Earthquake Swarm Area Research Articles

Detailed velocity ratio mapping during the aftershock sequence as a tool to monitor the fluid activity within the fault plane

The rheological properties of Earth materials are expressed by their seismic velocities and VP/VS ratio, which is easily obtained by the Wadati method. Its double-difference version based on cross-correlated waveforms enables focusing on very local structures and allows tracking, monitoring and analysing the fluid activity along faults. We applied the method to three 2014 mainshock–aftershock sequences in the West Bohemia/Vogtland (Czech Republic) earthquake swarm area and found pronounced VP/VS variations in time and space for different clusters of events located on a steeply dipping fault zone at depths ranging from 7 to 11 km. Each cluster reflects the spatial distribution of earthquakes along the fault plane but also the temporal evolution of the activity. Low values of VP/VS ratio down to 1.59±0.02 were identified in the deeper part of the fault zone whereas higher values up to 1.73±0.01 were estimated for clusters located on a shallower segment of the fault. Temporally the low VP/VS values are associated with the early aftershocks, while the higher VP/VS ratios are related only to later aftershocks. We interpret this behaviour as a result of saturation of the focal zone by compressible fluids: in the beginning the mainshock and early aftershocks driven by over-pressured fluids increased the porosity due to opening the fluid pathways. This process was associated with a decrease of the velocity ratio. In later stages the pressure and porosity decreased and the velocity ratio recovered to levels of 1.73, typical for a Poissonian medium and Earth's crust.

Coda Attenuation Analysis in the West Bohemia/Vogtland Earthquake Swarm Area

Seismic coda represents a valuable source of information about the attenuation of the high-frequency waves in the studied region. The quality factor Q derived from coda is an integral parameter of the volume surrounding the hypocenter and seismic station and, according to the applied method, represents the total attenuation or the intrinsic and scattering parts. We analyzed records of 13 selected earthquakes in the magnitude range 1.7–2.9 of the 2011 swarm from West Bohemian/Vogtland area (central Europe), which were recorded at epicentral distances from 7 to 50 km. Two methods were applied: coda method for estimation of the Qc and the Multiple Lapse Time Windows Analysis for separation of the scattering and intrinsic loss by estimation of Qi and Qsc. Careful selection of the analyzed events was necessary due to the frequent contamination of coda decays by the running seismic swarm activity. The resulting coda Qc is relatively high with respect to the geodynamic activity and varies between 100 and 2500 within the analyzed frequency range of 1–18 Hz. The intrinsic loss dominates over scattering attenuation with Qi increasing from 100 and 1850 and Qsc from 300 to 3400 in the same frequency range, which is consistent with the geodynamic activity of the region. We find that the intrinsic attenuation in West-Bohemia/Vogtland is higher that in neighboring Germany, which could be attributed to the heterogeneity of the crust in central Europe.

