P-wave Motion Research Articles

Normative expression and accurate understanding of focal mechanism parameters

This paper aims at the understanding of the relationship between the upper and lower hemisphere projection maps of the focal sphere, the correct reading of the focal mechanism parameters on the projection diagram, and to investigate if there is uncertainty about whether the slip angle has a unique solution when there are only two nodal strikes and dip angles. Based on the principle of stereographic projection, the following four expressions and understanding skills about focal mechanism parameters are refined: (1) according to the principle of stereographic projection, we analyzed the difference of the presentation of focal mechanism parameters on different projection maps. There is a 180° rotation relationship between the upper and the lower hemisphere projection map. (2) The method of determining the nodal strike on the upper and lower hemisphere projections: for the former, the observer faces the convex direction of the nodal arc, and the direction to the right of the nodal plane is the stike of the fault. The angle turned clockwise in the north direction is the strike angle of the nodal plane; For the latter, the observer faces the convex direction of the arc of the nodal plane, and the point corresponding to the point on the right side of the nodal plane plus 180° is the strike of the fault. (3) The relationship between the azimuth angles of the P, T and N-axis on upper and lower hemisphere projections: the azimuth angles of the P, T, and N-axis on the upper hemisphere projection map = the azimuth angles of the P, T, and N-axis on the lower hemisphere projection map + 180°. (4) Only the strike and dip angles of two nodal planes are used to derive the rake: according to the slip angle formula, two groups of rake solution can be calculated, indicating that the solution are not unique. The actual slip angle can be determined by combining the hanging wall and footwall and strike of fault, or by combining the actual P-axis and T-axis data, or the first motion of P-waves. Aims to aid accurate understanding and long-term memory of focal mechanism parameters with consistent and simple rules.

Micromechanisms of coal fracture: Insights from quantitative AE technique

Three-dimensional location and characterization of microcracks in controlled fracture tests of hard coal beams were experimentally conducted through the quantitative acoustic emission (AE) technique. The recorded AE events were physically interpreted as displacement discontinuities (DD), i.e. microcracks, involving only opening/closing and shear-sliding mechanisms to describe the fracture modes directly anticipated within a mechanically loaded material. Based on the AE sensor calibration using a scalar sensitivity parameter and the first motion of P-wave, the moment tensor components were determined through minimizing the errors between the measured and calculated displacements at each sensor location, subjected to a restriction from this “crack-only” source model. Source characterization results were presented in a statistical form, which showed mixed mode mechanisms of microcracks with approximately equal contribution of shear and tensile displacements locally due to the considerably tortuous crack path in the microstructural level of coal material. However, the microcrack volume decomposition under the global coordinate system indicated a mode-I opening micromechanisms dominantly, which was compatible with the expected mechanism from the three-point bending test. Additionally, the orientations of microcrack planes were more or less vertical, with an average angle of 20.4° from the global fracture plane. And around 79.6% of the AE sources with an angle, made by the displacement vectors with horizontal direction, less than 30°. AE magnitudes followed the Gutenberg-Richter relation with a b-value of around 1.3 during coal fracture. Owing to the physical basis of energy dissipation and the linear relationship with scalar moment, the so-called “crack volume” |b|ΔA was used as an index of the absolute AE energy. Energy dissipation region moved forward together with the crack propagation, and most of the energy was dissipated above the crack tip by separating the crack face.

DirectP-wave anisotropy measurements at Homestake Mine: implications for wave propagation in continental crust

