Seismogenic Fault Zone Research Articles

Seismic imaging of the Asian orogens and subduction zones

In this article we make a review of recent findings on seismic imaging of the Asian orogens and subduction zones. High-resolution images of seismic tomography and receiver functions of the Asian region are obtained, revealing significant lateral heterogeneities in the crust and upper mantle, which are caused by active plate subductions and continental orogeny. A significant advance in the seismic imaging is tomographic inversions for three-dimensional distribution of seismic anisotropy in the crust and mantle, which provides important new information on the lithospheric deformation and mantle convection associated with the continental orogeny and plate subductions. The intraplate volcanism in Northeast Asia is caused by hot and wet upwelling flows in the big mantle wedge above the stagnant Pacific slab in the mantle transition zone (MTZ). The age distribution of the subducting Pacific slab beneath East Asia is estimated, shedding new light on the evolution of the Pacific slab, as well as the East Asian tectonics during the Late Mesozoic to the Cenozoic. The nucleation of great earthquakes, such as the 2008 Wenchuan earthquake (M 8.0), the 2011 Tohoku-oki earthquake (M 9.0) and the 2015 Nepal earthquake (M 7.9), is controlled by structural heterogeneities in and around the seismogenic fault zones. It is considered that fluids are involved in the nucleation and rupture processes of all types of earthquakes. The cause of deep earthquakes is still not very clear, though transformational faulting triggered by metastable olivine transforming to spinel in the cold, stressed core of the subducting slab is a viable mechanism, and a metastable olivine wedge is revealed within the western Pacific subducting slab at the MTZ depths. The 2015 Bonin deep earthquake (M 7.9, ∼670km depth) occurred at the MTZ bottom within the vertical Pacific slab which is penetrating into the lower mantle. This very unusual deep event was caused by joint effects of several factors, including the slab’s fast deep subduction, slab tearing and thermal variation, stress changes and phase transformations in the slab, and complex interactions between the slab and the ambient mantle.

Paleoseismic study of the Kamishiro Fault on the northern segment of the Itoigawa\u2013Shizuoka Tectonic Line, Japan

The Mw 6.2 (Mj 6.8) Nagano (Japan) earthquake of 22 November 2014 produced a 9.3-km long surface rupture zone with a thrust-dominated displacement of up to 1.5 m, which duplicated the pre-existing Kamishiro Fault along the Itoigawa–Shizuoka Tectonic Line (ISTL), the plate-boundary between the Eurasian and North American plates, northern Nagano Prefecture, central Japan. To characterize the activity of the seismogenic fault zone, we conducted a paleoseismic study of the Kamishiro Fault. Field investigations and trench excavations revealed that seven morphogenic paleohistorical earthquakes (E2–E8) prior to the 2014 Mw 6.2 Nagano earthquake (E1) have occurred on the Kamishiro Fault during the last ca. 6000 years. Three of these events (E2–E4) are well constrained and correspond to historical earthquakes occurring in the last ca. 1200 years. This suggests an average recurrence interval of ca. 300–400 years on the seismogenic fault of the 2014 Kamishiro earthquake in the past 1200 years. The most recent event prior to the 2014 earthquakes (E1) is E2 and the penultimate and antepenultimate faulting events are E3 and E4, respectively. The penultimate faulting event (E3) occurred during the period of AD 1800–1400 and is associated with the 1791 Mw 6.8 earthquake. The antepenultimate faulting event (E4) is inferred to have occurred during the period of ca. AD 1000–700, likely corresponding to the AD 841 Mw 6.5 earthquake. The oldest faulting event (E8) in the study area is thought to have occurred during the period of ca. 5600–6000 years. The throw rate during the early Holocene is estimated to be 1.2–3.3 mm/a (average, 2.2 mm/a) with an average amount of characteristic offset of 0.7–1.1 m produced by individual event. When compared with active intraplate faults on Honshu Island, Japan, these slip rates and recurrence interval estimated for morphogenic earthquakes on the Kamishiro Fault along the ISTL appear high and short, respectively. This indicates that present activity on this fault is closely related to seismic faulting along the plate boundary between the Eurasian and North American plates.

