Fault Plane Orientations Research Articles

Evidence of multiple strain fields beneath the eastern flank of Mt. Etna volcano (Sicily, Italy) deduced from seismic and geodetic data during 2003–2004

We carried out a study of the seismicity and ground deformation occurring on Mt. Etna volcano after the end of the 2002–2003 eruption and before the onset of the 2004–2005 eruption. Data were recorded by the permanent local seismic network run by Istituto Nazionale di Geofisica e Vulcanologia – Sezione di Catania and by geodetic surveys carried out in July 2003 and July 2004 on the GPS network. Most earthquakes were grouped in two main clusters located in the northeastern and southeastern sectors of the volcano. The areal distribution of seismic energy associated with the recorded earthquakes allowed us to highlight the main seismogenic areas of Mt. Etna. In order to better understand the kinematic processes of the volcano, 3D seismic locations were used to compute fault plane solutions, and a selected dataset was inverted to determine stress and strain tensors. The focal mechanisms in the northeastern sector show clear left-lateral kinematics along an E-W fault plane, consistent with events occurring along the Pernicana Fault system. The fault plane solutions in the southeastern sector show mainly right-lateral kinematics along a NNE and ENE fault plane and left lateral-kinematics along NW fault planes that together suggest roughly E-W oriented compression. Surface ground deformation affecting Mt. Etna measured by GPS surveys highlighted a marked inflation during the same period and exceptionally strong seawards motion of its eastern flank. The 2D geodetic strain tensor distribution was calculated and the results show mainly ENE-WSW extension coupled with WNW-ESE contraction, indicating right-lateral shear along a NW-SE oriented fault plane. The different deformation of the eastern sector of the volcano, as measured by seismicity and ground deformation, must be interpreted by considering the different depths of the two signals. Seismic activity in the southeastern sector of volcano is located between 3 and 8 km b.s.l. and can be associated with a very strong additional E-W compression induced by a pressurizing source just westwards and at the same depth, located by inverting GPS data. Ground deformation, in contrast, is mainly affected by the shallower dynamics of the fast moving eastern flank which produces a shallower opposing E-W extension. The entire dataset shows that two different processes affect the eastern flank at the same time but at different depths; the boundary is clearly located at a depth of 3 km b.s.l. and could represent the decollement surface for the mobile flank.

Waveform inversion of broad-band data of local earthquakes in the Koyna-Warna region, western India

SUMMARY The Koyna–Warna region in western India is the best example of reservoir triggered seismicity. The world’s largest triggered earthquake of magnitude 6.3 occurred in Koyna in 1967, followed by several moderate to small earthquakes ever since. A digital seismograph network deployed for seismic monitoring during 2005 August to 2008 December has indicated a shift in concentration of seismicity towards south in the Warna region including a new zone of seismic activity to the southwest. During the observation period 13 earthquakes of magnitude 4 and greater have occurred of which 11 occurred near the Warna region while only two occurred in Koyna. In this study we modelled broad-band waveform data of six of these earthquakes near Warna using waveform inversion approach. Initially a new velocity model was determined using a joint hypocentral determination approach that simultaneously solves for the velocity structure as well as the hypocentral parameters. A trap thickness of 1.2km with a P-wave velocity of 4.40kms −1 and an upper crustal layer down to 10km with a velocity of 5.96kms −1 are obtained. The new model not only provides the minimum residual error for the traveltimes, but consistently provides the least mismatch error in the waveform inversion of all the events, performing better than any of the previously determined velocity models. In general, focal mechanisms of normal type with NS to NNW–SSE oriented fault planes are obtained for all these events that are correlated with probable faults inferred from satellite images and aeromagnetic anomalies. Focal depths in the range of 5–6km are obtained for earthquakes in the Warna region based on the sensitivity of whole waveform inversion at local distances. It is felt that joint inversion of waveform data of several earthquakes with a wider spatial distribution, along with the velocity structure would help in precisely characterizing the faulting mechanism and seismogenic depth range for the Koyna–Warna region in future.

Discussion on ‘Was the 2008 Wenchuan earthquake triggered by Zipingpu reservoir?’

