Shallow Seismic Events Research Articles

Hybrid broadband simulation of long-period ground motion in far-field basins based on group delay model

Shallow seismic events often induce long-period ground motions in distant sedimentary basins, which can cause damage to structures. The current design earthquake input is usually based on amplitude, while the equally essential phase characteristics of far-field basin seismic waves are often ignored. To address this issue, this paper proposes a semi-empirical model based on group delay, which integrates the effects of source, path, local site, and basin on group delay, we characterize randomness of group delay through a parametric Gaussian probability framework, and approximates the frequency non-stationarity of positive and negative dispersion by introducing piecewise spline gradient regression. As an improvement of the stochastic finite fault method, the proposed method solves the problem that the original method cannot sufficiently reflect the phase characteristics and frequency non-stationarity of basin-induced seismic waves. Based on this model, the stochastic finite fault method and the spectral element method are combined to perform the hybrid simulation of basin ground motion, and the earthquake of 2003 Tokachi-Oki verifies the reliability of the proposed method.

Local seismo-acoustic waves, but from the other side of the world

It is well understood that shallow seismic events near the Earth surface couple readily to the atmosphere where they can be observed as infrasonic waves. Typically, the observed sequence of infrasound arrivals includes a first arrival (i.e., “local infrasound”), which corresponds to the coupling of seismic body and surface waves to atmospheric infrasound, near an infrasound array. Under favorable atmospheric ducting conditions, additional coupled arrivals arrive later in time, having propagated throughout the (slower) atmosphere for part of the propagation path. In this presentation, we show that seismo-acoustic waves can couple from very deep earthquakes that occur on the other side of the world. This is demonstrated using seismo-acoustic observations from a M8.2 earthquake in Fiji that generated infrasound throughout Europe. Our analysis shows that various phases that reflected off the inner core boundary (ICB) were detected as pressure waves. This suggests that signals that are detectable on microbarometer arrays may have a surprising origin.

Induced seismicity due to hydraulic fracturing near Blackpool, UK: source modeling and event detection

Monitoring small magnitude induced seismicity requires a dense network of seismic stations and high-quality recordings in order to precisely determine events’ hypocentral parameters and mechanisms. However, microseismicity (e.g. swarm activity) can also occur in an area where a dense network is unavailable and recordings are limited to a few seismic stations at the surface. In this case, using advanced event detection techniques such as template matching can help to detect small magnitude shallow seismic events and give insights about the ongoing process at the subsurface giving rise to microseismicity. In this paper, we study shallow microseismic events caused by hydrofracking of the PNR-2 well near Blackpool, UK, in 2019 using recordings of a seismic network which was not designed to detect and locate such small events. By utilizing a sparse network of surface stations, small seismic events are detected using template matching technique. In addition, we apply a full-waveform moment tensor inversion to study the focal mechanisms of larger events (ML > 1) and used the double-difference location technique for events with high-quality and similar waveforms to obtain accurate relative locations. During the stimulation period, temporal changes in event detection rate were in agreement with injection times. Focal mechanisms of the events with high-quality recordings at multiple stations indicate a strike-slip mechanism, while a cross-section of 34 relocated events matches the dip angle of the active fault.

Rayleigh Wave Propagation in the Bighorn Mountains Region, Wyoming

ABSTRACT Short-period Rayleigh waves, Rg, provide strong constraints on the depth of shallow seismic events and are of interest for monitoring small explosions. Characterizing the seismic sources that generate Rg requires an understanding of how shallow crustal structure affects Rayleigh wave propagation. In support of these efforts, this study utilizes observed waveforms from small shallow explosions recorded on temporary seismic network deployments in the Bighorn region, Wyoming. We study regional near-surface geology by measuring changes in surface-wave amplitude and polarization during propagation through basins, foothills, and mountains. We develop additional insight by carrying out surface-wave eigenfunction analyses and numerical-wave simulations, which together reproduce many characteristics seen in the observed waveforms. Our results show how sedimentary basins in the Bighorn region allow for amplified prograde-polarized higher-mode and retrograde-polarized fundamental-mode Rayleigh waves, whereas adjacent mountains only support retrograde motion. These different modes provide distinct constraints on the Earth structure and source characteristics, potentially enabling targeted inversions in future studies. Our findings provide insight into Rg propagation through complex near-surface geology, improving our understanding of shallow propagation and source effects that are relevant to explosion monitoring efforts.

