Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

Finite-source inversions are performed using small earthquake waveforms as empirical Green's functions (eGf) to investigate the rupture process of repeating earthquakes along the San Andreas Fault in Parkfield, California. The eGf waveform inversion method is applied to a repeating M-w 2.1 Parkfield earthquake sequence using three-component velocity waveforms recorded by an array of borehole seismometers. The obtained models show a circular slip distribution with a similar to 20m radius, a 3.0-4.2cm average slip of the main asperity, and peak displacement of 10.6-13.5cm. The static stress drop distribution shows that the main asperity has a peak stress drop of 69.5-94.7MPa. The inversion results support an earlier finding by Dreger et al. (2007) that high-strength asperities exist in the rupture areas of the M-w 2.1 events at Parkfield. In addition, notable temporal peak slip and stress drop reduction was observed after the 2004 Parkfield event while the average value remains constant (similar to 12MPa) over time. These events may represent mechanically strong sections of the fault, surrounded by regions that are undergoing continuous deformation (creep), Given repeated loading of the strong asperities, it would be expected that these similar repeating earthquakes should also have very similar slip distributions since surrounding regions are deforming aseismically. There are small differences in the waveforms of these repeating earthquakes, and this could be because of rupture nucleation points not being in exactly the same location within the region of the fault that is capable of stick-slip behavior. Our result indicates that waveform slip inversion is needed to reveal spatial and temporal variations of the stress drop within the rupture area to improve understanding of fault healing and rupture mechanics.

DOI： 10.1002/2015JB012562

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

An earthquake with a reported magnitude of 4.4 (M-L) was detected on 13 June 2015 in western central Alberta, Canada. This event was the third felt earthquake this year near Fox Creek, a shale gas exploration region. Our results from full moment tensor inversions of regional broadband data show a strong strike-slip mechanism with near-vertical fault plane solutions. The decomposition of the moment tensor solution is overwhelmingly double couple, while only a modest (similar to 20%) contribution is attributed to compensated-linear-vector-dipole. The depth of this earthquake is 3-4 km, near the base of the sedimentary layer, and the moment magnitude (M = 3.9) of this event is considerably smaller than the initial reported ML value. The hypocenter location, depth, and mechanism are favorable to a possible association between this earthquake and hydraulic fracturing operations within the Duvernay shale.

DOI： 10.1002/2015GL066917

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SEISMOLOGICAL SOC AMER  

More than 60 small earthquakes (ML 0.7-3.0) were detected from December 2011 to March 2012 north of Cardston, Alberta, an area with little evidence for previous seismic activity. The timing of these events closely correlates (>99: 7% confidence) with hydraulic fracturing completions of the Devonian-Mississippian-age Exshaw Formation at a nearby horizontal well. Unanimous waveform multiplicity within the swarm suggests that the events share a similar origin and source mechanism. This observation is corroborated by the point-like collocation of hypocenters within the crystalline basement during robust, double-difference relocations. Furthermore, the presence of a pre-existing fault is confirmed via formation-top offset mapping and interpreted to be a Late Cretaceous extensional fault. The confirmation of this fault at depth provides a plausible pathway for rapid hydraulic communication from the fracturing interval into the crystalline basement. Consistent with structural interpretations and available stress information, moment tensor inversion of the largest magnitude event (M-w 3.0) indicates reactivation of a basement fault with normal slip. We conclude that the genesis of this earthquake swarm was likely caused by increased pore pressure, within the basement fault, as a result of fracturing stimulation.

DOI： 10.1785/0120150131

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SEISMOLOGICAL SOC AMER  

Understanding the source characteristics of hydraulic-fracturing-induced microearthquakes is expected to provide better understanding of both the fracturing process and the influence of pre-existing structures on the distribution of events. However, details of the source characteristics of microearthquakes remain largely unknown. One controversial issue is whether the mechanism of such events differs from natural tectonic events, that is, whether significant reductions of effective stress and/or volumetric change occur because of the stimulation. Herein, to address this question, we estimated the apparent stress of the 61 M-w <-0.6 microearthquakes observed during the hydraulic fracturing in the Carthage Cotton Valley gas field. We first obtained the apparent stress by estimating the radiated seismic energy from the duration of the relative moment rate function and then adjusted the values, considering the possibility of seismic energy underestimation. The initial value range of 0.001-0.02 MPa is consistent with other past studies of hydraulic-fracturing-induced microearthquakes. Although the adjustment increases the original value by a factor of 10, it is still within the lower bound of the range of constant apparent stress commonly observed for natural tectonic events. Furthermore, relatively lower apparent stress was observed for the events striking sub-parallel to the maximum horizontal compressional stress axis. This implies the mechanism of these events could be different from that of events optimally oriented for failure, although the uncertainty of their measurements was expected to be large compared with other events because of their small magnitude.