Structural Preconditions of West Bohemia Earthquake Swarms

The West Bohemia and adjacent Vogtland are well known for quasi-periodical earthquake swarms persisting for centuries. The seismogenic area near Nový Kostel involved about 90 % of overall earthquake activity clustered here in space and time. The latest major earthquake swarm took place in August–September 2011. In 1994 and 1997, two minor earthquake swarms appeared in another location, near Lazy. Recently, the depth-recursive tomography yielded a velocity image with an improved resolution along the CEL09 refraction profile passing between these swarm areas. The resolution, achieved in the velocity image and its agreement with the inverse gravity modeling along the collateral 9HR reflection profile, enabled us to reveal the key structural background of these West Bohemia earthquake swarms. The CEL09 velocity image detected two deeply rooted high-velocity bodies adjacent to the Nový Kostel and Lazy focal zones. They correspond to two Variscan mafic intrusions influenced by the SE inclined slab of Saxothuringian crust that subducted beneath the Tepla-Barrandian terrane in the Devonian era. In their uppermost SE inclined parts, they roof both focal zones. The high P-wave velocities of 6,100–6,200 m/s, detected in both roofing caps, indicate their relative compactness and impermeability. The focal domains themselves are located in the almost gradient-free zones with the swarm foci spread near the axial planes of profound velocity depressions. The lower velocities of 5,950–6,050 m/s, observed in the upper parts of focal zones, are indicative of less compact rock complexes corrugated and tectonically disturbed by the SE bordering magma ascents. The high-velocity/high-density caps obviously seal the swarm focal domains because almost no magmatic fluids of mantle origin occur in the Nový Kostel and Lazy seismogenic areas of the West Bohemia/Vogtland territory, otherwise rich in the mantle-derived fluids. This supports the hypothesis of the fluid triggering of earthquake swarms. The sealed focal domains retain ascending magmatic fluids until their critical pressure and volumes accumulated cause rock micro-fractures perceived as single earthquake bursts. During a swarm period, the focal depths of these sequential events become shallower while their magnitudes grow. We assume that coalescence of the induced micro-fractures forms temporary permeability zones in the final swarm phase and the accumulated fluids release into the overburden via the adjacent fault systems. The fluid release usually occurs after the shallowest events with the strongest magnitudes ML > 3. The seasonal summer declines of hydrostatic pressure in the Cheb Basin aquifer system seem to facilitate and trigger the fluid escape as happened for the 2000, 2008, and 2011 earthquake swarms. The temporary fluid release, known as the valve-fault action, influences the surface aquifer systems in various manners. In particular, we found three quantities, the strain, mantle-derived 3He content in CO2 surface sources and ground water levels, which display a 3–5 months decline before and then a similar restoration after each peak earthquake during the swarm activities. The revealed structure features are particularly important since the main Nový Kostel earthquake swarm area is proposed as a site for the ICDP project, ‘Eger Rift Drilling’.

Tomographic Determination of the Upper Crustal Structure in Jiashi Strong Earthquake Swarm Region‐Joint Inversion of Explosion and Earthquake Data

AbstractThe Jiashi strong earthquake swarm region is located at the juncture area of three tectonic units, i.e. the Tianshan fold zone, Pamir tectonic arc and Tarim block. Many strong earthquakes occurred recently in this area. In order to obtain the fine crustal velocity structures of this area, the data from a 3‐D seismic transmission experiment and a portable seismograph network acquired in the Jiashi strong earthquake swarm region in 1997 are processed. Joint inversion of explosion and earthquake data is carried out to reconstruct the 3‐D images of velocity perturbations of the upper crust under the seismic array. The temporary seismic array was used to record the seismic waves generated by 8 shots fired at different azimaths, and the additional 20 seismic stations were deployed in this area to record local earthquakes. The results show that the structure of the upper crust in the study area is characterized by obvious inhomogeneities both laterally and vertically. The low P wave velocity zone centered at Saruxi beneath the Jiashi earthquake swarm area is gradually replaced by high P wave velocity with the increase of depth. There is a high P wave velocity anomaly striking NNW at the depth of 12 km under the earthquake swarm area. It extends downwards to the depth of 16 km at least, and is surrounded by relatively low velocities. VP/VS ratios are also characterized by high values in this region. This kind of structural differences may be related to the occurrence of the Jiashi strong earthquake swarm. It can be found from the P wave velocity images at the depth of 16 km that the Jiashi strong earthquake swarm generally occurred in the regions with comparatively high P wave velocity perturbations or in the transition zones between high and low velocity perturbations. The existence of the high P wave velocity anomaly in the crust may play an important role in the seismogenic process.

Basement interface structural characteristics beneath Jiashi strong earthquake swarm area in Xinjiang, China

The seismic data obtained from high resolution seismic refraction profile in Jiashi strong earthquake swarm area in Xinjiang, China were further processed with ray hit analysis method and more complete basement interface structural characteristics beneath Jiashi strong earthquake swarm area were determined. The results show that there are two clear basement interfaces at the upper crust in Jiashi strong earthquake swarm area. The first one with buried depth ranging from 2.6 km to 3.3 km presents integral and continuous structure, and it appears an inclined plane interface and smoothly rises up toward Tianshan Mountain. The second basement interface with buried depth from 8.5 km to 11.8 km, is the antiquated crystalline basement of Tarim basin. Near the post number of 37 km, the buried depth of the crystalline basement changed abruptly by 2.5 km, which maybe result from an ultra crystalline basement fault. If taking this fault as a boundary, the crystalline basement could be divided into two parts, i.e. the southwestern segment with buried depth about 11.5 km, and the northeastern segment with buried depth approximately from 8.5 km to 9.0 km. That is to say, in each segment, the buried depth changes not too much. The northeast segment rises up as a whole and upheaves slightly from southwest to northeast, which reflects the upper crustal deformation characteristics under the special tectonic background at the northwestern edge of Tarim basin.

