Focal Mechanisms Of Microearthquakes Research Articles

3D architecture and complex behavior along the simple central San Andreas fault

The central San Andreas Fault (CSAF) exhibits a simple linear large-scale fault geometry, yet seismic and aseismic deformation features vary in a complex way along the fault. Here we investigate fault zone behaviors using geodetic observation, seismicity and microearthquake focal mechanisms. We employ an improved focal-mechanism characterization method using relative earthquake radiation patterns on 75,164 Ml ≥ 1 earthquakes along a 2-km-wide, 190-km-long segment of the CSAF, from 1984 to 2015. The data reveal the 3D fine-scale structure and interseismic kinematics of the CSAF. Our findings indicate that the first-order spatial variations in interseismic fault creep rate, creep direction, and the fault zone stress field can be explained by a simple fault coupling model. The inferred 3D mechanical properties of a mechanically weak and poorly coupled fault zone provide a unified understanding of the complex fine-scale kinematics, indicating distributed slip deficits facilitating small-to-moderate earthquakes, localized stress heterogeneities, and complex multi-scale ruptures along the fault. Through this detailed mapping, we aim to relate the fine-scale fault architecture to potential future faulting behavior along the CSAF.

Comparison of Single-Well Microseismic Focal Mechanism Inversions with Different Source Models

ABSTRACT Through downhole monitoring, the focal mechanisms of microearthquakes can be quantitatively determined, thus providing valuable information for characterizing the fracturing process and the in situ stress status. The double-couple (DC) and moment tensor (MT) source models are commonly used to study microearthquakes. However, the DC model fails to include non-DC mechanisms, and MT inversion from single-well data is still challenging. One possible way to address this is using the shear–tensile general dislocation (GD) source model. We provide a detailed comparison of the DC, GD, and MT models, and introduce the differences in their modeling and inversion theories. These three models are described by four, five, and six parameters, and correspond to a single point, a straight line, and the entire space in the Hudson source-type plots, respectively. Both the DC and GD models yield nonlinear inversions, whereas the MT inversion is linear. Synthetic tests set up from a field single-well monitoring case are performed to study the resolvability of the DC, GD, and MT models in single-well focal mechanism inversions. The results indicate that the inversion error increases from DC→GD→MT for a single-well acquisition system, and the GD and DC inversions are both stable, whereas the MT inversion deviates from the inputs in cases with a perfectly vertical receiver array, 5% model velocity perturbations, 10 m horizontal source location errors, or 40% noise levels. We also find that the focal mechanism inversion mainly depends on the horizontal source–receiver azimuth coverage, and that the nonvertical well direction is helpful for constraining single-well inversions. According to our study, focal mechanism inversions based on the GD model can obtain reliable solutions from near-vertical single-well data, which will help improve non-DC earthquake studies.

Seismogenic Structures in the Ma River Fault Zone, Vietnam: Insights from Focal Mechanisms of Microearthquakes and Local Stress States

Seismogenic Structures in the Ma River Fault Zone, Vietnam: Insights from Focal Mechanisms of Microearthquakes and Local Stress States

Complex microseismic activity and depth-dependent stress field changes in Wakayama, southwestern Japan

We examined the spatial relationship between seismicity and upper crustal structure in the Wakayama region, northwestern Kii Peninsula, Japan, by investigating microearthquake focal mechanisms and the local stress field. The focal mechanisms of most events studied fall into three categories: (1) normal faulting with N–S-oriented T-axes mainly occurring at shallow depths, (2) reverse faulting with E–W-oriented P-axes dominating at intermediate depths, and (3) strike-slip faulting with N–S-oriented T-axes and E–W-oriented P-axes mainly seen at greater depths. The stress field varies with depth: the shallow part is characterized by a strike-slip-type stress regime with N–S tension and E–W compression, while the deep part is characterized by an E–W compressional stress regime consistent with reverse faulting. The depth-dependent stress regime can be explained by thermal stress caused by a heat source, as expected from geophysical observations. Geologic faults, acting as weak planes, might contribute to generate shallow normal fault-type and deeper strike-slip fault-type microearthquakes.Graphical

