Earthquake Swarm Region Research Articles

Possible role of fluids in the process of earthquake swarm generation in the West Bohemia/Vogtland seismoactive region

The West Bohemia/Vogtland region in the western part of the Bohemian Massif, known for periodic occurrence of earthquake swarms, is also characterised by 0.2–0.5Ma old Quaternary volcanoes, rich CO2 emissions, anomalies of mantle-derived He, mineral springs and moffets. The detailed analysis of the time-space pattern of seismicity during the January 1997 swarm demonstrates the gradual and ordered migration of micro-seismic activity, suggesting the step-by-step penetration of fluids into a small fractured volume. This scenario is supported by the similarity of the source mechanisms of the natural events of the January 1997 swarm with artificial injection-induced micro-earthquakes during the German deep drilling project (KTB) in neighbouring NE Bavaria. Also the comparison of the West Bohemia/Vogtland earthquake swarm region with other intraplate earthquake swarm regions and with earthquake swarms induced by current volcanic processes indicates a role of intrusions of magma and related fluids in generation process of earthquake swarms.

Scenario of the January 1997 West Bohemia Earthquake Swarm

In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland earthquake swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the swarm. Two of them, typesAandBin our denotation, comprised all ML≥ 1.3 events and predominated in the swarm. TypeAwere pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For classBevents, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. TypeAevents predominated in the southern subcluster of the swarm, whereas most of typeBevents occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the swarm. The swarm essentially developed in four phases: in the first, typeAevents prevailed and the southern plane was active; during the second, characterised by the occurrence of both typeAandBevents (the former in the southern, the latter predominantly in the northern subcluster), the activity of the swarm culminated; in the third and fourth, the occurrence of typeBevents in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of typesAB,C,D,E,FandG, which were typical for ML≤1.2 events, occurred randomly throughout the swarm. TypeABevents were identified in both the southern and northern clusters, typeC,E,FandGmechanisms only southwards from NKC. TypeDevents exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 earthquake swarm and may, therefore, be typical for the region under study.

Layered Velocity Models of the Western Bohemia Region

A new robust and effective optimization algorithm – isometric algorithm – was used for the inversion of layered velocity models, with constant gradient in each layer, to find suitable 1-D models for the location of microearthquakes in the individual four subregions of the West Bohemian earthquake swarm region. Models which are considered as optimal yield the minimum sum of the absolute values of the travel-time residua in locating the whole group of earthquakes in the given subregion. The results obtained from the inversion of P and S waves and from P waves only are shown. For comparison, optimum homogeneous models derived by the grid search method, again using both P and S waves and P waves only, are given. The computations indicate that the models for the individual subregions differ from each other. For layered models the differences are more pronounced, as expected, in the upper parts, down to depths of about 5 km. In comparison with the subregions Nový Kostel and Plesna, the P and S wave velocities for subregion Lazy are relatively higher and the P and S velocities for subregion Klingenthal relatively lower. In the lower parts the differences are smaller and the velocities have practically identical gradients. The highest velocities were obtained for subregion Lazy and the lowest velocities for subregion Klingenthal, as well for the homogeneous models. The model that represents the whole swarm region was determined in a similar way. This model is compared with the previously published velocity-depth distribution, obtained from DSS profile VI/70 in the vicinity of the area under study.

Seismic regime of the west Bohemian earthquake swarm region: Preliminary results

The western part of the Bohemian Massif located between two tectonic units, the Moldanubian and the Saxo-Thuringian, is characterized by the re-occurrence of earthquake swarms. The focal region for these swarms includes the territory of West Bohemia and the adjacent territory of SE Saxony and NE Bavaria. During the most recent swarm in December 1985 – January 1986, more than 8000 small earthquakes were recorded; the two largest earthquakes with local magnitudes (ML) of 4·6 and 4·1. This paper presents a summary of the seismic energy release in space and time for the western part of the Bohemian Massif, based on seismic observations of permanent seismic stations established in West Bohemia since 1986. It was found out that microearthquake activity, mostly of a swarm-like character, persisted between two macroseismically observed swarms. The foci of the microearthquakes predominantly cluster in six main epicentral zones, four of which are located in West Bohemia or in its immediate vicinity in Saxony. The remaining two are in Saxony and in Bavaria. The four epicentral zones in West Bohemia were studied in detail. It was found that the individual zones differ in size, in depth of hypocentres, in geometry, as well as in temporal activity. Moreover, it was found that the seismicity in the most active epicentral zone is closely related to the system of principal tectonic faults referred to as the Krusne Hory fault and the Marianske Lazně fault.