DOI： 10.1785/0120140319

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Language：Japanese   Publishing type：Research paper (conference, symposium, etc.)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

We compute a series of finite-source parameter inversions of the fault rupture of the 2004 Parkfield earthquake based on 1 Hz GPS records only. We confirm that some of the co-seismic slip at shallow depth (<5 km) constrained by InSAR data processing results from early post-seismic deformation. We also show 1) that if located very close to the rupture, a GPS receiver can saturate while it remains possible to estimate the ground velocity (similar to 1.2 m/s) near the fault, 2) that GPS waveforms inversions constrain that the slip distribution at depth even when GPS monuments are not located directly above the ruptured areas and 3) the slip distribution at depth from our best models agree with that recovered from strong motion data. The 95th percentile of the slip amplitudes for rupture velocities ranging from 2 to 5 km/s is similar to 55 +/- 6 cm.

DOI： 10.1038/srep03646

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

Southeast (SE) Asia is a tectonically complex region surrounded by many active source regions, thus an ideal test bed for developments in seismic tomography. Much recent development in tomography has been based on 3-D sensitivity kernels based on the first-order Born approximation, but there are potential problems with this approach when applied to waveform data. In this study, we develop a radially anisotropic model of SE Asia using long-period multimode waveforms. We use a theoretical cascade approach, starting with a large-scale Eurasian model developed using 2-D Non-linear Asymptotic Coupling Theory (NACT) sensitivity kernels, and then using a modified Born approximation (nBorn), shown to be more accurate at modelling waveforms, to invert a subset of the data for structure in a subregion (longitude 75 degrees 150 degrees and latitude 0 degrees 45 degrees). In this subregion, the model is parametrized at a spherical spline level 6 (similar to 200 km). The data set is also inverted using NACT and purely linear 3-D Born kernels. All three final models fit the data well, with just under 80 per cent variance reduction as calculated using the corresponding theory, but the nBorn model shows more detailed structure than the NACT model throughout and has much better resolution at depths greater than 250 km. Based on variance analysis, the purely linear Born kernels do not provide as good a fit to the data due to deviations from linearity for the waveform data set used in this modelling. The nBorn isotropic model shows a stronger fast velocity anomaly beneath the Tibetan Plateau in the depth range of 150250 km, which disappears at greater depth, consistent with other studies. It also indicates moderate thinning of the high-velocity plate in the middle of Tibet, consistent with a model where Tibet is underplated by Indian lithosphere from the south and Eurasian lithosphere from the north, in contrast to a model with continuous underplating by Indian lithosphere across the entire plateau. The nBorn anisotropic model detects negative ? anomalies suggestive of vertical deformation associated with subducted slabs and convergent zones at the Himalayan front and Tien Shan at depths near 150 km.

DOI： 10.1111/j.1365-246X.2012.05489.x

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1190/1.3627488

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：SEISMOLOGICAL SOC AMER  

We performed 3D ground-motion simulations for 10 recent small to moderate earthquakes (M(w) 4.1-5.4) in the San Francisco Bay area to evaluate two versions of the USGS 3D velocity model (Brocher, 2005; Jachens et al., 2006; Brocher, 2008). Comparisons were made in terms of modeling phase arrival timing, peak ground-motion amplitudes, and the seismic waveforms. In the simulations we assumed the source parameters reported in the Berkeley Seismological Laboratory (BSL) Moment Tensor Catalog. Broadband seismic data from the Berkeley Digital Seismic Network (BDSN), and strong motion data from the USGS and the California Geological Survey (CGS) strong motion arrays were used in the analysis. The comparison of peak ground velocity (PGV) for both models reveals that both 3D models predict the observed PGV well over four orders of magnitude, and P- and S-wave timing and pseudospectral acceleration (PSA) are well modeled by the 3D structure. While the revised model (model 8.3.0) significantly improved the timing of the first arrival, and the waveform fit is generally good, there remain discrepancies in estimated amplitudes and durations that require improvements to the structure. Nevertheless, from our low-frequency (0.5 Hz) analysis we found that the 3D model is suitable for the simulation of PGV to assess the strong shaking hazard of future large earthquakes, because earthquakes larger than M 6 have PGV carried by waves of 1 to several seconds period.

