Earth Science Environment Geosphere Repost

Cascadia Unfolding: Anatomy of a Disaster

The time was twelve minutes after three in the morning on the west coast of the United States. A low-pressure system blew rain off the Pacific Ocean, and thick clouds covered western Oregon and Washington. The moon was almost full, but none of that light reached the cities, towns, and countryside. This weather system was the latest in a string of winter storms mercilessly pounding the west coast for two months. The soggy ground from Portland to Seattle was utterly saturated, and the Willamette River was at flood stage.

John Carrington slept soundly at his home in Cannon Beach on the Oregon coast. Gusts of wind slapped rain onto the windowpanes, but he slept on. His children and grandchildren had departed for Bend about noon the day before. It was just John and his wife now.

Northwest of John, far below the ocean’s surface, dangerous events unfolded. Off the coast of Washington and approximately thirty-five kilometers below the earth’s surface, the North American tectonic plate slid upward over the Juan de Fuca plate, creating a massive rupture of the Cascadia Fault Zone. The rupture zone is the hypocenter, and directly above it on the earth’s surface is the epicenter.

The initial rupture created a set of body waves. These are seismic waves that propagate spherically out from the hypocenter. Two types of body waves moved outward when the plates slipped along the fault: P waves and S waves. The P waves were the fastest of the two, and “P” technically stands for primary. These would be the first waves to reach the epicenter center at the surface and the first to reach the seismographs that initiate an early warning system.

P waves are also called pressure waves. They are like the ripple you get in a slinky when you push on one end of it. Pressure waves move by pushing particles in the direction of motion. When one particle moves, it bumps into its neighbor, causing the neighbor to move also. Your voice travels through the air via pressure waves when you speak. There is no noise in space since there are no particles to bump together.

About the same time the first body waves left the rupture zone, Gemma Roberts was standing at the bay window of her home in the Portland Hills, staring at the lights of the city. It was a restless night for her, and her two squawking budgies didn’t help. They were more agitated than usual, and she put a blanket over the cage to calm them down. Her house was off of Burnside Road, and she worried about the rain. Several years before, a small landslide at the edge of a cemetery had shut down the road. That incident had added thirty minutes to her daily commute.

She could see the back-porch lights of her neighbor’s house slightly down the hill to the right of her window. She and Joan sat there some mornings and talked over coffee. She wondered if Joan was up at this time of night too.

The P waves from the rupture traveled six kilometers per second and reached the surface epicenter in about six seconds. The S waves moved slower at about sixty percent the velocity of P waves, and it took ten seconds for them to reach the epicenter. S waves are secondary waves since they arrive after the primary waves (P waves). S waves are also known as shear waves. They are the kind of waves you make by tying one end of a rope to a tree and then shaking the other end to create big “S” shaped ripples. In reality, the S waves never reached the surface epicenter since the epicenter was on the surface of the ocean, and S waves can’t travel through water.

The first body waves reached John Carrington in 33 seconds by traveling a straight line distance of 200 kilometers from the hypocenter to Cannon Beach. The first jolt wasn’t too bad, but it was enough to wake John up. He had time to sit up before the S waves arrived 23 seconds later. The massive energy shock from the rupture zone then transitioned from body waves to surface waves, and the ground began to undulate. John rolled back into the bed dragging covers and pillows to cover his wife and himself. He wondered why he couldn’t hear any tsunami warnings sirens.

The National Tsunami Warning Center in Palmer, Alaska, serves the continental United States, Alaska, and Canada. Palmer is just northeast of Anchorage, and it is statistically part of the Anchorage metropolitan complex. Palmer is also 1,600 kilometers (1000 miles) northwest of the Washington Coast, where the Cascadia rupture occurred.

While John listened to windows breaking and bookshelves spilling their contents over the floor, the P waves were still on their four-minute and forty-four-second journey to the Palmer Center. Much destruction would be visited on Oregon and Washington before the first body waves reached Palmer. Even after the first waves reached the warning center, it would take time to process the data, locate the event epicenter, and issue a warning. But time was in short supply for Cannon Beach.

When the Cascadia fault zone ruptured, it initiated a series of events at the sea bottom. A large segment of sea-bottom, resting on the North American plate, thrust upward about twenty meters, elevating the water above it. As the surface of the ocean rose to twenty meters above normal, the land along the coast dropped by about two meters. This dual-action happened almost instantaneously, and the net effect was a massive energy imbalance that started self-correcting the moment it formed. The mountain of water collapsed under the influence of gravity, and a tsunami was born. This tsunami moved outward in every direction from the subduction zone. Powerful waves were on their way to Hawaii, Japan, Alaska, and the Oregon-Washington coast.

John didn’t wait for a tsunami warning. As soon as the shaking allowed, he and his wife put on the emergency boots they kept under each bed-side table. They went out the door in pajamas and coats, and they hurriedly walked up to the main road. John purchased his home knowing this time could come, and he knew that a ten-minute uphill walk would place him over fifty feet above sea level.

Fifteen to twenty minutes from when the quake occurred was the window of opportunity for living. This interval was the time it would take before twenty to fifty-foot waves inundated the coast, destroying everything in their path.

John fled the oncoming water, but Gemma wasn’t worrying about tsunami waves. She felt the S waves about the same time as John, but she had already positioned herself in the reinforced arch of her front doorway. She huddled against one of the walls holding a large couch pillow over her head. From her position, she could see the solar lights on Joan’s back porch through the glass side-panels of her front door. Surface waves slammed into her house, and she could hear the piercing sound of glass and porcelain falling like rain and shattering on Mexican tiles in the kitchen.