Simulation of seismic events induced by CO2 injection at In Salah, Algeria

Carbon capture and storage technology has the potential to reduce anthropogenic CO2 emissions. However, the geomechanical response of the reservoir and sealing caprocks must be modelled and monitored to ensure that injected CO2 is safely stored. To ensure confidence in model results, there is a clear need to develop ways of comparing model predictions with observations from the field. In this paper we develop an approach to simulate microseismic activity induced by injection, which allows us to compare geomechanical model predictions with observed microseismic activity. We apply this method to the In Salah CCS project, Algeria. A geomechanical reconstruction is used to simulate the locations, orientations and sizes of pre-existing fractures in the In Salah reservoir. The initial stress conditions, in combination with a history matched reservoir flow model, are used to determine when and where these fractures exceed Mohr–Coulomb limits, triggering failure. The sizes and orientations of fractures, and the stress conditions thereon, are used to determine the resulting micro-earthquake focal mechanisms and magnitudes. We compare our simulated event population with observations made at In Salah, finding good agreement between model and observations in terms of event locations, rates of seismicity, and event magnitudes.

Stress fields in and around metropolitan Osaka, Japan, deduced from microearthquake focal mechanisms

Present-day stress fields in and around metropolitan Osaka, Japan, have been investigated based on 238 microearthquake focal mechanism solutions from approximately the past 10years. The successful retrieval of the focal mechanisms in the noisy urban area owed much to the clean seismic data recorded at four borehole observatories deeper than 500m depth in Osaka prefecture, in which P-wave onset polarity of the microearthquake could be clearly identified. We found many microearthquakes with a pure reverse-faulting mechanism, which is consistent with the faulting style of major active faults within the area (42-km long east-dipping Uemachi and 38-km long east-dipping Ikoma fault zones); however, a considerable number of microearthquakes with strike-slip components were also found to occur. Most of the P-axes are oriented in the ESE–WNW to ENE–WSW directions sub-horizontally, conforming to the general tectonic trend of the area. The analysis of a stress tensor inversion delineated small but noticeable differences in stress regime, on a scale of at least 10km, where strike-slip faulting prevails throughout the region but the middle and southern sections of the Uemachi fault zone contain some reverse-faulting components. Based on the estimated stress fields, together with assumptions regarding the deep geometry of the Uemachi fault zone, we assessed the reactivation potential of the fault zone through slip-tendency analysis. The results indicate higher potential in the middle and southern sections of the fault zone, which is related to the locally increased reverse component detected there.

Orientations of the principal paleostress in the Western Bohemia sesmoactive region and their comparison with the recent stress

The palaeostress analyses based on the fault striae data were carried out in the Western Bohemia seismoactive region. The results of these analyses were compared with the recent orientations of the principal stresses determined from focal mechanisms of micro-earthquakes. The fault striae data were obtained mainly from the late-Variscan granites. No data for the palaeostress analyses were found in the post-Variscan (Tertiary) sedimentary cover.

Depth-dependent stress field in and around the Atotsugawa fault, central Japan, deduced from microearthquake focal mechanisms: Evidence for localized aseismic deformation in the downward extension of the fault

[1] Focal mechanisms have been determined from P wave polarity data as well as body wave amplitudes for 154 microearthquakes that occurred around the Atotsugawa fault in central Japan between 2002 and 2004. While we found many microearthquakes with a pure strike-slip mechanism that is similar to the faulting style of the Atotsugawa fault, a considerable number of microearthquakes with reverse faulting components are also occurring. Most of the P axes are horizontal and oriented in the WNW-ESE direction, which conforms to the general tectonic trend in this area. In contrast, the T axes have a wide range of plunge, suggesting that reverse-faulting-type earthquakes as well as strike-slip ones are occurring. The most conspicuous feature in the focal mechanism distribution is the depth dependence, where shallow earthquakes are primarily reverse faulting and strike-slip earthquakes become predominant with depth. A stress tensor inversion reveals that the shallower part is characterized by a mixture of reverse and strike-slip faulting regimes and that a pure strike-slip faulting regime appears only around the bottom of the seismogenic zone. Together with other geophysical observational evidence for the fault, we suggest that the existence of a localized aseismic deformation below the Atotsugawa fault is the simplest scenario that can explain the observed stress fields. This scenario provides a stress accumulation mechanism of disastrous shallow inland earthquakes, in which the localized aseismic deformation accumulates stress onto the fault plane in the seismogenic zone during an earthquake cycle and the main shock would occur when the failure stress is reached on the fault.

