scholarly journals Teleseismic P-residual study in the Italian region - inferences on large scale anisotropic structure of the subcrustal lithosphere

1998 ◽  
Vol 41 (1) ◽  
Author(s):  
J. Plomerová ◽  
V. Babuska ◽  
R. Scarpa
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Large Scale ◽  
Velocity Structure ◽  
P Wave ◽  
Anisotropic Structure ◽  
Low Velocity ◽  
Arrival Times ◽  
Station Corrections ◽  
Lateral Variations

Jeffreys-Bullen (absolute) and relative P-wave travel-time residuals were analyzed over Italy and its surrounding using P arrival times from the ISC bulletins supplemented by the data from local observatories. We analyzed the travel-time station corrections by two independent methods to obtain information on lateral variations of the velocity structure over the area and a view of possible upper mantle anisotropy. In the first method, the station corrections are computed as a constant and two cosine terms with appropriate phase shifts. Besides a static term, the second method allows us to study the relative residuals in dependence both on azimuths and incidence angles and thus to investigate their spatial variations and to map lateral variations of anisotropic structure of the subcrustal lithosphere. The high and low-velocity directions inferred from the spatial distribution of the relative residuals as well as the high- and low-velocity upper mantle heterogeneities reflect the geodynamic development of the region, governed by the collision between the African and Eurasian plates

scholarly journals P-wave crustal tomography of Greece with use of an accurate two-point ray tracer

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
G. Drakatos ◽  
G. Karantonis ◽  
G. N. Stavrakakis
Keyword(s):  
Large Scale ◽  
Velocity Structure ◽  
Three Dimensional ◽  
Seismic Energy ◽  
Aegean Sea ◽  
P Wave ◽  
Low Velocity ◽  
Aegean Area ◽  
Arrival Times ◽  
Horizontal Variations

The three-dimensional velocity structure of the crust in the Aegean sea and the surrounding regions (34.0º-42.OºN, 19.0ºE-29.0ºE) is investigated by inversion of about 10000 residuals of arrival times of P-wave from local events. The resulting velocity structure shows strong horizontal variations due to the complicated crustal structure and the variations of crustal thickness. The northern part of the region generally shows high velocities. In the inner part of the volcanic arc (Southern Aegean area), relatively low velocities are observed, suggesting a large-scale absorption of seismic energy as confirmed by the low seismicity of the region. A low velocity zone was observed along the subduction zone of the region, up to a depth of 4 km. The existence of such a zone could be due to granitic or other intrusions in the crust during the uplift of the region during Alpidic orogenesis.

scholarly journals Isochronal maps at Mt. Etna volcano (Italy): a simple and reliable tool for investigating large-scale heterogeneities

1997 ◽  
Vol 40 (5) ◽  
Author(s):  
G. Patanè ◽  
C. Centamore ◽  
S. La Delfa
Keyword(s):  
Large Scale ◽  
Volcanic Edifice ◽  
P Wave ◽  
Etna Volcano ◽  
Low Velocity ◽  
Seismic Stations ◽  
Arrival Times ◽  
Mt Etna ◽  
Wave Arrival ◽  
Feeding Systems

This paper analyses twelve etnean earthquakes which occurred at various depths and recorded at least by eleven stations. The seismic stations span a wide part of the volcanic edifice; therefore each set of direct P-wave arrival times at these stations can be considered appropriate for tracing isochronal curves. Using this simple methodology and the results obtained by previous studies the authors make a reconstruction of the geometry of the bodies inside the crust beneath Mt. Etna. These bodies are interpreted as a set of cooled magmatic masses, delimited by low-velocity discontinuities which can be considered, at present, the major feeding systems of the volcano.

scholarly journals Crustal velocity structure of the Apennines (Italy) from P-wave travel time tomography

1996 ◽  
Vol 39 (6) ◽  
Author(s):  
C. Chiarabba ◽  
A. Amato
Keyword(s):  
Velocity Structure ◽  
P Wave ◽  
Depth Interval ◽  
Low Velocity ◽  
Data Set ◽  
Arrival Times ◽  
Velocity Anomaly ◽  
Minimum Number ◽  
Velocity Images ◽  
Lower Crustal

In this paper we provide P-wave velocity images of the crust underneath the Apennines (Italy), focusing on the lower crustal structure and the Moho topography. We inverted P-wave arrival times of earthquakes which occurred from 1986 to 1993 within the Apenninic area. To overcome inversion instabilities due to noisy data (we used bulletin data) we decided to resolve a minimum number of velocity parameters, inverting for only two layers in the crust and one in the uppermost mantle underneath the Moho. A partial inversion of only 55% of the overall dataset yields velocity images similar to those obtained with the whole data set, indicating that the depicted tomograms are stable and fairly insensitive to the number of data used. We find a low-velocity anomaly in the lower crust extending underneath the whole Apenninic belt. This feature is segmented by a relative high-velocity zone in correspondence with the Ortona-Roccamonfina line, that separates the northern from the southern Apenninic arcs. The Moho has a variable depth in the study area, and is deeper (more than 37 km) in the Adriatic side of the Northern Apennines with respect to the Tyrrhenian side, where it is found in the depth interval 22-34 km.

