Determination Of Source Mechanisms Research Articles

Vulnerability in the Java northen region in association with earthquake sources of tectonic origin

While threats from the subduction zone off the Java south coast remain persistent, potential sources of a tectonic earthquake centered in the Java northern areas come into play. Despite their enigmatic presence and a clear shift of the sources from the mainland to the north from hypocentre relocation studies, this poses a danger to the community in the Java northern coastlines, increasing vulnerability to earthquake and tsunami hazards. The main aim of this study is thus to evaluate seismic hazards and corresponding analysis, hence assessing vulnerability with respect to the presence of active faults in the northern regions. The methods used here included a-value and b-value calculations inferred from seismogenic zonation made available for the study area and source mechanism determination derived from seismic inversion for two recent events recorded at the Java Sea. The results revealed that the a-value reached 8.55 in Zone 1, reflecting a high seismicity rate over the northern areas of West Java and Banten provinces, and the b-value was about 0.8 in Central Java and East Java, implying a relatively high-stress tectonic regime. The moment tensor solutions yielded sizes M w 6.7 at a depth of 539 km and M w 7.0 at a depth of 594 km with a major double-couple component for the normal faulting types of mechanism, consistent with reference. The results suggest the significance of awareness of seismic threats possible to occur in the Java northern regions; hence more investigations into detailed seismicity in the regions of interest for future research direction.

Detection, location, and source mechanism determination with large noise variations in surface microseismic monitoring

Microseismic monitoring aims at detecting as weak events as possible and providing reliable locations and source mechanisms for these events. Surface monitoring arrays suffer from significant variations of noise levels across receiver lines. When using a large monitoring array, we use a stacking technique to detect microseismic events through maximizing the signal-to-noise ratio (S/N) of the stack. But some receivers with a high noise level do not contribute to improving the S/N of the stack. We have derived a theoretical concept for the proper selection of receivers that best contribute to the stack for a constant strength of a signal across the array. This receiver selection criterion, based on the assumption of constant signal amplitude, provides a robust estimate of the noise threshold level, which could be used to discard or suppress contribution from the receivers that do not improve the S/N of the stack. We found that limiting the number of receivers for stacking improves the location accuracy and reduces the computational cost of data processing. Although the assumption of a constant signal never holds in real-life seismic applications, the noise level varies across the surface receivers in a significantly wider range than the signal amplitude. These noise variations can also increase the uncertainty of the source mechanism inversion and should be accounted for. Synthetic and field data examples show that weighted least-squares inversion with receiver weighting according to the noise level produces more accurate estimates for source mechanisms compared to the inversion that ignores information about noise.

Will your surface microseismic hydraulic fracture monitoring project be successful?

The benefits of microseismic monitoring of hydraulic fracturing are well understood, and continue to grow as acquisition, processing, and interpretation methods improve. Most surveys are conducted using downhole receivers placed close to the reservoir zone. However, for some surveys, a surface monitoring operation can produce good results and provide certain advantages compared to a downhole survey including: • No need for an observation well. • No problems with high reservoir temperatures. • Typically more accurate spatial locations (x,y) of events. • No spatial bias (or distance-to-observation-well bias), which is especially important for large multi-well projects. • Better cost scalability to multi-well projects, and probably lower overall costs for multi-well projects. • More robust source mechanism determination, especially compared to single observation well downhole monitoring. The disadvantages of a surface survey compared to a downhole survey include: • Less precise depth locations of events. • Typically higher costs for a single well project. • Lower sensitivity, and higher risk of failure. The risk of failure, especially in a new area or a new formation, probably deters people from using surface monitoring. Microseismic geophysicists who work for operators describe their discomfort in being unable to answer the question when asked by their management, ‘Will a surface project be successful?’ We present a summary of our experiences with successful and failed projects, we establish a set of metrics for the purposes of comparison, and we provide guidelines for planning successful projects.

Seismic shear waves as Foucault pendulum

AbstractEarth's rotation causes splitting of normal modes. Wave fronts and rays are, however, not affected by Earth's rotation, as we show theoretically and with observations made with USArray. We derive that the Coriolis force causes a small transverse component for P waves and a small longitudinal component for S waves. More importantly, Earth's rotation leads to a slow rotation of the transverse polarization of S waves; during the propagation of S waves the particle motion behaves just like a Foucault pendulum. The polarization plane of shear waves counteracts Earth's rotation and rotates clockwise in the Northern Hemisphere. The rotation rate is independent of the wave frequency and is purely geometric, like the Berry phase. Using the polarization of ScS and ScS2 waves, we show that the Foucault‐like rotation of the S wave polarization can be observed. This can affect the determination of source mechanisms and the interpretation of observed SKS splitting.

