Geological Implications of Crustal Thickness, Vp/Vs, and Poisson's Ratio Beneath the Western Mesopotamian Plain Edge, Iraq

Abstract


Introduction
The thickness (h), primary wave and velocity of secondary wave (Vp/Vs) ratio and Moho sharpness of the crust must all be known in order to investigate the nature and tectonic systems development inside it (Gao et al., 2004;Wei et al., 2011;Keller, 2013).The crust's genesis, structure, and development are controlled by Moho intensity, thickness, and Vp/Vs (Zhu and Kanamori, 2000;Chen et al., 2010;Liu and Gao 2010).Information regarding the composition of the earth's crust is provided by the crustal structure and the Poisson's ratio (σ) (Ji et al., 2009).The σ and the Vp/Vs have a strong relationship.The crustal structure shows how the crust thickens and thins, whereas the crustal σ reveals how the crust is made (Guo et al., 2019).The h and the Vp/Vs or Poisson's ratio (σ) obtained from the inversion of the receiver function were used by several authors to study the geological implications throughout the world,e.g., eastern Tibet (Yang et al., 2011), the central and western North China Craton (Wei et al., 2011), the coastal area of South China (Hai-Bo et al., 2014), NW Namibia (Heit et al., 2015), southwestern margin of northeast India (Saikia et al., 2017), South China (Guo et al., 2019), Dominican Republic ( Kumar et al., 2020), continental China (Cheng et al., 2022), the eastern South China Block (Zhang et. al., 2021), Myanmar (Nwe et al., 2021), Canada (Vervaet and Darbyshire, 2022).
The objectives of this study are to: (1) use the receiver function technique to determine the h, Vp/Vs, and σ beneath the western Mesopotamian Plain edge; and (2) interpret the values of the h, Vp/Vs, and σ in terms of crustal composition and structure.The current work represents the first effort to characterize the composition and crustal structure in Iraq in terms of h, Vp/Vs, and σ.

Tectonic Setting of the Study Area
The Arabian Shield and Platform regions comprise the Arabian Shelf.Iraq is located inside the platform region, and the Arabian Shelf in the Iraqi part is divided into two sections: stable and unstable shelf.The Stable Shelf contains the study area.A tectonically stable monocline, the Stable Shelf has mostly escaped damage during the Tertiary deformation of the Late Cretaceous (Jassim and Goff, 2006).The directions of the structures in this block are influenced by the geometry and faulting of the basement rocks, uplifting processes in the Paleozoic, and arcing in the Mesozoic.The stable shelf is divided into three tectonic divisions starting from east to west, Mesopotamian, Salman and Rutba-Jezirazones.The study region is situated on the western margin of the Mesopotamian zone of the stable continental shelf (Jassim and Goff, 2006).The three longitudinal divisions of the Mesopotamian zone are the Euphrates zone, the Tigris zone, and the Zubairzone.The Abu-Jir fault zone is a major structural feature in the studied region, as well as an underlying fault that extends from north to south.