Geological characterization of the Aquistore CO2 storage site from 3D seismic data

A 30km2 3D seismic survey was conducted in 2012 at the Aquistore CO2 storage site near Estevan, Saskatchewan, Canada. The purpose of the survey was to investigate the geological suitability of this site for the purposes of long-term CO2 storage. The resultant 3D seismic volume has been interpreted in conjunction with geological and geophysical logs from the 3400m deep CO2 injection well (01/5-6-2-8-W2M) that was subsequently drilled. The CO2 storage reservoir resides immediately above the Precambrian crystalline basement (3400m) and is part of a regionally extensive >200m-thick clastic interval (Winnipeg and Deadwood formations). The reservoir is capped by a 15m thick laterally-continuous shale unit (Icebox Member of the Winnipeg Formation). A regional evaporite at ∼2500m depth (Prairie Formation) provides a secondary barrier to vertical flow. It is >150m thick and shows no salt dissolution features. Above the Prairie Formation are 1500m of laterally continuous Middle Devonian to Lower Cretaceous strata and 1000m of Upper Cretaceous and younger sedimentary rocks, including additional regionally-extensive aquitards that provide tertiary seals: Watrous Formation (∼120m), Colorado Group (>185m), and Bearpaw Formation. There is no evidence of vertical faulting extending through the Devonian or deeper section. A local sub-vertical Precambrian basement fault is interpreted to exist. It lies beneath a flexure within the overlying Cambrian to Silurian strata. The fault is oriented at an azimuth of 75°–85° relative to the regional maximum horizontal stress making it less susceptible to reactivation during CO2 injection. There is no clear evidence that the strata in an overlying flexure are ruptured or faulted. Natural seismicity in the area is very low and the nearest known significant seismogenic fault zone is located ∼200km away.

Fracturing and rock pulverization along an exhumed seismogenic fault zone in dolostones: The Foiana Fault Zone (Southern Alps, Italy)

The Foiana Fault Zone (FFZ) is a major sinistral transpressive fault zone exhumed from <2km depth in the Italian Southern Alps. The fault zone crosscuts thick sequences of sedimentary dolostones and shows increasing cumulative throw (0.3–1.8km) moving from south to north along fault strike. The FFZ consists of variably fractured and fragmented dolostones locally cut by small-displacement (<0.5m) faults containing discrete, highly-reflective (so-called “mirror-like”) slip surfaces. The mirror-like slip surfaces are typically embedded within fine-grained cataclasite layers up to a few centimeters thick. Preservation of bedding planes in the fragmented dolostones indicates a lack of significant shear strain. Instead, the fragmented dolostones are affected by in-situ shattering from the centimeter down to the micrometer scale, resembling pulverized rocks in crystalline lithologies. Detailed field and aerial structural mapping reveals significant changes in the structure of the FFZ along strike. In particular, the fault zone exhibits large variations in thickness (from c. 100m in the north to more than 300m in the south) and changes in mean fault orientation and fault kinematics (from dominant oblique- and strike-slip in the north to dip-slip reverse in the south), together with the reactivation of preexisting anisotropies (i.e. bedding). Overall, the structure of the FFZ, when considered together with possible variable exhumation levels along strike, compares favorably to the predicted damage distribution in three-dimensional earthquake rupture simulations on strike-slip faults, as well as to the characteristics of active seismic sources hosted in carbonate rocks as illuminated by recent seismological studies.

