How can these inconsistencies about Jesus' tomb be explained?

The fact that two accounts don't include all the same details don't mean they contradict. It just means that one person mentioned a detail that another didn't.

So they didn't all mention the earthquake. Well, maybe it was mild enough and some of the writers far enough away at the time that they didn't feel it. Or maybe they just didn't find it necessary to include it. The story already had the amazing facts of angels and a huge stone rolled away and a man come back from the dead. Next to than an earthquake is small stuff.

suppose you read two news stories about the same event, say a political protest. Both say that the protestors marched down Main Street. Both say that they chanted a certain slogan. Both say that they assaulted bystanders. One says that some of the protestors broke some store windows and the other doesn't. Would you say that that is a "contradiction" and one of the reporters must be lying? Maybe the reporter who left it out just didn't think it was important enough to include. Or maybe he didn't see it. No one can report every single thing that happened, second by second.

RE one angel versus two: None of the Gospels says that there was only one angel. One says there were two angels, the others only mention one. Maybe they didn't think the number was important. Suppose I say, "I was at the hardware store yesterday and a clerk named Bob helped me find the power tools." Would you be justified in concluding that because I mentioned no other clerks, that Bob must be the only person who works at that store? Not necessarily. I might well have not mentioned other clerks because they weren't relevant to what I wanted to say.

One thing to keep in mind here is that we are relying on humans recalling events that had taken place in some cases as long 40 years prior. Setting aside inspiration for the moment (as we would expect a skeptic to do) makes it even easier, as it's just not reasonable to expect everything to line up perfectly.

What we do expect is for them to get the broad picture right. To be accurate on the big brush strokes. This is especially true with regards to the "fantastic and miraculous" events, because those are the things a person will tend to take note of and remember well later. Jesus' tomb was empty? You bet! Was an angel there? Sure. Was the angel outside or inside? I don't rightly recall, but I know it was there, so I better write about it.

Another good example is the Sermon on the Plain vs the Sermon on the Mount. How can you reconcile Matthew's record of the "Sermon on the Mount" with Luke's "Sermon on the Plain"? How were they able to record such a long discourse so accurately?

One explanation here is to remember that in those days, without tv or radio, public figures would travel from place to place and give the same speech everywhere they went. This was how they got their message out. Today, we call that a "stump" speech, and politicians today will still do that from time to time. This both explains differences between the sermons in Matthew and Luke (they were recounting different deliveries of the same speech) and explains how they were able to recall it so well: they heard it many times, because they traveled with Jesus, and possibly had the text reinforced in private. Even so, one would hardly expect perfect recollection.

So, in the first three cases (the Synoptic Gospels), you have:

1. an angel (or someone dressed in a gleaming white robe)
2. a stone rolled away

(And that's just one sentence)

In John, you have two angels, yes, but this purports to be the account of a different person. (the Synoptics have the disciples as the antecedent, John explicitly mentions Mary)

I suppose you could say that Matthew differs from Mark and Luke, in that one purports to narrate that the angel rolled it away, but before the disciples got there.

I'm not seeing the inconsistency -

There was one angel when the disciples came. There were two when Mary came.

In all instances, you have angels, a rolled away stone, and an empty tomb.

The Bridgeway Bible Commentary (by Don Fleming, I'm unsure of his denomination or theological lens) tries to harmonise the resurrection accounts in the following way (sorry this is a long quote):

Morning of the resurrection (Mt 28:1-15; Mk 16:1-11; Lk 24:1-12; Jn 20:1-18) It is not surprising that there are differences in the accounts of what people saw on the Sunday morning when Jesus rose from the dead. The sight of the empty tomb and the heavenly messengers produced a mixture of reactions - excitement, joy, anxiety, fear, wonder. There was confusion as people rushed here and there to tell others. One writer records what he heard from some, another what he heard from others. But there is no variation in the basic facts: the tomb was empty and Jesus had risen. The following summary suggests the possible order of events.

1. At the first sign of dawn two groups of women set out from separate places to take spices to anoint the body of Jesus. One group consisted of three women (Mary Magdalene, Mary the mother of James and Joses, and Salome the mother of the apostles James and John). The other group consisted of Joanna and some friends (Matt 28:1; Mark 16:1-3; Luke 24:1,10).

2. The group of three women arrived at the tomb first and found the stone rolled away. Mary Magdalene panicked and, without seeing the angel or hearing the voice, ran to tell Peter and John that the body had been stolen (John 20:1-2). But the other Mary and Salome remained. They met one angel sitting on the stone outside the tomb, and another sitting inside the tomb. Upon hearing that Jesus had risen and desired to be reunited with his disciples in Galilee, they rushed off to the place where the apostles were gathered, eager to pass on the exciting news (Matt 28:2-7; Mark 16:4-8).

