# Crustal structure of the Dead Sea Basin from local earthquake tomography

Rami Hofstetter, Catherine Dorbath, Marco Calo'

► To cite this version:

Rami Hofstetter, Catherine Dorbath, Marco Calo'. Crustal structure of the Dead Sea Basin from local earthquake tomography. Geophysical Journal International, 2012, 189 (1), pp.554-568. 10.1111/j.1365-246X.2012.05369.x. hal-00757087

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# Crustal structure of the Dead Sea Basin from local earthquake tomography

R. Hofstetter,<fnref n="1" /> C. Dorbath<fnref n="2" />,<fnref n="3" /> and M. Calò<fnref n="2" />

Accepted 2012 January 8. Received 2012 January 6; in original form 2010 November 2

## SUMMARY

New findings of the velocity structure of the crust across the Dead Sea Basin are obtained by applying tomography-based method to local earthquakes. We use P-wave traveltime of 614 earthquakes that occurred in the Dead Sea Basin in 1983–2009. At all depths, the Dead Sea Basin is characterized by lower velocities relative to both the eastern and western sides of the basin. There is significant seismic activity at a depth of about 20 km, mainly in the centre and the northern part of the basin.

At shallow depths (<15 km) there is more seismic activity on the eastern side of the basin than on the western side, and the northern basin is generally more active than the southern basin. Asymmetry is also observed in the faults that border the Dead Sea Basin. The Arava Fault on the eastern side, with nearly vertical dip faulting, appears to be a clear boundary at all depths down to about 20 km. The depth extension of the Jericho Fault on the western side of the basin is definitely limited to less than 15 km. At greater depths of 20 km or more, the western side is only partially bounded by a fault. The concentration of earthquakes in the central part of the basin at depths larger than 15 km suggests that the Dead Sea Fault at those depths acts as one single fault that is located in, or near, the central axis of the basin.

The existence of a number of clusters of earthquakes that spread from shallow depths of a few kilometres up to a depth of about 22 km, points to several defined faults that traverse the Dead Sea Basin. One such example is the aftershock sequence of the 2004 earthquake that occurred in the northern Dead Sea Basin. Seismic activity near Mt Sodom is relatively low and occurs at shallow depths down to 10 km, whereas at larger depths (≥15 km) it ceases. This implies that the whole structure is relatively shallow with no wide and deeper extension. Seismic activity near and within the Lisan Peninsula extends to somewhat larger depths (~15 km), and then it ceases.

The occurrence of earthquakes at large depths suggests that the upper and the lower crust are relatively cool, as was also suggested by earlier studies, pointing to the fact that the heat flow is significantly below the global average value.

**Key words:** Seismicity and tectonics; Seismic tomography.

## INTRODUCTION

The Dead Sea Basin is the largest basin within the Dead Sea Fault. Tectonics has shaped the Dead Sea Fault in general and the Dead Sea Basin in particular over the past few million years (e.g. Freund et al. 1970; Ben-Menahem et al. 1976). Various seismological, geophysical (i.e. refraction, reflection, gravity) and geological studies provided important information about the structure and the 1-D velocity model of the Dead Sea Basin and adjacent regions (e.g. Garfunkel et al. 1981; van Eck & Hofstetter 1989, 1990; Ben-Avraham & Ginzburg 1990; Hofstetter et al. 1991; Rotstein et al. 1991; Ben-Avraham 1997; Ginzburg & Ben-Avraham 1997;

Hofstetter et al. 2000; Al-Zoubi et al. 2002, 2006, 2007; Aldersons et al. 2003; Shamir et al. 2005; Lazar et al. 2006; Shamir 2006; Hofstetter et al. 2007; Mechie et al. 2009; Mohsen et al. 2011). The common model of the Dead Sea Basin, which describes its structure, is of a pull-apart basin, where the Jericho and Arava faults border it on the west and east, respectively (e.g. Neev & Hall 1979; Garfunkel 1981). These two faults overlap in the Dead Sea Basin to form two en-echelon strike-slip faults. The Dead Sea is usually divided into two main subbasins, a northern and a southern one, with the Lisan Peninsula, a large buried salt diapir, separating them (Fig. 1), where the Dead Sea Lake covers only part of the whole basin. A sedimentary sequences of about 6 and 13 km, characterizes

<footnote n="1">¹Geophysical Institute of Israel, Lod, Israel. E-mail: rami@seis.mni.gov.il</footnote>

<footnote n="2">²Ecole et Observatoire des Sciences de la Terre, Strasbourg, France</footnote>

<footnote n="3">³IRD, UMR 154, Toulouse, France</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Left-hand panel: schematic map of the Eastern Mediterranean Sea including the Dead Sea Fault; rectangle indicates area enlarged on the right. Right-hand panel: tectonic setting of the Dead Sea Basin and observed strong earthquakes ($5 \le M \le 6$ open circle; $6 < M$ solid circle) that occurred from 1900 to 2009 along with the main tectonic features, subbasins and schematic faults (solid or dashed where inferred) in the Dead Sea Basin (after Neev & Hall 1979; Bartov *et al.* 2002; Bartov & Sagy 2004; Ben-Avraham *et al.* 2008; Hofstetter *et al.* 2008). The division between the northern and southern subbasins separated by the Lisan diapir (lat. 31.25°N) is also marked, where the Dead Sea Lake covers only part of the whole basin.</figcaption></figure>

each subbasin, respectively (Ginzburg & Ben-Avraham 1997; Hofstetter *et al.* 2000).

Based on historical accounts and instrumental records, some describing destructive and devastating earthquakes, it is clear that seismic activity in the Dead Sea Basin poses a major threat to the population of Israel, Jordan and the Palestinian Authority. Furthermore, since the beginning of the 20th century, several strong earthquakes occurred in the Dead Sea Basin (see Fig. 1). The M 6.2 earthquake of 1927 July 11, resulted in 285 deaths, 940 wounded and extensive damage in many towns and villages on both sides of the Dead Sea Fault (Ben-Menahem *et al.* 1976; Shapira *et al.* 1992; Avni 1998). Throughout the 20th century, several other widely felt earthquakes occurred in 1928, 1956, 1970, 1979 and 2004 with a magnitude range of M 5.0–5.5 (Fig. 1), causing little or no damage in neighbouring regions (e.g. Arieh *et al.* 1982; Amiran *et al.* 1994; Salamon 2005; Al-Tarazi *et al.* 2006; Hofstetter *et al.* 2008). Despite the above-mentioned studies, the Dead Sea Fault is not clearly defined nor is the velocity model well known, rendering less accurate and reliable earthquake location.