Simultaneous microseismic event localization and source mechanism determination

Microseismic monitoring has become a tool of choice for the development and optimization of oil and gas production from unconventional reservoirs. The primary objective of (micro) seismic monitoring includes localization of (micro) seismic events and characterization of their source mechanisms. Most seismic events are of a nonexplosive nature, and thus, there are waveform (polarity) differences among receivers. Specifically, double-couple sources represented a challenge for migration-based localization techniques. We developed and applied a new migration-type location technique combined with source mechanism inversion that allowed for constructive interference of signal in seismic waveforms. The procedure included constructing image functions by stacking the amplitudes with compensated polarity changes. The compensation weights were calculated by using moment tensor inversion. This method did not require any picking of arrivals at individual receivers, but it required receivers to be distributed in multiple azimuths and offsets. This made the technique suitable for surface or near-surface monitoring, in which a low signal-to-noise ratio (S/N) can be overcome by stacking. Furthermore, the advantage of this technique was that in addition to the position in time and space, we also determined the source mechanism. We determined with numerical tests that the proposed technique can be used for detection and location of events with S/Ns as low as 0.05 at individual (prestacked) receivers. Furthermore, we found that other source mechanism parameters such as magnitude, volumetric, or shear components of the source mechanism were not suitable for the location. Finally, we applied the proposed technique to a microseismic event of moment magnitude [Formula: see text] induced during the hydraulic fracturing treatment of a gas shale reservoir in North America.

Detection, location and source mechanism determination of microseismic events: evaluating seismicity related to hydraulic fracturing

D. Anikiev, J. Valenta, F. Stanek and L. Eisner present a technique for processing continuous datasets from microseismic monitoring during hydraulic fracturing of a gas shale reservoir in North America.

The Use of Microseismicity To Understand Subsurface-Fracture Systems and To Increase the Effectiveness of Completions: Eagle Ford Shale, Texas

SummaryExisting natural fractures often have a significant impact on both the stimulation and the production of oil and gas wells. The effective exploitation of unconventional reservoirs requires understanding of the local tectonic history and the present-day stress regime. Signal strength; high-quality reflection seismic, microseismic imaging; and the moderate structural complexity of the liquids-rich gas and tight oil Eagle Ford Shale make it an excellent place to study hydraulic fracturing in tight rocks. Microseismic-monitoring results showed clear structural trends relating to the reactivation of existing faults and fractures, and rock-failure mechanisms determined through source mechanism inversion of events. These results provided critical information to the operator for optimizing the hydraulic-fracture design.Microseismic data collected by use of a surface array allowed the full geometry of the result to be viewed with no directional bias. The geometry of the microseismicity trends related to fracturing developed during the stimulation treatment was representative of the true geometry of the structure. The large aperture and wide azimuth of the monitoring array facilitated the determination of source mechanisms from every event detected, which provided full coverage of the focal sphere of each source mechanism. The events identified two different source mechanisms, indicating a failure mechanism for fractures that is different from that for reactivated faults.Microseismicity with a northeast/southwest (NE/SW) orientation is interpreted to be related to either induced or reactivated faults or fractures. Microseismicity also formed trends that are contiguous across more than one wellbore in an east-northeast/ west-southwest (ENE/WSW) direction. These trends are interpreted to have formed as a result of fault reactivation. Source mechanisms from fracturing parallel to SHmax have failure planes that strike NE/SW with normal dip-slip failure on steeply dipping planes. Those from fault reactivation have strike-slip failure on ENE/WSW-striking failure planes. The NE/SW-striking, dip-slip fractures are parallel to extensional Gulf of Mexico (GOM) growth faulting, and the ENE/WSW-striking, strike-slip faults are at an angle of approximately 25° to the dominant fracturing trends.Microseismicity trends associated with faults are used to project where faults will intersect adjacent wells. The identification of these faults in the reservoir by means of microseismic mapping allows operators to modify their treatment parameters and stage spacing to avoid geologic hazards. The operator combines the treatment-pump parameters for the wells with the additional structural understanding gained from the analysis of fracture trends and source mechanisms to identify zones that should be avoided in subsequent treatments. In addition, the mapped microseismicity provides critical information that was used to modify well spacing for subsequent wells, thereby optimizing the completion plan and dramatically cutting costs.