On the Source Parameters and Genesis of the 2017, Mw 4 Montesano Earthquake in the Outer Border of the Val d’Agri Oilfield (Italy)

On October 27, 2017, an Mw4 earthquake occurred close to the municipality of Montesano sulla Marcellana, less than 10 km external to the concession of the largest European onshore hydrocarbon reservoir—the Val d’Agri oilfield (Southern Italy). Being a weak event located outside the extended monitoring domain of the industrial concession, the relevance of this earthquake and the possible links with the hydrocarbon exploitation were not extensively discussed. Actually, the analysis of shallow seismic events close to subsurface exploitation domains plays a significant role in the definition of key parameters in order to discriminate between natural, triggered, and induced seismicity, especially in tectonically active regions. The study of weak-to-moderate earthquakes can improve the characterization of the potentially destructive seismic hazard of this particular area, already struck by M > 6.5 episodes in the past. In this work, we analyze the source parameters of this Mw4 earthquake by applying advanced seismological techniques to estimate the uncertainties derived from the moment tensor inversion and identify plausible directivity effects. The moment tensor is dominated by a NW–SE oriented normal faulting with a centroid depth of 14 km. A single ML2.1 aftershock was recorded and used as the empirical Green’s function to calculate the apparent source time function for the mainshock. Apparent durations (in the range 0.11–0.21 s, obtained from S-waves) define an azimuthal pattern, which reveals an asymmetric bilateral rupture with 70% of the rupture propagation in the N310°W direction, suggesting a rupture plane dipping to the SW. Our results tally with the activation of a deeper fault segment associated with the Eastern Agri Fault System close to the basement as the origin of the Montesano earthquake. Finally, the Coulomb stress rate induced by depletion of the oilfield is calculated to quantify the trigger potential estimated for the Montesano earthquake yielding relatively low probabilities below 10%. Our analyses point toward the conclusion that the Mw4 event was more likely due to the local natural tectonic stress, rather than induced or triggered by the long-term hydrocarbon extraction in the Val d’Agri oilfield.

Seismicity along the Magallanes-Fagnano fault system

The Magallanes-Fagnano fault (at ~51-56°S) is a continental NW-SE transform fault zone that marks the boundary between the South American Plate (north) and the Scotia Plate (South). In 1949, two crustal seismic events of estimated magnitude M > 7 were located in this area using teleseismic records. Although, this region is scarcely populated, such shallow seismicity could represent a considerable threat to the remote urban centers in this sector. The characterization of the local seismicity always lacked good quality records and permanent instrumentation. In the recent past years, permanent seismic stations (CSN and INPRES-CTBTO) were deployed in the Tierra del Fuego Island and Southern Patagonia, providing with continuous time, broadband records of waveform data. In order to obtain better insights about the local seismicity associated with the Magallanes-Fagnano fault, we jointly inverted teleseismic receiver functions with Rayleigh-wave dispersion data to build a calibrated velocity model for the study region. In parallel, we analyzed the continuous waveforms to automatically detect local earthquakes and locate them using our regional velocity model. Our results show: 1) Shallow seismic events distributed along the Magallanes-Fagnano fault. 2) Diffuse seismicity characterizing the South American crust, north of 53°S. Our observations suggest two mechanisms of crustal deformation. In the Tierra del Fuego Island the seismicity is mostly associated with the Magallanes-Fagnano fault system. In the Southern Patagonia, along the Patagonian Andes the seismicity would be mostly due to isostatic rebound following the retreat of the Patagonian Ice sheet.

