From a few examples on StackOverflow I put together this POC script for backing up and zipping Windows Security Logs to save them for compliance

Backup-SecurityEventLog.ps1

# Function to zip the archived log, requires .NET 4.5
function zipFiles($sourceDir, $zipFileName)
{
   Add-Type -Assembly System.IO.Compression.FileSystem
   $compressionLevel = [System.IO.Compression.CompressionLevel]::Optimal
   [System.IO.Compression.ZipFile]::CreateFromDirectory($sourceDir, $zipFileName, $compressionLevel, $false)
}

# Function to zip the archived log, requires 7zip (has command line version)
function create-7zip([String] $sourceDir, [String] $zipFileName)
{
   [string]$pathToZipExe = "$($Env:ProgramFiles)\7-Zip\7z.exe";
   [Array]$arguments = "a", "-tzip", "$zipFileName", "$sourceDir", "-r";
   & $pathToZipExe $arguments;
}

# Get the Security Event Log into an Object
$sEvents = Get-WmiObject -Class Win32_NTEventlogFile | where { $_.LogfileName -eq 'Security' }

# Test for Backup directory, create it if it doesn't exist
$backupDir = "C:\evt-backup\{0}_Security_{1:MM-dd-yyyy}"-f $env:COMPUTERNAME,[datetime]::Now
if (-not (Test-Path $backupDir))
{
   New-Item -Path $backupDir -ItemType Directory | Out-Null
}

# Build the Backup file name
$file = "{0}_Security_{1:MM-dd-yyyy}.evt" -f $env:COMPUTERNAME,[datetime]::Now

# Full path for log backup
$path = "$backupDir\$file"

# Arguements for zip file creation
$zipFileName = "$backupDir.zip"
$sourceDir = "$backupDir"

# Do the Backup and capture the returnvalue to chek for errors
$errBackup = ($sEvents.BackupEventlog($path)).ReturnValue

# If the backup is successfull, clear the log and verify ReturnValue for success
if ($errBackup -eq 0)
{
   Write-Verbose "Security log successfully archived"
   <#
   $errClear = ($sEvents.ClearEventlog()).ReturnValue
   if ($errClear -eq 0) {
     Write-Verbose "Security log successfully cleared"
   }
   #>
} 
else
{
   Write-Verbose "Backup failed with return code $errBackup"
}

#zipFiles $sourceDir $zipFileName
#create-7zip $sourceDir $zipFileName