High Seismic Wave Attenuation Research Articles

Dynamic Component of the Asthenosphere: Lateral Viscosity Variations Due To Dislocation Creep at the Base of Oceanic Plates

AbstractThe asthenosphere is commonly defined as an upper mantle zone with low velocities and high attenuation of seismic waves, and high electrical conductivity. These observations are usually explained by the presence of partial melt, or by a sharp contrast in the water content of the upper mantle. Low viscosity asthenosphere is an essential ingredient of functioning plate tectonics. We argue that a substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates. Numerical simulations of subduction show that dynamic weakening scales with the surface velocity both below the subducting and the overriding plate, and that the viscosity decrease reaches up to two orders of magnitude. The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations (LVV) below Earth's oceans. The obtained LVV help in explaining some of the long‐standing as well as recent problems in mantle viscosity inversions.

Evaluation of Lake Sediment Thickness from Water-Borne Electrical Resistivity Tomography Data.

Lakes are integrators of past climate and ecological change. This information is stored in the sediment record at the lake bottom, and to make it available for paleoclimate research, potential target sites with undisturbed and continuous sediment sequences need to be identified. Different geophysical methods are suitable to identify, explore, and characterize sediment layers prior to sediment core recovery. Due to the high resolution, reflection seismic methods have become standard for this purpose. However, seismic measurements cannot always provide a comprehensive image of lake-bottom sediments, e.g., due to lacking seismic contrasts between geological units or high attenuation of seismic waves. Here, we developed and tested a complementary method based on water-borne electrical-resistivity tomography (ERT) measurements. Our setup consisted of 13 floating electrodes (at 5 m spacing) used to collect ERT data with a dipole–dipole configuration. We used a 1D inversion to adjust a layered-earth model, which facilitates the implementation of constraints on water depth, water resistivity, and sediment resistivity as a priori information. The first two parameters were readily obtained from the echo-sounder and conductivity-probe measurements. The resistivity of sediment samples can also be determined in the laboratory. We applied this approach to process ERT data collected on a lake in southern Mexico. The direct comparison of ERT data with reflection seismic data collected with a sub-bottom profiler (SBP) showed that we can significantly improve the sediment-thickness estimates compared to unconstrained 2D inversions. Down to water depths of 20 m, our sediment thickness estimates were close to the sediment thickness derived from collocated SBP seismograms. Our approach represents an implementation of ERT measurements on lakes and complements the standard lake-bottom exploration by reflection seismic methods.

Data set | Evaluation of Lake Sediment Thickness from Water-Borne Electrical Resistivity Tomography Data

Lakes are integrators of past climate and ecological change. This information is stored in the sediment record at the lake bottom, and to make it available for paleoclimate research, potential target sites with undisturbed and continuous sediment sequences need to be identified. Different geophysical methods are suitable to identify, explore, and characterize sediment layers prior to sediment core recovery. Due to the high resolution, reflection seismic methods have become standard for this purpose. However, seismic measurements cannot always provide a comprehensive image of lake-bottom sediments, e.g., due to lacking seismic contrasts between geological units or high attenuation of seismic waves. Here, we developed and tested a complementary method based on water-borne electrical-resistivity tomography (ERT) measurements. Our setup consisted of 13 floating electrodes (at 5 m spacing) used to collect ERT data with a dipole–dipole configuration. We used a 1D inversion to adjust a layered-earth model, which facilitates the implementation of constraints on water depth, water resistivity, and sediment resistivity as a priori information. The first two parameters were readily obtained from the echo-sounder and conductivity-probe measurements. The resistivity of sediment samples can also be determined in the laboratory. We applied this approach to process ERT data collected on a lake in southern Mexico. The direct comparison of ERT data with reflection seismic data collected with a sub-bottom profiler (SBP) showed that we can significantly improve the sediment-thickness estimates compared to unconstrained 2D inversions. Down to water depths of 20 m, our sediment thickness estimates were close to the sediment thickness derived from collocated SBP seismograms. Our approach represents an implementation of ERT measurements on lakes and complements the standard lake-bottom exploration by reflection seismic methods.