Fine upper crustal structure of Jiashi strong earthquake swarm region in Xinjiang inferred from high resolution seismic refraction profile data

The data obtained from a high resolution seismic refraction profile, which was carried out in Jiashi, Xinjiang, strong earthquake swarm area, were processed with both finite difference inversion and Hagedoorn refractor wavefront imaging technique and the fine upper crustal structure was determined. The results show that the upper crustal structure is relatively well-distributed in laterally and obviously by layers vertically. From surface to 11.0 km depth, there are about four layers. The P wave velocity of top two layers range from 1.65 to 4.5 km/s and their bottom boundaries, the buried depths of which are 0.4, 2.96–3.0 km respectively, are almost horizontal; The third layer is comparatively complicated and its P wave velocity presents inhomogeneous in both laterally and vertically. The bottom boundary of third layer is crystalline basement and shows a little uplift, which seemly suggest that the upper crust had been resisted while the hard Tarim block inserting into Tianshan Mountain; The forth layer is relatively even and its P wave velocity is about 6.3 km/s. There are a lateral velocity variation at the depth of about 4.0 km, and suggest that it has something to do with the hidden Meigaiti fault and Meigaiti-Xiasuhong fault but there are no the structure features about these faults stretching to the surface and passing through the crystalline basement. The seismogenic tectonic of Jiashi strong earthquake swarm at least lies in middle or lower crust beneath 11.0 km depth.

Ground uplift detected by precise leveling in the Ontake earthquake swarm area, central Japan in 2002–2004

Repeated precise leveling in the earthquake swarm area of Ontake, central Japan has revealed uplift of 3–6 mm in proximity to the epicentral region of the most active earthquake cluster in 2002–2004. Although the uplift is small, the vertical displacement is significant even considering leveling error. This uplift is associated with increases in 3He/4He ratios and CO2 δ13C values at a mineral spring in the region, indicating an upper mantle contribution. A region of low resistivity at a depth of 2 km beneath the uplift area has also been inferred, suggesting that the observed uplift is related to changes in a shallow seismogenic layer due to increased hydrothermal input from the earthquake swarm area.

A33 御嶽火山群発地震域における精密水準測量による地殻上下変動 1999-2003 年

A33 御嶽火山群発地震域における精密水準測量による地殻上下変動 1999-2003 年

Study on basement structures of the northeast Pamirs

Basement structures and basement interfaces are obtained by finite-difference and time-term methods using Pg-wave data from two deep seismic sounding (DSS) profiles in the Artush-Jiashi strong earthquake area. The geological units differ considerably in basement depth. The basement structures of contact zones between two geological units also vary obviously, which marks the existence of boundary faults. Finally, we make a remark upon the relationship between characteristics of basement structures and seismicity in the Artush meizoseismal area and the Jiashi earthquake swarm area.

月齢と丹波山地の微小地震発生の相関について

In the Tamba Plateau, an earthquake swarm area in the Kinki district, Central Japan, seismicity was activated just after the Hyogo-ken Nanbu (Kobe) Earthquake (M7.3), which occurred in an adjacent area in 1995. We found that micro-earthquake activities in the Tamba Plateau corresponded to moon phase. Occurrences of micro-earthquakes increased after a new moon and a full moon during 1995 and 1996. Before 1995, such a correlation could not be found. The present study suggests a possibility that the stress change caused by the Hyogoken Nanbu Earthquake made seismicity in the Tamba Plateau sensitive to tidal forces.