Repeated seismic slips recorded in ultracataclastic veins along active faults of the Arima–Takatsuki Tectonic Line, southwest Japan

Field investigations, combined with meso- and microstructural analyses, reveal that numerous ultracataclastic veins are widely developed within a fault zone (<150 m wide) as simple veins, complex lenses, and networks, along active faults of the Arima–Takatsuki Tectonic Line, southwest Japan. These veins comprise mainly pseudotachylyte-like vein and weakly consolidated to unconsolidated fault gouge that is black, dark-brown, brown, gray, and brownish-red in color. Meso- and microstructural features show that these pseudotachylyte-like and fault gouge veins and networks formed during multiple stages, as earlier veins are generally cut and overprinted by younger veins, indicating that the vein-forming events occurred repeatedly and that ultracataclastic material was injected into networks of faults and fractures in the fault zone. The pseudotachylyte-like and fault gouge veins are characterized by an ultrafine- to fine-grained matrix and angular to subangular fragments of host granitic rocks of various sizes, ranging from submicron to millimeters. SEM–EDS (Scanning Electronic Microscope-Energy Dispersive X-ray) and powder X-ray diffraction analyses show that all the ultracataclastic veins are characterized by crystalline materials composed mainly of quartz and feldspar, similar to the host granitic rocks.The present results support the existing hypothesis that ultrafine- to fine-grained materials formed by comminution can be fluidized and injected rapidly into fracture networks located far from the source fault plane in a solid–fluid–gas system during seismic slip; therefore, such materials provide a record of paleoseismic faulting events that occurred repeatedly within the seismogenic fault zone.

Systematization of active faults for the assessment of the seismic hazard

A systematization of active faults has been developed based on the progress of scientists from the leading countries in the world in the study of seismotectonics and seismic hazard problems. It is underlain by the concept of the fault-block structure of the geological-geophysical environment governed by the interaction of differently oriented active faults, which are divided into two groups—seismogenic and nonseismogenic faults. In seismogenic fault zones, the tectonic stress accumulated is relieved by means of strong earthquakes. Nonseismogenic fault zones are characterized by creep displacement or short-term, oscillatory, and reciprocal movements, which are referred to local superintense deformations of the Earth’s crust (according to the terminology used by Yu.O. Kuz’min). For a situation when a strong earthquake happens, a subgroup of seismodistributing faults has been identified that surround the seismic source and affect the distribution of the seismic waves and, as a consequence, the pattern of the propagation of the coseismic deformations in the fault-block environment. Seismodistributing faults are divided into transit and sealing faults. Along transit faults, secondary coseismic effects (landfalls, landslides, ground fractures, liquefaction, etc) are intensified during earthquakes. In the case of sealing faults, enhancement of the coseismic effects can be observed on the disjunctive limb nearest to the epicenter, whereas, on the opposite limb, the intensity of such effects appreciably decreases. Seismogenic faults or their systems are associated with zones of earthquake source origination (ESO), which include concentrated seismicity regions. In such zones, each earthquake source is related to the evolution of a fault system. ESO zones also contain individual seismogenic sources being focuses of strong earthquakes with M of ≥5.5 in the form of ruptures, which can be graphically represented in 2D or 3D as a surface projection of the source. Depending on the type of data based on which they are identified, individual seismogenic sources are divided into geological-geophysical and macroseismic sources. The systematization presented is the theoretical basis for and the concept of the relational database that is being developed by the authors as an information system for the generation of seismotectonic GIS projects required for the subsequent analysis of the seismic hazard and the assessment of the probability of the origination of macroseismic earthquake effects in a predetermined location.

Field evidence of rupture of the Qingchuan Fault during the 2008 Mw7.9 Wenchuan earthquake, northeastern segment of the Longmen Shan Thrust Belt, China