Estimation of Seismic Attenuation of the Greenland Ice Sheet Using 3‐D Waveform Modeling

AbstractWe estimated the seismic attenuation (Q factor) of the Greenland Ice Sheet (GrIS) by comparing observed and theoretical Rayleigh waveforms. Observed waveforms are obtained by interfering noise waveforms in vertical‐component seismograms between stations belonging to the latest broadband seismic network distributed throughout Greenland (GLISN network). Theoretical waveforms are calculated by parallel computation using the latest 3‐D seismic waveform modeling method. Comparing the observed waveforms with the theoretical waveforms at different Q factors reveals that the GrIS has a low Q of 10 ≤ QP, QS ≤ 50, indicating very high attenuation of seismic waves due to the ice. This study is the first to establish the low Q factor of ice sheets via ultra‐long‐distance propagation (350−1,000 km). The Q factors obtained in this study are indispensable for estimating the thermal state and density distribution of the GrIS, as well as for interpreting the characteristics of seismic waveform that propagates through the GrIS.

New data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system

The investigation data on seismic wave attenuation in the lithosphere and upper mantle of the northeastern flank of the Baikal rift system obtained with a seismic coda envelope and sliding window are considered. Eleven local districts were described by one-dimensional attenuation models characterized by alternation of high and low attenuation layers, which are consistent with the results obtained previously by Yu.F. Kopnichev for the southwestern flank of the Baikal rift system [9]. The subcrust of the lithosphere contains a thin layer with high attenuation of seismic waves likely related to higher heterogeneity (fragmentation) and occurrence of fluids. The lithosphere basement depth varies from 100–120 km in the west within the Baikal folded area to 120–140 km in the east within the Siberian Platform. It is concluded that there are two asthenosphere layers. Based on specific features of the lithosphere and upper mantle structure, it can be assumed that they were subject to gradual modification involving fluidization processes and partial melting in the Late Cenozoic extension under the influence of distant tectogenesis sources.

Crustal flow pattern beneath the Tibetan Plateau constrained by regional Lg-wave Q tomography

As a prominent geophysical anomaly, unusually high seismic wave attenuation is observed in the crust and upper mantle of the Tibetan Plateau, particularly along its northern area. Theoretical and laboratory investigations show that the strong seismic attenuation can indicate high temperatures and partial melting, which may decrease the viscosity of the material and cause it to flow. Thus, seismic attenuation distribution may provide useful constraints to the crust flows if they exist. Using Lg-wave Q tomography, we construct a 0.05–10.0 Hz broadband high-resolution crust attenuation model for the Tibetan Plateau and its surrounding regions. The maximum spatial resolution is approximately 1.0°×1.0° in well-covered areas and for frequencies between 0.05 and 1.5 Hz. This attenuation model reveals an apparent low-QLg belt stretching along the northern and eastern Tibetan plateau. Combining the Lg-wave Q model with other geophysical data, two possible crust flow channels are found in the Tibetan Plateau. The main flow channel is from north to east and then turns to southeastern Tibet along the western edge of the rigid Sichuan basin, while a second channel starts from southern Tibet and crosses the Eastern Himalayan syntaxis.

A Study of Lg Coda Attenuation beneath North China: Seismic Imaging of Lg Coda Q0

AbstractBy using 560 Lg coda Q0 values measured on propagating paths, the Q0 image in the crust beneath the North China region is restructured with a stochastic modeling and the singular value decomposition. The result indicates that there is remarkable lateral heterogeneity on Lg coda Q0 distribution in the crust beneath the North China region, similar to the velocity image, and closely related with the known surface geological features: the high Q0 areas exist beneath the Taihang and Yanshan mountains and the Luxi uplift area, and low Q0 values appear below the Jizhong, Huanghua and Jiyang depressions and the plain areas. There is a clear division between the high and low Q0 values. Compared with the heat flow values in North China, most the low Q0 value areas are located in the high heat flow areas. It is possibly related with hot material activity in the crust below these areas. Although there is a clear difference between the high and low Q0 value areas in the crust of North China, as a whole North China belongs to low Q0 value abnormal regions. It means that the region is a high seismic wave attenuation area. Probably it is resulted from the interaction between the Eurasia and Pacific plates, which causes crust thinning and hot material upwelling.