Focal mechanisms of micro-earthquakes in the Dobrá Voda seismoactive area in the Malé Karpaty Mts. (Little Carpathians), Slovakia

We have analyzed 44 micro-earthquakes with magnitudes between 1.2 and 3.4, which occurred in the Dobrá Voda area, Slovakia, in the period 2001–2009. The epicentres of the micro-earthquakes form a cluster elongated in the ENE–WSW direction. This direction coincides with the orientation of the main fault systems in the area: Dobrá Voda and Brezová faults. The depths of the hypocentres vary from 1 km to 14 km. Three different methods were used to calculate the focal mechanisms: (a) a method using the polarities of Pg and Pn waves, (b) the P-wave amplitude inversion of moment tensors, and (c) the waveform inversion of moment tensors. The majority of the analyzed micro-earthquakes have a left-lateral strike-slip focal mechanism with weak normal or reverse components. The full moment tensors comprise significant non-double-couple (non-DC) components. The non-DC components are partly numerical errors of the inversion but might be also of a physical origin. The most accurate values of the non-DC components are obtained from the P-wave amplitude inversion. For this inversion, the isotropic component (ISO) and the compensated linear vector dipole component (CLVD) are mostly positive and well correlated. This might indicate tensile faulting. Adopting the model of tensile faulting, we estimated the mean ratio of P to S wave velocities in the focal area from the values of ISO and CLVD, v P / v S = 1.5–1.6. The three different datasets of the focal mechanisms have been inverted for the present-day tectonic stress in the Dobrá Voda area. The slip shear stress component criterion was applied in the stress inversion. The results of the three inversions are well-consistent and point to a high reliability and good accuracy of the inverted stress. The orientations of the principal stresses are (azimuth/plunge): σ 1 = 210–220°/5–25°, σ 2 = 70–105°/55–75°, and σ 3 = 305–315°/15–25°, and the shape ratio is R = 0.45–0.60. The azimuth is measured clockwise from the north and the plunge downwards from the horizontal plane. The retrieved maximum compression lies along the belt of the Malé Karpaty Mts. The local tectonic stress reflects complex tectonic conditions in the area. The presence of tensile faulting might point to an extensional stress regime in the area.

Earthquakes, stress, and strain along an obliquely divergent plate boundary: Reykjanes Peninsula, southwest Iceland

We investigate the seismicity and the state of stress along the obliquely divergent Reykjanes Peninsula plate boundary and compare the directions of stress from inversion of earthquake focal mechanisms with the directions of strain rate from GPS data. The seismicity on the peninsula since early instrumental recordings in 1926 shows a systematic change from primarily earthquake swarms in the west to main shock–aftershock sequences in the east. The largest earthquakes on the Reykjanes Peninsula typically occur by right‐lateral slip on N‐S faults and reach magnitude 6 on the eastern part of the peninsula. During 1997–2006 most earthquakes on the Reykjanes Peninsula were located in two areas, Fagradalsfjall and Krísuvík on the central part of the peninsula, as recorded by the South Iceland Lowland (SIL) seismic network. The state of stress estimated by inversion of microearthquake focal mechanisms from the SIL catalogue is mainly oblique strike slip, with a tendency toward a normal stress state. Mapping the directions of the least compressive horizontal stress (Shmin) shows an average Shmin direction of N(120±6)°E and a remarkable agreement with the directions of greatest extensional strain rate (Hmax) derived from GPS velocities during 2000–2006. The agreement between the directions of stress at depth and strain rate observed at the surface indicate that the earthquakes are primarily driven by plate motion.