Local earthquake tomography can serve as an effective tool for studying the seismic properties of the crust and the upper mantle along with improving event locations. The tomographical code of Zhang & Thurber (2003), known as tomoDD, was successfully applied for crustal study of California and other parts of the world, for example, in Algeria (Ayadi *et al.* 2008), and New Zealand (Bannister *et al.* 2006), and also for studying the subducting lithosphere, in Japan (Zhang *et al.* 2004), Chile (Dorbath *et al.* 2008a) and Sicily (Calò *et al.* 2009). In our study, we analyse local earthquake data from almost three decades (1983–2009) recorded by the Israeli and Jordanian seismic networks located in the Dead Sea Basin and adjacent regions.

Taking into account historical seismic activity, this study aims at improving our understanding of the dynamic processes and enhancing the seismic hazard assessment due to activity in the Dead Sea Basin. The main contributions of this study are: (1) a new 3-D velocity model of the crust and the upper part of the upper mantle, which has never been calculated before; (2) improved location of earthquakes within the Dead Sea Basin and (3) a more accurate determination of the structure and location of the Dead Sea Fault within the Dead Sea Basin.

## DATA ACQUISITION AND PROCESSING

Seismic activity in the Dead Sea Basin and adjacent regions is monitored by the Israel Seismograph Network (ISN; GII Seismological Bulletins 1983–2009) and the Jordan Seismological Observatory (JSO; Seismological Bulletins 1983–2009), which include short-period and broad-band stations (Fig. 2a). Each short-period station is equipped with a 1-Hz natural frequency seismometer, with an effective relatively flat operating response range between 0.4 Hz and about 20 Hz. A general description of a short-period station is given by Lee & Stewart (1981), and a similar description of a typical ISN station is given in GII Bulletins (1984–2009) and by van Eck & Hofstetter (1989). Each broad-band station is composed of an STS2 seismometer with a Quanterra data logger and a flat response in the frequency range from 0.01 Hz to about 80 Hz.

We use a compilation of 614 earthquakes (of 1202), which occurred in the Dead Sea Basin or in its adjacent vicinity (Fig. 2a), with an average of nine stations for each event of a total of 45 stations that operated at various periods of time. A total of 4009 P-wave arrivals and 1521 S-wave arrivals, picked from the recordings
<figure><img src="image_3.png" /><figcaption>**Figure 2.** (a) and (b) present the locations of earthquakes before and after the application of the tomographical code, respectively. Seismic stations that were in operation for less than 10 yr and for more than 10 yr are presented by open and solid triangles, respectively; (c) number of earthquakes as a function of the depth for the entire Dead Sea Basin (circles), northern Dead Sea Basin (triangles) and southern Dead Sea Basin (squares). Horizontal dashed lines divide the crust into upper and lower units, as well as the upper mantle, following the velocity models of Feigin & Shapira (1994) and Gitterman *et al.* (2005).</figcaption></figure>

of the short-period and broad-band seismic stations, are reported from 1983 to 2009 by the ISN and JSO bulletins. All traces of each event were visually compared and correlated using distinct phases, such as *P* and *S*. The reading error is estimated to be less than 0.05 s. We discarded doubtful readings, that is, unclear emergent arrivals or poor signal-to-noise ratios. Each of the analysed events was recorded by at least six phases, and most of them included over 10 phases, implying a small scatter of errors due to varying number of observations for a given event. The initial velocity model includes six layers at depths of 5, 10, 15, 20, 25 and 30 km, with velocities of 5.6, 5.9, 6.2, 6.6, 7.1 and 7.6 km s⁻¹, respectively.

The hypocentres and the velocity model of the Dead Sea Basin were obtained using the tomoDD algorithm of Zhang & Thurber (2003). The algorithm determines a 3-D velocity model jointly with the absolute and relative event locations. This approach has the advantage of including relative arrival times with their quality values along with absolute arrival times, thereby not discarding valuable information by only using adjusted picks. At the same time, this approach dispenses with simplifying assumptions about ray path geometries, or path anomalies and produces absolute and not just

relative, locations. The velocity model obtained with double difference (DD) tomography should also be superior to that acquired by standard tomography (Zhang & Thurber 2003). Differential data have been collected linking pairs of events with initial interevent distances of less than 10 km and recorded by stations that are up to 100 km away from the epicentre. The total resulting data set consists of 42 603 pairs comprising 34 610 *P*-phase pairs and 7993 *S*-phase pairs. The tomoDD algorithm has been applied to both *P*- and *S*-wave picks. Because the number of *S*-wave picks is rather small we present results that are only related to *P*-wave velocities.

To facilitate the presentation of results, the initial inversion grid used (Fig. 3a) had an origin of a Cartesian system centred at (31.3°N;35.5°E). All distances are expressed in kilometres relative to this point. The grid spacing along each direction is 10, 8 and 5 km, in *X*, *Y* and *Z*, respectively. Thus, the inverted gridpoints are at *X* (longitude) = −63, −48, −33, −23, −13, −3, 7, 17, 27, 37, 52, 67 km, at *Y* (latitude) = −90, −75, −60, −50, −40, −32, −24, −16, −8, 0, 8, 16, 24, 32, 40, 48, 56, 64, 74 km and at *Z* (depth) = 0, 5, 10, 15, 20, 25, 30, 35 km. The final velocity model is further optimized by applying the Weighted Average Model (WAM)
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Application of the WAM method to the Dead Sea Basin: (a) the grid in the Dead Sea Basin used for inversion; (b) two inversions of a total of 14, where the first grid is aligned in the N–S direction and the second is slightly rotated clockwise. The inset presents an example of the distribution of gridpoints of all 14 inversions in the southern part of the Dead Sea Basin. Inset (lower right side): solid crosses show the original gridpoints and the other crosses represent the gridpoints for the additional inversions.</figcaption></figure>

