Fault Creep

The San Andreas Fault may be the most famous fault on Earth. For roughly 750 miles (1200 km), it creases California and marks part of the tectonic boundary between the North American and Pacific plates. It creates tangible examples for us to see plate tectonics in action.

Aerial view of landscape with fault line at center right.

The San Andreas Fault cleaves the land on the Carrizo plain. Photo courtesy of Ikluft and Wikipedia.

For about 75 miles, California State Route 25 (CA 25) roughly traces the path of the San Andreas Fault as the highway passes through an open valley filled with cattle ranches. (If you’re visiting the east side of Pinnacles National Park, you’ll drive this road.) From the ground, the fault is relatively hidden in most places even though the highway crosses it several times. On Google Earth, it shows a bit more clearly.

Google Earth image of creek valley with buildings at center.

A group of buildings, sitting just to the east of CA 25, is bisected by the San Andreas Fault. The red line marks the fault’s approximate location.

This part of the fault creeps along at a slow rate, maybe an inch per year. When covered by soil and vegetation, the resulting displacement would be nearly invisible on a yearly basis. When we pave the landscape with asphalt or concrete, however, the fault’s movement can manifest itself in ways that are easy to see.

About a ten-minute drive north of Pinnacles National Park’s east entrance the San Andreas Fault crosses CA 25. Here, the San Andreas Fault is slowly tearing the pavement apart.

Road with crack running from middle left to lower right.

This is essentially the boundary between the Pacific and North American tectonic plates. Land and water on the fault’s west and south side is moving north relative to the North American continent.

Person standing on road. Land to right is North American plate. Land on lower left is Pacific plate.

Yours truly straddles the plate boundary between North America and the Pacific.

According to Greg Hayes on his Geotripper blog, this section of road was repaved in 2008. When he visited this site in 2017, the yellow center line paint had not yet split. When I stopped on the morning of January 31, 2018, the paint was clearly cracked.

Crack in pavement across yellow line.

View is looking north.

This movement has been going on for millions of years. The rocks of Pinnacles National Park, now most famous for scenery and condors, are part of a volcanic field that erupted almost 200 miles to the south. Since then, movement along the San Andreas has displaced the rocks northward, leaving about a third of the volcanic field behind.

Road pavement with crack. Text reads "To Alaska" and "North"

Land on the south and west side of the San Andreas Fault is on track to meet Alaska in a couple hundred million years.

The crack in the pavement is the current surface expression of the fault’s movement. Fault creep is evident elsewhere in California. In Hayward, creep along the Hayward Fault is splitting the city hall in half.

This section of the San Andreas provides a rare opportunity to observe the Earth’s tectonic plates in motion. Because it happens over immense time scales, geologic change is most often undramatic and unnoticed. It happens slowly in rivulets of erosion on a hillside, waves reworking sand on a beach, dust blown in the wind, and creep along faults. As passengers on Earth’s brittle crust, we’re always moving relatively speaking.

Google Earth image of road moving north to south.

You can visit this site on CA 25 at 36°35’54.27″N, 121°11’40.19″W. Please be cautious though; this is a busy highway with a high speed limit. It’s also surrounded by private land, but you can find a couple of small pullouts about a hundred yards from the fault.

Earth Time Lapse

Nothing is completely static on geologic timescales, but some features—like volcanoes, barrier islands, glaciers, and human development—change faster than others. To see these changes, I’ve been playing around with Google’s Earth Engine. By combining over 30 years of Landsat imagery it offers a remarkable look at how Earth’s surface has changed recently. I found reason for concern, but was reminded just how beautiful the planet is.


Volcanoes are the most dynamic landforms on Earth. While the above GIF’s imagery starts a few years after Mount Saint Helen’s 1980 eruption, it captures the volcano’s awakening from 2004-2008 when a large lava dome grew in the crater. Very cool.

Surtseyan eruptions are island forming eruptions that happen in shallow water. In 2011 and 2012, you can see new islands suddenly appear on the sea’s surface in the Zubair Group, volcanic islands in the Red Sea. Few people have seen new islands form, but there they are. Super cool.

Barrier Islands

Barrier islands move, often rapidly. Toms Cove Hook is protected as part of Assateague Island National Seashore and Chincoteague National Wildlife Refuge. Unlike most barrier islands on the Atlantic coast of the U.S., Assateague is almost free of permanent structures and roads. Toms Cove Hook has no development at all. Here is an increasingly rare opportunity to watch a barrier island dance. Beautiful.

Fire Island is a national seashore off of Long Island in New York. Before the 2012 breach, watch the sand slowly creep across the island. Barrier islands move shoreward when sea levels rise. They are not permanent features. When allowed to move naturally I find these islands exceptionally beautiful, but they are no place for permanent roads or structures. It’s human folly to build on these islands. They simply change too quickly.


Carbon Glacier carves the north slope of Mount Rainer. Worldwide, glaciers have undergone significant declines over the last 30 years and Carbon Glacier is no exception, but that doesn’t mean glaciers still don’t flow downhill. Even receding glaciers continue to erode the land as Carbon Glacier demonstrates. It carries sediments downhill in a conveyor belt-like manner while its terminus shrinks. I find a glacier’s flow mesmerizing.

Human Development

This is, sadly, probably the easiest example of rapid change to find on Earth today. Cranberry Township, Butler County, PA is a typically example of the development much of the U.S. has experienced in my lifetime. I grew up about 20 miles north of here and witnessed its semi-rural farmland transform into a maze of tract homes and strip malls.

That rainforest destruction I heard about as a kid really hasn’t abated either. GIF showing destruction of Amazon rainforest

Who knew it would be so easy to watch decades of change? Tools like the Earth Engine are amazing and they allow me to see the world like never before. As I browsed I was awed by power and beauty of natural changes and saddened by the rate humans are altering the planet. Hopefully, we can use information like this to inspire everyone to protect what we have left.