A study of microearthquake seismicity and focal mechanisms within the Sea of Marmara (NW Turkey) using ocean bottom seismometers (OBSs)

We have carried out seismological observations within the Sea of Marmara (NW Turkey) in order to investigate the seismicity induced after Gölcük–İzmit (Kocaeli) earthquake ( M w 7.4) of August 17, 1999, using ocean bottom seismometers (OBSs). High-resolution hypocenters and focal mechanisms of microearthquakes have been investigated during this Marmara Sea OBS project involving deployment of 10 OBSs within the Çınarcık (eastern Marmara Sea) and Central-Tekirdağ (western Marmara Sea) basins during April–July 2000. Little was known about microearthquake activity and their source mechanisms in the Marmara Sea. We have detected numerous microearthquakes within the main basins of the Sea of Marmara along the imaged strands of the North Anatolian Fault (NAF). We obtained more than 350 well-constrained hypocenters and nine composite focal mechanisms during 70 days of observation. Microseismicity mainly occurred along the Main Marmara Fault (MMF) in the Marmara Sea. There are a few events along the Southern Shelf. Seismic activity along the Main Marmara Fault is quite high, and focal depth distribution was shallower than 20 km along the western part of this fault, and shallower than 15 km along its eastern part. From high-resolution relative relocation studies of some of the microearthquake clusters, we suggest that the western Main Marmara Fault is subvertical and the eastern Main Marmara Fault dips to south at ∼45°. Composite focal mechanisms show a strike-slip regime on the western Main Marmara Fault and complex faulting (strike-slip and normal faulting) on the eastern Main Marmara Fault.

New insights into Kilauea's volcano dynamics brought by large‐scale relative relocation of microearthquakes

We investigated the microseismicity recorded in an active volcano to infer information concerning the volcano structure and long‐term dynamics, by using relative relocations and focal mechanisms of microearthquakes. There were 32,000 earthquakes of the Mauna Loa and Kilauea volcanoes recorded by more than eight stations of the Hawaiian Volcano Observatory seismic network between 1988 and 1999. We studied 17,000 of these events and relocated more than 70%, with an accuracy ranging from 10 to 500 m. About 75% of these relocated events are located in the vicinity of subhorizontal decollement planes, at a depth of 8–11 km. However, the striking features revealed by these relocation results are steep southeast dipping fault planes working as reverse faults, clearly located below the decollement plane and which intersect it. If this decollement plane coincides with the pre‐Mauna Loa seafloor, as hypothesized by numerous authors, such reverse faults rupture the pre‐Mauna Loa oceanic crust. The weight of the volcano and pressure in the magma storage system are possible causes of these ruptures, fully compatible with the local stress tensor computed by Gillard et al. [1996]. Reverse faults are suspected of producing scarps revealed by kilometer‐long horizontal slip‐perpendicular lineations along the decollement surface and therefore large‐scale roughness, asperities, and normal stress variations. These are capable of generating stick‐slip, large–magnitude earthquakes, the spatial microseismic pattern observed in the south flank of Kilauea volcano, and Hilina‐type instabilities. Rupture intersecting the decollement surface, causing its large‐scale roughness, may be an important parameter controlling the growth of Hawaiian volcanoes.

大阪盆地域での微小地震の発震機構

大阪盆地域での微小地震の発震機構

Microearthquake seismicity and focal mechanisms at the Rodriguez Triple Junction in the Indian Ocean using ocean bottom seismometers

Hypocenters and focal mechanisms of microearthquakes have been investigated at the Rodriguez Triple Junction in the Indian Ocean. Little was known on microearthquake activity in this region. We deployed 18 ocean bottom seismographs during the KH93–3 cruise of the R/V Hakuho‐Maru (Ocean Research Institute, University of Tokyo) from July 30 to August 20, 1993. We obtained 579 well‐constrained hypocenters and 13 focal mechanisms. Microearthquakes were found to be active along all of the three ridges: the Central Indian Ridge, the Southeastern Indian Ridge, and the Southwestern Indian Ridge. Especially at the triple junction there was an earthquake swarm within narrow area of approximately 15×5 km2. All of the 13 focal mechanisms showed normal or strike‐slip faultings, which means that the extensional stress field characterizes this region.

