumental data for the calculation of the “size” of the deformation in a seismic zone and of all reliable fault plane solutions which are available for the broader seismic belt for the determination of the “shape” of the deformation. The Aegean and its surroundings has been divided into 11 such seismic belts which may consist of one or more seismic zones that share earthquakes with similar focal mechanisms. This data analysis showed that along the coastal region of Albania, Yugoslavia and western Greece the deformation is taken up by compression in a direction perpendicular to the coast line (47°E) at a rate of about 2 mm/a. In the Ionian islands (Leukada, Cephalonia, Zante) compression occurs at a rate of 10 mm/a in an almost EW direction (N83°E) and extension at a rate of 11 mm/a in an almost NS direction (N174°E). Along the convex side of the Hellenicarc (south of Peloponese, Crete, Rodos), the upper crust is compressed at a rate of about 6 mm/a in a direction N34°E. In the Aegean Sea and the surrounding lands (mainland of western and northern Greece, southern Yugoslavia and Bulgaria, western Turkey) the seismic deformation is taken up by an almost NS extension at an average rate of 5 mm/a. In northwestern Anatolia and the northern Aegean fault zones deformation is controlled by the westward movement along the North Anatolian fault. Northern Anatolia is undergoing a N115°E compression at a rate of 22 mm/a which is relieved by a N25°E extension at a rate of 19 mm/a, and the Northern Aegean is undergoing EW compression at a rate of 16 mm/a and NS extension at a rate of 8 mm/a. The vertical crustal thickening along the external compressional zones ranges from 0.2 to 0.5 mm/a with an average of 0.3 mm/a and the vertical crustal thinning in the inner back-arc extensional area ranges from 0.1 to 2.4 mm/a with an average equal to 0.8 mm/a. In the western part of the area and between the external compressional field and the internal extensional field, a belt exists where the deformation is expressed as extension at an average rate of 1.6 mm/a in a N112°E direction.

Seismic faults in the Aegean area

Reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic, geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the Aegean and surrounding area. Thrust faults having an about NW-SE strike occur in the outer seismic zone along western Albania-westernmost part of mainland of Greece-Ionian Sea-south of Crete-south of Rhodes. The inner part of the area is dominated by strike-slip and normal faulting. Strike-slip with an about NE-SW slip direction occurs in the inner part of the Hellenic arc along the line Peloponnesus-Cyclades-Dodecanese-southwest Turkey as well as along a zone which is associated with the northern Aegean trough and the northwesternmost part of Anatolia. All other regions in the inner part of the area are characterized by normal faulting. The slip direction of the normal faults has an about SW-NE direction in Crete (N38°E) and an about E-W direction (N81°E) in a zone which trends N-S in eastern Albania and its extension to western mainland of Greece. In all other regions (central Greece-southern Yugoslavia and Bulgaria, western Turkey) the slip of the normal faults has an about N-S direction.

Some reliable fault plane solutions

The reliability of earthquake fault plane solutions usingP andPKP first motion data is discussed. Effects of the usefulness of the individual data, of the number and distribution of data around the focus, of great velocity contrasts in the focal region, and of the kind of projection technique are considered. The increase of the number of inconsistent data by a change in position of the three principal axes can be used as a more or less objective measure for the reliability of a solution.