Shallow Seismicity and the Classification of Structures in the Lau Back‐Arc Basin

AbstractBack‐arc basins open in response to subduction processes, which cause extension in the upper plate, usually along trench‐parallel spreading axes. However, global seismic databases reveal that the majority of seismic events in the Lau Basin occur along transcurrent (strike‐slip) rather than extensional faults. To better characterize active deformation in this region, we compared centroid moment tensors (CMTs), calculated for large (Mw > 5), shallow (<30 km) seismic events, to the orientations of seafloor lineaments mapped throughout the Lau Basin. Ship‐based multibeam and satellite altimetry were combined with vertical gravity gradient data to create the lineament map. By comparing the possible focal planes of the CMTs to the orientations of the lineaments, the most likely fault plane solutions were selected, thus classifying the faults and establishing the nature of the highly variable stress regimes in the basin. We resolved the strike, dip, and dip direction of 308 faults and classified 258 additional structures by fault type. The analysis highlights a stress regime that is dominated by a combination of left‐lateral and right‐lateral strike‐slip faults, large‐scale transcurrent motion along rigid crustal‐scale fault zones, and nonrigid diffuse deformation along preexisting seafloor structures, with extension mainly limited to the tips of propagating rifts and spreading centers. By resolving many of the uncertain motions on the mapped lineaments of the Lau Basin, the CMT analysis addresses a number of questions concerning basin‐scale stress regimes and microplate development, complementing GPS measurements, and providing a more complete picture of the complexities of back‐arc basin development.

The 25 October 2018 Mw = 6.7 Zakynthos earthquake (Ionian Sea, Greece): A low-angle fault model based on GNSS data, relocated seismicity, small tsunami and implications for the seismic hazard in the west Hellenic Arc

We present a joint analysis of seismological and Global Navigation Satellite System (GNSS) data of the seismic sequence that started on October 25, 2018 with a shallow Mw = 6.7 earthquake offshore Zakynthos (Ionian Sea, Greece). The inversion of the GNSS data shows the activation of a N-S striking thrust/oblique-slip fault at the African-Aegean plate interface with a length of ∼26 km, and depth shallower than 15 km. The fault-plane geometry is well constrained by GNSS with a low-dip angle (23°), and dip-direction towards east. This is consistent with the distribution of the relocated aftershocks (1811 events). Our analysis indicates that the October 25, 2018 event ruptured the Hellenic megathrust. This event highlights the high degree of seismic coupling in the western region of the Hellenic Arc. It also highlights the “strong” nature of the subducting slab with the occurrence of “locked” patches under the Ionian seafloor that fail during large, reverse/oblique-slip earthquakes. The latest, shallow seismic events of the western Hellenic arc (1976 Zakynthos, 1997 Strofades, 2018 Zakynthos) point to a sequence of low-angle events along the plate interface with most of co-seismic deformation taken up by the upper (Aegean) plate. The GNSS data also point to a similar pattern between the co-seismic strain release and the long-term (interseismic) strain accumulation along the west Hellenic Arc.

Focal mechanisms of the 2017 North Korean nuclear test and its early collapse event

SUMMARYNorth Korea conducted sixth underground nuclear test on 3 September 2017. Unlike its previous tests, a rare subsequent collapse event occurred after about 8.5 min. As two types of distinctive shallow seismic events, accurate inversion of their focal mechanisms is important for event identification for CTBT. In this paper, we carry out moment tensor inversion of the nuclear test and the collapse event with gCAP using waveform data from dense regional seismic stations. And their focal mechanisms are further constrained with surface wave amplitude ratio. The results show that the surface wave amplitude ratio has further constraints for screening the waveform inversion results. The resolution of the focal mechanism inversion for the nuclear test is high, which is close to a Crack source. However, the resolution for the collapse event inversion is not so high and the source type is difficult to be accurately determined. One reason of the poor resolution for the collapse event may be due to the limited availability of high quality data, and complexity of the source process might be another factor.