SUMMARYWe measured anisotropic seismic properties of schists of the Homestake Formation located at a depth of 1478 m in the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota, USA. We deployed a 24-element linear array of three-component geophones in an area in the Homestake Mine called 19-ledge. An airless jackhammer source was used to shoot two profiles: (1) a walkaway survey to appraise any distance dependence and (2) a fan shot profile to measure variations with azimuth. Slowness estimates from the fan shot profile show a statistically significant deviation with azimuth with the expected 180° variation with azimuth. We measured P-wave particle motion deviations from data rotated to ray coordinates using three methods: (1) a conventional principal component method, (2) a novel grid search method that maximized longitudinal motion over a range of search angles and (3) the multiwavelet method. The multiwavelet results were computed in two frequency bands of 200–600 and 100–300 Hz. Results were binned by azimuth and averaged with a robust estimation method with error bars estimated by a bootstrap method. The particle motion results show large, statistically significant variations with azimuth with a 180° cyclicity. We modelled the azimuthal variations in compressional wave speed and angular deviation from purely longitudinal particle motion of P-waves using an elastic tensor method to appraise the relative importance of crystalline fabric relative to fracturing parallel to foliation. The model used bulk averages of crystal fabric measured for an analogous schist sample from southeast Vermont rotated to the Homestake Formation foliation directions supplied by SURF from old mine records. We found with average crustal crack densities crack induced anisotropy had only a small effect on the observables. We found strong agreement in the traveltime data. The observed amplitudes of deviations of P particle motion showed significantly larger variation than the model predictions and a 20° phase shift in azimuth. We attribute the inadequacies of the model fit to the particle motion data to inadequacies in the analogue rock and/or near receiver distortions from smaller scale heterogeneity. We discuss the surprising variability of signals recorded in this experimental data. We show clear examples of unexplained resonances and unexpected variations on a scale much smaller than a wavelength that has broad implications for wave propagation in real rocks.

Citizen Seismology in Taiwan: Development, Outreach, and Formative Assessment of Near-Real Time Earthquake Game Competition Activities

Citizen seismology encourages the public involvement in data collection, analysis, and reporting, and has the potential to greatly improve the emergency response to seismic hazard. We believe in the value of citizen seismology and started with distributing Quake-Catcher Network (QCN) sensors at schools in Taiwan. Unlike most of western countries, Earth Sciences is a required course in junior and senior high schools of Taiwan (Ministry of Education, 2014). Low-cost seismometer is potentially a powerful tool in classrooms to teach earthquake science, which makes school teachers our targeted users. We work closely with school teachers and students to understand how can a citizen seismology project can help them to encourage successful engagement. In this study, we establish the achievements gained and problems encountered in different phases of the project since 2013. The main tasks carried out here include (1) refinement of Citizen Seismology Literacy (CSL) into three dimensions: Awareness, and Willingness, and Technology fluency (2) development of an interactive competition platform called the Near-Real Time Earthquake Game Competition (EGCnrt) that allows citizen seismologists to report earthquake information by processing P- and S-wave arrivals, peak ground motion, and first motion of P-waves for every inland M≥4 earthquakes in Taiwan; (3) development of the formative assessment for the 1.5-hour-long, non-lecture-based game activities. Based on 565 student surveys completed after our 2016 summer outreach activity, we found that all three CSL dimensions have an influence on students’ score in the training activity. The final score in EGCnrt is also found to highly correlate with the performance in the three training sections. We therefore, propose that the CSL can act as a powerful indicator for the performance and engagement in earthquake learning activities. The game-based, non-lecture-based learning approach can be effective in promoting citizen seismology in the future.

A Stress Condition in Aquifer Rock for Detecting Anomalous Radon Decline Precursory to an Earthquake

Recurrent groundwater radon anomalous declines were observed from well measurements in the Antung hot spring area (eastern Taiwan) prior to five of six earthquakes that occurred between 2003 and 2011 (M w range 5.0–6.8). The relationship between the detectability of radon anomalies and the first motions of P-waves was investigated. Based on the first motions of P-waves recorded near the investigated well, a precursory decrease in groundwater radon can be detected only when the first motion is compression. No precursory change in groundwater radon concentration was observed for the downward first motion of P-waves.