Receiver Functions Method
Receiver functions (RFs) analysis is one of the methods used to investigate the Earth's crust.Teleseismic P-waves, which provide information on the crustal structures beneath a single station, are the central principle of the receiver function technique.Depending on the nature of the contact and the wave's incident angle, wave energy may be transmitted, reflected, or converted in this instance.As a result, the receiver functions technique is a time series acquired by the three components of a seismometer that provide the Earth's corresponding change in the structure of the crust a seismic station's base (Stein and Wysession, 2003).Moho boundary is among the interactions that body waves contact with.Over this border, the velocity of both P and S waves varies significantly.Teleseismic waves propagate spherically in all directions when an earthquake occurs.The P and S waves will contact with Moho boundary and go further into the crust, where the seismometer will record it as a P-wave.The Pwave will arrive to the seismic station as direct P and Ps waves especially given the fact that portion of it will be turned into an S-wave and then traveled (Ammon, 1991).The basic concept of receiver functions analysis is shown to be dependent on P and S wave velocity differences across the Moho border, P phase conversion into S phase at the crust-mantle interface, moreover, the seismic station records several P-to-S phase reverberation reflections in the crust (Stein and Wysession, 2003).Receiving functions result from disintegrating a vertical component of a P teleseismic wave into its horizontal components (Langston, 1979).A Gaussian filter is employed to eliminate high frequencies, and deconvolution stability is ensured using a water-level parameter (Ammon, 1991).An effective high-frequency limit of about 0.5 Hz was achieved in the P-wave data by setting the waterlevel value to 0.01, and the Gaussian filter parameter to 2.5.The most recent version of Herrmann's Computer Programs in Seismology (CPS) (2013) was used to analyze the receiver function of the teleseismic earthquake recorded by seismic stations in the study area.Receiving functions result from disintegrating a vertical component of a teleseismic P wave into its horizontal components (Langston, 1979).A Gaussian filter is employed to eliminate high frequencies, and deconvolution stability is ensured using a water-level parameter (Ammon, 1991).An effective highfrequency limit of about 0.5 Hz was achieved in the P-wave data by setting the water-level value to 0.01, and the Gaussian filter parameter to 2.5.The last version of Computer Programs in Seismology (CPS) prepared by Herrmann (2013) was used to analyze the receiver function of the teleseismic earthquake rrecorded by seismic stations in the study area.

Data Preparation
To adequately depict the interior of the Earth, the receiver function technique requires a number of conditions to be met.One of the requirements is a high magnitude since it gives a high signal-to-noise ratio.The used events' epicentral distances must be between 30 and 90 degrees in order to ensure the arrival of a useful propagation path (Rondenay, 2009).The study area includes two seismic stations belonging to the Mesopotamian Network (MP), University of Basrah, namely: ANB1 and KAR2 seismic stations.The epicentral distances of 30° to 90° and seismic events with magnitudes greater than 7 Mw were selected as suitable events for this study.The earthquakes recorded in the two stations for the period from 2018 to 2021 were analyzed.The European-Mediterranean Seismological Centre's open access to metadata was used to extract the event's details, which included the time, date, latitude, longitude, magnitude, and depth of the focus.Earthquakes recorded in the two stations that do not meet the above-mentioned requirements are removed from the database.Seventeen teleseismic events matching the above criteria were selected, Table 1 and Fig   The receiver function approach isolates the crustal structure at the receiver from the influence of the source's crustal structure and remote structure influences (Shearer, 2009).The deconvolution method eliminates the impact of functions of source time and structure of near-source (Ligorria and Ammon, 1999).Deconvolving the vertical (Z) and radial (R) components of teleseismic waveforms produces radial waveforms for receiver functions, which isolate the effects at receiver position from other sources (Langston, 1979).This technique has proved useful in estimating crustal thickness (h) and the ratio of P-wave velocity to S-wave velocity under seismic stations (Zhu and Kanamori, 2000).For data processing and quality assurance, the following guidelines should be adhered: (1) choosing earthquakes with suitableepicentral distances and magnitudes; (2)identifying the first P-wave arrival time of event reached the station; and (3) manually analyzing the waveforms of the three components, events that produced a lot of noise were deleted to obtain adequate signal-to-noise ratios.The record of waveform was rotated from the premier Z, N, and E coordination system to the Z, R, and T, and the receiver functions were calculated by a time domain deconvolution approach (Ammon, 1991).The six panels exhibit the R and T components, which were filtered with three different Gaussian filters (0.5, 1.0, and 2.5 Hz).The R and T components receiver functions of one of the selected events are shown in Fig. 3.