Transport of mantle volatiles through the crust traced by seismically released fluids: a natural experiment in the earthquake swarm area Vogtland/NW Bohemia, Central Europe

Vogtland, which is located on the Czech–German border near the crossing of the Eger Rift and the Mariánské Lázně fault zone, is characterised by CO2-rich mineral springs fed by a magmatic source and swarm earthquake activity. The free gas of the Eisenquelle mineral spring was studied weekly from April 1994 to June 1996, a period which included a quake swarm on December 4 and 5, 1994, comprising approximately 500 individual events Mlocal≤2.2. Co-seismic shifts in He content after about a month as well as in the δ13C of CO2 and 3He/4He after about 220days were resolved from seasonal background variations. The He concentration shift is assumed to originate near the discharge, whereas the isotopic shifts are probably caused by the addition of crustal fluids released by crushing rocks within the hypocentre. From the distance of 14km between the hypocentre to the mineral spring, the transport velocity of the released crustal fluids was determined to be at least 50m/day. Such a high speed indicates gaseous fluid transport along channel-like conduits, rather than being predominantly controlled by dissolution in water.Using a simple mass balance it is shown that the amount of crustal fluids potentially released by seismic activity strongly exceeds that required for the observed shifts in 3He/4He and δ13C. This fact along with the coincidence of both shifts supports the hypothesis that magmatic fluids occurring in mineral waters of the Eisenquelle environs ‘flush’ the hypocentral crustal block by means of advectional transport.

松代群発地震地域に湧出する深層地下水

松代群発地震地域に湧出する深層地下水

Depth‐dependent crustal anisotropy at Midwestern Honshu, Japan

Aligned cracks, possibly induced by tectonic stress, produce anisotropy in the Earth's crust. Permeability and state of stress at several depths in the crust can be estimated from the state of crack distribution. We present clear evidence that crustal anisotropy changes with depth at the Inagawa earthquake swarm area, mid‐western Honshu, Japan. The observed fast shear‐wave directions are parallel to the maximum horizontal compressional axes obtained from a hydro‐fracturing test at a depth of 0.80 km and focal mechanism solution at depths 4–8 km in the same area. These observations provide us the evidence that the crustal anisotropy in this region is controlled by the tectonic stress. We estimated the degree of anisotropy, that is, the ratio of the difference between fast and slow S‐wave velocities, as a function of depth using the layer‐stripping method. The obtained value is 2% or less at most depths but anisotropy of 11% is observed at depths 6–8 km. This high‐anisotropy layer has a local gradient in seismicity rates. We infer that vertical fluid flow from the high‐anisotropy layer would be trapped in the low‐anisotropy layer above, pore pressure would increase, and it could generate earthquakes. This correspondence suggests that the degree of anisotropy in the upper crust changes with depth and has some relation to seismicity.

In-situ stress measurement in an earthquake focal area

A 2-km-deep borehole was drilled into granitic rock where many shallow earthquakes, with focal depths from 2 to 15 km, have occurred. The drill site, Ashio, is 100 km north of Tokyo. Downhole testing and measurements were conducted five times: four times after each 500 m drilling and the fifth time after completing the 2000 m borehole. Measurements of in-situ stress orientation and magnitude were conducted by the hydraulic fracturing method, stress-induced well bore breakout analysis, and drilling-mud pressure induced hydraulic fracturing analysis. Breakouts and mud pressure induced hydraulic fractures were observed below 650 m and 1250 m, respectively. The circular well bore is maintained only in limited spots below 650 m because of breakouts indicating a large differential stress condition between the maximum and the minimum principal stresses. The differential stress is calculated at 90 ± 20 MPa at the depth of 2000 m based on the condition under which the breakout with some degree of width appears. It is interpreted that this large differential stress is representative of the regional crustal stress condition in the earthquake swarm area. Each spot of the circular well bore is always adjacent to a fracture zone. This suggests that the fracture zone has small differential stress. The stress values were measured where the well bore is circular by the hydraulic fracturing method. For example, the maximum and the minimum horizontal compressive stresses are about 35 MPa and about 25 MPa, respectively, at the depth of 1650 m; giving the differential stress of 10 MPa. The water pressure in pre-existing fractures was also measured, and found that they were nearly equal to the hydrostatic water pressure at the corresponding depths. The stress direction estimated from the azimuth of the breakouts and the hydraulic fracture is consistent with that estimated from the earthquake focal mechanisms. These results support the following conclusions. The differential stress is large in the earthquake swarm region. But, it is extremely small at narrow zones adjoining fracture zones. This is due to the relaxation of the differential stress at fractures of low frictional strength. However, water pressure is not so large that the low differential stress causes sliding of the fractures. Clay minerals seems to play an important role in decreasing of the frictional strength.

