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Iwo Jima was a Japanese fortress. It was designed for attrition.
First, there were artillery, mortars, and rockets on the foot and slopes of Mount Suribachi. They had walls of reinforced concrete, four feet thick. In addition, they had reinforced steel doors. These were well-protected from bombardment. They were meticulously aimed and when the Americans showed up, the Japanese just opened fire.
The bombardment did destroy all buildings, aircraft, and waters-edge defenses, but that was it. The Japanese knew these would be destroyed, so they were only a minor (if not nominal) part of their defensive strategy.
In addition, there was a huge network of underground tunnels, almost completely safe and untouched by any bombardment of any size or length. They were deep underground and the bulk of the Japanese garrison were immune down there. There were several chambers large enough to hold 300 plus men. All major chambers were connected by over eleven miles of tunnels, all deep enough to be safe from any bombardment.
During the actual bombardment, there was no Japanese response. The Americans had no idea where the Japanese artillery was. And there was an enormous amount of Japanese artillery:
General Kuribayashi had available to him 361 artillery pieces of 75 mm or larger caliber, a dozen 320 mm mortars, 65 medium (150 mm) and light (81 mm) mortars, 33 naval guns 80 mm or larger, and 94 anti-aircraft guns 75 mm or larger. In addition to this formidable array of large caliber guns, the Iwo Jima defenses could boast more than 200 20 mm and 25 mm anti-aircraft guns and 69 37 mm and 47 mm antitank guns.
The firepower of the artillery was further augmented with a variety of rockets varying from an eight-inch type that weighed 90 kg and could travel 2β3 km, to a giant 250 kg projectile that had a range of more than 7 km. Altogether, 70 rocket guns and their crews reached Iwo Jima. -- Wikipedia
As I mentioned earlier Japanese firepower was aimed before the battle. They were all concentrated on various spots about 500 meters inland. When Americans reached these points, they were hit by enormous amounts of concentrated artillery fire, prepared weeks in advance.
The Japanese knew that when the Americans attacked, they would start with a bombardment. They were ready for it and designed their defense specifically against it. No artillery technology at the time could bust into the Japanese defenses, regardless of the length of the sustained barrage.