Although many studies of the 2008 Mw 7.9 Wenchuan earthquake have described the ground deformation features, rupture mechanism, and structural features of the seismogenic fault zone associated with this event, debate remains concerning the total length of the co-seismic surface rupture zone and whether the earthquake ruptured the Qingchuan Fault in the northeastern segment of the Longmen Shan Thrust Belt (LSTB), China. In this paper, we present new field evidence that the Qingchuan Fault was ruptured by the 2008 Wenchuan earthquake and that the total length of the co-seismic surface rupture zone is up to 285–300km. Field investigations reveal that the earthquake produced a ≥60-km-long surface rupture zone along the pre-existing Qingchuan Fault, with the offset being mainly right-lateral strike–slip and a distinct component of vertical slip. Co-seismic surface ruptures are characterized by faults and extensional cracks. Field measurements indicate co-seismic right-lateral strike–slip displacements along the Qingchuan Fault of 0.3–0.6m and vertical offsets of 0.2–0.5m, which differs to the displacements observed along the central and southwestern segments of the Wenchuan surface rupture zone in the displacement amount and sense. The change in slip sense from thrust-dominated slip in the central and southwestern segments of the LSTB to right-lateral strike–slip-dominated displacement along the Qingchuan Fault (northeastern segment of the LSTB) reflects a change in the orientation of compressive stress along the LSTB, associated with eastward extrusion of the Tibetan Plateau as it accommodates the ongoing penetration of the Indian Plate into the Eurasian Plate.

Seismic slip recorded by fluidized ultracataclastic veins formed in a coseismic shear zone during the 2008 Mw 7.9 Wenchuan earthquake

Dark, aphanitic material occurs as a thin ( w 7.9 Wenchuan, China, earthquake. It also forms injection veins in a >5-cm-wide coseismic shear zone within the Longmen Shan thrust belt. Powder X-ray diffraction data and microstructural analysis indicate that the veins contain little or no glassy material, and are composed of fine-grained fragments set in an ultrafine-grained matrix, all sourced from the country rock of interbedded siltstone and mudstone. On the basis of X-ray diffraction and mesostructural and microstructural features, it is concluded that these veins are ultracataclastic veins generated by coseismic comminution with little melting, accompanied by rapid fluidization in the coseismic shear zone during the 2008 Wenchuan earthquake. The findings support earlier suggestions that (1) ultracataclastic veins that resemble pseudotachylyte veins in appearance can be generated by crushing and rapid injection accompanied by fluidization of fine-grained material during seismic faulting, with little or no melting; and (2) ultracataclastic veins formed in this way may record seismic slip events in seismogenic fault zones, acting as earthquake fossils in much the same way as pseudotachylyte veins.

Temperature estimates of coseismic heating in clay-rich fault gouges, the Chelungpu fault zones, Taiwan

To investigate the coseismic frictional temperature in seismogenic fault zones, we examine the characteristics of clays in the Chelungpu-fault zones with isothermal heating experiments, scanning electron microscope coupled to an energy dispersive spectrometer (SEM/EDX), and thermogravimetry analysis (TGA). In the TCDP case (Taiwan Chelungpu fault Drilling Project), three fault zones of the Chelungpu-fault system were identified at the depth of 1111 m, 1153 m, and 1222 m (described as FZ1111, FZ1153, and FZ1222 hereafter), respectively. The clay mineral assemblages of FZ1111 show evidence of melting, and the temperature in a ~ 2 cm band within the black gouge zone is estimated to be from 900 °C to 1100 °C by comparing the SEM images of in situ natural samples with those of heated materials, and the finding of no recrystallization of kaolinite-amorphous aluminosilicates-spinel in the fault samples. The clay mineral assemblages of FZ1153 suggested that kaolinite has been broken down by the thermal decomposition/dehydroxylation but chlorite has not. The clay characteristics and results of SEM/EDX and TGA constrain the faulting temperature from 500 °C to 900 °C, with a spatial distribution up to ~ 1.3 m. The clay characteristics of FZ1222 indicated that clays were changed by experiencing high temperature acid fluids, instead of thermal decomposition/dehydroxylation processes, and that the temperature is localized in ~ 2 cm and ranges from 350 °C to 500 °C, the lowest temperature among three fault zones. The estimates of temperature ranges, and thermal anomaly intervals among three fault zones provide important information and constraints on the physical and chemical processes, coseismic dynamic weakening mechanism, and earthquake energy budget in the future.

