<?php

add_filter('getarchives_where','filtro_getarchives_where');
add_filter('getarchives_join','filtro_getarchives_join');

function filtro_getarchives_join($join) {
    global $wpdb;
    if(is_category()) {
        $join .= " LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id) LEFT JOIN         $wpdb->term_taxonomy taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = taxonomy.term_taxonomy_id) ";
    }
    return $join;
}

function filtro_getarchives_where($where) {

    if(is_category()) {

        $category = get_query_var('cat');
        $where .= " AND taxonomy.taxonomy = 'category' AND taxonomy.term_id = ". $category;

    }

    return $where;
}

?>