method (Calò 2009 and web page; Calò *et al.* 2008, 2009). The WAM method is a post-processing technique that can be applied to velocity models obtained by any tomographic inversion code, and it is based on resampling of the model, compatible with the data set used and the synthesis of a new and more reliable model. In our case, we resampled the space of velocity models 14 times; each time it was either rotated by a small angle (10 times) or translated by 1/3 of a grid distance to the sides (four times). At each node, we obtained a vector of velocity values with dimensions equal to the number of inversions carried out. The velocity WAM is composed of the weighted mean of velocities obtained from each inversion. Only velocity values for which we obtained Derivative Weight Sum (DWS) > 50 (Toomey & Foulger 1989) were considered in the construction of the average model. In general, the larger the DWS the more limited the resolved area is. This is a conservative value compared with other studies, that is, DWS > 10 used by Dorbath *et al.* (2008b), and DWS > 25 used by Bannister *et al.* (2006) and Ayadi *et al.* (2008). Fig. 3(a) illustrates the initial grid in the Dead Sea Basin. Fig. 3(b) shows two inversions of a total 14, where the first grid is aligned in the N–S direction and the second is slightly rotated clockwise. The inset presents an example of the distribution of gridpoints of all the 14 inversions in the southern part of the Dead Sea Basin.

To assess the reliability of our results and to show the resolution power of the data and method, we constructed a checkerboard model characterized by alternating positive and negative velocity changes of ±2.5 per cent with respect to the initial 1-D velocity model, with dimensions of 20, 16 and 10 km along the longitude, latitude and depth, respectively. For this model, with the same configuration of earthquakes and stations as in the real inversion, we calculated synthetic traveltimes. Possible traveltime errors were simulated by adding a vector of random error with a standard deviation of 0.1 s. The 1-D initial model was then used as the starting velocity structure for the inversion of the perturbed synthetic database. Fig. 4 shows the checkerboard test results for the layers at depths of 5 and 20 km, two E–W cross-sections at latitudes 42.5 and –20 km, and one N–S cross-section along longitude –2 km (see Fig. 4a for locations of the cross-sections). The best resolved areas are located in the centre

of the domain, with a depth ranging between 2.5 and 20 km, in correspondence with the largest ray path lengths and densely criss-crossed grid cells. However, the anomaly pattern is sufficiently reproduced for most part of the investigated volume, at least down to a depth of about 25 km.

## RESULTS

Figs 2(a) and (b) present observed earthquakes and reporting stations in the Dead Sea Basin and adjacent areas used in this study before and after applying the tomographical code (Zhang & Thurber 2003; Calò *et al.* 2009). The new locations of earthquakes are less scattered and there are a few denser and clear clusters of events within the Dead Sea Basin itself. The average rms of the initial located events was 0.37 s whereas after the relocation with the 3-D model it showed a significant reduction to a value of 0.11 s. The majority of earthquakes occurred in the northern part of the basin. Seismic activity in this area can be divided into two distinct groups, one in the northernmost part of the basin, and from the central part southward towards the Lisan. Another area of seismic activity is located in the southernmost part of the Dead Sea Lake. There is also a spread of seismic activity south and north of the Dead Sea Lake, near its flanks and in between the above-mentioned groups. Fig. 2(c) presents the number of earthquakes as a function of depth for the entire Dead Sea Basin. There are clear peaks of seismic activities at depth of 15 km for the entire basin and the two subbasins, in agreement with observations of Aldersons *et al.* (2003). This means that a large part of seismic activity in both subbasins occurs either in the lower part of the upper crust or in the upper part of the lower crust.

Later, we describe a series of horizontal and vertical cross-sections (see locations in Fig. 5a) within the Dead Sea Basin, along with relocated earthquakes. Fig. 5(a) presents projected seismicity both on N–S and E–W cross-sections. The N–S cross-section exhibits two interesting features: (1) there are several clear clusters of seismic activity about every 20–22 km that start at shallow depths and continue to a depth of about 25 km and (2) a large part of seismic
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Application of the checkerboard test along with changes of P-wave velocity: (a) depth of 5 km. The short lines at latitudes 42.5 and −20 and longitude −2 mark the cross-sections in C, D and E, respectively. Earthquakes at these depths are presented by black circles, and the stations on the surface (red triangles) are shown for reference; (b) similar to (a) but for a depth of 20 km; (c) and (d) E–W cross-sections along latitudes 42.5, and −20 (see Fig. 4a for location); (e) N–S cross-section along longitude −10 (see Fig. 4a).</figcaption></figure>

activity is focused in the northern subbasin. The 2004 earthquake was the only strong one with a sequence of aftershocks during this period of time (lat. 40 in Fig. 5). This sequence was rather short time-wise and also in the number of aftershocks. The rest of the seismic activity in the Dead Sea Basin was neither intense nor concentrated in a specific location, and occurred throughout the whole period of observation. Thus the clusters, that is, latitudes 20 and −22 in Fig. 5, refer to concentrated activity in space that occurred over many years (>10 yr). Although the hypocentres are projected of a single plane, the E–W cross-section shows a large concentration

of seismic activity eastward of the centre of the basin relative to the western side. Below, we discussed these observations.

Fig. 5 presents E–W and N–S depth–velocity cross-sections in the Dead Sea Basin (see locations of cross-sections in Fig. 5a). For each vertical or horizontal section, only the best resolved area (DWS > 50) is shown. Figs 5(b)–(g) presents a series of horizontal velocities at the following depths: 2.5, 5 and 10 km for the upper crust, 15, 20 and 25 km for the lower crust. The range of each scale is ±1 km s⁻¹ where the central velocity value presents the initial velocity of the specific layer that is used in the inversion, assuming a
<figure><img src="image_6.png" /><figcaption>**Figure 5.** (a) Final locations of earthquakes in the Dead Sea Basin along with their N–S and E–W depth projections. Lines 1–7 mark the locations of the E–W and N–S cross-sections, shown in Fig. 6; (b–g) velocities of layers at depths of 2.5, 5, 10, 15, 20 and 25 km, along with earthquakes (black square) in a layer of ±2.5 km on both sides of the central depth. The seismic stations (red triangles) and the coast of the Dead Sea Lake are shown for reference. The range of each scale is ±1 km s⁻¹ where the central velocity value presents the initial velocity of the specific layer that was used in the inversion, assuming a gradual increase of velocity with depth. The spacing between two adjacent isolines is 0.5 km s⁻¹.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 5. (Continued.)</figcaption></figure>

gradual increase of velocity with depth. The layer at depth 2.5 km is not part of the inversion grid and it is shown here for demonstrating the very shallow seismic activity of the basin. The located earthquakes are presented in the cross-sections as well, where the

thickness of the earthquake layers on both sides of a given horizontal and vertical cross-section is ±2.5 and ±4 km, respectively. At large depths (>25 km), the number of earthquakes is small and the velocities are poorly resolved and therefore they are ignored. Based
on the checkerboard test, for all depths the best resolved areas are located inside the Dead Sea Basin and its immediate vicinity. Some parts of the bordering regions present pronounced low and high-velocity anomalies, where the amplitudes are poorly resolved and therefore ignored. In our analysis, especially at shallow depths, we also relate to the prominent salt domes of Mt Sodom and the Lisan Peninsula, which are clear on-surface land marks (see Fig. 1).

The layer of velocities at 2.5 km includes earthquakes in the depth range from 0 to 5 km, that is, the upper part of the upper crust (Fig. 5b). Earthquakes are distributed throughout the entire basin, however it is possible to identify a few diffused clusters of seismic activity: north of the Lisan Peninsula at latitude 20, the northern part of the Lisan, and southern part of the Dead Sea Lake. Some sparse activity is located outside the basin. The whole Dead Sea Basin is characterized by low velocities with some moderate undulations of relatively low-velocity anomalies south of the Lisan area and south of the lake itself, and two similar velocity values in the middle of the northern Dead Sea Basin and north of the lake. On the other hand, both sides of the basin exhibit high-velocity anomalies, which can be explained by the relatively high velocity of the hard limestone and dolomite sequence on the western side and the sandstone and granitic sequence on the eastern side. Mt Sodom (long. -10; lat. -20; see Fig. 1) is located at the southwest corner of the southern Dead Sea subbasin, with no clear tectonic feature, and is characterized by velocity values of 4.8-5.0 km s⁻¹. Salt is anomalously fast if we consider an equivalent light density material, but it is still slow relative to the carbonatic (i.e. dolomite) and granitic sequences on the flanks of the basin. These values are higher than the common velocity values of salt—about 4.5 km s⁻¹ (i.e, Dobrin 1952). The diapir may stem from a salt layer that is located at a depth of 5-6 km with thickness of less than 1 km (Gardosh et al. 1997; Ginzburg et al. 2006). Given the resolution of our study it is unlikely that such a layer can be observed. The Lisan diapir, a larger on-land feature relative to Mt Sodom with a thickness of 5 km (Abu Ajamieh et al. 1989), is characterized by velocity values of 4.3-4.6 km s⁻¹, which are related to the salt dome.

Fig. 5(c) shows the velocities at depth of 5 km (middle upper crust). The main features in this layer are similar to those of 2.5 km. The Dead Sea Basin and its margins are characterized by relatively low and high-velocity values, respectively, with the continuation of the above-mentioned anomalies that are observed at a depth of 2.5 km (Fig. 5b) down to this depth. The low-velocity anomalies north and south of the Dead Sea Lake are poorly resolved in the checkerboard test (Fig. 4). The velocity values of 4.8-5.0 km s⁻¹ in the Lisan area are somewhat higher than common salt values, which imply that the salt diapir does not continue to a depth of 5 km. While earthquakes are distributed throughout the entire basin, it is possible to identify a few dispersed clusters of seismic activity in the basin: the location of the 2004 earthquake and its aftershocks around latitude 45 (see also Fig. 2), around latitude 20, the northern part of the Lisan, and the southern Dead Sea Basin around latitude -20. Especially in the northern basin, earthquakes are distributed along E-W trending lineaments. There is also some diffused activity outside the basin mainly on the eastern side.

Fig. 5(d) presents the velocity anomalies at a depth of 10 km (lower part of upper crust). The Dead Sea Basin is characterized by alternating high and low-velocity anomalies. The high-velocity anomalies on the eastern side (latitudes 25 and -10) and the low-velocity anomalies west and north of the Dead Sea Lake are poorly resolved in the checkerboard test. The earthquakes are distributed throughout the whole basin with some diffused clusters in several

distinct areas: (1) northern part of the Dead Sea in the same area of the locations of the 2004 earthquake and its aftershocks (Hofstetter et al. 2008), essentially along latitude 45; (2) two distinct areas in the central part of the northern subbasin, just north of the Lisan Peninsula (latitudes 17 and 28); (3) the Lisan Peninsula and west of it (latitude 0) and (4) the southern Dead Sea and especially the southeastern edge of the basin (latitude -25).

Fig. 5(e) presents the velocity anomalies at a depth of 15 km (upper part of lower crust). The Dead Sea Lake and south of it are characterized by undulations of low and high-velocity anomalies. These pronounced anomalies on the eastern side and the northwest corner of the bordered area are poorly resolved in the checkerboard test. The western side is characterized by alternating velocity anomalies suggesting that the Jericho Fault on the western side of the pull-apart basin is limited to this depth or shallower. Unlike the shallower depths, the earthquakes are more concentrated in the central part of the basin, and are mainly distributed in three distinct areas: (1) northern part of the Dead Sea Basin (around latitude 45) in the same area of the locations of the 2004 February 11, earthquake and its aftershocks; (2) widely distributed in the central part of the basin, just north of the Lisan Peninsula (latitude range from 10 to 25) and (3) southeastern most edge of the Dead Sea Basin (latitude -25). There is also some diffused activity north and south of the basin.

Fig. 5(f) shows the velocity anomalies at a depth of 20 km (middle part of lower crust). The Dead Sea Lake is characterized by fairly resolved low-velocity anomalies. The high-velocity anomalies on the western and eastern sides are properly and poorly resolved, respectively (see Fig. 4b). Earthquakes are mainly concentrated in the central part of the basin, and are distributed in four distinct areas: (1) northern part of the Dead Sea in the same area of the locations of the earthquake on 2004 February 11, and its aftershocks (latitude 45); (2) central part of the basin (latitude 20); (3) just north of the Lisan Peninsula (latitude 10) and (4) west of the Lisan Peninsula (latitude -5). There is also some diffused activity north of the basin and somewhat more concentrated activity south of the basin (latitude -40).

Fig. 5(g) presents the velocity anomalies at depths of 25 km (lower part of lower crust), which are fairly resolved between north of the Lisan and the southern Dead Sea subbasin (between latitudes -22 and 13) and in the northern Dead Sea subbasin near latitude 45. These areas are characterized by both low and high-velocity anomalies, respectively. Anomalies along the margins of the basin are poorly resolved. Earthquakes are distributed along the north-south central axis of the northern Dead Sea subbasin with a distinct concentration west and north of the Lisan Peninsula. The southern Dead Sea subbasin is depleted of seismic activity.

The E-W cross-sections (#1-4, Fig. 6) are selected for parts of the Dead Sea Basin that exhibit significant seismic activity. In general, earthquakes occurring at the shallow depths (≤10 km) are spread close to the flanks of the Dead Sea, whereas at deeper depths (≥15 km) most of seismic activity occurs along the central part of the basin. The basin's sides are essentially depleted of deep seismicity (see also Fig. 5). The red Y-shape lines (Fig. 6) represent the most likely faulting in each cross-section based on seismicity. We note that at shallow depths there is more seismic activity on the eastern side of the basin than on the western side (see also Fig. 5a). The continuation of the Dead Sea Fault to larger depths is not necessarily vertical and there can be some changes in the dip of the fault. For example, the E-W cross-section #3 in Fig. 6 cuts the Lisan Peninsula. In this case, as shown in Fig. 5, most of the seismic activity is shifted westwards to the straits of the Lisan.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** N–S and E–W cross-sections in the Dead Sea Basin (see Fig. 5 for locations). Solid triangles and squares represent seismic stations and earthquakes, respectively. The red Y-shape lines represent the most likely faulting in each cross-section based on seismic activity.</figcaption></figure>

In all N–S cross-sections (#5–7, Fig. 6), we observe several clusters of earthquakes at distinct latitudes that are spread from shallow depths (about 5 km) to greater depths (≥20 km), that is, at latitudes -25, 0, 20 and 40. There are also several clearly defined clusters at large depths, that is, at latitudes 0 and a depth of 25

km in cross-section 5, and latitudes 20 and 45 and a depth of 20 km in cross-section 6. These can be related to activity in the central part of the basin. As the Dead Sea is slightly rotated in a clockwise direction, the northern and southern parts of N–S cross-sections 5 and 7 are outside the basin area, respectively. However, the
cross-sections still they present the seismic activity near the flanks of the basin. Again, we can see that at shallow depths there is more seismic activity on the eastern side of the basin than on the western side. The shallow upper layers (<7 km) and the deep part of the lower crust (>20 km) are laterally stratified. On the other hand, the middle layers (7 km < depth < 20 km) exhibit some horizontal velocity variations, that is, at latitudes 42 and -5 (N-S and E-W cross-sections in Fig. 6).

## DISCUSSION

The discussion section is divided into a few subsections dealing with different topics: the velocity structure of the Dead Sea Basin, seismic activity inside the basin and adjacent regions, the Dead Sea Fault and the northern and southern terminations of the Dead Sea Basin.

### Velocity structure of the Dead Sea Basin

The entire Dead Sea Basin is characterized at all depths down to 25 km by lower velocity anomalies relative to the high-velocity anomalies of the bounding flanks (Fig. 5). These low-velocity anomalies are not homogenous throughout the basin and there are some clear anomaly variations. The very low Bouguer gravity anomalies, reaching -130 mGal in the Lisan Peninsula, and negative throughout the entire basin (ten Brink et al. 1993, 2001) are well correlated with the observed low-velocity anomalies, which imply low mass density in the Dead Sea Basin from the surface to a depth of about 10 km.

Based on geological and geophysical evidence, the Dead Sea Basin is divided into a southern and a northern subbasin, which are separated by the Lisan Peninsula that acts as a buffer zone. Subsequently, each subbasin is further divided into at least two smaller subbasins (e.g. Neev & Hall 1979; ten-Brink & Ben-Avraham 1989; Ben-Avraham 1997; Ginzburg & Ben-Avraham 1997; Garfunkel & Ben-Avraham 2001; Lazar et al. 2006; Ben-Avraham et al. 2008). The northern part of the southern subbasin includes industrial evaporation ponds on the Jordanian and Israeli sides (Southern Lake Basin) and is bounded in the north by the Lisan Peninsula and the Amazzyahu Fault in the south. The southern part, known as the Idan Basin, is located between the Amazzyahu and Idan faults in the central Arava.

The northern subbasin is divided into a series of smaller subbasins (Garfunkel 1997; Lazar et al. 2006; Ben-Avraham et al. 2008). The joint lake-land Kalia subbasin exhibits low seismic activity. Down to a depth of 10 km, it has lower velocity values in comparison with the sides of the basin suggesting relatively low-density material. The transverse Ein-Gedi (Neev & Hall 1979) and Kalia (Lazar et al. 2006) faults at latitudes 20 and 45, respectively, which bound the Arnon sink are aligned in the E-W directions and exhibit high seismic activity from depths of 10 km to 20 km with less activity at shallower or larger depths (Fig. 5).

ten-Brink et al. (2006; fig. 1 therein) presented detailed results of a refraction study based on a long NW-SE cross-section through the Dead Sea rift near the Amazzyahu Fault and the northern part of the Idan Basin. Mechie et al. (2009) conducted a wide-angle refraction/reflection (WRR) study based on a long E-W cross-section through the Lisan Peninsula in the Dead Sea Basin (lat. -5 in Fig. 5). Mohsen et al. (2011) examined the WRR cross-section of Mechie et al. (2009) using receiver function data. The observed sedimentary sequence, which was reported by Ginzburg & Ben-Avraham

(1997), Hofstetter et al. (2000) and Gardosh et al. (1997), based on the Sdom Deep borehole, is similar to the structure in the southern part of the lake, about 10 km north of the cross-section of ten Brink et al. (2006) and just south of the cross-section of Mechie et al. (2009). At depths from 20 km to the Moho at about 32 km, they reported that there are no changes of the lower crust and any perturbation of Moho topography is within the data resolution. Fig. 5 shows a low-velocity anomaly, relative to the other parts of the basin for any given depth from 0 to 20 km. The results of velocity changes with depth in the sedimentary sequence of ten Brink et al. (2006) are in very good agreement with our results as presented in Fig. 5, where down to a depth of 15 km differences in velocity values do not exceed 0.2 km s⁻¹. At larger depths (>15 km), velocity differences are slightly larger and reach 0.4–0.6 km s⁻¹, as low-velocity anomalies are still observed.

We can compare the results of the refraction cross-section outside the Dead Sea Basin on the Israeli side northwest of this low-velocity anomaly. However, this is not possible on the Jordanian side as the number of reporting stations southeast of this anomaly is relatively small, thus limiting the resolved area (see Fig. 5). Outside the rift, the agreement with results of the refraction study is even better than that inside the rift, and velocity differences at all depths are smaller and do not exceed 0.2 km s⁻¹. The results of variations in velocity with depth in the sedimentary sequence, upper crust and lower crust of Mechie et al. (2009) are slightly lower by 0.1–0.3 km s⁻¹ at all depths compared with our results as presented in Fig. 5. Due to the relative left-lateral movement of about 105 km between both sides of the Dead Sea rift, the bounding flanks are geologically different. A sequence of sandstone overlies the granitic rocks on the eastern side whereas a limestone-dolomite sequence is found on the western side. These geological differences are apparent also in the determined velocity values. At all depths, from shallow down to about 20 km, the eastern side has equal or higher velocity values than the western side, which is in very good agreement with the refraction results of ten Brink et al. (2006) and Mechie et al. (2009). We cannot compare the velocities at larger depths (>20 km) due to the limited resolution on the eastern flank.

### Seismic activity in the Dead Sea Basin

In this study, we used a compilation of 614 earthquakes that occurred in the Dead Sea Basin or in its adjacent vicinity (Fig. 2b). The 2004 earthquake $M_d$ 5.1 was the only strong one with a short temporal sequence of aftershocks (lat. 40 in Fig. 5). Through the observation period of 26 yr, the rest of the seismic activity in the Dead Sea Basin was neither intense nor concentrated in a specific location during relatively short times. Thus the clusters, that is, at latitudes 20 and -22 in Fig. 5, refer to concentrated activity in space that slowly spread over many years (>10 yr). It is obvious that a longer observation period, containing a larger number of earthquakes, would be better but even the presented seismic activity can provide good insight into the characteristic activity in the Dead Sea Basin.

The distribution of relocated seismicity enables us to better identify active faults in the basin. The faults on the eastern and the western sides clearly border the Dead Sea Basin at depths of up to 20 and 10 km, respectively, rooted in the middle lower crust and lower part of the upper crust (Fig. 5). This suggests an asymmetry in the structure of the Dead Sea Basin, which is in agreement with the asymmetrical structure that was presented by Zak & Freund (1981) and Wdowinsky & Zilberman (1996) based on geological
observations. The fact that the Arava Fault on the eastern side appears to be a clear boundary of the basin at almost the same location at all depths down to about 20 km suggests a nearly vertical dip faulting to this depth. The depth extension of the Jericho Fault on the western side of the basin is definitely limited to less than 15 km. We note that at shallow depths (<15 km) there is more seismic activity on the eastern side of the basin than on the western side (Figs 2a and 5) and the northern basin is generally more active than the southern basin. Those observations can be explained by the model of Garfunkel (1981) in which the geometric relations of the structures along the transform define an Eulerian pole of relative plate motion at (32.8°N; 22.6°E). Furthermore, much of the transform is leaky and in a rift valley, which is delimited by normal faults, with a small component of transverse extension, and hardly any local transverse compression.

The clearly defined clusters of earthquakes that are spread from shallow depths of a few kilometres up to depths of 20 km and more (see Figs 2 and 5) suggest the existence of several active faults that traverse the Dead Sea Basin. For example, from north to south: the aftershock sequence of the 2004 earthquake in the northern Dead Sea Basin illustrates such activity (Hofstetter et al. 2008), the activity in the middle of the northern subbasin, which is related to the Ein-Gedi Fault (Neev & Hall 1979; Lazar et al. 2006), and the activity in the middle of the southern subbasin, near the termination of the lake itself (also reported by van Eck & Hofstetter, 1989, 1990).

Seismic activity in the Idan Basin, which is situated between latitudes -25 (southern end of the lake) and -65 in Fig. 5, is diffused at all depths and does not delineate any clear fault. The pronounced seismic activity at the southern end of the lake extends in an E-W direction and is located just north of Amazyahu Fault, where the latter appears to be almost inactive. Geophysical studies conducted by Ginzburg & Ben-Avraham (1997) and Hofstetter et al. (2000) indicated a thick sedimentary sequence reaching 13 km and more in the Southern Lake Basin. The northern border of this basin was identified by Ben-Avraham & Schubert (2006) and Ben-Avraham et al. (2008) as the Bokek Fault, which is the southern border of the Lisan salt diapir and traverses the basin in an SE-NW direction. Furthermore, the authors suggested that the basin is still subsiding. Seismic activity along the supposed fault is sparse at depths from 5 to 15 km and occurs mainly on the eastern side (latitudes -10 to -15 in Fig. 5), which can be equally related to the Arava Fault. The lack of clear seismic activity surrounding the above-mentioned basin, except along its southern border, points to the fact that currently no major subsidence is taking place.

We note that an overwhelming majority of earthquakes in the Dead Sea Basin (>90 per cent) occur within the lake itself (Fig. 2). South of the lake, there is a clear velocity anomaly at all depths from 2.5 to 20 km that is characterized by some limited seismic activity (Figs 2, 5 and 6). To the north, there is some diffused activity at all depths while outside the lake, in other directions, activity is minimal. Most of seismic activity that takes place in the northern subbasin is spread at all depths, whereas activity in the southern subbasin is largely at depths of 7-17 km with some activity at shallower and deeper depths (Fig. 2c). This depth range, from 7 to 17 km, fits very well the upper crust model as presented by ten Brink et al. (2006; fig. 1 therein), in which most or all deformation in the crust is taking place. According to the authors, there is no indication for a low-velocity anomaly that extends into the lower crust. However, the existence of shallow or deep seismic activity and the determined velocity changes in deeper layers, for example, at 20 km, point to the fact that some deformation takes place not only in the sedimentary

section above the crystalline upper crust but also in the lower crust at about 20 km (Figs 5 and 6).

