Rock Structure Explains Slow Seismic Waves

Analysis of Stress Drop Variations in Fault and Subduction Zones of Maluku and Halmahera Earthquakes in 2019

Jurnal Penelitian Fisika dan Aplikasinya (JPFA) ◽

10.26740/jpfa.v9n2.p152-162 ◽

2019 ◽

Vol 9 (2) ◽

pp. 152

Author(s):

Rahmat Setyo Yuliatmoko ◽

Telly Kurniawan

Keyword(s):

Subduction Zone ◽

Stress Drop ◽

Subduction Zones ◽

Slip Rate ◽

Disaster Mitigation ◽

Mitigation Measures ◽

Reverse Fault ◽

Nonlinear Inversion ◽

Rock Structure ◽

Geological Conditions

The amount of stress released by an earthquake can be calculated with a stress drop, the stress ratio before and after an earthquake where the stress accumulated in a fault or a subduction zone is immediately released during an earthquake. The purpose of this research is to calculate the amount of stress drop in faults and subduction in Maluku and Halmahera and their variations and relate them to the geological conditions in the area so that the tectonic characteristics in the area can be identified. This research employed mathematical analysis and the Nelder Mead Simplex nonlinear inversion methods. The results show that Maluku and Halmahera are the area with complex tectonic conditions and large earthquake impacts. The Maluku sea earthquake generated a stress drop of 0.81 MPa with a reverse fault mechanism in the zone of subduction, while for the Halmahera earthquake the stress drop value was 52.72 MPa, a typical strike-slip mechanism in the fault zone. It can be concluded that there is a difference in the stress drop between the subduction and fault zones; the stress drop in the fault was greater than that in the subduction zone due to different rock structure and faulting mechanisms as well as differences in the move slip rate that plays a role in the process of holding out the stress on a rock. This information is very important to know the amount of pressure released from the earthquake which has a very large impact as part of disaster mitigation measures.

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Crosscoherence-based interferometry for the retrieval of first arrivals and subsequent tomographic imaging of differential weathering

Geophysics ◽

10.1190/geo2018-0405.1 ◽

2019 ◽

Vol 84 (4) ◽

pp. Q37-Q48 ◽

Cited By ~ 3

Author(s):

Joachim Place ◽

Deyan Draganov ◽

Alireza Malehmir ◽

Christopher Juhlin ◽

Chris Wijns

Keyword(s):

Seismic Waves ◽

Seismic Velocity ◽

Signal To Noise Ratio ◽

Seismic Profile ◽

Seismic Interferometry ◽

Near Surface ◽

Tomographic Images ◽

Traveltime Tomography ◽

Seismic Methods ◽

Source Signal

Exhumation of crust exposes rocks to weathering agents that weaken the rocks’ mechanical strength. Weakened rocks will have lower seismic velocity than intact rocks and can therefore be mapped using seismic methods. However, if the rocks are heavily weathered, they will attenuate controlled-source seismic waves to such a degree that the recorded wavefield would become dominated by ambient noise and/or surface waves. Therefore, we have examined the structure of differential weathering by first-break traveltime tomography over a seismic profile extending approximately 3.5 km and acquired at a mining site in Zambia using explosive sources and a source based on the swept-impact seismic technique (SIST). Seismic interferometry has been tested for the retrieval of supervirtual first arrivals masked by uncorrelated noise. However, use of crosscorrelation in the retrieval process makes the method vulnerable to changes in the source signal (explosives and SIST). Thus, we have developed a crosscoherence-based seismic-interferometry method to tackle this shortcoming. We investigate the method’s efficiency in retrieving first arrivals and, simultaneously, correctly handling variations in the source signal. Our results illustrate the superiority of the crosscoherence- over crosscorrelation-based method for retrieval of the first arrivals, especially in alleviating spurious ringyness and in terms of the signal-to-noise ratio. These benefits are observable in the greater penetration depth and the improved resolution of the tomography sections. The tomographic images indicate isolated bodies of higher velocities, which may be interpreted as fresh rocks embedded into a heavily weathered regolith, providing a conspicuous example of differential weathering. Our study advances the potential of seismic methods for providing better images of the near surface (the critical zone).