The seismogenic fault of the 2010 Efpalion moderate-size seismic sequence (western Corinth gulf, Central Greece)

The 2010 Efpalion seismic sequence with two main moderate earthquake events occurred in the northwestern part of the Corinth Gulf (Central Greece)—a region that has been intensely stretched due to an on-going N–S extensional stress regime. Previous studies assign these two events to activations of (a) two faults dipping to the north with low angles; (b) two faults dipping at high angles, the first dipping to the south, and the second to the north; and (c) two faults dipping at high angles, but the first dips to the north, and the second to the south. The recently proposed TR method for focal mechanisms that identifies the seismogenic fault of an earthquake sequence is applied on the available focal mechanisms of the sequence, and its results are interrelated with the geology of the region, and previous contradictory interpretations. The focal mechanisms constructed with MT inversion define a steep north-dipping normal fault, whereas those constructed with first motions of P-waves define the activation of two adjoining faults that dip with high angles to the SSE and south, respectively, and which are characterized by strain (slip) compatibility. The latter option fits well with the geology of the region that is dominated by a SE to S-dipping horse-tail splay fault zone which exists at the eastern tip of the Nafpaktos Mountain Front. The application of the TR method reveals that the usage itself of different methods for the construction of the focal mechanisms complicates the problem of correctly identifying the seismogenic fault.

1909 Taipei Earthquake Ground Motion Simulation

The 1909 Taipei earthquake (M 7.3) occurred beneath the Taipei metropolitan area (TMA) causing substantial damage according to the historical literature. According to the hypocenter relocation and tectonic implications provided in a previous study, we simulated ground motions within the TMA using a hybrid simulation method involving the spectral-element method (SEM) and the empirical Green's function method (EGFM). We used the SEM for simulating low-frequency components and the EGFM for simulating high-frequency components. These high and low frequency components were subsequently combined. For the EGFM we used the records from a recent M_L 4.9 earthquake (11 October 2013, depth = 143.8 km) in the Taipei area as the empirical Green's function. According to the historical literature, the observed PGA (peak ground acceleration) values are 59.2 and 67.0 gal at ancient stations TAP and KEE, with periods of 1.21 and 1.34 s, respectively. By comparing the simulated PGA values at modern stations TAPB and WFSB to the historical documented ones for 12 different models, our result suggests that the 1909 Taipei earthquake was an event with a magnitude of about M_w 7.3 and stress drop of approximately 30 bars, or a smaller equivalent magnitude between M_w 6.8 - 7.3 but with much higher average stress drop of more than 100 bars. For a deep event beneath TMA a larger vertical P-wave motion and longer period shaking wave, as addressed in the historical literature, might be expected with prolonged shaking as found in the simulation. A seismic hazard assessment is necessary for metropolitan Taipei to better understand the long period shaking from deep subduction zone intra plate events.

A study of the focal mechanisms of small volcanic earthquakes during the precursory period and the occurrence of the 2008–2009 eruption on Koryakskii Volcano

This is the first-ever study to determine the focal mechanisms for very small volcanic earthquakes (magnitudes approximately 1). We used a swarm of small volcanic earthquakes in the Koryakskii Volcano area during the seismic precursory period before the eruption (March to December 2008) and during the subsequent volcanic activity (December 28, 2008 through April 2009). Statistically significant estimates of mechanism parameters were found for the volcanic earthquakes in this swarm. The mechanisms were determined using the polarities of the first motions of P-waves. The resulting rupture planes strike along azimuths that are consistent with the fault plane of the nearly north–south zone where the 2008–2009 seismicity of Koryakskii Volcano occurred. It was shown that the earthquakes in and around the fault zone can be classified into those south of the Koryakskii summit (zone I) and north of it (zone II). The azimuths of rupture planes in zones I and II are significantly different (212° ± 12° and 5° ± 12°, respectively).

Earthquakes focal mechanism and stress field pattern in the northeastern part of Egypt

Egypt is characterized by moderate size seismicity where earthquakes are distributed within several active regions. In the present study, we investigated the source mechanism of earthquakes using the digital waveform data recorded by the Egyptian National Seismic Network (ENSN) during the period from 2004 to 2008. The focal mechanisms are constructed with high reliability based on the polarity of the first motion of P-wave.These solutions are used to examine the mode of tectonic deformation and the present-day stress field pattern affecting on different tectonic provinces in the northern part of Egypt. The results demonstrate mainly a normal faulting mechanism with minor strike slip component generally trending parallel to the northern Red Sea, the Suez rift, Aqaba rift with their connection with the great rift system of the Red Sea and the Gulf of Suez and Cairo-Alexandria trend.The inversion technique scheme is used also in the present study for determining the regional stress field parameters for earthquake focal mechanism solutions based on the grid search method of Gephart and Forsyth (1984). The Results of the stress tensor using focal mechanisms of recent earthquakes show a prevailed tension stress field in N52°E, N41°E and N52°E for the northern Red Sea, Gulf of Suez and Gulf of Aqaba zone respectively.