Upper mantle structure along a profile from Oslo (NORESS) to Helsinki to Leningrad, based on explosion seismology

1990 ◽  
Vol 80 (6B) ◽  
pp. 2194-2213
Author(s):  
Vladislav Ryaboy
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Profile Analysis ◽  
Time Estimation ◽  
Seismic Profile ◽  
P Wave ◽  
Apparent Velocity ◽  
Low Velocity ◽  
Time Curve ◽  
Volcanic System

Abstract Waveforms from the NORESS array were analyzed for 147 industrial explosions during the 1985 to 1988 period, along a profile running east from Oslo (NORESS) to Helsinki to Leningrad (OHL profile). The events were 250 to 1300 km from NORESS and had local magnitude in the range 2.0 to 3.5. Event locations and origin times constrained by the University of Helsinki's regional seismic network provide a reliable basis for travel-time estimation at NORESS. We also used data recorded by NORSAR in 1979 for three shots on the FENNOLORA north-south, long-range seismic profile, which were near the OHL profile. Analysis of mantle P-wave signals from the explosions showed that first arrivals could be traced continuously to a distance of 750 to 800 km, where there is a cutoff and shift of approximately 2.0 to 2.5 sec in the travel-time curve and an increase in average apparent velocity. Interpretation of the observed travel times and waveforms for this profile suggests a low-velocity zone from approximately 105 to 135 km depth. Combined analysis of the seismic data with a Bouguer gravity map indicates the presence in the upper mantle of a high-velocity, high-density body of linear extent approximately from 200 to 300 to 500 to 600 km east of the NORESS array. It is postulated that this body may represent the root of an ancient volcanic system, in which lighter, silicic constituents were depleted from the upper mantle during the eruptive phase.

scholarly journals Insights from the P Wave Travel Time Tomography in the Upper Mantle Beneath the Central Philippines

2021 ◽  
Vol 13 (13) ◽  
pp. 2449
Author(s):  
Huiyan Shi ◽  
Tonglin Li ◽  
Rui Sun ◽  
Gongbo Zhang ◽  
Rongzhe Zhang ◽  
...  
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
High Velocity ◽  
Velocity Structure ◽  
The Philippines ◽  
P Wave ◽  
The South ◽  
Teleseismic Tomography ◽  
The North ◽  
Velocity Anomalies

In this paper, we present a high resolution 3-D tomographic model of the upper mantle obtained from a large number of teleseismic travel time data from the ISC in the central Philippines. There are 2921 teleseismic events and 32,224 useful relative travel time residuals picked to compute the velocity structure in the upper mantle, which was recorded by 87 receivers and satisfied the requirements of teleseismic tomography. Crustal correction was conducted to these data before inversion. The fast-marching method (FMM) and a subspace method were adopted in the forward step and inversion step, respectively. The present tomographic model clearly images steeply subducting high velocity anomalies along the Manila trench in the South China Sea (SCS), which reveals a gradual changing of the subduction angle and a gradual shallowing of the subduction depth from the north to the south. It is speculated that the change in its subduction depth and angle indicates the cessation of the SCS spreading from the north to the south, which also implies that the northern part of the SCS opened earlier than the southern part. Subduction of the Philippine Sea (PS) plate is exhibited between 14° N and 9° N, with its subduction direction changing from westward to eastward near 13° N. In the range of 11° N–9° N, the subduction of the Sulu Sea (SS) lies on the west side of PS plate. It is notable that obvious high velocity anomalies are imaged in the mantle transition zone (MTZ) between 14° N and 9° N, which are identified as the proto-SCS (PSCS) slabs and paleo-Pacific (PP) plate. It extends the location of the paleo-suture of PSCS-PP eastward from Borneo to the Philippines, which should be considered in studying the mechanism of the SCS and the tectonic evolution in SE Asia.

Crustal velocity structure beneath the NW Dinarides from 1-D hypocenter-velocity inversion

2021 ◽  
Author(s):  
Gregor Rajh ◽  
Josip Stipčević ◽  
Mladen Živčić ◽  
Marijan Herak ◽  
Andrej Gosar
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Depth Range ◽  
Velocity Inversion ◽  
Arrival Times ◽  
Simultaneous Inversion ◽  
Velocity Modeling ◽  
Velocity Models ◽  
Station Corrections