Some recent publications presented a result suggesting that Zipingpu reservoir hastened the occurrence of the 2008 MS8.0 Wenchuan earthquake by tens to hundreds of years. Their researches calculated the Coulomb stress change induced by Zipingpu reservoir on the rupturing fault of Wenchuan earthquake. Their results, however, are critically dependent upon the 3-D event location, reservoir location, and the fault plane orientation. We repeated Ge et al.’s work in this paper and found that an improper dip angle parameter of their 2-D fault model might lead to a wrong conclusion. Both the modeling results based on the 2-D model and 3-D model with proper fault parameters will show Coulomb stress changes alone were neither large enough nor had the correct orientation to affect the occurrence of Wenchuan earthquake, which supports our recent argument based on the local seismicity analysis and the induced Coulomb stress change calculation with a 3-D model.

Numerical modelling of ground motion in the Taipei Basin: basin and source effects

SUMMARY The Taipei basin in northern Taiwan is located in a high seismicity region and was affected by several earthquakes in the past (ML= 7.3 on 1909 April 15; ML= 6.8 on 1986 November 15; the Chi-Chi ML= 7.3 earthquake on 1999 September 21 and ML= 6.8 on 2002 March 31). The main characteristic of the Taipei basin is its complex shape with a deep western and shallow eastern part. The uppermost Sungshan formation with its low shear wave velocities (90–200 m s−1) is also a distinct feature of the basin. Based on the large data base of earthquake records obtained from the Taiwan Strong Motion Instrumentation Program network, many studies on ground motion within the Taipei basin exist. However, the influence of the various subsurface structures on the observed ground motions as well as the variability of ground motion with respect to earthquake location is not fully understood. We apply a 3-D finite-difference method to simulate wave propagation up to 1 Hz for a small earthquake close to the basin in order to resolve these open questions. By varying source and structural parameters, we explore the variability of ground motion. Our study includes a subsurface model that is based on recent studies on the basin structure and on the crustal structure of Taiwan. From our simulations we find a good fit between simulated and observed waveforms and peak ground accelerations for the considered small earthquake near the basin. We also explore the influence of fault plane orientation, hypocentre location, deep basin structure and soft soil surface layers of the Sungshan formation by varying the subsurface structure and earthquake position. Our studies reveal that the basin structure produces an amplification factor of about 4 compared to hard rock conditions. Additionally, the soft soil Sungshan formation produce amplification of a factor of 2. This results in a maximum amplification of the basin structure of about 8, which is in good comparison with amplification values larger than 5 found from the analysis of observed earthquakes. These values clearly exceed the amplification values of about 2–3 obtained when applying standard 1-D site effect analysis. Our simulations for different earthquake positions show that ground motion depends strongly on earthquake location and fault orientation. Therefore, the application of average values of spectral amplification obtained from the analysis of recorded data from distant earthquakes with different azimuths and fault planes may significantly underestimate future ground motions of possible earthquakes on known faults close to the Taipei basin. The simulation of a small earthquake near the Taipei basin presented in this study will help to set up adequate simulation parameters for a possible large earthquake close to the Taipei basin. Such a simulation of a scenario earthquake close to the Taipei basin would allow to significantly improve hazard assessment as no observations of strong earthquakes in the vicinity of the basin exist.

Measurement of differential rupture duration as constraints on the source finiteness of deep‐focus earthquakes: 2. Synthetic tests to estimate errors in rupture vectors and their effect on fault plane identification