On the Portability of ML–Mc as a Depth Discriminant for Small Seismic Events Recorded at Local Distances

AbstractWe show that ML–Mc is a viable and regionally portable depth discriminant, and therefore may contribute to nuclear test ban treaty verification. A recent study found that the difference between local magnitude (ML) and coda duration magnitude (Mc) discriminates shallow seismic events (mining blasts, mining‐induced earthquakes, and shallow tectonic earthquakes) from deeper tectonic earthquakes in the Utah region. The shallow seismic events had anomalously high Mc values, with increasingly negative ML–Mc values as depth decreased. Here, we evaluate the performance of ML–Mc as a depth discriminant in three new regions and find that ML–Mc increases between 0 and 9 km depth in all cases. Initially, we investigated ML–Mc as a function of depth for naturally occurring earthquakes in the region around Yellowstone National Park, as recorded by the University of Utah Seismograph Stations. For 3358 Yellowstone earthquakes with well‐constrained depths, we found ML–Mc increased 0.030±0.007 magnitude units (m.u.) for each 1 km increase in depth up to 10 km depth. Next, we examined ML–Mc values for anthropogenic seismicity in northern Oklahoma and southern Kansas, as recorded by the National Earthquake Information Center. For 1628 events with well‐constrained depths, we computed a slope for ML–Mc of 0.022±0.010 m.u./km. Finally, we analyzed ML–Mc for 28,722 well‐located earthquakes in Italy, as recorded by the National Institute of Geophysics and Volcanology, and found an ML–Mc slope of 0.018±0.001 m.u./km. In each case, the quoted error bounds represent 95% confidence regions that exclude zero, implying that the depth dependence of ML–Mc is statistically significant. We performed several robustness tests in which we varied the criterion used to define a well‐constrained depth and the depth range used in the linear fit. In nearly all cases, we found a positive slope for ML–Mc versus depth at a confidence level above 95%.

Seismic detection of strong ground motions by MW5.6 North Korean nuclear explosion

The North Korean nuclear explosion test site in Punggye-ri is located in a seismically quiescent region on a stable Precambrian basement. The 3 September 2017 MW5.6 North Korean underground nuclear explosion (UNE) test produced unprecedented strong ground motions. The peak ground accelerations might reach tens to hundreds m/s2 on the surface of the UNE test site, decaying exponentially with distance. Ten shallow events with magnitudes greater than or equal to ML2.5 and source depths less than 3 km followed the 2017 UNE for 5 months in an area with a radius of 15 km from the UNE where strong ground shaking was experienced. The largest event with MW3.7 occurred 20 days after the 2017 UNE test at shallow depths less than 3 km. Its moment tensor solution indicates a combined source behavior with comparable strengths of double-couple and compensated linear vector dipole (CLVD) components, suggesting an unusual event different from typical natural earthquakes in the Korean Peninsula. The clustered shallow seismic events appeared to have occurred in damaged media that were effectively perturbed by the strong ground motions of the UNE.

Relative Moment Tensor Inversion with Application to Shallow Underground Explosions and Earthquakes

Research Article| August 14, 2018 Relative Moment Tensor Inversion with Application to Shallow Underground Explosions and Earthquakes Anastasia Stroujkova Anastasia Stroujkova aWeston Geophysical Corp., 181 Bedford Street Suite 1, Lexington, Massachusetts 02420, ana@westongeo.com Search for other works by this author on: GSW Google Scholar Author and Article Information Anastasia Stroujkova aWeston Geophysical Corp., 181 Bedford Street Suite 1, Lexington, Massachusetts 02420, ana@westongeo.com Publisher: Seismological Society of America First Online: 14 Aug 2018 Online Issn: 1943-3573 Print Issn: 0037-1106 © Seismological Society of America Bulletin of the Seismological Society of America (2018) 108 (5A): 2724–2738. https://doi.org/10.1785/0120180082 Article history First Online: 14 Aug 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Anastasia Stroujkova; Relative Moment Tensor Inversion with Application to Shallow Underground Explosions and Earthquakes. Bulletin of the Seismological Society of America 2018;; 108 (5A): 2724–2738. doi: https://doi.org/10.1785/0120180082 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Single‐source moment tensor (MT) techniques require calculation of the Green’s functions. This limits applicability of the approach to low frequencies because Green’s function calculations are very sensitive to media heterogeneities, especially at high frequencies. Relative moment tensor inversion (RMTI) developed by Dahm (1996) eliminates the need for calculation of the Green’s functions. In this article, the RMTI method is modified to perform MT inversion for a group of closely located shallow seismic events (e.g., underground explosions) by incorporating amplitudes for short‐period fundamental‐mode Rayleigh waves (Rg). The main benefits of the modified method are (1) it can be used in any frequency range provided the signal‐to‐noise ratio is high enough in that range, and (2) the method allows the calculation of the Green’s functions in any frequency range, self‐consistently, after the MT inversion is complete. The robustness of the approach for MT inversion and Green’s function estimation for the small shallow events was tested using synthetic data as well as a small chemical explosion dataset (e.g., Stroujkova et al., 2017). You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The role of asthenospheric flow during rift propagation and breakup