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.

Discrimination between local microearthquakes and quarry blasts by multi-layer perceptrons and Kohonen maps

Abstract The results of the application of artificial neural nets (ANNs) to discriminating microearthquakes from quarry and mining blasts in the West Bohemia earthquake swarm region are presented and discussed. Input vectors consisting of seven spectral and seven amplitude parameters, automatically extracted from local three-component digital broadband (0.6 to 60 Hz) velocigrams, have been employed for training of different ANN configurations. Multi-layer perceptrons (MLP) trained in supervised mode by different subsets of a representative set of 312 events have been used as discriminators, and unsupervised Kohonen self-organizing feature maps (SOFM) have been used as complementary reliability estimators. The reason for comparative application of both techniques was to increase the reliability of the discrimination: complementary information that a pattern has been recognized as a member of a conflict cluster allows detecting problematic patterns that an MLP may not be able to classify correctly. The optimal MLP, trained by one randomly selected half of the complete set of 312 input vectors and tested by the other half-set, and vice versa, correctly classified, on average, 99% of all events. The optimal SOFM correctly classified as problematic patterns all events misinterpreted by the MLP, and about 20% of all events were classified by them as ambiguous cases. The obtained results evidence that a relatively small number of spectral and amplitude parameters of observed ground velocity may suffice for a reliable discrimination between local microearthquakes and quarry blasts by means of small neural nets. The MLP/SOFM combination discussed in this article has attained a discrimination reliability that allows it to be employed routinely in observatory practice.

Topographic anisotropy of slickenside from deep borehole sample in an earthquake swarm region and its generation mechanism : Y. Kuwahara, N. Oyagi & H. Takahashi, Journal of Physics of the Earth, 41(2), 1993, pp 75–85

A slickenside was discovered from a boring core sample 1, 880m deep in the Matsushiro earthquake swarm region in Nagano Prefecture, Japan. The slickenside has been considered to be created by earthquake faulting during the earthquake swarm period, because it accompanied fresh rock powder produced by earthquake faulting. The power spectral density of the topography of the slickenside was measured with a stylus profilometer over the wavelength range from 10-4 to 10-2m. Each profile of the surface has a red noise power spectrum over the wavelength range studied, with the power falling off on average between 2 and 3 orders of magnitude per decade decrease in wavelength. Meanwhile, the topography of the slickenside is found to show anisotropy in the spectral amplitude; the amplitude perpendicular to the slip direction is two to four times as large as that parallel to the slip direction. A generation mechanism for such topographic anisotropy is numerically modeled from a viewpoint of the fracture mechanics. It is supposed that the crack tip stress distribution in the plane parallel to the slip direction is represented by a model of a plane strain shear crack (Mode II crack) and that the stress distribution in the plane perpendicular to the slip direction is represented by an antiplane strain shear crack (Mode III crack). The stress field around the crack tip in the Mode III case shows a broader distribution with respect to the azimuth than that in the Mode II case. This difference of a stress field at the crack tip between the Mode II and the Mode III is a possible cause of the topographic anisotropy.

Topographic Anisotropy of Slickenside from Deep Borehole Sample in an Earthquake Swarm Region and Its Generation Mechanism.