The Volcanic Earthquake Swarm of October 20, 2009 in the Tatun Area of Northern Taiwan

On October 20, 2009, a series of felt earthquakes with local magnitudes ranging from 2.8-3.2 occurred in the Tatun volcanic area off the northern tip of Taiwan. Although there was no damage caused by those earthquakes, many residents in the Taipei metropolitan area, particularly for people who live near the Yangminshan National Park, felt strong ground shaking. In order to know what the possible mechanisms were that generated those earthquakes, we carefully examined seismic data recorded by a dense seismic array in the Tatun volcanic area. During the period between October 18 and 22, 2009 we detected at least 202 micro-earthquakes. Most of the earthquakes were relocated using the double-difference method and were clustered in the shallow crust beneath the Dayoukeng area, which is the strongest fumarole in the Tatun volcanic area. Among these earthquakes, 72 focal mechanisms were determined by polarizing the first P-wave motion. Most earthquakes belonged to normal faulting. An extremely high b-value of 2.17 was obtained from those earthquakes. Based on the seismic variations in both the temporary and spatial distribution as well as an extremely high b-value, we conclude that the earthquake sequence on October 20, 2009 was a typically seismic swarm associated with possible active volcanism in the Tatun volcanic area.

Investigation of ground rotational motions caused by direct and scattered P-waves from the 4 March 2008 TAIGER explosion experiment

High-frequency rotational motions of P-waves and coda waves were analysed using rotation rate sensors and strong motion array data from the 4 March 2008 TAiwan Integrated GEodynamics Research (TAIGER) explosion experiment in northeastern Taiwan. Theoretical and observational investigations focussed on the effects of this experiment on the free surface. The main goal of this study was to explore possible applications of combined measurements of artificial explosion-derived translational and rotational motions. Also investigated was the consistent ground rotation observed directly by rotation rate sensors and derived using translational seismic arrays. Common near-source high-frequency rotational motion observations and array-recorded translational motions from one shallow borehole explosion are analysed in this study. Using a half-space assumption of plane P-wave propagation across the recording site, we conclude that: (1) rotational motions induced by direct P-waves interacting with a free surface in theory can be used to estimate wave radial direction, velocity and anisotropic properties; (2) rotational motions derived from scattering are predominant among the observed rotations during the TAIGER explosion experiments and allow us to image the heterogeneous structure of the medium at the investigated site; and (3) rotation sensor measurements undertaken during TAIGER explosion experiments may be affected by cross-axis sensitivities, which need to be considered when using the data obtained during these experiments.

Determination of regional earthquake source parameters in wavelet domain

Investigating source parameters of small and moderate earthquakes plays an important role in seismology research. For small and moderate earthquakes, the mechanisms are usually obtained by first motion of P-Wave, surface wave spectra method in frequency-domain or the waveform inversion in time-domain, based on the regional waveform records. We applied the wavelet domain inversion method to determine mechanism of regional earthquake. Using the wavelet coefficients of different scales can give more information to constrain the inversion. We determined the mechanisms of three earthquakes occurred in California, the United States. They are consistent with the previous results (Harvard Centroid Moment Tensor and United States Geological Service). This proves that the wavelet domain inversion method is an efficient method to determine the source parameters of small and moderate earthquakes, especially the strong aftershocks after a large, disastrous earthquake.