Crustal structure in the Matsushiro Earthquake Swarm area

The crustal structure of the Matsushiro area, Central Japan, was studied in two profiles, A and B, with the explosion seismic method to obtain a better understanding of the physical processes of the Matsushiro Swarm Earthquakes. The layer with a velocity of 6.0 km/sec is extremely shallow and becomes deeper west of Chikuma River and around the southeastern end of profile B; there exists a faultlike structure in the most active area. The comparison of hypocenter distributions with the crustal structure shows that almost all swarm earthquakes have their hypocenters below the 6.0 km/sec layer and are confined to the region where this 6.0 km/sec layer is shallow. The velocity gradient in the 6.0 km/ sec layer is determined with certainty by the time-term analysis. In the seismically most active region the anomalous structure is derived not only from the traveltime analysis but also from the amplitude studies; that is, the velocity and the Q-value are smaller than in other regions.

Relation between tectonic stress, great earthquakes and earthquake swarms

Along the northern boundary of the Philippine Sea plate are a remarkable earthquake swarm area and several epicenters of great earthquakes which occurred in the past. The temporal variation of the swarm activity shows a remarkable relation to the occurrence of great earthquakes: the swarm activity falls off when a great earthquake occurs at the boundary between the Philippine Sea plate and the continental block that includes the swarm area, but it rises when a great earthquake occurs at a boundary between the Philippine Sea plate and one of the other continental blocks. On the basis of the detailed mechanism studies on these great earthquakes, a simple model is constructed to explain this relation between the swarm earthquake and the great earthquakes. In this model, several independent crnstal blocks are opposing the motion of the Philippine Sea plate; each block is stressed by the motion of the Philippine Sea plate through a friction-coupled interface where great earthquakes are supposed to occur. When a great earthquake occurs at the boundary between the plate and the crustal block that includes the swarm area, the stress in this block is released and the swarm activity falls off. On the contrary, the stress in this block increases when a great earthquake occurs, as a rebound, at the boundary between the Philippine Sea plate and one of the other crustal blocks, because such rebound removes the force opposing the motion of the Philippine Sea plate, propels its motion, and therefore increases the stress in the block that includes the swarm area. Since the rise of the activity is found to precede the earthquake, probably because of some premonitory creep-like deformation at depths, a practical means for predicting an impending great earthquake in this region may be provided by monitoring the swarm activity.

TIME-TERM ANALYSES OF EXPLOSION SEISMIC DATA

Explosion seismic data in two regions are studied by utilizing the time-term method. The results of two profiles in the Matsushiro earthquake swarm area agree very well with the previous models by ASANO et al. In one profile, the travel-time residuals plotted in the ordinate to distances in the abscissa show a convex tendency which strongly suggests vertical velocity gradient in the basement layer. The distance-dependent velocity is determined accurately with the modified time-term method.Some defects in the model by HASHIZUME et al. for the Kesennuma-Oga profile are removed by the model obtained with the time-term method in the present study. It is particularly remarkable that the possibility of the lateral change in the Pn velocity near the Honshu-Japan Sea boundary is pointed out.

ANOMALOUS ATTENUATION OF P WAVES IN THE MATSUSHIRO EARTHQUAKE SWARM AREA

The ratios of the spectral amplitude at 10cps to that at 20cps of the compressional waves obtained from the explosions in the Matsushiro earthquake swarm area are used to study the latera1 variation of specific attenuation factor Q in this area. The head waves through the 6 km/sec-1ayer observed at the observation sites on Profiles A and B are analyzed for this purpose. This analysis showed that the location of the low Q region coincided with the seismically most active region, and the Q value in this region was extremely low, 30 to 50, which is 1/20 to 1/50 of the surrounding region.