Continental rifting precedes the breakup of continents, leading to the formation of passive margins and oceanic lithosphere. Although rifting dynamics is classically described in terms of either active rifting caused by active mantle upwelling, or passive rifting caused by far-field extensional stresses, it was proposed that a transition from passive to active rifting can result from changes in buoyancy forces due to localized thinning of the lithosphere. Three-dimensional numerical experiments of rifting near an Euler pole allow the quantification of these buoyancy forces and show that gravitational stresses are strong enough not only to sustain rifting and drive axis-parallel motion in the asthenosphere dome, but also to promote along-axis asthenospheric flow and to drive the propagation of the rift tip toward its rotation pole. We show that gradients of gravitational potential energy due to the presence of the dome of asthenosphere induce time-dependent phases of compressional and transcurrent stress regimes, despite an overall divergent plate setting. Our experiments predict overdeepened bathymetry at the tip of the propagating rift, as well as the variability of focal mechanisms of shallow seismic events similar to those observed in such a setting. We also explain the episodes of basin inversion documented in many rifted continental margins.

Causes of earthquake spatial distribution beneath the Izu-Bonin-Mariana Arc

Statistics about the occurrence frequency of earthquakes (1973–2015) at shallow, intermediate and great depths along the Izu-Bonin-Mariana (IBM) Arc is presented and a percent perturbation relative to P-wave mean value (LLNL-G3Dv3) is adopted to show the deep structure. The correlation coefficient between the subduction rate and the frequency of shallow seismic events along the IBM is 0.605, proving that the subduction rate is an important factor for shallow seismic events. The relationship between relief amplitudes of the seafloor and earthquake occurrences implies that some seamount chains riding on the Pacific seafloor may have an effect on intermediate-depth seismic events along the IBM. A probable hypothesis is proposed that the seamounts or surrounding seafloor with high degree of fracture may bring numerous hydrous minerals into the deep and may result in a different thermal structure compared to the seafloor where no seamounts are subducted. Fluids from the seamounts or surrounding seafloor are released to trigger earthquakes at intermediate-depth. Deep events in the northern and southern Mariana arc are likely affected by a horizontal propagating tear parallel to the trench.

A study of the energy released by great (M ≥ 7) deep focus seismic events with allowance for the M w 8.3 earthquake of May 24, 2013 in the Sea of Okhotsk, Russia