The prominent salt diapirs of Mt Sodom and the Lisan Peninsula are clear on-surface landmarks. Seismic activity near Mt Sodom is relatively low and occurs at shallow depths down to 10 km, whereas at larger depths (≥15 km) it ceases. This implies that the whole structure is relatively shallow with no deeper roots. Seismic activity near and within the Lisan Peninsula extends to somewhat larger depths (~15 km), and then it ceases. The shallow seismic activity can be related to upward movement of the salt diapirs of Mt Sodom and the Lisan Peninsula (ten Brink & Ben-Avraham 1989; Pe'eri et al. 2004; Weinberger et al. 2006; Frumkin 2009). Activity west of the peninsula at depths of 20 km and deeper is probably correlated with the Dead Sea Fault and not the updoming of the salt diapir.

We note that there is a significant number of observed earthquakes at depths of 20 km and more. Over 50 per cent of seismicity in the entire basin occurs in the depth interval between 10 and 20 km, and about 30 per cent occurs in the shallower part of the crust (Fig. 2c). The relatively deep seismicity suggests that the upper and lower crusts are relatively cool. This fact is supported by earlier studies (Ben-Avraham et al. 1978; Eckstein 1978; Eckstein & Simmons 1978; Feinstein 1987) in which the heat flow was shown to be significantly below the global average value. Based on a 1-D velocity model obtained from local earthquake tomography of the Dead Sea Basin and the Jericho Valley, and heat flow values, Aldersons et al. (2003) proposed a possible composition of the Dead Sea Basin. The results of this study, based on a larger database, more accurate earthquake locations and taking into account the distribution of the earthquakes within the subbasins, suggest that other models should be considered (see later).

Fig. 1 presents several strong earthquakes ($M > 5$) that occurred in the Dead Sea Basin or just north of it (such as the 1928 earthquake). We note that close to the epicentres of the earthquakes from 1956, 1970, 1979 and 2004 there is significant seismic activity. However, diffused activity occurs near the epicentre of the 1928 earthquake while the location of the 1927, $M = 6.2$, earthquake along the Jericho Fault is depleted of any seismic activity. These observations should be further studied as station coverage of the Jordanian and the Israeli seismic networks from 1983 to 2009 has been relatively uniform and any relatively small event of $M \sim 2$ (or smaller) should have been recorded. Arieh et al. (1982) determined the focal mechanism of the 1979 earthquake to be strike-slip with two nodal planes striking essentially in N-S and E-W directions. Given the location and depth of the earthquake it is likely that it was initiated along the main Dead Sea Fault and thus favours a left-lateral strike-slip mechanism.

The overall low seismic activity in the southern subbasin relative to that in the northern subbasins (Fig. 2c) may have a few implications: (1) the southern Dead Sea subbasin is probably close to a state of equilibrium, otherwise one would expect to see more seismic activity in general and more uniform activity in various parts associated with vertical movements along the flanks of the basin or of inner blocks; (2) the fact that the water level in the northern subbasin has been constantly dropping (over 30 m in the last 30 yr), implies a different load of mass column and stress regime, which is probably manifested in seismic activity. The relation of changes in water level (normally artificial reservoir loading) and seismic activity was studied in several cases, such as Oroville Reservoir, California (Bell & Nur 1978), Koyna Reservoir, India (Gupta 1983; Gupta & Rambabu 1990) and Aswan Lake, Egypt (Awad & Mizoue 1995; Hassoup 2002), among many others. However, there are several reports for clear changes in the seismic activity, including some
moderate to strong events, that are correlated with seasonal changes of water levels and even with the decrease of the water level, that is, Koyna Reservoir, India (Gupta & Rastogi 1976; Talwani 1997), Sea of Galilee just north of the Dead Sea Basin (Kafri & Shapira 1990) and Lake Van, Turkey (Utkucu 2006), among others. (3) On the other hand, the southern subbasin was in historic times (at least 2500 years ago) subaerial as it is today, and no major change of the mass column has occurred there. Changes in water levels, which are more pronounced in the last 30–50 years, can explain part of the seismic activity in the northern subbasin but definitely the tectonic setting plays an important role, as evident in the southern subbasin or north and south of the water body.

The lack of a well-documented seismic catalogue that includes small earthquakes, starting as early as the end of the 19th century or the beginning of the 20th century, prior to the extensive usage of water in the Jordan Valley and the decrease of the water level of the Dead Sea, hampers the ability to make a reliable comparison between time periods. Nevertheless, a rough estimation of the contribution of the tectonic and the mass-column components can be obtained. The distribution of seismicity as a function of depth shows a clear peak at 15 km for both subbasins (Fig. 2c). This peak is likely due to tectonic activity and less (or not at all) to changes in water level, especially in the case of the subaerial southern subbasin. If we assume that the increase of seismic activity in the northern subbasin due to changes in the water level takes place in the first 10 km (i.e. only in the upper crust, left of the dashed line in Fig. 2c), then the level of activity in the southern subbasin can be considered as the ‘zero level’. For the depth range of 0–10 km, the difference in the number of events per each depth point between both basins is on the average 10 km, which is estimated as the contribution due to changes in water level. This contribution is comparable with that of the tectonic setting. Taking into account the fact that the northern subbasin is about twice as large as the area of the southern subbasin, only five additional events per each depth point were found. Activity in the lower crust of the northern subbasin, which is assumed to be of tectonic origin, is about three times larger than the equivalent of the southern subbasin (right side of the dashed line in Fig. 2c). The pronounced peak at about 15–20 km implies active tectonic faulting in the northern subbasin. In the unlikely case where the contribution of the changes in water level goes as deep as the lower crust then even by removing the water level effect of five events per depth point, the tectonic contribution is still significant, especially at depths of 15–20 km. In conclusion, the tectonic contribution is significant, especially in the deeper part of the basin.