Recent developments in earthquake prediction research in Japan

Recent research on earthquake prediction in Japan has been carried out mainly in the framework of the “5-year Program on Earthquake Prediction Research” which was first founded by the government in 1965. Some of the latest experiments are precise observation of explosions for detecting a possible velocity change in a tectonically active area, laboratory study of the migration of micro-fractures in rocks under stress, and a deep drilling in the Matsushiro earthquake swarm area. Based upon the outcome of the 5-year program and the experience of several destructive earthquakes encountered during the period, a new 5-year program starting from 1969 was proposed in 1968. The new program which aims at actual predictions of destructive earthquakes emphasizes augmented instrumentation, strategy, and organization.

Explosion Seismic Studies of the Underground Structure in the Matsushiro Earthquake Swarm Area

The underground structure in the Matsushiro Earthquake Swarm Area was derived from the data of explosion seismic observations in profiles A and B. First, the number of layers and the velocity in each layer are determined by using the T' curve. Then on the assumption that the dip of each interface is small, the structure of the first approximation was derived. The final models were derived by the method of trials and errors so as to reduce (O-C), the difference between observed and calculated travel times. The underground structure in profile A consists of the following three layers on the whole: the first layer : 1.6-2.3 km/s the second layer : 3.3-4.75 km/s the third layer : 5.9-6.0 km/s In profile B the underground structure also consists of three layers, but there is a surface layer in some places: the surface layer : 0.36-1.2 km/s the first layer : 1.7-3.2 km/s the second layer : 4.0-4.4 km/s the third layer : 6.0 km/s The underground structures derived by using above velocities are given in Fig. 2 for profile A and in Fig. 3 for profile B. The underground structure in profile A, which is in the Central Belt of Uplift named geologically and is parallel to its strike, is relatively simple. Generally speaking, the top of the third layer (with the velocity of 6.0km/s) is unusually shallow, about 1.5km from the earth's surface even at the deepest, and has a tendency getting shallower a little toward the southwest. Or it may be said that the depth to the top of the third layer is smaller by 0.3-0.5km in the portion from the middle of E2 to D8 than the other portions. The velocity in the third layer is 6.0km/s in the area northeast of Matsushiro and 5.9km/s in the area southwest of Matsushiro. There is a possibility of existence of anomalous structure or a fault between E4 and E5 although there are no observation points in this area because of the topography. This anomalous struc-ture may correspond to the low Bouguer anomaly around Mt. Minakami. The underground structure in profile B, which passes the central part of the epicentral area and crosses the Central Belt of Uplift, is fairly complicated. The velocity in the first layer is 1.7-3.2km/s and this layer becomes thick abruptly near the Chikuma River toward the northwest. The velocity in the second layer is 4.0-4.4km/s and it is noteworthy that the thickness of this layer increases by about 3km almost discon-tinuously around the middle of E1 near Nagano City. From another point of view, the top of the third layer becomes very shallow in the central part of the profile and reaches only about 1km from the earth's surface. This interface becomes deep toward the southeastern end of the profile, so that the deep structure of the Central Belt of Uplift may be defined. At least the layer with the velocity of 6.0km/s might be related to the formation of the Central Belt of Uplift, or it might be said that this layer played an important role for its formation. This profile passes the foot of Mt. Mina-kami at D7 and the underground structure has anomaly near Mt. Minakami which may correspond to low Bouguer anomaly. There is a large possibility that the anomalous structure in this profile is of the same origin with that in profie A since the locations of anomalous structures are close with each other and their manners are quite similar. It is quite interesting that there exist anomalous underground structures in the area where Matsushiro swarm earthquakes have occurred most frequently. Also it is interest-ing to note that there is no layer with the velocity of about 5.5km/s in the area concerned. On the other hand by the comparison of the underground structure in this paper with results from other geophysical and geological investigations, several interesting relations are pointed out.