FIELD OF THE INVENTION [0001] The present invention relates to metallocene compounds useful as olefin polymerization catalysts or catalyst components, and to olefin polymerization processes with an olefin polymerization catalyst containing the metallocene compound. In detail, the invention relates to olefin polymerization catalysts that can catalyze with high polymerization activity the production of olefin polymers having high melt tension, excellent mechanical strength and good particle properties, and relates to olefin polymerization processes using the catalysts. Further, the invention relates to ethylene polymers obtained by the polymerization processes that have good processability and particularly excellent mechanical strength compared to conventional ethylene polymers, and relates to thermoplastic resin compositions containing the ethylene polymers. In more detail, the invention is concerned with shaped articles ical strength compaor films that are obtained from the ethylene polymers or the thermoplastic resin compositions containing the ethylene polymers, or relates to laminate films including the films. Furthermore, the invention is concerned with ethylene polymers that have good processability and easy-opening properties compared to conventional ethylene polymers, and thermoplastic resin compositions containing the ethylene polymers . In more detail, the invention is concerned with shaped articles or films that are obtained from the ethylene polymers or the thermoplastic resin compositions containing the ethylene polymers, or relates to laminate films including the films. BACKGROUND OF THE INVENTION [0002] Olefin polymers are shaped by various methods and used in wide-ranged applications. For example, ethylene polymers are extruded into films or sheets for use in the packaging of foods, liquids or daily sundries. Olefin polymers require various properties depending on the shaping methods or use applications. In the case of T-die extrusion as an example, they require performances such as stable processability even at high speed (high-speed film-forming properties) and small neck-in. [0003] Low density polyethylenes (LDPE) by high-pressure radical polymerization have a high melt tension because of their complicated long-chain branched structures, and show good shaping processability such as small neck-in, thereby finding various uses. However, shaped articles therefrom still have low mechanical strength properties such as tensile strength, tear strength and impact resistant strength. Further, these polymers show poor high-speed film-forming properties in T-die extrusion. [0004] In contrast to LDPE, Ziegler-catalyzed or metallocene-catalyzed ethylene polymers possess high tensile strength, tear strength and impact resistant strength due to their molecular structures, and they are used in applications requiring mechanical strength. However, these polymers have a low melt tension and consequent poor shaping processability. [0005] To solve these problems, [1] LDPE is blended with a Ziegler-catalyzed or metallocene-catalyzed ethylene polymer (Patent Document 1); [2] the molecular weight distribution is broadened by multistage polymerization (Patent Document 2); [3] a long-chain branched ethylene polymer is produced with a chromium catalyst; [4] a long-chain branched ethylene polymer is produced with a specific metallocene catalyst (Patent Document 3) ; [5] macro monomers are copolymerized with use of a specific metallocene catalyst to give a long-chain branched ethylene polymer (Patent Document 4) ; or [6] ethylene and diene are copolymerized with use of a specific metallocene catalyst to afford a long-chain branched ethylene polymer (Patent Documents 6 and 7). However, the method [1] greatly increases costs in the blending of the polymers, and the ethylene polymers obtained by the methods [2], [3], [4] and [5] have a small number of long-chain branches and do not have a sufficient melt tension or shaping processability. Further, the method [6] can deteriorate mechanical characteristics inherent to polymers or can result in gelation when the diene is used in large amounts. [0006] Patent Documents 8 and 9 teach the use of two or more kinds of metallocene compounds or organometallic complexes in order to produce more long-chain branches or to increase the melt tension. However, the number of long-chain branches is still insufficient and problems remain in terms of shaping processability. Further, the catalytic activity is far below the industrial level. [0007] As discussed above, it has been difficult to produce resins having high melt tension and excellent mechanical strength inexpensively and efficiently by means of the conventional catalyst systems or by blending resins. In other words, the development of efficient production processes for ethylene polymers having high melt tension and excellent mechanical strength is important and highly valuable in the industrial production. [0008] When ethylene polymers are used as sealants in packaging materials, the polymers require mechanical strength such as heat seal strength or pack breakage strength to protect the contents. However, packaging materials that are easily opened (have easy openability) attract attention out of consideration for elderly people, infants and disabled people. One of the approaches for easy openability is to appropriately