The distribution of the hypocenters of great seismic events with M >= 7.0 and, consequently, the seismic energy released in their sources is asymmetric along the Earth radius. According to our estimates, 90% of the energy is released relatively close to the Earth's surface, at the average depth of 50 km. The bulk of the remaining 10% is associated with seismic events that take place at large depths, on average 630-640 km above the boundary between the transition zone and lower mantle. These very deep earthquakes (depth >= 350 km) significantly differ from the shallow events. Their sources, in contrast to the shallow focus events, are located inside the plate. The examination of seven seismic zones described in the present study, except for the Honsu-Kamchatka zone which accommodates both the shallow and deep M >= 7.0 earthquakes, shows that the linear distribution of the hypocenters of deep earthquakes is considerably shorter than that of the shallow earthquakes, which, in turn, determines the length of the seismic zones at depth. In the zones of the Solomon Islands and the Izu-Bonin-Mariana arc, there are no seismic events with M >= 7.0 deeper than 450 km. In the zones of Indonesia, Philippines, Tonga-Kermadec, and Chile-Peru, the mentioned length's shortening at the top of the lower mantle (660-km discontinuity) relative to the length of the zone observed close to the surface is unequivocal. The relationship between the lithospheric plates is supported by the spatial distribution of the hypocenters. The position of the foci of very deep (>= 500 km) earthquakes indicates where the descending lithospheric plates conflict with the upper boundary of the lower mantle, and where in some cases they cross it. This passage generates the compression and elongation inside the slab. A comparison of the time distribution of the shallow and deep seismic events suggests the absence of direct relationship between these two different earthquake activities. For studying the fairly uncommon deep earthquakes, important additional information was provided by the largest of the deep earthquakes, the May 24, 2013 M 8.3 event beneath the Sea of Okhotsk, in an area where significant deep earthquakes have already occurred. Based on our studies of the records provided by the Russian and Hungarian national seismological networks, we concluded that this seismic event was preceded by an earthquake swarm, which consisted of 58 M >= 5 events between May 15 and 24, 2013 in the higher part of the sinking slab east of Kamchatka within the segment of increased historical seismicity. Most probably, the interaction of two distinct active source zones took place. The aftershock activity beneath the Sea of Okhotsk was moderate: thirteen events with magnitudes above De >= 4.0 were observed by June 27, 2013. Nevertheless, the unusually small number of aftershocks determined a fault area (2.64 x 10(4) km(2)), generally similar to that in the case of the assumed shallow M 8.3 event.

3D angle gathers with plane-wave reverse-time migration

Angle domain common image gathers (ADCIGs) from reverse-time migration (RTM) provide new tools for imaging complex geologic structures, such as salt flank or subsalt areas, characterized by multiarrivals. Compared with common image gathers from Kirchoff or Beam migration, the ADCIGs from RTM have the advantage of relying on wave propagation, providing a more reliable input for tomography or amplitude analysis. In practice however, most current wide azimuth surveys (e.g., deep-water regions of the Gulf of Mexico) are acquired with coarsely sampled shot and receiver locations on the surface, which leads to severe angular undersampling. This phenomenon is frequently observed on shallow seismic events in high-resolution 3D ADCIGs from common-shot RTM. In addition, because small offsets are frequently not recorded, seismic events are missing at small incidence angles in angle gathers. We have made a detailed study of the angular sampling issue in 3D angle gathers. We then used plane-wave RTM to generate 3D angle gathers. Plane-wave RTM, with its low-cost and automatic angular interpolation, is a promising solution for improving the quality of 3D angle gathers.

Mechanical and statistical evidence of the causality of human-made mass shifts on the Earth’s upper crust and the occurrence of earthquakes

A global catalog of small- to large-sized earthquakes was systematically analyzed to identify causality and correlatives between human-made mass shifts in the upper Earth’s crust and the occurrence of earthquakes. The mass shifts, ranging between 1 kt and 1 Tt, result from large-scale geoengineering operations, including mining, water reservoirs, hydrocarbon production, fluid injection/extractions, deep geothermal energy production and coastal management. This article shows evidence that geomechanical relationships exist with statistical significance between (a) seismic moment magnitudes M of observed earthquakes, (b) lateral distances of the earthquake hypocenters to the geoengineering “operation points” and (c) mass removals or accumulations on the Earth’s crust. Statistical findings depend on uncertainties, in particular, of source parameter estimations of seismic events before instrumental recoding. Statistical observations, however, indicate that every second, seismic event tends to occur after a decade. The chance of an earthquake to nucleate after 2 or 20 years near an area with a significant mass shift is 25 or 75 %, respectively. Moreover, causative effects of seismic activities highly depend on the tectonic stress regime in which the operations take place (i.e., extensive, transverse or compressive). Results are summarized as follows: First, seismic moment magnitudes increase the more mass is locally shifted on the Earth’s crust. Second, seismic moment magnitudes increase the larger the area in the crust is geomechanically polluted. Third, reverse faults tend to be more trigger-sensitive than normal faults due to a stronger alteration of the minimum vertical principal stress component. Pure strike-slip faults seem to rupture randomly and independently from the magnitude of the mass changes. Finally, mainly due to high estimation uncertainties of source parameters and, in particular, of shallow seismic events ( M6) seem to be triggered. The rupture propagation of triggered events might be dominated by pre-existing tectonic stress conditions.