Calculating of Poisson's Ratio
Poisson's ratio is theoretically dependent on Vp/Vs.Depending on the results of inversion of receiver functions; the Vp/Vs ratio was calculated.The following equation was used to calculate Poisson's ratio (Dugda et al., 2005): (1) Where σ is Poisson's ratio and (k) is the Vp/Vs ratio.1).Three values-0.5,1.0, and 2.5-were used to filter the R and T components Fig. 4, shows the receiver functions for the selected earthquakes as a function of the back azimuth.The alignment of events is identified by the back azimuth number.

Results and Discussion
Using surface registrations from seismic stations, the receiver function technique generates a time series that emphasizes velocity variations between layers.Converting the converted phase arrival times at a discontinuity, like the Moho, to depths is how receiver functions are inverted.To transform seismic phases into depths, iterative inversion has to know the seismic Earth velocities.The S-wave velocity variance and discontinuities are the primary determinants of receiver functions (Abdulnaby, 2013).Using the half-space homogeneous earth structure model and an initial S-wave velocity (Vs) of 3.5 km/sec, the receiver function values were inverted.The half-space shows initial velocity model has a 400 km depth and 65 layers.Multiple layers were created from the 65 layers.The layers we employed in the study are 24 uppermost layers with a depth of 48 km.Each layer has a thickness of 2 km and a constant velocity.The inversion approach was carried out using the rftn96 software from the CPS package.By minimizing the variance between the expected and observed receiver functions, this program relies on linearized iterative inversion (Herrmann and Ammon, 2007).Each event was subjected to twelve iterations in order to develop the receiver functions.The crustal seismic structure under the research region was calculated using the receiver function inversion for seventeen events.For the inversion of receiver functions, Gaussian filter values of 0.5, 1, and 2.5 were used.The ultimate velocity model is determined by the best agreement between the observed and calculated receiver functions.Results of the inversion of receiver functions for the selected earthquakes recorded at ANB1 and KAR2 seismic stations are listed in Table 2.The average crustal velocity structure beneath the western edge of Mesopotamian zone is shown in Fig. 5.

Geological Implications of Crustal Structure
Understanding the composition, structure, tectonic setting, and development of the crust requires knowledge of the h, the Vp/Vs ratio, and σ (Zandt and Ammon, 1995;Zhu and Kanamori, 2000;Chevrot and Van der Hilst, 2000;Yang et al., 2011).The obtained values of h, Vp/Vs ratio, and σ for the study area are listed in Table 4. Due to the lack of local studies related to estimating the Vp/Vs ratio and σ, the results of the current study will be compared with the results of global studies.

Geological implication of crustal thickness
Geological processes including plate convergence and geological collision produced the crustal thickness, which is a key indicator for classifying the various types of global plates and regional blocks (Huang et al., 2014).The thickening and thinning of the crust is described by its thickness (Guo et al., 2019).Results of inversion of the receiver functions of teleseismic earthquakes recorded by ANB1 and KAR2 seismic stations revealed that the study area's crustal thickness ranged from 44 to 46 km with an average value of 45 km.Rafea et al.(2022) found that the crustal thickness beneath the Anbar seismic station was 44km.Albakr et al. (2022) analyzed the seismic ambient noise recorded by ANB1 and KAR2 stations to determine the crustal structure beneath the Dhafriyah and Kumaite Oil fields region.They found the crustal thickness is 46 km.The crustal thickness underneath the western edge of the Mesopotamian plain zone is higher than the average continental crust thickness measured globally, which is 41 km (Mooney, 2007).It is within the range of the crustal thickness of the platform region, 39.96 km ± 7.03 (Mooney et al. 1998).Tectonic deformation, thermal expansion, the addition of volcanic products to the crust's surface, the insertion of magmatic intrusions deep within the crust, and sedimentation all cause the crust to thicken (Yang and Liu, 2009).Due to the lower crust's ductile character during deformation, orogeny zones exhibit crustal thickening largely in the lower crust (Condie, 2011).
The collapse of the magma chamber, ductile flowage, thermal uplift, surface and subcrustal erosion, injection of dense material, and lateral movements like rifting and transverse shearing cause the crust to thin (Thybo and Nielsen, 2008).The high thickness of sedimentary cover beneath the study area and the thinning of the lower crustal layer indicate that the thickening of the crust beneath the research area may be due to the sedimentation processes.

