re few markers far apart, so we set it low.
var heat = L.heatLayer([], { maxZoom: 12 }).addTo(map);

// We're just using a featureLayer to pull marker data from Mapbox -
// this is not added to the map.
var layer = L.mapbox.featureLayer('{{site.baseurl}}/assets/data/earthquakes.geojson').on('ready', function() {
    // Zoom the map to the bounds of the markers.
    map.fitBounds(layer.getBounds());
    // Add each marker point to the heatmap.
    layer.eachLayer(function(l) {
        heat.addLatLng(l.getLat