Investigations of the Aftershocks of July 4, 1968 Earthquake in Epidavros

The spatial and tlie time distribution of the aftershocks of the earthquake of July 1968 in Epidavros (Greece) has been studied. It was found that the value of b in Gutenberg-Richter's relationship was nearly the same for the primary as well as the secondary or second order aftershocks of the sequence. The focal mechanism of the second order aftershocks as observed from the sense of first motion of P-waves showed some difference from that of the main shock or its primary aftershock.

Preliminary outlining of the seismological active zones in Syria

The aim of this study was to outline the seismologically active zones in Syria using the focal mechanisms of the largest events recorded by the Syrian National Seismological Network over the last decade. A dataset of fault-plane solutions was obtained for 49 events with magnitude ≥3.5 using the first P-wave motions. Most of these events had strike‐slip mechanisms in agreement with the configuration of the seismogenic belts in Syria. Normal mechanisms were more scarce and were restricted to certain areas, such as the coastal ranges. These data show that despite the relatively small magnitudes of the events studied, they provide a coherent picture of the deformation that has currently been taking place along the active faults. However, some other faults were inactive during the period of this study.

The mechanism of induced seismicity at mosul reservoir based on the first motion analysis

In 1986 shortly after the impounding of Mosul reservoir, shallow earthquakes began occurring in the immediate reservoir vicinity, with magnitudes up to ML 3.0, at rates of up to 3 events per week. These events were almost certainly reservoir-induced and coincided with steadily increasing water levels. Cluster of epicenters was observed in the area located within a complex fault zone called the Sinjar-Dohouk-Kuchuk fault system. The presence of such fault system considers a potential source of earthquakes. A composite fault plane solution, based on first p-wave motion analyses, indicates that the mechanisms of seismicity were right-lateral strike-slip faulting along N44°E plane dipping 58° NW, in conformity with the local tectonics.

Historical Review of the Study on Intermediate and Deep Earthquakes

The deep earthquake was first detected by Herbert Hall Turner in 1922, and confirmed by Wadati in 1928, Scrase in 1931 and Stechschulte in 1932. Wadati demonstrated the inclined intermediatedeep earthquake zone under the Japanese islands and neighborhoods in 1935. Such earthquake zones were found in other areas in the circum-Pacific regions. Benioff (1954) classified them morphologically into two types, and ascribed them to thrusts, following Suess's idea. Richter (1958) critisized the thrust hypothesis, as most of deep earthquakes were not accompanied by reverse faults, but by normal faults which were deduced from the initial motion of P-waves. Gutenberg and Richter (1954) pointed out the regular arrangement of geology and geophysical phenomena from an outer oceanic trench to an inner belt of deep earthquakes in the circum-Pacific region. Wadati (1928) demonstrated the vertical relation between shallow earthquakes and deep ones in central Honshu of the Japanese islands. Suzuki (1975) showed a similar relationship in northeast Honshu, and suggested the role of vertical deep faults. Bemmelen (1972) attributed the inclined intermediate-deep earthquake zone to the upward driving force derived from the deep mantle. Suzuki et al. (1978) tried a simulation experiment using the finite element method to propagate the inclined intermediate-deep earthquake zone by the differential vertical displacement at the base of mantle, and the result was compared with the earthquake-generating stresses.

Types of focal mechanism solutions and parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces

Based on P-and S-wave amplitudes and some clear initial P-wave motion data, we calculated focal mechanism solutions of 928 M≥2.5 earthquakes (1994–2005) in four sub-blocks of Sichuan and Yunnan Provinces, namely Sichuan-Qinghai, Yajiang, Central Sichuan and Central Yunnan blocks. Combining these calculation results with those of the focal mechanism solutions of moderately strong earthquakes, we analyzed the stress field characteristics and dislocation types of seismogenic faults that are distributed in the four sub-blocks. The orientation of principal compressive stress for each block is: EW in Sichuan-Qinghai, ESE or SE in Yajiang, Central Sichuan and Central Yunnan blocks. Based on a great deal of focal mechanism data, we designed a program and calculated the directions of the principal stress tensors, σ1, σ2 and σ3, for the four blocks. Meanwhile, we estimated the difference (also referred to as consistency parameter $$\bar \theta $$ ) between the force axis direction of focal mechanism solution and the direction of the mean stress tensor of each block. Then we further analyzed the variation of $$\bar \theta $$ versus time and the dislocation types of seismogenic faults. Through determination of focal mechanism solutions for each block, we present information on the variation in $$\bar \theta $$ value and dislocation types of seismogenic faults.