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Ambient lower mantle structure and composition inferred from seismic tomography, convection models, and geochemistry.

10.5194/egusphere-egu2020-11806 ◽

2020 ◽

Author(s):

Grace E. Shephard ◽

John Hernlund ◽

Christine Houser ◽

Reidar Trønnes ◽

Fabio Crameri

Keyword(s):

Lower Mantle ◽

Subduction Zones ◽

Large Scale ◽

Seismic Velocity ◽

Seismic Signal ◽

High Viscosity ◽

S Wave ◽

Velocity Models ◽

Reference Velocity ◽

Seismic Anomalies

The lower mantle can be grouped into high, low, and average (i.e., ambient) seismic velocity domains at each depth, based on the amplitude and polarity of wavespeed perturbations (% &#948;lnVs, % &#948;lnVp). Many studies focus on elucidating the thermo-chemical and structural origins of fast and slow domains, in particular. Subducted slabs are associated with fast seismic anomalies throughout the mantle, and reconstructed palaeo-positions of Cenozoic to Mesozoic subduction zones agrees with seismically imaged deep slabs. Conversely, slow wavespeed domains account for the two antipodal LLSVPs in the lowermost mantle, which are potentially long-lived features, as well as rising hot mantle above the LLSVPs and discrete mantle plumes. However, low-amplitude wavespeeds (close to the reference velocity models) are often overlooked By comparing multiple P- and S-wave tomographic models individually, and through &#8220;vote maps&#8221;, we reveal the depth-dependent characteristics and the geometry of ambient structures, and compare them to numerical convection models. The ambient velocity domains may contain early refractory and bridgmantic mantle with elevated Si/(Mg+Fe) and Mg/Fe ratios (BEAMS; bridgmanite-enriched mantle structures). They could have formed by early basal magma ocean (BMO) fractionation during a period of core-BMO exchange of SiO2 (from core to BMO) and FeO (from BMO to core), or represent cumulates of BMO crystallization with bridgmanite as the liquidus phase. The high viscosity of bridgmanitic material may promote its convective aggregation and stabilise the large-scale, degree-2 convection pattern. Despite its high viscosity, bridgmanitic material, representing a primitive and refractory reservoir for primordial-like He and Ne components, might be entrained in vigorous, deep-rooted plumes. The restriction of a weak seismic signal, ascribed to iron spin-pairing in ferropericlase, to the fast and slow domains, supports the notion that the ambient lower mantle domains are bridgmanitic.

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Bringing slow slip processes into focus

10.5194/egusphere-egu2020-18869 ◽

2020 ◽

Author(s):

Rebecca Bell

Keyword(s):

High Resolution ◽

Subduction Zones ◽

Seismic Velocity ◽

Numerical Models ◽

Fluid Pressure ◽

Geophysical Methods ◽

Rock Properties ◽

Slow Slip ◽

Velocity Models ◽

The North

The discovery of slow slip events (SSEs) at subduction margins in the last two decades has changed our understanding of how stress is released at subduction zones. Fault slip is now viewed as a continuum of different slip modes between regular earthquakes and aseismic creep, and an appreciation of seismic hazard can only be realised by understanding the full spectrum of slip. SSEs may have the potential to trigger destructive earthquakes and tsunami on faults nearby, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. Laboratory and numerical models suggest that slow slip can be spontaneously generated under conditions of very low effective stresses, facilitated by high pore fluid pressure, but it has also been suggested that variations in frictional behaviour, potentially caused by very heterogeneous fault zone lithology, may be required to promote slow slip.Testing these hypotheses is difficult as it requires resolving rock properties at a high resolution many km below the seabed sometimes in km&#8217;s of water, where drilling is technically challenging and expensive. Traditional geophysical methods like travel-time tomography cannot provide fine-scale enough velocity models to probe the rock properties in fault zones specifically. In the last decade, however, computational power has improved to the point where 3D full-waveform inversion (FWI) methods make it possible to use the full wavefield rather than just travel times to produce seismic velocity models with a resolution an order of magnitude better than conventional models. Although the hydrocarbon industry have demonstrated many successful examples of 3D FWI the method requires extremely high density arrays of instruments, very different to the 2D transect data collection style which is still commonly employed at subduction zones.&#160;The north Hikurangi subduction zone, New Zealand is special, as it hosts the world&#8217;s most well characterised shallow SSEs (<2 km to 15 km below the seabed).&#160; This makes it an ideal location to collect 3D data optimally for FWI to resolve rock properties in the slow slip zone. In 2017-2018 an unprecedentedly large 3D experiment including 3D multi-channel seismic reflection, 99 ocean bottom seismometers and 194 onshore seismometers was conducted along the north Hikurangi margin in an 100 km x 15 km area, with an average 2 km instrument spacing. In addition, IODP Expeditions 372 and 375 collected logging-while drilling and core data, and deployed two bore-hole observatories to target slow slip in the same area. In this presentation I will introduce you to this world class 3D dataset and preliminary results, which will enable high resolution 3D models of physical properties to be made to bring slow slip processes into focus. &#160;