A slickenside was discovered from a boring core sample 1, 880m deep in the Matsushiro earthquake swarm region in Nagano Prefecture, Japan. The slickenside has been considered to be created by earthquake faulting during the earthquake swarm period, because it accompanied fresh rock powder produced by earthquake faulting. The power spectral density of the topography of the slickenside was measured with a stylus profilometer over the wavelength range from 10-4 to 10-2m. Each profile of the surface has a red noise power spectrum over the wavelength range studied, with the power falling off on average between 2 and 3 orders of magnitude per decade decrease in wavelength. Meanwhile, the topography of the slickenside is found to show anisotropy in the spectral amplitude; the amplitude perpendicular to the slip direction is two to four times as large as that parallel to the slip direction. A generation mechanism for such topographic anisotropy is numerically modeled from a viewpoint of the fracture mechanics. It is supposed that the crack tip stress distribution in the plane parallel to the slip direction is represented by a model of a plane strain shear crack (Mode II crack) and that the stress distribution in the plane perpendicular to the slip direction is represented by an antiplane strain shear crack (Mode III crack). The stress field around the crack tip in the Mode III case shows a broader distribution with respect to the azimuth than that in the Mode II case. This difference of a stress field at the crack tip between the Mode II and the Mode III is a possible cause of the topographic anisotropy.

PANDA: A simple, portable seismic array for local- to regional-scale seismic experiments

Abstract A portable array for numerical data acquisition (PANDA) was designed and built in-house at the Center for Earthquake Research and Information (CERI), Memphis State University. The design goal was a transportable set of instruments that could record high-quality seismic data. By making modifications to traditional seismic network technology and using a super-microcomputer workstation for digital data acquisition, this array provides a set of reliable and cost-effective instruments for the collection and analysis of high-resolution seismic data. The field equipment and central recording system are designed to overcome several major limitations in existing seismic network technologies, such as low dynamic range, single-component recording, lack of a common time base between stations, topographic constraints in radio telemetry, and lack of field processing capabilities. PANDA overcomes these limitations. To increase the dynamic range and record full three-component data, each PANDA station has six channels divided into two three-component sets. One set is operated at high gain and the other at low gain, giving a minimum of 90 dB dynamic range. Modular design of the PANDA station electronics facilitates field maintenance and also permits the stations to be run in several additional modes (e.g., with both sets of three channels operating at high gain) or with different sensors on each set. Each telemetry link to the central recording site carries data from two stations (12 channels) by repeating data from an “outer” seismic station through an “inner” seismic station and repeater combination. In addition, several repeat-only stations were built. This ability to repeat signals increases the aperture of PANDA to more than 150 km and overcomes topographic limitations on station locations. Other design features include low power consumption, solar power operation, and transmission of daily calibration and diagnostic signals. These features enable the stations to operate unattended for long periods and provide verification of station performance from the central recording site. The central recording system is based on a high-performance super-micro-computer workstation with a 256 channel A/D front end, which allows both real time digital recording with a common time base and immediate processing of the data. A 40-station network with 6 channels per station uses 240 channels, leaving 16 channels for auxiliary inputs such as time codes and spares. PANDA was field tested in the Arkansas earthquake swarm region from October 1986 to April 1987. It was then deployed in San Juan, Argentina, from August 1987 to May 1988, and in Jujuy, Argentina, from September 1988 to May 1989. A total of more than 20,000 earthquakes were recorded during the two Argentina experiments.

青森県西海岸 (岩崎村付近) の群発地震 (I)

An active earthquake swarm have been occurring in and near Iwasaki, Aomori Prefecture, the northeast of Honshu, Japan from September 16, 1978 with precursory earthquake swarm before 3 and 9 days. The magnitude of the largest event was 4.3. A temporal seismic network using wireless telemetry was developed to precisely determine the distribution of hypocenter locations. Hypocenter locations of 2000 events were determined for the period of the observation over 80 days. Most of hypocenters were located at a distance less than 2km from the coast line and in a small region beneath the Sea of Japan of water depth shallower than 50m. An extent of the earthquake swarm region was about 2km in the north-south and 4km in the east-west direction. We found hypocenter distribution of shallow earthquakes having planed structure, which had not been found in the Tohoku district so far. Hypocenter locations projected on the vertical section in the northwest-southeast direction clearly showed that the events occured in a region of planed structure dipping about 40 degrees in the direction of the northwest. The events were at depths ranging from 4km to 7km. A high seismic activity was found at deeper focal depth from 6km to 7km in the first stage of the swarm and decreased rapidly from the latter half of October. However, the seismic activity of shallower depth did not decrease till the end of November.