The SIL data acquisition system—at present and beyond year 2000

The South Iceland Lowland (SIL) data acquisition system presently consists of 33 digital, three component seismic stations connected to a common data center. The automatic, on-line, earthquake analysis performed by the SIL network can be divided into three categories. (1) Single-station analysis performed at the site stations producing information about all incoming phases. A short message with data on the phase is sent to the center. (2) Multi-station analysis done at the center, using the phase reports from the stations and producing information about all detected events including estimates of location, magnitude and fault plane solutions. (3) Alert reporting to notify the operators of the network in cases of a priori defined changes in parameters derived from the single- and multi-station analysis. The system is designed for maximum automatic operation and minimum operational cost and has shown to be capable of automatic evaluation of more then 1000 earthquakes per day or episodically several earthquakes per minute. While no attempt is made to detect and locate teleseismic events, teleseismic data is automatically saved, based on e-mail messages from global seismological networks. Groups of events are analyzed using correlation techniques to obtain accurate absolute and relative locations of earthquakes with similar waveforms. In some areas within the network, most of the earthquakes correlate very highly with each other. Based on this a new approach is being taken regarding the automatic operation of the network. A geographically indexed data base will be created where different classes of earthquakes are stored. As new earthquakes are recorded by the network the system automatically looks for similar waveforms in this data base and, if found, takes the onset and first motion direction picks from there. The algorithm is planned to be implemented in late 1999. New methods have been developed to estimate the stress tensor based only on the microearthquake focal mechanisms and accurate relative locations. This is planned to be implemented into the automatic on-line procedures. Methods and related software are being developed for real-time monitoring of fault movements based on the high accuracy locations and fault plane solutions.

Recognition of active strike-slip faulting from high-resolution marine seismic reflection profiles: Eastern Marlborough fault system, New Zealand

Major strike-slip faults within the eastern part of the Marlborough fault system of New Zealand extend offshore beneath the continental shelf, into the southern end of the Hikurangi margin. Six major submarine faults, each tens of kilometers in length and exhibiting the three-dimensional structural characteristics typical of strike-slip deformation zones, are mapped in detail using closely spaced high-resolution seismic-reflection profiles. Each fault displaces late Quaternary sediments that have been interpreted within the framework of the sequence stratigraphic model and the established history of glacio-eustatic sea-level cyclicity. Some of the faults are associated with strike-slip microearthquake focal mechanisms. The faults are within a complex structural high that underlies much of the outer continental shelf, and within the Flaxbourne basin between the structural high and the coast. The plan-view pattern of faulting recorded in the Quaternary sediments reflects only the major surface traces that have propagated upsection as contemporaneous sedimentation has blanketed most of the evolving structures. Regionally, the late Quaternary faults compose two groups that bound blocks with rhomboid surface areas on a scale of tens of square kilometers to hundreds of square kilometers. One group strikes typically between 023° and 057°, i.e., 22°–56° from the plate motion vector (079°), and includes dextral, oblique-slip thrust faults as well as inherited, steeply dipping, pre-Pliocene strike-slip faults that bound thick sedimentary basins. The second group comprises possibly young (<1 Ma), steeply dipping strike-slip faults that commonly exhibit normal-slip separation, and strike between 067° and 085°, subparallel to the azimuth of the instantaneous Pacific-Australian plate motion vector.