Hydrothermal alteration as a trigger mechanism for earthquake swarms: the Vogtland/NW Bohemia region as a case study

Earthquake swarms occur mostly in regions with CO2-enriched pore fluids. It is generally accepted that both geodynamic stress accumulation and critical pore fluid pressures act as a triggering mechanism for most seismic events. The new thesis presented here is that hydrothermal alteration processes in fault zones help facilitate the shear failure propagation due to mechanical weakening and dissolution of the wall rock, in addition to the normal shear stress and fluid overpressure. The basic idea that stress corrosion cracking results from chemical weakening and comminution has been discussed for many years. However, it has not yet been applied to explain the earthquake swarm phenomenon. Studies of extensive alteration as well as the latest investigations of CO2 sequestration give evidence that these high dissolution rates of wall rock in contact with an acid fluid phase exist in seismogenic fault zones. Several indications support the assumption that in the Vogtland/NW Bohemia region, the weakening of stressed fault zones by hydrothermal alteration could take place at seismogenic depths and could generate earthquake swarms. Investigations of quartz samples from the fracture zones by means of cathodoluminescence as well as spatiotemporal analysis of seismicity and numerical modelling of alteration-induced earthquake swarms support this hypothesis.

Crustal shear-wave splitting in the epicentral zone of the 2001 Mw 7.7 Bhuj earthquake, Gujarat, India

Abstract We report here crustal shear-wave anisotropy, ranging from 1% to 10.76% with an average of 2.4% in the aftershock zone of the 2001 Bhuj earthquake, Gujarat, India, from a study of leading shear-wave polarization directions (LPSDs), which vary on average from NNW–SSE to E–W with a delay of 0.07–0.14 s. The delays in the NNW–SSE to NE–SW directions observed at seven stations, near the seismogenic fault, suggest cracks parallel to the direction of the maximum horizontal regional compressional stress prevailing in the region, suggesting a dilatancy-induced anisotropy resulting from approximately stress-aligned parallel vertical micro-cracks. In contrast, the LPSDs at Ramvav, Rapar and Vondh stations, away from the seismogenic fault, are fault parallel, approximately E–W and almost orthogonal to the stress-aligned polarizations inferred elsewhere. The maximum average time delay of 0.14 s is observed at Lodai, where the fast polarization direction is found to be N338°W. This has been observed from anisotropic poro-elastic (APE) modelling and observations that these are 90° flips in shear-wave polarization, resulting from propagation through micro-cracks containing fluids at critically high pore-fluid pressure surrounding the hypocenter of the 2001 mainshock. The presence of high pore-fluid pressure in the seismogenic fault zone could also explain the observed scatter in shear-wave time delays. Further, the coincidence of the N–S trending intrusive bodies (as inferred from tomographic studies in the area) with the N–S direction of regional maximum horizontal compressional stress supports the interpretation of stress-aligned vertical extensive-dilatant anisotropic (EDA) cracks. The depth distribution of the estimated anisotropy (1–10.76%), b -values and stress drop values suggests an increase at 18–30 km depths, which could be attributed to high pore-fluid pressures resulting from a fluid-filled fractured rock matrix or open micro-cracks (characterized by high crack density and high porosity) coinciding with a low velocity zone (at 18–30 km depths) as delineated from tomographic studies in the area.

Geodetic displacements and aftershocks following the 2001 Mw = 8.4 Peru earthquake: Implications for the mechanics of the earthquake cycle along subduction zones

We analyzed aftershocks and postseismic deformation recorded by the continuous GPS station AREQ following the Mw = 8.4, 23 June 2001 Peru earthquake. This station moved by 50 cm trenchward, in a N235°E direction during the coseismic phase, and continued to move in the same direction for an additional 15 cm over the next 2 years. We compare observations with the prediction of a simple one‐dimensional (1‐D) system of springs, sliders, and dashpot loaded by a constant force, meant to simulate stress transfer during the seismic cycle. The model incorporates a seismogenic fault zone, obeying rate‐weakening friction, a zone of deep afterslip, the brittle creep fault zone (BCFZ) obeying rate‐strengthening friction, and a zone of viscous flow at depth, the ductile fault zone (DFZ). This simple model captures the main features of the temporal evolution of seismicity and deformation. Our results imply that crustal strain associated with stress accumulation during the interseismic period is probably not stationary over most of the interseismic period. The BCFZ appears to control the early postseismic response (afterslip and aftershocks), although an immediate increase, by a factor of about 1.77, of ductile shear rate is required, placing constraints on the effective viscosity of the DFZ. Following a large subduction earthquake, displacement of inland sites is trenchward in the early phase of the seismic cycle and reverse to landward after a time ti for which an analytical expression is given. This study adds support to the view that the decay rate of aftershocks may be controlled by reloading due to deep afterslip. Given the ratio of preseismic to postseismic viscous creep, we deduce that frictional stresses along the subduction interface account for probably 70% of the force transmitted along the plate interface.

GPS monitoring of vertical ground motion in northern Ardenne–Eifel: five campaigns (1999–2003) of the HARD project

We present the HARD project of GPS monitoring of vertical ground motion in NE Ardenne and Eifel (western Europe). Its main purposes are to get a better insight into the present-day rates of vertical ground motion in intraplate settings and to identify the various causes of these motions. Since 1999, we have carried out yearly campaigns of simultaneous GPS measurements at 12 sites situated so as to sample the different tectonic subunits of the study area and especially to record potential displacements across the seismogenic Hockai fault zone. Five campaigns (1999–2003) have been processed currently. Key issues of the data processing with the Gamit software are discussed and first results are presented. Though temporally consistent in many cases, the obtained vertical motion rates are spatially highly variable. They are also much too high (several mm/year) to support a tectonic interpretation, and a long-term influence of groundwater level variations is proposed to account for the observed motions. This influence should be distinguished from seasonal variations and from inter-survey variations linked to the varying degree of soil and subsoil drying off during the successive spring surveys.

Geoscience. What caused the great Lisbon earthquake?

In 1755, the great Lisbon earthquake (estimated magnitude 8.7) killed 60,000 people in southern Iberia and northwest Morocco and generated a 5- to 10-m-high tsunami wave. In his Perspective, Gutscher discusses the current research on the tectonic source of the earthquake. Recent evidence suggests that an eastward-dipping subduction zone beneath Gibraltar may have caused the earthquake. Though no interplate earthquakes have been recorded here, evidence from mud volcanoes, seismic images of deformed sediments, and heat flow data suggest that subduction is active and that the seismogenic fault zone may be locked. Numerous cruises planned in 2004 and 2005 may help to provide a definitive answer.

A lithospheric-scale seismogenic thrust in central Italy

In this paper, the relatively deep (up to 90 km) seismicity of central Italy is considered in comparison with the upper crust seismicity at the outer front of the Apennine fold-and-thrust belt system and with the major crust-scale tectonic discontinuities outlined by deep crust seismic reflection and refraction profiles. In particular, attention is given to the presence of a Plio-Quaternary SW-dipping thrust zone cutting the entire Adriatic crust and emerging at the outer front of the Apennine thrust system, along the Po Plain-Adriatic belt. This discontinuity, detectable from the CROP 03 near-vertical seismic reflection profile, but only partially highlighted in previous studies, is here named Adriatic Thrust (AT). The geometry of the AT is compared with the distribution and kinematics of the Coastal-Adriatic upper crust seismicity, of the Marche Foothill lower crust seismicity and of the Apennine deep seismicity (base of the crust and upper mantle). A good spatial correspondence between the down-dip fault geometry and the seismicity is observed. The rheological stratification of the lithosphere controls the location of the seismic activity at different depth intervals along the fault plane (0–10, 15–25, 35–45 km). The reconstructed rheological conditions do not allow to justify in terms of brittle behavior the deepest earthquakes, located at depths from 55 to 70 km within the possible down-dip intra-mantle prosecution of the AT. These events might represent either co-seismic plastic instabilities due to high strain concentrations within the AT shear zone or seismic failure of the continental Adriatic lithosphere due to a strong increase in pore fluid pressure (infiltration from the asthenosphere of hydroxil-CO 2 rich volatiles). The proposed interpretation of the AT as a lithosphere-scale seismogenic fault zone, has several implications: (1) the Po Plain-Adriatic front of the Apennine thrust system is still active; (2) the Apennine deep seismicity is not associated with a SW-dipping subduction plane; (3) seismogenic deformation in association with lithospheric failure is possible.

Fault zone fabrics and geofluid properties as indicators of rock deformation modes

In this paper we present the results of a geostructural study on active faults in central Italy, where seismogenic fault zones occur as part of a Quaternary network dissecting and/or inverting earlier tectonic features of the central Apennines fold and thrust belt. In our work we focus on the possibility of using structurally-oriented quantitative analysis of fault fabrics and fluid inclusion studies for assessing the hydraulic properties and scaling relations of fault zones in order to evaluate the role and effects of the interaction between rock and fluids in the brittle deformation of strained crustal rock volumes. The results of our study show that this approach is appropriate for (i) assessing the structural permeability of faulted and fractured rock volumes, (ii) defining the conduit/barrier behaviour of fault zones to fluid flow, (iii) mapping spatial variations of the fluid pressure across different fault segments, (iv) evaluating the maturity of a structural network and the degree of interaction of linked structural discontinuities, (v) assessing fluid composition and the conditions of deformation by means of microstructural and fluid inclusion data.

Fault slip rates at depth from recurrence intervals of repeating microearthquakes

Unique attributes in sequences of recurring, similar microearthquakes at Parkfield, California, provide a means for inferring slip rate at depth throughout the active fault surface from the time intervals between sequence events. Application of the method using an 11-year microseismicity record revealed systematic spatial and temporal changes in the slip rate that were synchronous with earthquake activity and other independent measures of fault-zone slip. If this phenomenon is found to be generally common behavior in active faults, it forms the basis for a method to monitor the changing strain field throughout a seismogenic fault zone.

Seismotectonic significance of the 29 January 1989 Etne earthquake, SW Norway

The integrated analysis of geological, seismological and field observations with lineament data derived from satellite images allows the identification of a possible seismogenic fault zone for an earthquake which occurred near Etne in southwestern Norway, on 29 February 1989. The hypocentre of the earthquake was located at the mid-crust at a depth of 13.8±0.9 km which is typical of small intraplate earthquakes. The Etne earthquake occurred as a result of normal faulting with a dextral strike-slip component on a NW–SE trending fault. Available geological and lineament data indicate correlation of the inferred seismogenic fault with the NW–SE trending Etne fault zone. An aeromagnetic anomaly related to the Etne fault zone forms a regional feature intersecting both Precambrian basement and allochthonous Caledonian rocks. Based on these associations the occurrence of the Etne event is ascribed to the reactivation of a zone of weakness along the Etne fault zone. Slope-instabilities developed in the superficial deposits during the Etne event demonstrate the existence of potentially hazardous secondary-effects of such earthquakes even in low seismicity areas such as southwestern Norway.

Seismic-Wave Attenuation Associated with Crustal Faults in the New Madrid Seismic Zone

The attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the New Madrid seismic zone in the central Mississippi Valley. The waves that have the strongest attenuation pass through the seismic trend along the axis of the Reelfoot rift in the area of the Blytheville arch. Defocusing of the waves in a low-velocity zone and/or seismic scattering and absorption could cause the attenuation; these effects are most likely associated with the highly deformed rocks along the arch. Consequently, strong seismic-wave attenuation may be a useful criterion for identifying seismogenic fault zones.