Geological implication of Vp/Vs ratio
The obtained average value of theVp/Vs ratio beneath the study area is 1.79, Table 3.This value is comparable to the average value for the world (1.78) of the continental crust and the average Vp/Vs value of the platform crust, 1.78 ± 0.01, (Zandt and Ammon, 1995;Christensen, 1996).The Vp/Vs ratio value estimated for the study area was compared with the results of studies carried out in different regions around the world due to lack of the local studies.The obtained Vp/Vs value is higher than reported in Southern African cratons, 1.74 (Nair et al., 2006), and in Australian cratons, 1.76 (Chevort and van der Hilst, 2000).The Vp/Vs average of the Iberian crust is 1.74 ± 0.05 (Julià and Mejía 2004).The crustal Vp/Vs ratios ranged between 1.74 and 1.83 along the earthquake sources from Memphis, Tennessee, to St. Louis, Missouri (Catchings, 1999).The crustal Vp/Vs ratio in the eastern South China Block is 1.72 on average (Zhang et al., 2021).There is a large fluctuationof Vp/Vs ratio value (1.68 -1.93) in the central and western North China Craton (Wei et al., 2011).In the southwestern margin of Northeast India, the crustal Vp/Vs ratio value ranges from 1.69 to 1.75 (Saikia et al., 2017).The crust of South China has an average Vp/Vs ratio of 1.697 (Chen et al.,2019).The average Vp/Vs ratio value of the crust beneath South China was 1.697 (Chen et al., 2019).The crustal Vp/Vs ratio beneath continental China ranged between 1.75 and 1.85 (Cheng et al., 2022).In Canada, the crustal Vp/Vs ratio beneath the eastern Superior craton margins ranged from 1.66 to 1.76 except in an area dominated by anthrosite massifs is 1.85 (Vervaet and Derbyshire, 2022).In the Dominican Republic, the crustal Up/Vs ratios vary from 1.58 to 1.99 with a median of 1.79 (Kumar et  , 2020).The crustal Vp/Vs in the western Bengal Basin, India is 1.73 (Mitra et al., 2008).In the East Anatolian volcanic belt, a low average crustal Up/Vs ratio (less than 1.71) was reported, indicating a felsic crustal composition at the southwest of the study region, whereas a high average crustal Vp/Vs ratio (1.82-1.87)was found in close proximity to volcanic centers, indicating a mafic crustal composition (Alkan, 2022).The results of laboratory experiments and the seismic surveys have confirmed that the Vp/Vs ratio is more useful than Vp or Vs alone (Zandt and Ammon, 1995).Because the relationship between Vp and composition is constrained by the similar P-wave velocities for many common crustal rock types, field measurements of the crustal Vp/Vs ratio offer important restrictions on crustal composition.(Christensen and Mooney, 1995).The mineral composition of rocks plays a major role in the difference in Vp/Vs ratio (Christensen, 1996).There is a strong correlation between the Vp/Vs ratio value and the quartz and feldspar contents in the crust (Zhang et al., 2021).The Vp/Vs ratio value is directly proportional to the plagioclase content and inversely to quartz content (Chen et al., 2010).The Vp/Vs ratio value is affected to a limited extent by the change in pressure or temperature (Christensen, 1996) and increases due to the fluid content and partial melting (Watanabe, 1993).
The study area is classified as a part of the Arabian platform area and is located on the western edge of the Mesopotamian plain (Jassim and Goff, 2006).The obtained crustal Vp/Vs value for the study area (1.79) is typical for the platform regions.The granitic, andesitic, and basaltic rocks have Vp/Vs ratios of 1.71, 1.78, and 1.87, respectively (Tarkov and Vavakin, 1982).The estimated Vp/Vs ratio for the study area is typical for andesitic rocks and shows a small difference between the crust's felsic and mafic components.