Attenuation of shallow microearthquakes at Abu Simbel area, south Egypt

In the last few years Abu Simbel area received a special attention from the Egyptian government from the tourism point of view as well as the construction of new projects, cities and cultivation of more new areas. As this area is seismically active we decided to carry out studies on attenuation of high-frequency seismic waves at this area. The quality factor (Q) of P-wave was calculated using the Spectral ratio known as the Single-Station method. A selected data set of 14 shallow seismic events recorded by a local network operated around the area was used to calculate QP for four stations. Magnitude of these earthquakes varied between 1.7 and 3.6. The results show variations in QP as a function of frequency according to the power law Q=Q0fn, with frequency dependence coefficient n ranging between 0.4 and 1.2. Strong azimuthal variations of QP were found. It is proposed that this variation indicates presence of local strong lateral heterogeneities and/or fluid filled cracks.

Detection and modeling of Rayleigh wave induced patterns in the ionosphere

[1] Global Positioning System (GPS) allows the detection of ionospheric disturbances associated with the vertical displacements of most of the major shallow seismic events. We describe a method to model the time and space distributions of Rayleigh wave induced total electron content (TEC) patterns detected by a dense GPS array. We highlight the conditions for which a part of the ionospheric pattern can be directly measured, at teleseismic distance and above the epicenter. In particular, a satellite elevation angle lower than 40° is a favorable condition to detect Rayleigh wave induced ionospheric waves. The coupling between the solid Earth and its atmosphere is modeled by computing the normal modes of the solid Earth–atmosphere system. We show the dependency of the coupling efficiency on various atmospheric conditions. By summation of the normal modes we model the atmospheric perturbation triggered by a given earthquake. This shows that a part of the observation is a Rayleigh-induced radiation pattern and therefore characteristic of the seismic rupture. Through atmosphere-ionosphere coupling, we model the ionospheric perturbation. After the description of the local geomagnetic field anisotropic effects, we show how the observation geometry is strongly affecting the radiation pattern. This study deals with the related data for two earthquakes with far-field and near-field observations using the Japanese GPS network GEONET: after the 12 May 2008 Wenchuan earthquake (China) and after the 25 September 2003 Tokachi-Oki earthquake (Japan), respectively. Waveforms and patterns are compared with the observed TEC perturbations, providing a new step toward the use of ionospheric data in seismological applications.

Constraining Event Depths and Crustal Velocities Using Regional Depth Phases

A method for determining average crustal seismic velocities above a cluster of shallow seismic events using depth phases observed at regional and teleseismic arrays has been developed. The method uses travel-time differences between the direct and the surface-reflected phases to simultaneously find the event depths and the best-fitting seismic velocities in the hypocentral area. Simplicity of the method makes it suitable for rapid data analysis as it requires a relatively small amount of data and may provide useful information about the seismic velocities in remote regions with shallow seismicity. However, the method requires accurate depth phase identification, which is often a difficult task. The technique has been applied to find above-source seismic velocities and depths of 14 clustered events from the Xiuyan (1999–2000) sequence in China. The estimated depths of the events are between 5 and 8 km. The obtained velocities are ![Graphic][1] and ![Graphic][2] , which corresponds to a slightly elevated V P / V S ratio of 1.86±0.12 as compared to the value predicted from IASP91 ( V P / V S =1.72; see Kennett and Engdahl, 1991). [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif