Add filter to Orderby Parameter using Array

Using WP-Query, I'm setting the Orderby parameter using an array:

$events_args = array(
    'post_status' => 'publish',
    'post_type' => 'events',
    'meta_key' => 'start_time',
    'meta_type' => 'DATETIME',
    'orderby' => array( 'meta_value_num' => 'DESC', 'title' => 'ASC' ),
    'order' => 'ASC',
    'posts_per_page'    => -1,
    'meta_query' => array(
        array(
            'key'       => 'event_date',
            'compare'   => '=',
            'value'     => 20150225,
        )
      )
);

I need to filter the first item in the array (meta_value_num). Using add_filter , how can I access the first item of the Orderby array to make the adjustments I need?

Thank you!