Surface waves are one of the real agents of destruction in an earthquake. They are different from body waves, but like body waves, there are two types: Rayleigh waves and Love waves. Love waves are not as cozy as they sound. They take their name from mathematician A.E.H Love who, in 1911, developed a mathematical model to describe this type of wave. Love waves are faster than Rayleigh waves, and thus they are the first to arrive. Their motion is entirely horizontal as they undulated from side to side like a snake winding across the earth’s surface. When the front of your house moves to the right, the back may be moving to the left. This motion is why Gemma watched a vertical crack slice open one of her living room walls.

The Rayleigh waves act like waves on the ocean, moving everything up and down in an orbital motion as they pass. Chunks of drywall fell from the ceiling, and Gemma tried to shrink closer to the wall. The budgies shrieked frantically in the background. From the corner of her eye, she saw Joan’s solar lights again, and she had the illusion that she was shooting uphill. The reality was that Joan was moving downhill as her house detached along with a chunk of the hillside and slipped downslope.

The rain and surface waves had done their job. The water-saturated soils were heavy and weak. The soil’s resistance to shearing was minimal, and the surface waves initiated slip planes on the hillsides. Tree roots gave way, and gravity reshaped the topography. Shaking damage is usually not as destructive as landslides. Soil and earth were moving throughout the Portland Hills, and as the hillsides collapsed, they took all the homes in their path with them.

While Gemma pressed her back against the wall, Officer Jenkins lay on the ground in an open area between Naito Parkway and the Willamette River. The Tom McCall Waterfront Park by the river is a thin, long strip of land that had become home to a sizable transient population of homeless souls. Officer Jenkins could see them trying to drag their belongings from beneath the Burnside Bridge. He had been responding to a call and inspecting an area around the Japanese American Historical Plaza when the shaking started. He quickly moved to an open area away from any potential falling objects, and then the force of the surface waves knocked him to the ground.

Officer Jenkins was witness to a catastrophe. He watched as several support columns on the Burnside Bridge collapsed, and portions of the decking plummeted into the river. The Steel Bridge to the north of him had already disappeared. His mind registered that at quarter after three in the morning, Portland was being spared the human carnage that would have occurred if rush hour traffic was in full swing.

The flat areas of the city near the rivers were taking the worst beating. Portions of the city at the base of the Portland Hills and between the Willamette and Columbia rivers are part of the Portland Basin. This is a tectonic feature created by the geologic creep of the coastal range in a northward direction. Shearing motion created by lateral movement of Coastal range caused a large, 2000 square kilometer block of the earth’s surface to start sinking about 20 million years ago. Nature abhors a vacuum, and the Columbia river obliged by filling in this hole with its sediment load. Sediments in the basin were saturated with water from the rivers and the rain.

The sediments in the Portland Basin, particularly those nearest the surface, are not solid rock. They are composed of mud, silt, sand, and gravel deposits. They are firm enough under normal conditions, but as the earthquake unfolded, shaking from the surface waves disaggregated the sediment pile and effectively turned it into patches of quicksand. The process is known as liquefaction. Structural support failed for buildings whose foundations were not up to code. If they didn’t immediately collapse, they partially sank into the quicksand mixture and tilted dangerously.

The shaking at Gemma Robert’s house was terrible, but it was worse in the Portland Basin. The loose, water-saturated alluvium of the basin was much less dense than the bedrock of the surrounding hills. As the surface waves exited from solid rock and entered the basin, they slowed down. Slower waves sound good at first glance, but they are not. The issue is energy conservation. The transition from solid rock to alluvium does not decrease the total energy, and the wave must increase in amplitude to counteract the loss of velocity. The net effect is that the surface waves get bigger and more dangerous.

Officer Jenkins could see a parking garage with the northeast corner missing as if a huge bite had removed part of the structure. Several other buildings tilted at odd angles, and older brick buildings near the waterfront quickly turned into piles of rubble. Thick muddy water was spurting out of a fissure in the street. Officer Jenkins thought it was a broken water main, but he was wrong. Liquefaction had turned the ground into a viscous fluid, and pressures generated by the ground shake forced this liquid earth to the surface, where it bubbled from the ground.

When Officer Jenkins could finally stand up, only five minutes had elapsed since the initial rupture on the Cascadia fault zone. Most of the lights in the city were out from a collapsed and disrupted power transmission system. Flames from ruptured gas lines were visible in some areas. Those five minutes would lead to five years of reconstruction after burying the dead and treating survivors.

Conditions were terrible in Portland, but the worst was still to come for Cannon Beach. Killer waves were gliding silently over the Pacific Ocean in the darkness of the night. Twelve minutes after Officer Jenkins was able to stand up from the ground, a thirty-foot tsunami slammed into the Oregon coast at Cannon Beach. John and his wife viewed the destruction from a position of relative safety. They were more fortunate than many of the residents in this oceanside resort. When the full series of waves finished pounding the coast, there was very little to see in Cannon beach other than foundations.

The Cascadia subduction zone lies a mere 50 miles off of the Oregon coast. The physics of destruction from a major earthquake along the Cascadia Fault allows for precious little lead time between rupture and the ensuing chaos. This fact does not mean that each individual shouldn’t prepare; it merely means it could happen without warning.

The last major earthquake on the Cascadia Fault occurred on January 26th, 1700, at about nine in the evening. The scenario created above has a reasonable chance of happening between now and 2220. Risk exposure to this type of disaster is what residents trade in exchange for the opportunity to live and work in the wild and beautiful Pacific Northwest.

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.