Geophysical Institute of Israel

Abstract New geophysical profiles across the central Dead Sea Transform (DST) near the Sea of Galilee, Israel, and surrounding highlands, augmented by static stress modeling, allow us to study continental transform plate deformation. The DST separates a ∼10 km thick sedimentary column above a thinned (16–23 km) crust to the west from a ∼7 km column above a ∼30‐km thick crust to the east. Crustal thinning starts under the DST, as observed also farther south, indicating that the DST is indeed located along the boundary between the Arabian plate and its continental margin. Moho step here is gradual. The DST's eastern shoulder dips westward toward the DST unlike the upward flexed shoulder observed farther south, perhaps delineating the northern limit of a thinner and hotter lithosphere. The shape of the Sea of Galilee is modeled as an asymmetric pull‐apart basin formed by a left‐lateral stepover of 2.6 km between slightly divergent and underlapping strike‐slip fault strands dipping 70° to the west. Reflection data indicate that these strands are not connected. Several fault traces within the Sea of Galilee have previously been suggested to carry part of the relative plate motion. However, given slip along the main DST faults, Coulomb stress will increase only on fault portions in the northern part of the lake, in accord with the geographical distribution of seismicity, suggesting that these faults are likely secondary. Mismatch between the DST strand locations in the geophysical profiles and the subsidence model, may reflect temporal changes in fault geometry.

Detection and imaging of subwavelength features in the subsurface using diffracted waves are rapidly gaining momentum in the oil and gas industry as well as in the fields of engineering, archeology, and homeland security. Most of the methods include coherent summation of the recorded wavefield along diffraction traveltime surfaces from point scatterers. The summation focuses energy onto point-like diffractors that appear at the resulting images as prominent anomalies. However, in cases in which the target is an elongated object such as a fault plane, fracture, tunnel, or elongated cave, a more efficient imaging method can be constructed. We have developed an algorithm for detecting and characterizing linear subsurface elements using a linear-diffractor operator. Our algorithm is based on the coherent summation of the edge diffraction generated by the entire lineament and on the analysis of the calculated coherence measure (semblance). The advantages and limitations of our method are discussed, and the results are compared to conventional point-diffractor-based techniques. Synthetic and real data examples demonstrate that using a linear-diffractor-based algorithm can dramatically improve the detection of linear objects.

We studied whether: 1) the carotid and forearm diastolic pressure-decay time constants (tau) are similar and 2) the associated Windkessel (WK)-derived compliance and stiffness values are also similar and are related to arterial stiffness. Ambulatory normotensive and hypertensive subjects were studied after 30 minutes of supine rest with radial and carotid arterial tonometry (Sphygmocor), standard oscillometric cuff BP, and central (heart-femoral, hf) and peripheral (femoral-ankle, fa) pulse wave velocity (PWV, Colin VP1000). Brachial and carotid Doppler flow studies were performed in a subset. Tonograms were photo-digitized and the pressure in late diastole was modeled as an exponential decay to an asymptote A with time constant tau and start-decay time t0. [Resistance = mean pressure/flow; WK compliance = tau/resistance and WK stiffness = 1/WK compliance.] Tonometry and PWV data were available in 98 individuals; carotid and forearm blood flow in 22: (mean[SD]) age 50[20] years, weight 81[17] kg, BP 134/77[17/12] mmHg, resting HR 66[12], 38% female. Data are presented in the Table; hfPWV and faPWV were well correlated (p<0.001) but neither was correlated with carotid or forearm tau or the corresponding regional WK stiffness. We conclude that diastolic pressure decay (tau) and WK stiffness (or compliance) are not systemic indicators but rather unique regional characteristics not related to central or peripheral large artery stiffness. These findings suggest that single-WK models do not adequately represent the arterial circulation.

This research examines the interrelations in a complex hydrogeological system, consisting of a multi-layered coastal aquifer, the sea, and a surface reservoir (fish ponds) and the importance of the specific connection between the aquifer and the sea. The paper combines offshore geophysical surveys (CHIRP) and on land TDEM (Time Domain Electro Magnetic), together with hydrological measurements and numerical simulation. The Quaternary aquifer at the southern Carmel plain is sub-divided into three units, a sandy phreatic unit, and two calcareous sandstone (‘Kurkar’) confined units. The salinity in the different units is affected by their connection with the sea. We show that differences in the seaward extent of its clayey roof, as illustrated in the CHIRP survey, result in a varying extent of seawater intrusion due to pumping from the confined units. FEFLOW simulations indicate that the FSI (Fresh Saline water Interface) reached the coastline just a few years after pumping has begun, where the roof terminates ~100 m from shore, while no seawater intrusion occurred in an area where the roof is continuous farther offshore. This was found to be consistent with borehole observations and TDEM data from our study sites. The water level in the coastal aquifer was generally stable with surprisingly no indication for significant seawater intrusion although the aquifer is extensively pumped very close to shore. This is explained by contribution from the underlying Late Cretaceous aquifer, which increased with the pumping rate, as is also indicated by the numerical simulations.