<p>The investigated area of the NW Dinarides is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin at the NE corner of the Adriatic Sea. Its complex crustal structure is the result of interactions among different tectonic units. Despite numerous seismic studies taking place in this region, there still exists a need for a detailed, smaller scale study focusing mainly on the brittle part of the Earth's crust. Therefore, we decided to investigate the velocity structure of the crust using concepts of local earthquake tomography (LET) and minimum 1-D velocity model. Here, we present the results of the 1-D velocity modeling and the catalogue of the relocated seismicity. A minimum 1-D velocity model is computed by simultaneous inversion for hypocentral and velocity parameters together with seismic station corrections and represents the best fit to the observed arrival times.</p><p>We used 15,579 routinely picked P wave arrival times from 631 well-located earthquakes that occurred in Slovenia and in its immediate surroundings (mainly NW Croatia). Various initial 1-D velocity models, differing in velocity and layering, were used as input for velocity inversion in the VELEST program. We also varied several inversion parameters during the inversion runs. Most of the computed 1-D velocity models converged to a stable solution in the depth range between 0 and 25 km. We evaluated the inversion results using rigorous testing procedures and selected two best performing velocity models. Each of these models will be used independently as the initial model in the simultaneous hypocenter-velocity inversion for a 3-D velocity structure in LET. Based on the results of the 1-D velocity modeling, seismicity distribution, and tectonics, we divided the study area into three parts, redefined the earthquake-station geometry, and performed the inversion for each part separately. This way, we gained a better insight into the shallow velocity structure of each subregion and were able to demonstrate the differences among them.</p><p>Besides general structural implications and a potential to improve the results of LET, the new 1-D velocity models along with station corrections can also be used in fast routine earthquake location and to detect systematic travel time errors in seismological bulletins, as already shown by some studies using similar methods.</p>

Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

1992 ◽  
Vol 29 (7) ◽  
pp. 1530-1548 ◽  
Author(s):  
B. C. Zelt ◽  
R. M. Ellis ◽  
R. M. Clowes ◽  
E. R. Kanasewich ◽  
I. Asudeh ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Velocity Structure ◽  
P Wave ◽  
Seismic Reflection Profile ◽  
Wide Angle ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Middle Crust ◽  
Refraction Data

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

The Study of Crustal Velocity Structure Characteristics in Yangjiang Area of South China

2020 ◽  
Author(s):  
Xiaona Wang ◽  
Zhihui Deng ◽  
Xiuwei Ye ◽  
Liwei Wang
Keyword(s):  
South China ◽  
Seismic Data ◽  
Velocity Structure ◽  
Seismic Network ◽  
P Wave ◽  
Adjacent Area ◽  
Low Velocity ◽  
Arrival Times ◽  
P Wave Velocity ◽  
Buried Fault

<p>This paper collects 43,225 absolute first arrival P wave arrival times and 422,956 high quality relative P arrival times of 6,390 events occurred in Yangjiang and its adjacent area from Jan, 1990 to Aug, 2019, these seismic data is recorded by 49 stations from Guangdong seismic network, Guangxi seismic network and Hainan seismic network. Based on the seismic data above, we simultaneously determine the crustal 3D P wave velocity structure and the hypocenter parameters of 6255 events in Yangjiang and its adjacent area by applying Double-Difference seismic tomography. The result shows that, shallow P wave velocity in Yangjiang area is higher due to the thinner sedimentary layer and widely exposed Yanshanian granite, Indosinian granite and Cambrian metamorphic rocks. There are obvious correspondences between the distribution of shallow velocity and fault structure as well as geological structure. A wide range of low velocity anomaly exists in 20km depth, which verifies the low velocity layer in the middle crust at Yangjiang area of South China continent. The velocity image from land to ocean in 30km depth shows low velocity in NW side and high velocity in SE side, which verifies the characteristic of crust thinning in South China coastal continent. The NEE seismic belt from Yangbianhai to Pinggang is speculated to locate in a buried fault of southwest segment of Pinggang fault. The buried thrust fault is a N78°E strike fault, dip to NW with a dip angle of 85 °. In addition, the buried fault locates in the abnormal junction of high velocity on the NW side and low velocity on the SE side, which reflects the tectonic activity characteristic of NW plate uplifting and SE plate declining from Miocene period. The characteristic of activity in the buried fault shows thrust movement with a small strike slip component, which is consistent with the focal mechanism of M4.9 earthquake occurred in 2004.</p>

Subduction zone Calibration and teleseismic relocation of thrust zone events in the central Aleutian Islands

1981 ◽  
Vol 71 (6) ◽  
pp. 1805-1828
Author(s):  
Kazuya Fujita ◽  
E. R. Engdahl ◽  
Norman H. Sleep
Keyword(s):  
Travel Time ◽  
Velocity Structure ◽  
P Wave ◽  
Aleutian Islands ◽  
Local Network ◽  
Origin Time ◽  
Station Corrections ◽  
The North ◽  
Thrust Zone ◽  
Local Station

Abstract Teleseismically determined epicenters for thrust zone earthquakes in the central Aleutian Islands are systematically mislocated by about 40 km to the north relative to local network solutions. We attribute the mislocation to travel-time anomalies produced by the higher P-wave velocity in the subducting slab. These travel-time anomalies can be estimated by seismic ray tracing through a thermally modeled velocity structure for the slab. Observed teleseismic station residuals and slab travel-time anomalies computed from local network hypocenters, plotted as a function of event distance to the center of curvature of the island arc, show good agreement. This distance versus residual relationship can be used to validate the local network hypocenters and to constrain the length of subducting lithosphere. Computed slab source corrections, when incorporated in a standard location program, reduce teleseismic mislocations to 20 km; the addition of empirical station corrections reduces the mislocation to about 10 to 15 km. Depth and origin time estimates can be further improved through use of better station corrections, a local station within 2°, and depth phases.

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