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Recent advances in rock physics

Geophysics ◽

10.1190/1.1441879 ◽

1985 ◽

Vol 50 (12) ◽

pp. 2480-2491 ◽

Cited By ~ 26

Author(s):

David P. Yale

Keyword(s):

Electrical Properties ◽

Seismic Waves ◽

Seismic Velocity ◽

Pore Space ◽

Rock Physics ◽

Rock Properties ◽

Bright Spot ◽

Geometrical Parameters ◽

S Wave ◽

Velocity Models

The need to extract more information about the subsurface from geophysical and petrophysical measurements has led to a great interest in the study of the effect of rock and fluid properties on geophysical and petrophysical measurements. Rock physics research in the last few years has been concerned with studying the effect of lithology, fluids, pore geometry, and fractures on velocity; the mechanisms of attenuation of seismic waves; the effect of anisotropy; and the electrical and dielectric properties of rocks. Understanding the interrelationships between rock properties and their expression in geophysical and petrophysical data is necessary to integrate geophysical, petrophysical, and engineering data for the enhanced exploration and characterization of petroleum reservoirs. The use of amplitude offsets, S‐wave seismic data, and full‐waveform sonic data will help in the discrimination of lithology. The effect of in situ temperatures and pressures must be taken into account, especially in fractured and unconsolidated reservoirs. Fluids have a strong effect on seismic velocities, through their compressibility, density, and chemical effects on grain and clay surfaces. S‐wave measurements should help in bright spot analysis for gas reservoirs, but theoretical considerations still show that a deep, consolidated reservoir will not have any appreciable impedance contrast due to gas. The attenuation of seismic waves has received a great deal of attention recently. The idea that Q is independent of frequency has been challenged by experimental and theoretical findings of large peaks in attenuation in the low kHz and hundreds of kHz regions. The attenuation is thought to be due to fluid‐flow mechanisms and theories suggest that there may be large attenuation due to small amounts of gas in the pore space even at seismic frequencies. Models of the effect of pores, cracks, and fractures on seismic velocity have also been studied. The thin‐crack velocity models appear to be better suited for representing fractures than pores. The anisotropy of seismic waves, especially the splitting of polarized S‐waves, may be diagnostic of sets of oriented fractures in the crust. The electrical properties of rocks are strongly dependent upon the frequency of the energy and logging is presently being done at various frequencies. The effects of frequency, fluid salinity, clays, and pore‐grain geometry on electrical properties have been studied. Models of porous media have been used extensively to study the electrical and elastic properties of rocks. There has been great interest in extracting geometrical parameters about the rock and pore space directly from microscopic observation. Other models have focused on modeling several different properties to find relationships between rock properties.

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SEISMIC TOMOGRAPHY USING METROPOLIS METHOD OF VELOCITY FIELDS PARAMETERIZED BY HAAR WAVELET SERIES

Brazilian Journal of Geophysics ◽

10.22564/rbgf.v34i2.812 ◽

2016 ◽

Vol 34 (2) ◽

Author(s):

Alexsandro Guerra Cerqueira ◽

Wilson Mouzer Figueiró ◽

Paulo Eduardo Miranda Cunha

Keyword(s):