The directivity method of Warren and Silver (2006) has been used to distinguish the fault planes of deep earthquakes in the Tonga and Middle America subduction zones. These studies identified exclusively subhorizontal fault planes between 100 and 300 km depth, raising the question of whether the observations represent new constraints on the physical mechanism of the earthquakes or a bias in the methodology. Here, the strengths, weaknesses, and biases of the method are investigated through the comprehensive analysis of 120 synthetic earthquakes with varying depth, rupture vector orientation, rupture complexity, signal‐to‐noise ratio, and station distribution. These synthetic tests, which allow the evaluation of the effect of each of the varied parameters on the rupture vector determination, show that fault planes can be identified for a wide variety of conditions. The method underestimates rupture velocities by 76%–94%, but this does not affect the determination of the orientation of the rupture vector. The most important parameter for determining the orientation of the rupture vector, and thereby allowing identification of the fault plane, is the distribution of stations recording the earthquake; better coverage around the earthquakes results in tighter constraints on the rupture vector. The broader distribution of recording stations for deeper earthquakes results in tighter constraints on the rupture vectors for deeper earthquakes and, therefore, more ease in identifying the fault plane. However, the difference in resolution does not systematically prohibit the identification of certain orientations of fault planes at shallower depths and suggests that previous observations of exclusively subhorizontal fault planes between 100 and 300 km depth are not the result of systematic bias in the methodology. Instead, the observed dominance of subhorizontal fault planes provides important constraints on the physical mechanism of intermediate‐depth earthquakes. The observed subhorizontal fault orientation is inconsistent with the reactivation of the dominant trenchward‐dipping outer rise normal faults.

Observation of surface seismic activity changes of an Alpine glacier during a glacier‐dammed lake outburst

During the 2004 outburst flood of Gornersee, Switzerland, we observed surface microseismic activity (so‐called icequakes) near the glacier‐dammed lake on Gornergletscher. Using surface waves, we located near‐surface tensile fracturing events by inverting time delays using a quasi‐Newton scheme. We located 8520 near‐surface events with uncertainties of less than 10 m. Seismic activity increased during the lake drainage, and the icequake epicenters migrated downglacier. Furthermore, we identified events whose fault plane orientations are nearly perpendicular to the local pattern of surface crevassing. These observations can be explained by the rotation of principal strains at the glacier surface deduced from local ice flow measurement which occurred during the flotation of the ice dam at the onset of the lake drainage. In general, we suggest that our technique is a useful tool to locate large data sets of microseismic events and can be used to monitor the temporal evolution of fracture processes and their dependence on glacier flow and glacier hydrology.

Evidence for Near-Horizontal Tensile Faulting at the Base of Gornergletscher, a Swiss Alpine Glacier

Abstract Using 3D Green’s functions we determine full and constrained moment tensor solutions of icequakes near the base of Gornergletscher, Switzerland. The seismic events were recorded in the summer of 2004 using a high-density seismometer array. The seismic velocity model used in the generation of Green’s functions is based on radio-echo soundings to approximate the basal topography, which beneath the study site exhibits a strong inclination. As the basal conditions are not well known, we try moment tensor inversions with seismic velocity profiles consisting of two and three media. The former case consists of homogeneous ice resting on bedrock, whereas the latter case includes a thin basal layer with slow seismic velocities representing eroded material or highly fractured ice. Effects of errors in Green’s functions are estimated by sensitivity studies in which we invert 1D and 3D synthetics using Green’s functions of wrong velocity models. The results show that calculations of source types and fault plane orientations of tensile cracks are rather robust with respect to errors in Green’s functions. However, the quality of the waveform fits depends on strike and dip of the synthetic source. When inverting seismograms, Green’s functions of the seismic model that includes the basal slow velocity layer are found to give the most realistic source types as well as the best waveform fits. The fault mechanisms derived from constrained moment tensor inversions are near-horizontal tensile cracks, which suggest a complex time-dependent basal stress field.

Intraplate earthquake swarm in Belo Jardim, NE Brazil: reactivation of a major Neoproterozoic shear zone (Pernambuco Lineament)

SUMMARY Intraplate earthquakes in stable continental areas have been explained basically by reactivation of pre-existing zones of weakness, stress concentration, or both. Zones of weakness are usually identified as sites of the last major orogeny, provinces of recent alkaline intrusions, or stretched crust in ancient rifts. However, it is difficult to identify specific zones of weakness and intraplate fault zones are not always easily correlated with known geological features. Although Northeastern Brazil is one of the most seismically active areas in the country (magnitudes 5 roughly every 5 yr), with hypocentral depths shallower than ∼10 km and seismic zones as long as 30–40 km, no clear relationship with the known surface geology can be usually established with confidence, and a clear identification of zones of weakness has not yet been possible. Here we present the first clear case of seismic activity occurring as reactivation of an old structure in Brazil: the Pernambuco Lineament, a major Neoproterozoic shear zone. The 2004 earthquake swarm of Belo Jardim (magnitudes up to 3.1) and the recurrent activities in the nearby towns of Sao Caetano and Caruaru (magnitudes up to 4.0 and 3.8), show that the Pernambuco Lineament is a weak zone. A local seismic network showed that the Belo Jardim swarm of 2004 November occurred by normal faulting on a North dipping, E–W oriented fault plane in close agreement with the E–W trending structures within the Pernambuco Lineament. The Belo Jardim activity was concentrated in a 1.5 km (E–W) by 2 km (downdip) fault area, and average depth of 4.5 km. The nearby Caruaru activity occurs as both strike-slip and normal faulting, also consistent with local structures of the Pernambuco Lineament. The focal mechanisms of Belo Jardim, Caruaru and S. Caetano, indicate E–W compressional and N–S extensional principal stresses. The NS extension of this stress field is larger than that predicted by numerical models such as those of Coblentz & Richardson and we propose that additional factors such as flexural stresses from the nearby Sergipe-Alagoas marginal basin could also affect the current stress field in the Pernambuco Lineament.

Faulting Parameters of Earthquakes (4.1 <= ML <= 5.3) in the Peninsular Ranges of Baja California, Mexico

Seismic moment ( M ) is a parameter related to the physical process that takes place at the source (Aki 1966; Aki and Richards 1980). Sometimes it is possible to calculate M from the constant spectral amplitude level at low frequencies, but in other cases this is not possible. For instance, for M > 4.0 earthquakes (recorded at short distances), it is difficult to determine the flat portion of the spectrum due to instrumental limitations. For these cases, waveform modeling is an alternative way to evaluate M . In addition to determining this source parameter, it is also possible to obtain the faulting geometry and the depth of the hypocenter through waveform modeling. In the Peninsular Ranges of Baja California (PRBC), few studies have been conducted to determine earthquake faulting parameters. Some examples are the studies of Nava and Brune (1983) (1975 ML 5.1 earthquake) and Gonzalez (1987) (1985 ML 5.0 earthquake). However, studies of the seismic source have not been attempted for earthquakes of lower magnitude ( M < 5). We computed the faulting parameters of five small-to-moderate earthquakes that occurred between 1991 and 1996 in the PRBC, via waveform modeling of broadband seismograms recorded at regional distances (180 to 245 km). Our modeling was important in constraining the fault plane orientation obtained from first-motion polarities recorded for those earthquakes. In this modeling, we considered epicenter-to-station paths of the Peninsular Ranges of southern California and Baja California tectonic regions. Historically, moderate size earthquakes (5 ≤ M ≤ 7) have occurred in the PRBC, mainly along the San Miguel fault. Examples of these earthquakes are the 1954 earthquakes ( M 6.0 and 6.4; Leeds 1979) and the 1956 earthquakes ( ML 6.8, 6.4 and 6.3; Shor and Roberts 1958). Castro (1983) used synthetic seismograms to calculate the seismic …

Multiplet Focal Mechanisms from Polarities and Relative Locations: The Iznajar Swarm in Southern Spain

Abstract In April 1998, a swarm of ∼1800 microearthquakes near the village of Iznajar (southern Spain) was recorded at the Granada basin short-period seismic network. Focal mechanisms from local P -wave polarities are poorly constrained and cannot characterize the seismotectonics of the series. Here we combine polarity information and multiplet relocation to address this issue. We use waveform cross correlation on P and S arrivals to identify events with highly similar seismograms, group our detections into multiplet clusters, and invert the cross-correlation time delays to obtain precise relative locations. Relative locations have errors of several tens to a few hundreds of meters horizontally and vertically, and define strike and dip of active fault patches with an accuracy of ∼20 ° –30 ° . We introduce the multiplet fault plane orientations into focal mechanism inversion, now yielding mostly well-constrained solutions, in addition to resolving the nodal plane symmetry. We observe mainly north-south left-lateral strike-slip faulting and a few north-northwest–south-southeast normal faulting solutions, illustrating the kinematic complexity of the swarm, and pointing to a local deformation style different from the nearby Granada basin.