The IMS Belbasi Seismic Array (BRAR) in Central Turkey

In this paper we give detailed station specifications as well as a preliminary study of the S -wave velocity structure beneath the Belbasi seismic array. The Belbasi seismic array is one of the first seismic arrays in the world; it was established in 1951 in the Belbasi suburb of the city of Ankara in central Turkey. This array had been jointly operated by the Turkish General Command (TGC) and the Air Force Technical Application Center (AFTAC) of the USA until February 2000. Recently, Bogazici University Kandilli Observatory and Earthquake Research Institute (KOERI) took over the operation of this array. In the near future, the entire array will be relocated to the town of Keskin, about 120 km east of the present location. This new array will be a primary seismic station of the International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO). In addition to this short-period array in Keskin, a new long-period array and one broadband station will be deployed around the city of Ankara. Despite its new location, the entire array, including broadband, short-period, and new long-period stations, will still be named the Belbasi array but with the station code of BRTR. Turkey, where the array is located, and the surrounding regions are seismically active and rapidly deforming and are characterized by high seismic wave attenuation in the crust ( Lg ) and uppermost mantle ( Sn ). The high attenuation of the regional seismic waves and the relatively sparse IMS station distribution in the region combine to lessen the detection capabilities in the area. This makes it essential that effective seismic arrays are deployed in the Middle East. Furthermore, the event location capabilities of the IMS have been selected as one of the major research priorities by the Preparatory Commission for the CTBTO. Data collected …

Evidence of low Q below Popocatépetl Volcano, and its implication to seismic hazard in Mexico City

Seismograms recorded in Mexico City reveal that the amplitudes of seismic waves whose wavepaths pass below Popocatépetl, presently an volcano, before reaching the city are diminished by a factor of about one‐third at frequencies greater than 1 Hz as compared to those which do not cross the volcano. The high attenuation of seismic waves below the volcano may be attributed to the presence of magma and partial melting of rocks. Q of shear waves below the volcano is roughly estimated as 60. A consequence of the large attenuation of high‐frequency seismic waves is a decrease in the seismic hazard to low‐rise buildings in Mexico City from intraplate earthquakes whose wavepaths cross the volcano.

Estimation of attenuation structure and local earthquake magnitude based on acceleration records in Greece

Strong-motion accelerograms are used for the estimation of attenuation coefficient in the upper crust in Greece, on the assumption that direct S-waves are responsible for the strong near-source motion. This coefficient was then used to derive a calibration formula for the estimation of local earthquake magnitude for epicentral distances Δ < 130 km and 4.0 ≤ M L ≤ 6.1. Q s values are calculated by the spectral decay of the log spectrum of the accelerograms. The resulting values exhibit the high attenuation of seismic waves in Greece and indicate the existence of a highly attenuative surface “sedimentary” layer which strongly controls the total wave attenuation.

Microseismicity in the middle Okinawa trough

We conducted a microearthquake survey with ocean bottom seismometers (OBS) in the middle Okinawa trough in 1984. During about one month of operation, very active microseismicity was observed. The microearthquake activity was characterized by the ceaseless occurrence of numerous small earthquakes in the vicinity of OBS stations. This activity was recorded by almost every OBS station but not by land‐based instruments. Swarm activities also occurred at many places. Average activity levels of earthquakes whose durations are longer than 30 s, recorded at individual OBSs, are from 4 to 20 events per day, which seem higher than those of ordinary mid‐oceanic ridge‐fracture zone systems and other back arc basins. Apparently, the number of smaller size earthquakes is fewer than expected. This might indicate the rather high attenuation of seismic waves or low‐Q of the crust around OBS stations, which makes it difficult to detect small size events at a distant range. Attenuation of seismic waves was large beyond the epicentral distance of about 120 km. This seems to indicate low‐Q in the lower crust. Also, a particularly low‐Q area was found at the northern most part of the research area where high heat flow measurements were obtained. This suggests that igneous activity reaches to the surface in that vicinity from the bottom of the crust where magmatic activity has been implied in other studies. At least two types of swarm activities are indicated for this area. Some have no dominant events, show high “b” value properties, and seem to be volcanic. Others have dominant events and appear to be tectonic in nature. Among the swarm activity, two kinds of signals closely resembling volcanic tremors were observed. From these features, we conclude that the crust of this area is highly fractured and subject to intense tectonic activities, as well as some kind of igneous processes.

Seismic wave attenuation in the upper Mantle beneath the Aegean region

A detailed study of P and S waves from earthquakes located along the Hellenic arc and recorded by the Greek seismological network, shows an abnormal distribution of the seismic waves in the central Aegean Sea. The data indicate a zone of anomalously high seismic wave attenuation in the Upper Mantle beneath the Cyclades plateau and the inner part of the volcanic arc. Several independent observations suggest the presence of magma beneath the Cyclades massif. In addition, geophysical data indicate the presence of low-density and rigidity material. Theoretical consideration of the propagation of elastic waves corroborate the observed absence of shear waves.