Infrasonic signals propagate from an atmospheric source via media with stochastic and fast space-varying conditions. Hence, their travel time, the amplitude at sensor recordings and even manifestation in the so-called “shadow zones” are random. Therefore, the traditional least-squares technique for locating infrasonic sources is often not effective, and the problem for the best solution must be formulated in probabilistic terms. Recently, a series of papers has been published about Bayesian Infrasonic Source Localization (BISL) method based on the computation of the posterior probability density function (PPDF) of the source location, as a convolution of a priori probability distribution function (APDF) of the propagation model parameters with likelihood function (LF) of observations. The present study is devoted to the further development of BISL for higher accuracy and stability of the source location results and decreasing of computational load. We critically analyse previous algorithms and propose several new ones. First of all, we describe the general PPDF formulation and demonstrate that this relatively slow algorithm might be among the most accurate algorithms, provided the adequate APDF and LF are used. Then, we suggest using summation instead of integration in a general PPDF calculation for increased robustness, but this leads us to the 3D space-time optimization problem. Two different forms of APDF approximation are considered and applied for the PPDF calculation in our study. One of them is previously suggested, but not yet properly used is the so-called “celerity-range histograms” (CRHs). Another is the outcome from previous findings of linear mean travel time for the four first infrasonic phases in the overlapping consecutive distance ranges. This stochastic model is extended here to the regional distance of 1000 km, and the APDF introduced is the probabilistic form of the junction between this travel time model and range-dependent probability distributions of the phase arrival time picks. To illustrate the improvements in both computation time and location accuracy achieved, we compare location results for the new algorithms, previously published BISL-type algorithms and the least-squares location technique. This comparison is provided via a case study of different typical spatial data distributions and statistical experiment using the database of 36 ground-truth explosions from the Utah Test and Training Range (UTTR) recorded during the US summer season at USArray transportable seismic stations when they were near the site between 2006 and 2008.

Estimating possible site effect is an integral part of evaluation of the seismic hazard and reduction of earthquake damages. In regions with low or moderate seismicity as in Israel, the site response should be determined by analytical tools. These computations require knowledge of the subsurface geological structure in terms of shear-wave velocity (Vs) profile down to seismic bedrock. Conventionally, this problem is resolved by joint implementation of Horizontal-to-Vertical Spectral Ratios (HVSR or Nakamura’s) technique, which is based on ambient noise measurements and seismic methods such as S-wave refraction or Multichannel Analysis of Surface Waves (MASW) method. The first one does not allow deep penetration of seismic waves because of its weak source. The MASW method using 4.5 Hz geophones is restricted in penetration depth of surface waves because of frequency (wavelength) limitations. In this study, we have applied 2.5 Hz geophones and special data processing to provide constructing Vs section to a depth of 100 m and deeper. In combination with HVSR measurements, MASW enables constructing reliable subsurface model, which could be integrated into the seismic hazard assessment. Testing of this combined methodology was carried out at a number of sites with differing geological structures in Israel.

We have investigated earthquake source parameters and seismic moment-magnitude relations from 103 regional and local earthquakes with moment magnitude 2.6 to 7.2, which occurred in a distance range from 4.5 to 550 km during 1995–2012 by applying Brune’s seismic source model (J Geophys Res 75:4997–5009, 1970, J Geophys Res 76:5002, 1971) for P- and S/Lg-wave displacement spectra. Considering P- and S-wave data separately, we first studied the empirical dependence of the Fourier spectral amplitudes Ω due to the geometrical spreading and the inelastic attenuation and of the corner frequency, f 0, with the epicentral distances, R. We found the distance correction parameters, Re 0.0042R and R 0.8333 e 0.00365R for the low-frequency spectral amplitudes and f 0 = f 0 ′ e 0.00043R and f 0 = f 0 ′ e 0.00044R for the corner frequency at the source, f 0, and observed at the station, f 0 ′ , from P-wave and S-wave spectra, respectively. Applying the distance correction procedure, we determined the source displacement spectrum of P and S waves for each earthquake to estimate the seismic moment, M 0; the moment magnitude, M W; the source radius, r; and the stress drop, Δσ. The seismic moments range from 1.06 × 1013 to 7.67 × 1019 N m, and their corresponding moment magnitudes are in the range of 2.6–7.2. Values of stress drop Δσ vary from 0.1 to 44 MPa. It was found that the stress drop increases with the increasing seismic moment in the range of 1013–1016 N m and possibly becomes constant at higher magnitudes, reaching a maximum value of about 40–45 MPa. We demonstrate that the values of the M 0 and M W estimated from P-wave and S-wave analysis are consistent and confirmed by the results of waveform inversions, i.e., centroid moment tensor (CMT) solution.

Lowermost mantle velocity in the area 15°S–70°N latitude/60°W–5° W longitude is estimated using two groups of observations, complementary to each other. There are 894 Pdif observations at stations in the Balkan and Eastern Mediterranean areas from 15 major earthquakes in Central and South America. Another 218 Pdif observations are associated with four earthquakes in Greece/Turkey and one event in Africa, recorded by American stations. A Pdif slowness tomographic approach of the structures immediately above the core-to-mantle boundary (CMB) is used, incorporating corrections for ellipticity, station elevation and velocity perturbations along the ray path. A low-velocity zone above CMB with a large geographical extent, approximately in the area (35–65°N) × (40–20°W), appears to have the velocity perturbations exceeding the value actually assumed by some global models. Most likely, it is extended beneath western Africa. A high-velocity area is observed west of the low-velocity zone. The results suggest that both Cape Verde and Azorean islands are located near transition areas from low-to-high velocity values in the lowermost mantle.