Difference in the initial slopes of the P-wave envelope waveforms of the main shock M7.4 and foreshock M7.1 of the 2004 off the Kii peninsula earthquakes

Abstract We made a close investigation about the initial rising slopes of the envelope waveforms of the P-wave ground motion by using data for recent earthquakes. We have found that the P-wave initial slopes decrease almost linearly with increasing epicentral distance over the wide range of distance. This linear relation looks to be independent of the earthquake magnitudes and focal depths though the dispersion of data is somewhat large. We have found that the P-wave initial slopes of the foreshock M7.1 of the 2004 off the Kii peninsula earthquakes are very sharp. To the contrary, the slope of the main shock M7.4 is quite gentle. The main shock was preceded by a small event that occurred about 6.7 seconds before the main shock. This pre-event was generated near the initiation point of the main fracture. We are speculating that the difference in the initial slope between the foreshock and main shock may reflect the difference in the time variation of the stress drop at the early stage of the fracture and that the small pre-shock may be responsible for the slow rising of the initial P-wave motion of the main shock.

Automatic Location of Swarm Earthquakes from Local Network Data

An advanced method of automated seismic phase picking and exact location and magnitude determination of swarm micro-earthquakes from local network data is presented. The phase picker is applied in two steps: first, S-wave groups are identified using a polarisation detector, and then corresponding P-wave groups are searched for. The times of maximum P- and S-amplitudes are then used as starting points for the determination of accurate P- and S-arrival times. The maximum S-wave amplitudes are utilised for determining local magnitudes. The whole procedure is checked by simultaneous preliminary hypocentre location providing estimates of local magnitudes and a compatibility check of the candidate P- and S-phases. The closest station to the earthquake cluster is used as a master, and the phase search at the remaining stations is governed by the P- and S-phases identified at the master station. Thanks to the use of apriori information on the approximate position of hypocentres, the procedure is also capable of picking the individual P- and S-phases of sequences of overlapping swarm events. The performance of the procedure was tested by comparison of the automatically and interactively created catalogues of the January 1997 NW-Bohemia micro-earthquake swarm. With stations located at epicentral distances between 0 and 20 km, the difference between hypocentre coordinates obtained by automatic and interactive processing did not exceed 80 m for 86% events. All events above magnitude 0.5 were identified, and the automatically determined polarity of first P-wave motion proved to be correct in 89% of them.

Earthquake source parameters as inferred from the body waveform modeling, southern Turkey

Focal mechanism parameters of some significant earthquakes from southern Turkey have been estimated using either the body waveforms or the first motions of P-waves. It is observed that fault plane solutions derived from first motions may not be well constrained and may often be in error because of poor signal-to-noise ratios at some stations, poorly distributed recording stations in azimuth, lack of recognition of the reversed polarities at some stations, and positions of stations in terms of the crustal velocity structure, focal depth and, hence, take-off angles in the source area. The use of the inversion method introduces a considerable improvement in the focal parameters estimated in previous studies that have been used to construct seismotectonic models in the study area. The focal depths inferred from the inversion of body waveforms for the earthquakes occurring in southwestern Turkey are compatible with those reported by the ISC bulletins and other previous studies while the focal depths of earthquakes from the Antalya Bay are found to be less than those given by different agencies. Waveform inversion of earthquakes that have occurred in and around the Antalya Bay implies a low velocity layer showing azimuthal dependence. Hence, depth of rupture and lateral extent of velocity structure are of importance in the investigation of earthquake faulting process in this area. The source time duration of shallow earthquakes is generally longer than that for the deep focus earthquakes from the Antalya Bay and the Rhodes area. The events with relatively short and simple time functions may be interpreted as low stress drop events relative to the crustal ones.