Geological implication of Poisson's ratio
The obtained crustal σ of the study area is of 0.27.This value is consistent with the average σ value for the continental crust, 0.27 and with that for the platform regions, 0.27 (Zandt and Ammon, 1995).The obtained crustal Poisson's ratio value was compared with that reported in global studies.The low crustal Poisson ratio value (σ = 0.249) was reported for the Precambrian cratons in eastern China (Chen et al. 2010).In East Africa, the average crustal σ value for all terrains of Precambrian crust is 0.25 (Tugume et al., 2012).The crustal σ value in South China ranges from 0.20 to 0.31 (Guo et al., 2019).Across earthquake source zones from Memphis, Tennessee, to St. Louis, Missouri, the crustal σ value ranges from about 0.26 to 0.33 ( Catchings, 1999).The crustal σ in eastern Tibet ranges from 0.26 to 0.28 (Yang et al., 2011).The crustal σ over mainland China is about 0.249 (Chen et al., 2010).
A number of authors have interpreted the σ values in terms of the compositionof the earth's crust (Zandt and Ammon, 1995;Christensen, 1996;Zhu and Kanamori, 2000;Ji et al., 2009).They observed that a low σ (0.26) indicates that the crust has a large proportion of felsic minerals and a low proportion of mafic minerals.The balance of felsic and mafic minerals is suggested by the intermediate σ value of 0.26 to 0.28.The σ value of 0.28 to 0.30 suggests a high concentration of mafic minerals and a low concentration of felsic minerals in the crust.The σ values greater than 0.30 have been explained by the existence of serpentinized fault zones, fracture zones with fluid infiltration, or partial melting of rocks within the crust ( Ji et al., 2009).
The estimated crustal σ in the study area (σ = 0.27) indicates that the felsic mineral content is slightly different from the mafic mineral content in the crust, which in turn, points to the andesitic composition of the rock.

Conclusions
From the obtained results, we conclude the following: • The seismic structure of the crust under the study area consists of three layers: the sedimentary cover layer with a thickness of 12 km, the upper crust layer with a thickness of 26 km, and the lower crust layer with a thickness of 7 km .• The average crustal thickness (the depth of the Moho discontinuity) is 45 km, which represents the typical value of the platform area in which the study area is located.The high thickness of sedimentary cover beneath the study area and the thinning of the lower crustal layer indicate that the thickening of the crust beneath the study area may be attributed to the sedimentation processes.• The obtained crustal Vp/Vs value for the study area (1.79) is typical for the platform regions and is typical for andesitic rocks and it shows a small difference between the crust's felsic and mafic components.
• The obtained crustal σ of the study area is 0.27.This value is consistent with the average σ value for the continental crust and with that for the platform regions.The estimated crustal σ value indicates that the felsic mineral content is slightly different from the mafic mineral content in the crust, which in turn, refers to the andesitic composition of the rock.

Fig. 1 .
Fig.1.Location map of ANBI and KAR2 seismic stations (red triangle) . 2 illustrate the distribution of the used earthquakes.

Fig. 4 .
Fig.4.Receiver functions as a function of back azimuth for 9 events recorded by ANB1 station and 8 events recorded by KAR2 station.The one event is shown to overlap with four events

Fig. 5 .
Fig. 5. Mean crustal velocity structure beneath the study area

Table 1 .
Earthquake information recorded in Anbar and Karbala seismic stations used in the analysis of receiver functions * Same earthquake was recorded in both stations.

Table 2 .
The crustal structure parameters beneath ANB1 and KAR2 seismic statio

Table 3 .
The crustal layers thickness (h), Vp/Vs ratio and σ beneath the study area