DOI： 10.1785/0120090076

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER GEOPHYSICAL UNION  

[1] The 28 September 2004 Parkfield earthquake, arguably the best recorded earthquake ever, allows for detailed investigation of finite-source models and their resolution. We have developed finite- source models using GPS and interferometric synthetic aperture radar geodetic data and seismic strong motion waveform data (f <= 0.5 Hz) both independently and combined. The preferred model shows that the rupture is predominantly unilateral to the NW with a small component to the SE. Slip is concentrated into two primary patches, one near the hypocenter and the other between 10 and 23 km to the NW within a narrow depth range (5 - 13 km). The scalar seismic moment is 1.3 X 10(18) N m (Mw 6.0), the overall rupture length is 23 km, the peak slip is 0.45 m, and the rupture velocity is 2.6 km/ s. The average static stress drop obtained from the spatially distributed slip model is 2.3 MPa, and peak values are 10 MPa. A detailed sensitivity and resolution analysis shows that the recovered slip, the average rupture velocity, risetime, and slip velocity are well resolved.

DOI： 10.1029/2007JB005115

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Language：English   Publisher：The Society of Exploration Geophysicists of Japan  

We introduce the 3-D attenuation tomographic approach that combines the frequency-domain inversions scheme consisting of a set of sub-inversions and the site effect estimation scheme based on the residual spectra. We apply it to data observed at the Japanese Islands to estimate three-dimensional P wave attenuation structure beneath the Japanese islands. The results show that the Qp-1 distribution is highly variable over the entire Japanese Islands, with the spatial wavelength of 2-3 degrees. This heterogeneous material property of the underground structure may be related to a tectonic state beneath the Japanese Islands.

DOI： 10.3124/segj.58.617

CiNii Books

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Language：Japanese   Publisher：京都大学防災研究所  

CiNii Books

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Language：Japanese   Publisher：The Volcanological Society of Japan  

DOI： 10.18940/vsj.2022.0_128

CiNii Research

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J-GLOBAL

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Publishing type：Internal/External technical report, pre-print, etc.  

Recognizing seismic waves immediately is very important for the realization<br />
of efficient disaster prevention. Generally these systems consist of a network<br />
of seismic detectors that send real time data to a central server. The server<br />
elaborates the data and attempts to recognize the first signs of an earthquake.<br />
The current problem with this approach is that it is subject to false alarms. A<br />
critical trade-off exists between sensitivity of the system and error rate. To<br />
overcame this problems, an artificial neural network based intelligent learning<br />
systems can be used. However, conventional supervised ANN systems are difficult<br />
to train, CPU intensive and prone to false alarms. To surpass these problems,<br />
here we attempt to use a next-generation unsupervised cortical algorithm HTM.<br />
This novel approach does not learn particular waveforms, but adapts to<br />
continuously fed data reaching the ability to discriminate between normality<br />
(seismic sensor background noise in no-earthquake conditions) and anomaly<br />
(sensor response to a jitter or an earthquake). Main goal of this study is test<br />
the ability of the HTM algorithm to be used to signal earthquakes automatically<br />
in a feasible disaster prevention system. We describe the methodology used and<br />
give the first qualitative assessments of the recognition ability of the<br />
system. Our preliminary results show that the cortical algorithm used is very<br />
robust to noise and that can successfully recognize synthetic earthquake-like<br />
signals efficiently and reliably.

arXiv

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Presentation type：Poster presentation  

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Language：English   Presentation type：Poster presentation  

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Language：English   Presentation type：Poster presentation  

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Language：English   Presentation type：Poster presentation  

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Language：English   Presentation type：Poster presentation  

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Language：English   Presentation type：Oral presentation (invited, special)  

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