The two main subbasins can be further divided into smaller size subbasins, such as the Arnon Basin (Neev & Hall 1979). Bartov & Sagy (2004) reported on the Mor structure, an active small pull-apart basin located within the western part of the Dead Sea Basin, west of the Lisan Peninsula, which exhibits recent basin-parallel extension and strike-slip faulting (see Fig. 1). This structure was active from Late Pleistocene times to the present day. Earthquakes located at all depths and especially between 10 and 20 km support these geological observations of present-day activity.

## Dead Sea Fault

The red Y-shape faults plotted on the four E–W cross-sections in Fig. 6 present the general trend of earthquakes in the Dead Sea Basin. At shallow depths of up to 10 km, the earthquakes are located near the sides of the basin (shown by the red branches), whereas at

depths of about 15 km and deeper they are concentrated in or near the middle of the basin. This suggests that the Dead Sea Fault at great depths (>15 km) acts as a one single fault along the entire basin, as was suggested earlier by Lazar (2004). The continuation of the Dead Sea Fault into larger depths is not necessarily vertical and there are some changes in the dip of the fault, as can be seen, for example, on E-W cross-section #3 (in Fig. 6). Previous studies of the Dead Sea Basin, including E–W cross-sections, suggested an overall asymmetric basin with the deepest part close to the eastern border fault, a relatively steep eastern side and a western side comprised of a series of down-faulted blocks (i.e. Kashai & Croker 1987; Ben-Avraham 1992; ten Brink et al. 1993; Garfunkel & Ben-Avraham 1996; and many others). The clear boundaries on the eastern and western sides down to depths of about 20–25 km and about 10 km (Fig. 5), respectively, and the earthquake distribution in the upper 10–15 km represented by the Y-shape faults (Fig. 6) are in accordance with the above-mentioned structure of the basin.

The fact that the Dead Sea Fault at large depths acts as one unit has implications for viewing the Dead Sea Basin as a rhomb-shaped graben or a pull-apart basin or alternatively for applying a shearing model. Normally, pull-apart basins are described as elongated depressions that developed along stepped segments of strike-slip faults, and may involve substantial horizontal extension parallel to the strike-slip segment and subsidence of the overlapping zone (e.g. Freund 1965; Aydin & Nur 1982; Mann et al. 1983; Christie Blick & Biddle 1985; Reches 1987; Garfunkel & Ben-Avraham 1996). Various studies, such as Garfunkel (1981), Garfunkel et al. (1981), Reches (1987), Ben-Avraham & ten Brink (1989), ten Brink & Ben-Avraham (1989), ten Brink et al. (1993), Lazar et al. (2006), Petrunin & Sobolev (2006, 2008) and Ben-Avraham et al. (2008), modelled the Dead Sea pull-apart with the extension of transverse normal faults, confined by two strike-slip segments, based on gravitational and reflection data. Based on the WRR results discussed earlier, Mechie et al. (2009) suggested that the Dead Sea pull-apart basin is essentially an upper crustal feature with upper crustal extension. Below about 20 km the two plates move past each other in a shearing motion. Kashai & Croker (1987) questioned the implication of a truly pull-apart graben for the Dead Sea Basin or its subbasins, suggesting another explanation using shear motion that occurs along a largely south-north trending, slightly arcuate line and consisting of a series of en-echelon left-stepping left-lateral master faults of varying lengths. Where overlap of the two faults occurs, one gets narrow and deep grabens, and the width can be equal to the depth. At large depths, the faults coalesce into a single fault trace in the lower crust (Segall & Pollard 1980). For the southern Dead Sea Basin, the Ein-Bokek Fault (seismically inactive) and seismicity along latitude –20 (see Fig. 5) are the northern and southern sides, respectively, of the pull-apart basin or ‘drop-down’ basin, as suggested by Ben-Avraham & Schubert (2006), and the seismicity on the eastern and western sides is minimal. Seismic activity in the northern subbasin, although widely spaced, is in three main E–W transverse faults along the latitudes: 10, 20 and 40 (Fig. 5) and the eastern and western borders are essentially inactive. The model of Kashai & Croker (1987) does not explain seismic activity along the E–W transverse faults across the basin (see E–W cross-sections in Fig. 6) where most of the earthquakes were rather small (except the 2004 February 11 event). Activity occurred mainly along the E–W transverse faults and was minimal along the main N–S basin bordering faults. Thus, one cannot expect all faults to be seismically active all the times, but the low to inactive seismicity points to the need for a longer period of observations beyond 26 yr, or a more refined model should be sought.
## Northern and southern terminations of the Dead Sea Basin

Fig. 5 shows velocity anomalies and seismic activity north and south of the Dead Sea Lake down to a depth of 20 km and even deeper in the southern part. These areas are not as active as the area of the lake itself however they may be a southward or a northward continuation of the Dead Sea Basin. The location of the termination of the Dead Sea basin on the northern side was discussed in a series of studies (i.e. Kashai & Croker 1987; Rotstein et al. 1991; ten Brink et al. 1993; Garfunkel & Ben-Avraham 2001), suggesting different locations and modes of termination. Although no delimiting transverse fault was determined all studies postulated, based on indirect evidence, that the northernmost part of the basin is somewhere between the lake to the town of Jericho. Recently, Lazar et al. (2006), based on seismic profiles, indicated the existence of the Jericho and Fazael subbasins north of the lake thus suggesting that the northern termination of the Dead Sea Basin is about 10–12 km north of Jericho. This means a northward extension of the whole Dead Sea Basin by another 15 km relative to all former studies. According to their model, the end of the Dead Sea Basin is set by the eastern and western converging boundary faults of the basin. A similar mode of termination probably exists south of the Idan subbasin in the southern part of the Dead Sea Basin (Ben-Avraham et al. 2008). In general, the spatial distribution of seismicity south and north of the Dead Sea Lake and the velocity anomalies (Fig. 5a) can support such a model. However, it seems that a more detailed study of these areas is needed to verify this type of termination mode.

## ACKNOWLEDGMENTS

The study was supported by EOST, Univ. Louis Pasteur, Strasbourg, France and the Earth Sciences and Research Administration, Ministry of the National Infrastructure, Israel. We are grateful to Zvi Garfunkel, Amir Sagy and Michael Lazar for constructive comments and discussion. Some figures in this report were prepared using the GMT program (Wessel & Smith 1991).

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