Velocity Field ◽

Seismic Tomography ◽

Seismic Waves ◽

Seismic Velocity ◽

Haar Wavelet ◽

Velocity Fields ◽

Metropolis Method ◽

Geological Models ◽

Qualitative Models ◽

Wavelet Series

ABSTRACT. The representation of compressional seismic waves velocity ﬁelds from geological models through numerical parameters has a strong geophysical importance, because, it makes possible to quantify such qualitative models, allowing its mathematical manipulation. In this way, the parameterization by Haar wavelet series may be seen as an attractive alternative.Keywords: parameterization, Haar wavelet series, pyramid algorithm, seismic tomography, seismic velocity ﬁeld, traveltime data, Metropolis method. RESUMO. A representação de campos de velocidades sísmicas compressionais, através de parâmetros numéricos, é de importância básica na geofísica, pois torna possível a quantiﬁcação de modelos, antes qualitativos, permitindo assim que sejam matematicamente manipulados. A parametrização por série ondaleta Haar pode ser vista como uma alternativa atrativa para quantiﬁcar tais modelos...Palavras-chave: parametrização, série ondaleta Haar, inversão sísmica tomográﬁca, campo de velocidade sísmica, dados de tempo de trânsito, método Metropolis.

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The temperature dependence of seismic waves in ice

Journal of Glaciology ◽

10.1017/s0022143000023467 ◽

1974 ◽

Vol 13 (67) ◽

pp. 144-147 ◽

Cited By ~ 73

Author(s):

Heinz Kohnen

Keyword(s):

Temperature Dependence ◽

Seismic Waves ◽

Seismic Velocity ◽

Seismic Velocities ◽

Velocity Data

In this paper all available seismic velocity data from Greenland and Antarctica are used to determine the relation between seismic velocities and temperatures in the ice.

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Evaluation of Excavation-Damaged Zone around Underground Tunnels by Theoretical Calculation and Field Test Methods

Energies ◽

10.3390/en12091682 ◽

2019 ◽

Vol 12 (9) ◽

pp. 1682 ◽

Cited By ~ 5

Author(s):

Ming Tao ◽

Zhixian Hong ◽

Kang Peng ◽

Pengwei Sun ◽

Mingyu Cao ◽

...

Keyword(s):

Seismic Waves ◽

Seismic Velocity ◽

Underground Mining ◽

Hydrostatic Stress ◽

Friction Angle ◽

Geological Strength Index ◽

Excavation Damaged Zone ◽

Disturbance Factor ◽

Damaged Zone

Excavation-damaged zones (EDZs) induced in underground mining and civil engineering potentially threaten tunnel safety and stability, and increase construction and support costs. In this paper, an investigation of the excavation damaged zone (EDZ) around roadways in Fankou lead-zinc mine in Guangzhou, China is performed by applying a seismic velocity method accompanied by SET-PLT-01 nonmetallic ultrasonic detector. Meanwhile, the in situ stress in the mining area was measured based on the stress relief method with the Swedish high-precision LUT system. The results indicate that the stress field is dominated by the maximum horizontal tectonic stress, and the extents of the EDZ on the roof-floor region are greater than that on the sidewall. In addition, both of the in situ stresses and EDZs show an increasing trend with an increase of depth. Analytical solutions of EDZ around circular openings in the brittle rock mass subjected to non-hydrostatic stress fields are presented in terms of the Mohr–Coulomb and generalized Hoek–Brown criteria, and validated by several cases mentioned above. The extents of EDZ solved by closed-form solutions were found to be in a great agreement with those obtained in the field. Finally, a series of parametric studies are conducted to investigate the effects of cohesion (c), friction angle (φ), geological strength index (GSI), mi, uniaxial compressive strength (σc), and disturbance factor (D) on EDZ. It is shown that the effects of c, φ, GSI, and σc are significant; however, more attention should be paid to consider the dynamic disturbances induced by mechanical drilling, blasting, and seismic waves in tunnel excavations or operations.

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Seismic velocity recovery following the 2015 Mw 7.8 Gorkha earthquake, Nepal: Towards a coupled vision of damage and hydrological-induced velocity variations

10.5194/egusphere-egu21-12714 ◽

2021 ◽

Author(s):

Luc Illien ◽

Christoph Sens-Schönfelder ◽

Christoff Andermann ◽

Odin Marc ◽

Kristen Cook ◽

...