A method to determine microearthquake focal mechanisms in the presence of seismic anisotropy

We use three-component S waves together with P waves from microearthquakes in an isolated intraplate sequence in Arkansas, USA, to determine individual focal mechanisms, taking due account of shear-wave splitting if this is present. The method uses an extension of the relative amplitude method for microearthquakes, in which the polarities and relative amplitudes of two- or three- component direct S waves are measured in addition to P-wave polarities. For each earthquake, the identification of any apparent shear-wave splitting is confirmed by comparing the mutual compatibility of observations at all stations under different interpretations of the supposed slow S wave. This provides evidence of the correct identification of shear-wave splitting which does not rely simply on the analysis of the seismic waveforms themselves. In this example, it is concluded that the supposed shear-wave splitting is real, and hence that anisotropy is present. Well-constrained oblique strike slip focal mechanisms are determined for six events, and these agree with the composite focal mechanisms of other workers who used only P-wave polarities. This success demonstrates the feasibility of determining focal mechanisms for individual microearthquakes using this method. This is important as it removes the need to synthesise composite focal mechanisms under the assumption that all events have the same source orientation, and makes it realistic to examine the distribution of focal mechanisms within a population of microearthquakes.

Seismic Anisotropy In the Crust At the Mid-Atlantic Plate Boundary In South-West Iceland

Seismograms of microearthquakes recorded on stations of the SIL network in south-western Iceland exhibit strong shear-wave splitting, and are consistent with being caused by aligned parallel cracks in the uppermost crust. Splitting times of 0.1–0.3 s are observed, with the larger values (0.2–0.3 s) occurring beneath stations in the highly lineated Western Volcanic Zone, and the smaller values (0.1 s) in the younger, less fractured South Iceland Seismic Zone. Five of the stations have a fast shear-wave polarization azimuth of N30°E-N40°E. parallel to the axis of the Western Volcanic Zone (i.e. The Mid-Atlantic Ridge in Iceland) and in the direction of maximum horizontal compression defined by fault, dyke and fissure strikes and microearthquake focal mechanisms. Station GYG, in the north-western part of the network, has the significantly different azimuth of N70°E. This is parallel to one of the local strike-slip fault trends, and is probably due to a lineated rock fabric caused by those faults.

SEISMIC ACTIVITY IN THE NORTHEASTERN JAPAN ARC

The most recent data obtained from the high-gain seismograph network of Tohoku University are used to make an investigation on seismic activity and tectonics in the northeastern Japan arc. Seismic activity is closely related to the tectonic development of island arcs, and detailed analyses of the spatial distribution of microearthquakes in the northeastern Japan arc have led to conclusive evidence that almost all the shallow microearthquakes in the land area occur only in the layer with a P-wave velocity of 5.9 km/sec which was determined from explosion seismic observations. The configuration of the deep seismic zone in this region is discussed in relation to the plate subduction beneath the arc. The accurate determination of the hypocenters and focal mechanisms of microearthquakes has made it possible to investigate the subduction process beneath the arc.

阿寺断層付近の低い地震活動

An observation of microearthquakes was carried out near the Atera Fault for two and a half months, from July 14 to October 2, 1971, since the fault has been known as one of the most active faults in Japan.In the south-western side of the Atera Fault, the focal depths of earthquakes were deeper than 30km, that is, earthquakes occurred in the mantle but not in the crust. In the northeastern side, however, only a few earthquakes were detected. Although three direct analogue recording systems of high sensitivity were used near the fault, very few earthquakes could be located near the fault. A comparison of daily frequency of earthquakes near the NeoValley Fault with the present observation showed that the seismicity near the Atera Fault was undoubtedly lower than that near the Neo-Valley Fault. The same tendency has been also observed for larger earthquakes. Number of earthquakes detected at one of the three stations was exceptionally large, though the sensitivity was the same for all stations. But most of distant earthquakes were equally detected.Considering these facts, it might be concluded that the present seismic activity is very low and, in addition, seismic waves of high frequency are attenuated in the crust near the Atera Fault. Some focal mechanisms of microearthquakes were calculated. The pressure axis thus obtained was E-W direction as in the case studied by MATSUDA (1969) and ICHIKAWA (1971).