OCEANIC CRUST IN THE DYNAMICS OF PLATE MOTION AND BACK-ARC SPREADING

Some of the fundamental features of plate tectonics are interpreted in connection with the behavior of oceanic crust. It is shown to be likely that the oceanic crust which is produced at the mid-ocean ridge by chemical differentiation may be removed from the downgoing slab by melting at the depth of asthenosphere behind the deep-sea trench. The melting of crustal material after the subduction is made possible by an efficient supply of heat through the well-developed asthenosphere with a low-velocity and high-attenuation of seismic waves. The removal of subducted oceanic crust from the slab is consistent with the positive gravity anomaly behind trenches and the double Benioff zone recently discovered. We propose new type of driving forces of plate motion, which arises from the density contrast between the crust and mantle when the oceanic crust is either created or destructed. The proposed driving mechanism is consistent with the non-uniform size and shape of individual plates, the migration of mid-ocean ridges and compressional intraplate stress, while these facts are difficult to understand in the framework of conventional models. A continuous accumulation of basaltic magma beneath the trench-arc system results in a catastrophic overflow of material, which corresponds to back-arc spreading. The picture presented in this paper explains the evolution of marginal basins that is characterized by the presence of remnant arcs, the changes in stress field and the dip angle of the slab, and the anomalous depth-age relationships.

Seismic wave attenuation in the upper mantle beneath the southwest Pacific

A detailed study of P and S seismic waves produced by hundreds of earthquakes located along the New Hebrides, Fiji plateau, Solomon, New Britain and New Guinea seismic zones and recorded at stations in Fiji, New Hebrides and New Caledonia shows significant variations in the amplitudes and frequencies of the waves. The data define zones of anomalously high seismic wave attenuation in the uppermost mantle beneath the Fiji plateau and the Woodlark basin. The active volcanoes of the New Hebrides arc seem to mark the western boundary of the high-attenuation zone that exists beneath the Fiji plateau. Our observations are in agreement with the hypothesis that the Fiji plateau and the Woodlark basin are young oceanic features formed by crustal extension. In addition, a zone of high attenuation exists in the uppermost mantle somewhere between the northeastern coast of Australia and the New Hebrides arc; the data further require that this zone be located not closer than about 400 km to the New Hebrides trench. Observations at New Caledonia stations of very low frequency shear waves from intermediate-depth earthquakes in the westward dipping Tonga—Kermadec seismic zone yield an estimate of Q s of about 10 in the zone of extremely high attenuation located beneath the Lau basin, the actively spreading inter-arc basin behind the Tonga arc.

Travel times of seismic waves between the New Hebrides and Fiji Islands: A zone of low velocity beneath the Fiji Plateau

Shallow earthquakes that occurred during 10 years between the New Hebrides and Fiji islands are relocated by using a digital computer. The spatial distribution of the earthquakes may outline plate boundaries in the Fiji plateau; these boundaries, however, are diffuse and could be broad zones of deformation. In the center of the plateau west of Fiji times of P and S waves traveling in the uppermost mantle indicate velocities of 7.70 and 4.30 km/sec, respectively. Along the seismically active margins of the plateau P velocities are 7.30–7.40 km/sec. These velocities are considerably lower than P and S velocities of about 8.45 and 4.75 km/sec, respectively, of the normal oceanic basins of the Pacific plate to the north and east of the plateau. The zone of low velocity beneath the Fiji plateau and its boundaries seems to coincide with a high seismic wave attenuation zone that exists in the uppermost mantle between the Fiji and New Hebrides islands. These observations and other geophysical and geological aspects of the Fiji plateau clearly imply that the different lithospheric plates between the two opposite-facing lithospheric consumption zones of Tonga and New Hebrides arcs were recently generated and are not part of the oceanic Pacific plate.

PandStraveltimes in the Tonga-Fiji region: A zone of low velocity in the uppermost mantle behind the Tonga Island Arc

The principal purpose of this study is to report the existence of a zone of extremely low compressional-wave velocities in the uppermost mantle beneath most of the Lau basin, an interarc basin located west of or behind the Tonga Island arc. Velocities beneath the basin appear to be as low as 7.1 km/sec. In contrast, times of P and S waves traveling beneath and parallel to the Tonga-Kermadec ridge indicate velocities of 8.45 and 4.75 km/sec, respectively. Although the lateral boundaries of the zone of low velocity beneath the Lau basin are not well defined, they coincide approximately with the boundaries of the zone of high seismic-wave attenuation existing beneath the Lau basin. The large difference (up to 15%) between P wave velocities beneath the Lau basin and those in areas adjacent to it probably requires partial melting in the upper mantle beneath the Lau basin. The P and S velocities measured parallel and approximately perpendicular to the Tonga trench do not differ significantly and hence provide no evidence for anisotropy in the Pacific lithosphere.