Earthquake source mechanism analysis for events between 1992 and 1997 using sparse New Zealand broadband data

Data from the permanent broadband seismic station SNZO and temporary broadband deployments have been used to test the operation of the regional, full waveform inversion source mechanism determination technique of Dreger and Helmberger (1991) in New Zealand. An amplitude ratio technique similar to the AMPRAT technique, which is widely used in New Zealand, was also tested with the same data. Tests using synthetic data showed that the waveform inversion is not reliable if data from only one station are available, but that the reliability of the technique improves when a second station is added, and the reliability is directly proportional to the azimuthal separation of the stations. Addition of a third station further improves the results. To test the applicability of these synthetic results, regional earthquakes with moment tensors published between 1992 and 1997, and which were recorded at SNZO, were investigated using the waveform inversion technique. Where possible, additional data from temporary broadband deployments were also included in the inversion. The results of these tests on the 11 events support the synthetic results. While two of the events returned source mechanisms consistent with the published solutions, there was no known way to separate the consistent from the inconsistent mechanisms, making it impossible to use the technique reliably with data from only one station. When further stations were available for addition to the inversion, the results were improved. However, there were insufficient events recorded at multiple stations to reach any firm conclusions. The same events were analysed using an amplitude ratio technique plus first motion polarities from the New Zealand short period network, and similar results were found. The amplitude ratio technique was more reliable than the waveform inversion when only one station (plus first motions) was used, with six of the events returning source mechanisms consistent with the published solutions. This was not sufficiently reliable to guarantee good results. Addition of further stations again improved the reliability of the results. In addition to the tests of the techniques, new source mechanisms are presented for the 1994 June 18 Arthur's Pass and 1995 November 24 Cass earthquakes.

The determination of source mechanisms of small earthquakes and revised models of local crustal structure by moment tensor inversion

We describe a procedure for retrieving the source mechanism of small earthquakes and a revised model of crustal structure in heterogeneous media based on interactive inversion of high frequency seismograms and demonstrate its application to three local earthquakes. The earthquakes are located along the Rukwa rift, south-western Tanzania, and were recorded by a local network in the rift. Due to strong lateral crustal heterogeneity in the area, the procedure involves a single-station interactive inversion for moment tensor solution and modelling of the crustal structure along the propagation path between the source and the station. Based on this source solution and the a priori crustal structure, synthetic seismograms are generated and compared with observed displacement seismograms at other stations where the event has been recorded. Deviations between the calculated and observed seismograms are indicated by time shifts and amplitude mismatches on prominent phases; these are employed to correct the phase arrivals used to invert for the partial multistation source solution. The moment tensor inversion of high frequency seismogram data in this study is based on a point source approach. The process includes generation of synthetic seismograms using the frequency-wavenumber integration technique for an elastic, horizontally-layered, medium. Moment tensor inversion is carried out by minimizing the L2 norm of the difference between synthetic and observed displacement data under a deviatoric condition. Results for the three small earthquakes show good agreement between single station and multistation source solutions. The inversions for these events indicate dominantly dip-slip mechanisms with seismic moment magnitudes in the range 2.0 ≤Mw≤ 3.3.

Application of genetic algorithms to focal mechanism determination

Genetic algorithms are a new class of methods for global optimization. They resemble Monte Carlo techniques, but search for solutions more efficiently than uniform Monte Carlo sampling. In the field of geophysics, genetic algorithms have recently been used to solve some non‐linear inverse problems (e.g., earthquake location, waveform inversion, migration velocity estimation). We present an application of genetic algorithms to focal mechanism determination from first‐motion polarities of P‐waves and apply our method to two recent large events, the Kushiro‐oki earthquake of January 15, 1993 and the SW Hokkaido (Japan Sea) earthquake of July 12, 1993. Initial solution and curvature information of the objective function that gradient methods need are not required in our approach. Moreover globally optimal solutions can be efficiently obtained. Calculation of polarities based on double‐couple models is the most time‐consuming part of the source mechanism determination. The amount of calculations required by the method designed in this study is much less than that of previous grid search methods.

Rapid source mechanism determination of large (Mw≥5) earthquakes in the western United States

Inversion of complete long‐period ground motions is used for rapid determination of source mechanisms of Mw ≥ 5 earthquakes in the western United States. The first few minutes of ground motions on local and regional distance waveforms are inverted for point‐source centroid moment tensors. Waveforms recorded at regional distances (300–1500 km) can be adequately modeled at periods longer than 50 s using the laterally homogeneous PREM model [Dziewonski and Anderson, 1981]; at closer distances (<300 km) waves with periods as short as 35 s can be analyzed using PREM. Best double‐couple focal mechanisms for 18 recent large western U.S. earthquakes compare very well with those obtained by other methods. Our procedure, interfaced with automated data retrieval and source location methods, is a viable tool for reliable determination of seismic moment and fault mechanism within 30 minutes after an earthquake.