Keyword(s):

Seismic Waves ◽

Seismic Velocity ◽

Monsoon Season ◽

Seismic Interferometry ◽

Linear Superposition ◽

Gorkha Earthquake ◽

Ground Shaking ◽

Velocity Variations ◽

Velocity Changes ◽

Induced Velocity

Following the passage of seismic waves, most geomaterials experience non-linear mesoscopic elasticity (NLME). This is described by a drop in elastic moduli that precedes a subsequent recovery of physical properties over a relaxation timescale. Thanks to the development of seismic interferometry techniques that allows for the continuous monitoring of relative seismic velocity changes &#948;v in the subsurface, observations of NLME (&#948;vNLME) in the field are now numerous. In parallel, a growing community uses seismic interferometry to monitor velocity changes induced by seasonal hydrological variations (&#948;vhydro). Monitoring of these variations are often independently done and a linear superposition of both effects is mostly assumed when decomposing the observed &#948;v signal (&#948;v =&#160; &#948;vNLME + &#948;vhydro). However, transient hydrological behaviour following co-seismic ground shaking has been widely reported in boreholes measurements and streamflow, which suggests that&#160; &#948;vhydro may be impacted by the transient variation of material properties caused by NLME. In this presentation, we attempt to characterize the relative seismic velocity variations &#948;v retrieved from a small dense seismic array in Nepal that was deployed in the aftermath of the &#160;2015 Mw 7.8 Gorkha earthquake and that is prone to highly variable hydrological conditions. We first investigated the effect of aftershocks in computing &#948;v at a 10-minute resolution centered around significant ground shaking events. After correcting &#948;v for NLME caused by the Gorkha earthquake and its subsequent aftershocks, we test whether the corresponding residuals are in agreement with the background hydrological behaviour which we inferred from a calibrated hydrological model. This is not the case and we find that transient hydrological properties improve the data description in the early phase after the mainshock. We report three distinct relaxation time scales that are relevant for the recovery of seismic velocity at our field site:&#160; 1. A long time scale activated by the main shock of the Gorkha earthquake (~1 year) 2. A relatively short timescale (1-3 days) that occurs after moderate aftershocks. 3. An intermediate timescale (4-6 months) during the 2015 monsoon season that corresponds to the recovery of the hydrological system. This timescale could correspond to an enhanced permeability caused by Gorkha ground shaking. Our study demonstrates the capability of seismic interferometry to monitor transient hydrological properties after earthquakes at a spatial scale that is not available with classical hydrological measurements. This investigation demands calibrated hydrological models and a framework in which the different forcing of &#948;v are coupled.

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Monitoring transient changes within overpressured regions of subduction zones using ambient seismic noise

Science Advances ◽

10.1126/sciadv.1501289 ◽

2016 ◽

Vol 2 (1) ◽

pp. e1501289 ◽

Cited By ~ 23

Author(s):

Esteban J. Chaves ◽

Susan Y. Schwartz

Keyword(s):

Seismic Noise ◽

Subduction Zones ◽

Seismic Velocity ◽

Fluid Pressure ◽

Pore Fluid ◽

Ambient Seismic Noise ◽

Pore Fluid Pressure ◽

Near Surface ◽

Velocity Reduction ◽

Pressurized Fluids

In subduction zones, elevated pore fluid pressure, generally linked to metamorphic dehydration reactions, has a profound influence on the mechanical behavior of the plate interface and forearc crust through its control on effective stress. We use seismic noise–based monitoring to characterize seismic velocity variations following the 2012 Nicoya Peninsula, Costa Rica earthquake [Mw(moment magnitude) 7.6] that we attribute to the presence of pressurized pore fluids. Our study reveals a strong velocity reduction (~0.6%) in a region where previous work identified high forearc pore fluid pressure. The depth of this velocity reduction is constrained to be below 5 km and therefore not the result of near-surface damage due to strong ground motions; rather, we posit that it is caused by fracturing of the fluid-pressurized weakened crust due to dynamic stresses. Although pressurized fluids have been implicated in causing coseismic velocity reductions beneath the Japanese volcanic arc, this is the first report of a similar phenomenon in a subduction zone setting. It demonstrates the potential to identify pressurized fluids in subduction zones using temporal variations of seismic velocity inferred from ambient seismic noise correlations.

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