Fault plane orientations of microearthquakes at Mt. Etna from the inversion of P-wave rise times

A crucial point in the analysis of tectonic earthquakes occurring in a volcanic area is the inference of the orientation of the structures along which the ruptures occur. These structures represent zones of weakness which could favor the migration of melt toward the surface and the assessment of their geometry is a fundamental step toward efficient evaluation of volcanic risk. We analyzed a high-quality dataset of 171 low-magnitude, tectonic earthquakes that occurred at Mt. Etna during the 2002–2003 eruption. We applied a recently developed technique aimed at inferring the source parameters (source size, dip and strike fault) and the intrinsic quality factor Q p of P waves from the inversion of rise times. The technique is based on numerically calibrated relationships among the rise time of first P waves and the source parameters for a circular crack rupturing at a constant velocity. For the most of the events the directivity source effect did not allow us to constrain the fault plane orientation. For a subset of 45 events with well constrained focal mechanisms we were able to constrain the “true” fault plane orientation. The level of resolution of the fault planes was assessed through a non linear analysis based on the random deviates technique. The significance of the retrieved fault plane solutions and the fit of the assumed source model to data were assessed through a χ-square test. Most of the retrieved fault plane solutions agree with the geometrical trend of known surface faults. The inferred source parameters and Q p are in agreement with the results of previous studies.

Spatiotemporal distribution of aftershocks of the 2004 December 5ML= 5.4 Waldkirch (Germany) earthquake

SUMMARY The Waldkirch earthquake on 2004 December 5 at 1:52 UTC with ML= 5.4 was the strongest earthquake in Baden-Wurttemberg since 1978. It was followed by a large number of aftershocks. We were operational in field only 14 hr after the main shock for recording aftershocks. The aim of that field campaign was to test the performance of our newly developed system and to compare the results obtained by one small aperture seismic array to a standard aftershock network. More than 700 microearthquakes were recorded within a period of about 39 hr, the largest with magnitude ML= 3.0. Events with negative as well as positive polarities were identified as a result of alternating radiation patterns of the focal mechanisms. The analysis of the events was carried out with a statistical approach because of the high amount of events. Similarity matrices sorted by time reveal no significant event clustering, whereas matrices, rearranged by dendrograms according to the cross-correlation coefficients indicate that a part of the aftershocks are spatially clustered. An investigation of the chronological occurrence of these spatially clustered events shows a continuous seismic activity for the duration of measurement. The mean depth of the seismogenic zone could be determined to 11.1 km using tS–tP time differences. A relative location of aftershocks provides evidence for a WNW–ESE oriented fault plane direction and confirms the orientation of a nodal plane of the main shock. This Subhercynian direction is a predominant structure in the Black Forest and can be observed on many faults and mineral dykes. Our results coincide well with observations from the permanent and a temporarily installed network, which was operational 2 d after the main shock.

Combined kinematic and paleostress analysis of fault-slip data: The Multiple-slip method

We describe the Multiple-slip method for the construction of the displacement gradient tensor, which describes the faulting-related deformation of the region. Three elements must be known for each fault: (1) the fault plane orientation, (2) the slip direction, and (3) the number of parallel faults in the same size range. The data on the orientation of faults and the direction of slip along them define the geometric moment tensor for each fault, while the data on the number of parallel faults belonging to a particular fault-set constrain the weighting factor for each fault. In the Multiple-slip method, the weighting factors also depend on the stress state that produced the displacement along the faults. The stress state on the observed faults at the time of faulting can be estimated from the paleostress analysis, which finds the stress tensor capable of explaining the slip direction along the faults. Therefore, the Multiple-slip method is a combination of kinematic and paleostress techniques, in which the paleostress analysis must be performed prior to the kinematic analysis. The Multiple-slip method allows for calculating (1) the direction of kinematic axes (directions of maximum shortening and extension), (2) the direction and relative magnitude of rotation, and (3) the ratio between the principal strains.

Seismicity and seismotectonics of West Saxony, Germany — new insights from recent seismicity observed with the Saxonian seismic network

A new network of permanently recording seismic stations in West Saxony has considerably improved detection threshold, location accuracy and depth determination in this seismically active region. Between 2001 and 2007 more than 900 events have been located. Seismicity mainly occurred along a band stretching north-south between Leipzig and Vogtland/NW Bohemia area with local magnitudes ranging between −0.8 and 2.8. Seismicity clearly delineates the Leipzig-Regensburg (L-R) fault zone striking N-S, and the Gera-Jachymov (G-J) fault zone striking roughly NNW-SSE. The hypocentral depths can be divided into two depth ranges, one at depths below 10 km, and a second at less than 10 km depth that only extends S-N from the Vogtland until the crossing between L-R and G-J fault zones. A small earthquake sequence that occurred near Werdau/Zwickau in August 2006 at almost the same epicenters as an earlier sequence 1997/98 seems to confirm this finding: a relative localization of 15 events with the double-difference technique clearly reveals two distinct subclusters at about 6 and 12–14 km depth. With the improved station coverage 33 new fault plane solutions from events along the L-R fault zone north of the swarmquake area could be determined from P-polarities and P/S ratios. They do not differ significantly from solutions in the Vogtland/NW-Bohemia area and are mostly compatible with a N-S oriented fault plane. Strike slip mechanisms with or without a dip slip component dominate.

Post 2000-swarm microearthquake activity in the principal focal zone of West Bohemia/Vogtland: Space-time distribution and waveform similarity analysis

We present the pattern of seismic activity in the period between 2001 and 2007 for the Nový Kostel focal zone, which is recently the most active zone of the West-Bohemia/Vogtland earthquake swarm region. While the year 2001 was characterized by dying out of the 2000-swarm activity in the form of a few microswarms, almost no seismicity occurred in the period between 2002 and 2003. Since 2004 an elevated seismic activity occurs in the form of repeating microearthquake swarms. We used a relative location method to relate the hypocenter positions of the post-swarm activity to the geometry of the 2000-swarm cluster. We found that the activity has concentrated in several clusters, which have been repeatedly activated. Some clusters coincide with the position of the previous activity; the others have activated so far inactive deep segments at the southern edge of the Nový Kostel fault. Besides the shift of the hypocenters to the edges of the previously active area we observe a southward migration of the activity and an increase of maximum depths of earthquakes from 10 to 13 km. The waveform similarity analysis disclosed that some fault patches consist of only a single, repeatedly activated fault plane, while the others consist of multiple, differently oriented fault planes activated almost simultaneously. Most of the focal mechanisms are consistent with the geometry of hypocenters showing NNW-SSE trending steep fault planes with left-lateral strike-slip mechanisms and varying dip-slip component.

Ruptures in a Highly Fractured Upper Crust

The 1999 Chiayi M 6.4 earthquake sequence that occurred about one month after the 1999 Chi-Chi M 7.6 earthquake is remarkably similar to the Ruey-Li M 6.2 earthquake sequence which occurred one year before the big Chi-Chi event. Both sequences were energetic and short in duration; with events confined in small source volumes, and the fault-plane orientation of individual events in these sequences varied abruptly during the development of the dynamic rupture. Both occurred on the southern extension of the large Chelungpu rupture causal to the Chi-Chi event, and in an area of highly fractured upper crust. We consider that this faulting behavior is due to the rupture-induced dynamic stress that interacts with the pre-existing highly fractured upper crust and continually perturbs the local stress field. This phenomenon appears to be typical of particularly active plate boundary regions with a highly fractured upper crust under intense tectonic loading.

Fault plane orientations of intermediate‐depth earthquakes in the Middle America Trench

Intermediate‐depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones. The orientation of presubduction faults is particularly well known offshore of Middle America, where seismic reflection profiles show outer rise faults dipping toward the trench and extending &gt;20 km into the lithosphere. If water is transported along these faults and incorporated into hydrous minerals, the faults may be reactivated later when the minerals dehydrate. In this case, the fault plane orientations should be the same in the outer rise and at depth, after accounting for the angle of subduction. To test this hypothesis, we analyze the directivity of 54 large (MW ≥ 5.7) earthquakes between 35 and 220 km depth in the Middle America Trench. For 12 of these earthquakes, the directivity vector allows us to identify the fault plane of the focal mechanism. Between 35 and 85 km depth, we observe both subhorizontal and subvertical fault planes. The subvertical fault planes are consistent with the reactivation of outer rise faults, whereas the subhorizontal fault planes suggest the formation of new faults. Deeper than 85 km, we only observe subhorizontal faults, indicating that the outer rise faults are no longer being reactivated. The similarity with previous results from the colder Tonga‐Kermadec subduction zone suggests that the mechanism generating these earthquakes, and controlling fault plane orientations, depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate‐depth seismicity. Exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes.