Source retrieval from broadband regional seismograms: Hindu Kush region

A method of relocation and source characterization of small earthquakes using one modern regional station is developed and tested. First, we model teleseismic body waves of two events, which are used as masters. Short period depth phases, pP and sP, are used to establish the epicentral depth, and the events are relocated using calibrated stations and a mantle model derived for this region, TIP. The events moved upward by 12 and 31 km, respectively. The regional waveforms recorded at the IRIS station GAR from the best-determined source are forward modeled to establish a local crustal model. A four-layered model with a thickness of 65 km proves effective. Synthetics from this crustal model can then be compared with the data from other events where the depths are poorly known, i.e. PDE depths of 33 km. Forward modeling of the short period phases allows better estimates of depth and makes relocation possible. Next, we perform a long period inversion of whole and partial waveform data to obtain source mechanism and moment, and repeat the procedure if necessary. This technique is applied to events in the tectonically active Pamir-Hindu Kush region to test its usefulness. Five crustal events sampling various azimuths are presented as examples of relocation and determination of source mechanisms from small events. The smallest event has a moment of 5.1 × 10 22 dyn cm. For the seven events we studied, the average depth correction to PDE is 19 km, and the location correction is 11 km on average. The method can be used to identify earthquakes and thus it lowers the threshold below present m b: M S discrimination, since these events are too small to be seen teleseismically. Secondly, these events can be used as ‘masters’ in the calibration of other systems, case-based event characterizations, etc.

長周期地震波による近地地震震源メカニズムの決定

長周期地震波による近地地震震源メカニズムの決定

Use of long-period surface waves for rapid determination of earthquake source parameters 2. Preliminary determination of source mechanisms of large earthquakes ( MS ⩾ 6.5) in 1980

An inversion method for fast retrieval of earthquake source parameters developed earlier has been applied to 28 large ( M S ⩾ 6.5), shallow earthquakes which occurred in 1980. Rayleigh waves recorded by the IDA (International Deployment of Accelerographs) network were used for the analysis. For very shallow events, the inversion is ill-conditioned and two of the moment-tensor elements, M zx and M zy , are unresolvable. Of the 28 events, a solution is obtained for 25 with the constraints M zx = M zy = 0. Although these constraints limit the solution to be either vertical strike-slip or 45° dip-slip, the method provides a useful first approximation to the mechanism solution. For deeper events, all of the elements of the moment tensor can be determined. After the seismograms have been retrieved, the solution can be obtained in ∼10 min for most events. The method would be adequate for preliminary cataloging purposes.

Earthquake source mechanism determination by use of body-wave amplitudes—An application to Swedish earthquakes

abstractA method for earthquake source studies based on a combined use of polarity and amplitude observations is proposed. It makes use of the impulses of the observed body waves to avoid the introduction of additional source parameters besides the orientation angles and seismic moments.A statistical model is proposed and the method is applied to five small earthquakes in Sweden recorded by 3 to 5 stations in the distance range of 20 to 100 km. The seismic moments of these earthquakes are in the range of 0.6 to 10 × 1011 N-m (1 N-m = 107 dyne-cm).The resulting mechanisms are statistically significant and well defined in spite of the fact that for four of the earthquakes, only two or less polarities are used. Furthermore, the solutions are close to the mechanism of the major (Mo = 3.5 × 1015 N-m) earthquake in the Gulf of Finland 1976. They all indicate horizontal principal compressive stresses in the sector E-W to SSE-NNW.The successful use of amplitude observations presented in this paper shows that source mechanism inversion can be made based on recordings at only a very few close stations.

The fate of the downgoing slab: A study of the moment tensors from body waves of complex deep-focus earthquakes

Two large, complex deep-focus earthquakes are analyzed, using body waves, to determine the relative locations and equivalent forces of the component events. Details of the source mechanism determination are given, with special emphasis on the means of comparing results for double-couple and more general models. The formalism includes a description of a method for calculating the confidence ellipses around the principal axes of the moment tensor. An interpretation of the results suggests that the mechanism of deep-focus earthquakes is controlled by the buoyancy of the slab and the change in viscosity at the 650 km discontinuity.