weaken the heat seal strength at the sealed portion. Accordingly, there is a need for ethylene polymers having appropriately low heat seal strength. [0009] The present inventors studied diligently in view of the problems in the art as describe above. They have then found that a single or plural kinds of bridged metallocene compounds having differing cyclopentadienyl-derived groups can afford macromonomers that are a source of long-chain branches and can simultaneously catalyze the repolymerization of the macromonomers into olefin polymers having a large number of long-chain branches, small neck-in in the T-die extrusion, small take-up surge and superior mechanical strength or olefin polymers having small neck-in in the T-die extrusion, small take-up surge and easy opening properties. Such compounds as olefin polymerization catalysts and polymerization processes using the compounds have been found to be capable of efficiently producing the olefin polymers as described above. The present invention has been completed based on the findings. Patent Document 1: WO 99/046325 Patent Document 2: JP-A-H02-53811 Patent Document 3: JP-A-H04-213306 Patent Document 4: JP-A-H08-502303 Patent Document 5: JP-A-H04-213306 Patent Document 6: JP-A-H09-227626 Patent Document 7: JP-A-H04-506372 Patent Document 8: JP-A-H07-252311 Patent Document 9: JP-A-2006-2057 SUMMARY OF THE INVENTION [0010] The present invention has been made in view of the background art as discussed above. It is therefore an object of the invention to provide bridged metallocene compounds for olefin polymerization that can afford with high polymerization activity a relatively low molecular weight olefin polymer (macromonomer) having a higher proportion of terminal double bonds than produced by conventional metallocene compounds. It is another object to provide olefin polymerization catalysts containing the bridged metallocene compounds, in detail olefin polymerization catalysts that can catalyze with high polymerization activity the production of olefin polymers having high melt tension, excellent mechanical strength and good particle properties, and to provide polymerization processes using the catalysts. It is a further object of the invention to provide ethylene homopolymers or copolymers that have good processability and particularly excellent mechanical strength compared to conventional ethylene polymers, and to provide thermoplastic resin compositions containing the polymers. It is a still further object to provide shaped articles or films that are obtained from the polymers or the thermoplastic resin compositions, or to provide laminate films including the films. Furthermore, the invention has an object of providing ethylene homopolymers or copolymers that have good processability and easy-opening properties compared to conventional ethylene polymers, and thermoplastic resin compositions containing the ethylene polymers. It is a still further object of the invention to provide shaped articles or films that are obtained from the polymers or the thermoplastic resin compositions, or to provide laminate films including the films. [0011] A bridged metallocene compound according to the present invention is represented by Formula [1] below: [0012] (Formula Removed) [0013] wherein R1, R2, R3 and R4 are selected from a hydrogen atom, hydrocarbon groups, silicon-containing groups, heteroatom-containing groups and halogen-containing groups and are the same or different from one another; R1, R2, R3 and R4 are not all hydrogen atoms and at least one of these groups is an ethyl group or a group represented by any of Formulae [2] to [7] below; neighboring substituent groups among R1 to R4 may be linked together to form an aliphatic ring; Q1 is selected from Cl-20 hydrocarbon groups, halogen-containing groups, silicon-containing groups, germanium-containing groups and tin-containing groups; X independently at each occurrence is a group selected from a hydrogen atom, halogen atoms, hydrocarbon groups, halogen-containing groups, silicon-containing groups, oxygen-containing groups, sulfur-containing groups, nitrogen-containing groups and phosphorus-containing groups; and M is a titanium atom, a zirconium atom or a hafnium atom; [0014] (Formula Removed) [0015] (Formula Removed) [0016] (Formula Removed) [0018] (Formula Removed) [0019] (Formula Removed) [0020] wherein R7 to R16 are selected from a hydrogen atom, hydrocarbon groups, silicon-containing groups, heteroatom-containing groups and halogen-containing groups and are the same or different from one another, but they are not aryl groups; D and E are each a divalent heteroatom; G and L are each a trivalent heteroatom; and T and W are each a tetravalent heteroatom or a carbon atom. An olefin polymerization catalyst (a) according to the present invention comprises the following components (A) and (C) : [0021] Component (A): the bridged metallocene compound represented by Formula (1) above; [0022] Component (C): at least one compound selected from the group consisting of: (c-1) organometallic compounds represented by Formulae [11], [12] and [13] below; (c-2) organoaluminum oxy-compounds; and (c-3) compounds that react with the component (A) to form an ion pair; [0023] (Formula Removed) [0024] wherein Ra and Rb are each a Cl-15 hydrocarbon group and are the same or different from each other; X is a halogen atom; 0