Source analysis of the February 12th 2007, Mw 6.0 Horseshoe earthquake: Implications for the 1755 Lisbon earthquake

While very large earthquakes are generally confined to subduction zones, the SW Iberian margin –setting of the famous Mw 8.5–8.7, 1755 Lisbon tsunami earthquake‐ may be an exception to this rule. Evidence for active subduction is not conclusive here, but instead plate convergence in old oceanic lithosphere with large brittle layer thickness can account for the occurrence of great earthquakes along moderate‐length faults. We estimate the source parameters of the February 12th 2007, Horseshoe earthquake. Regional moment tensor inversion yields an Mw 6.0, reverse to strike‐slip faulting source in the upper mantle. Modelling teleseismic, surface‐reflected body waves (pP, pwP, sP) indicates a source depth of 40 km beneath the seafloor. Analysing apparent source time functions allows identifying the preferred fault plane (strike N245°E/ dip 55°/ rake 50°), and estimating rupture area (53 km2) and average slip (0.27 m). Scaling the source characteristics to the size of the 1755 earthquake suggests a fault length of 230–315 km, being compatible with the length of mapped faults in the area.

Earthquake mechanics and deformation in the Tonga‐Kermadec subduction zone from fault plane orientations of intermediate‐ and deep‐focus earthquakes

We make use of rupture directivity to analyze 82 deep earthquakes (≥100 km depth) in the Tonga‐Kermadec subduction zone. Identifying the fault planes for 25 of them, we are able to place new constraints on both the physical mechanism of intermediate‐ and deep‐focus earthquakes and deformation within the subducting slab. We find that half of deep earthquakes with MW ≥ 6 have detectable directivity. We compare the obtained fault orientations with those expected for the reactivation of outer‐rise normal faults and with those expected for the creation of new faults in response to the ambient stress field. Earthquakes &gt;300 km depth match the patterns expected for the creation of a new system of faults: we observe both subhorizontal and subvertical fault planes consistent with a downdip‐compressional stress field. Slip along these faults causes the slab to thicken. Rupture propagation shows no systematic directional pattern. In contrast, at intermediate depths (100–300 km), all ruptures propagate subhorizontally and all identified fault planes, whether in the upper or lower region of the double seismic zone, are subhorizontal. Rupture propagation tends to be directed away from the top surface of the slab. After accounting for the angle of subduction, the subhorizontal fault plane orientation is inconsistent with the orientation of outer‐rise normal faults, allowing us to rule out mechanisms that require the reactivation of these large surface faults. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation. If all deformation takes place on these subhorizontal faults, it would cause the slab to thin. Assuming the slab is incompressible, this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths.

Seismically active fracture zones in the continental wedge above the Andean subduction zone in the Arica Elbow region

In the continental lithosphere overlying the Wadati-Benioff zone in the Arica Elbow region eleven seismically active fracture zones were delineated on the basis of distribution of earthquake foci. The position and width of the trace, thickness, dip and maximum depth of individual fracture zones were estimated and complemented by tectonic and geological evidence. Existing fault plane solutions were attributed to the pertinent fracture zones and confronted with their orientation, inclination and tectonic function. The analysis shows normal faulting in the eastward dipping fractures, bordering the Andes in the west, and reverse faulting in the westward dipping fractures, limiting the Andes in the east. Fault plane orientations and slip directions indicated by earthquake focal mechanisms were used for the analysis of the state of stress in the fracture zones. The results point to the necessity of presence of fluids for the generation of earthquakes along faults in the western fracture zones. The position, inclination and stress conditions in the westernmost fracture system might influence the shift of active volcanism westwards from the surface projection of the intermediate depth aseismic gap in the Wadati-Benioff zone and cause the channeling of magma through tectonically disintegrated seismically active fracture zones.