3. Meanwhile the Roman guards fled the tomb and hurried across the city to tell the chief priests what had happened. These priests were the ones who had set the guard in the first place, and their purpose was to prevent Jesus’ followers from stealing the body. Now the same priests bribed the guards to spread the story that Jesus’ followers stole the body while the guards slept. The priests had earlier been worried that Jesus’ disciples might deceive people, but now they themselves were the deceivers (Matt 28:11-13; cf. 27:62-66). If Pilate heard the story of the guards sleeping on duty, the Jewish leaders promised to protect them by bribing Pilate (Matt 28:14-15).

4. Back at the tomb, a few minutes after the first group of women had departed, Joanna and her friends arrived. They went inside, met two angels, heard the news of Jesus’ resurrection, and hurried off to tell the apostles (Luke 24:2-8).

5. Soon after the women left the tomb, Peter and John arrived, went inside and saw the linen cloth lying neatly folded. They believed the evidence they saw that Jesus must have risen from the dead, but they left the tomb confused, not understanding the significance of the event (John 20:3-10; Luke 24:12).

6. Mary Magdalene, who followed Peter and John back to the tomb, arrived after they had left. She remained there alone, weeping. Then she saw the two angels inside the tomb and, on turning round, saw a man whom she did not immediately recognize (Mark 16:9; John 20:11-15). When she discovered that the man was Jesus, she took hold of him as if not wanting to let him go. Jesus told her she had no need to cling to him in this way, as he was not ascending to heaven immediately (though he would within a few weeks). She should not become dependent on his physical presence, otherwise she would be disappointed again. She was to go and tell the apostles what he had told her (John 20:16-17).

7. Shortly after appearing to Mary Magdalene, Jesus appeared to the other women of her group (the other Mary and Salome) as they were on their way to tell the apostles of their discovery (Matt 28:8-10).

8. The two groups of women reached the house of the apostles about the same time, followed soon after by Mary Magdalene. They told the apostles of what they had seen at the tomb and of their separate meetings with the risen Jesus, but the apostles believed neither Mary nor the other women (Mark 16:10-11; Luke 24:9-11; John 20:18). (All the events summarized in sections 1 to 8 above probably happened within the space of an hour or so.)

The Treasury of Scripture Knowledge (by R. A. Torrey - American fundamentalist) on the angles in Mark 16:5 says:

This appears to have been a different angel from that mentioned by Matthew. The latter sat in the porch of the tomb, and had assumed a terrible appearance to overawe the guard. (Mt 28:1;) but this appeared as a young man, within the sepulchre, in the inner apartment. The two angels spoken of by John (ch. 20:11) appeared some time after these; but whether they were the same or different cannot be ascertained; nor whether the angels which manifested themselves to the second party of women, recorded by Luke, (ch. 24:4,) were the same or different.

Matthew Poole (English Nonconformist/Puritan) on Mark 16:5 concerning the number of angels says:

Both Luke and John mention two angels in the habit of young men. Matthew speaks of one sitting upon the stone. They might see him sitting upon the stone, and yet find him within also, the motions of angels are quick and undiscernible to our sense, or the stone might be rolled inward. That they were affrighted is no wonder, considering how apt we are to be frightened by any apparitions.

John Gill (English Baptist Calvinist) on Mark 16:5 on the number of angels says:

nor is this any contradiction to John's account, who says there were two angels, one at the head, and another at the feet, Joh 20:12; since Mark does not say there was no more than one;

John Gill (English Baptist Calvinist) on Matt 28:2 on the angel says:

This posture of the angel does not contradict what other evangelists say of this, and the other angel, that they stood by the women, and also were sitting in the sepulchre, Mr 16:5, for each was true: when the women first came, the angel sat upon the stone; after that, with the other, stood by them; when having invited them to the grave, placed themselves, sitting the one at the head, and the other at the feet, where the body of Christ had lain.

Interestingly the The Pulpit Commentary (edited by Joseph Exell and Henry Donald Maurice Spence-Jones who were conservative Church of England folk) on Luke 24:4 and the number of angels says:

To one company of women one angel appeared: to another, two. Mary Magdalene, a little later, saw two angels in white sitting, as it were keeping watch and ward over the sepulchre for a short time after the sacred form had left it. The words which these beings from another sphere spoke to the mourning women were slightly different, but the teaching was the same in each case: "He is not here, but is risen. Do you not remember what he told you when he was yet with you?" Van Oosterzee and Farrar repeat a beautiful passage from Lessing on this: "Cold discrepancy-mongers, do you not, then, see that the evangelists do not count the angels?... There were not only two angels there were millions of them. They appeared not always one and the same, not always the same two; sometimes this one appeared, sometimes that; sometimes on this place, sometimes on that; sometimes alone, sometimes in company; sometimes they said this, sometimes they said that."