EVCNB hosts tsunami lecture

A standing-room-only crowd packed the Pine Grove Community House in Manzanita on July 28, to attend a lecture on tsunami modeling hosted by the Emergency Volunteer Corps of Nehalem Bay.

The lecture by Dr. Jonathan Allan from Oregon’s Department of Geology and Mineral Industries detailed how experts at the department use clues from past tsunamis to predict future outcomes.

Allan has worked for the Department of Geology and Mineral Industries (DOGAMI) since 2001 and currently serves as the head of its tsunami modeling and mapping program.

Scientists studying tsunamis look at the historical record of tsunamis, in a practice known as paleoseismology, combined with the more recent recorded events to predict what will happen in future tsunamis.

Paleoseismologists look for evidence of sediment deposits caused by tsunami inundation in soil records, areas where land subsidence can be observed and ghost forests, where low-lying trees were submerged in salt water after a seismic event, to indicate an area’s seismic activity.

After observing those indicators, scientists look at recent, well-documented earthquakes and tsunamis to develop inundation models. Those include the 1960 Chilean earthquake, which at a magnitude 9.5 was the largest in recorded history, as well as the 1964 Alaskan earthquake and 2011 Japanese event.

Allan then discussed the situation on the Oregon coast, which sits near the Cascadia Subduction Zone and is susceptible to tsunamis from farther afield. Allan said that tsunami generation requires an earthquake with a magnitude of at least 7.0 on the Richter scale.

Distant tsunamis are those that are generated in fault zones elsewhere on the pacific rim and take between four and 12 hours to arrive on Oregon shores. Since 1850, 30 distant tsunamis have impacted the Oregon coast, with substantial damage caused in Cannon Beach and Seaside by the 1964 Alaskan quake and tsunami and in Brookings by the 2011 Japan earthquake.

The more serious scenario for the Oregon coast is a Cascadia subduction zone event, which would cause exponentially more damage due to its proximity.

The Cascadia subduction zone is formed where the Juan de Fuca tectonic plate slides eastward under the North American plate just off the coast of northern California, Oregon and Washington. The zone’s existence was not discovered until the 1990s, when researchers pieced together indigenous accounts, geologic evidence and Japanese records to determine that a major tsunami had occurred in 1700.

Since the subduction zone’s discovery, researchers have examined soil records both on land and under the ocean to develop a more detailed picture of the zone’s record. That research shows that 26 partial-margin ruptures, causing quakes around a magnitude 7, have occurred, while full-margin ruptures have occurred on average every 500 years.

Allan then discussed the experience of going through and impacts of a Cascadia subduction zone earthquake and tsunami.

Depending on the size of the rupture along the fault and location, shaking from the earthquake would last between tens of seconds and five minutes. The southern coast is located closer to the fault itself and is projected to have severe to violent shaking, while the north coast is expected to experience strong to severe shaking. Allan said that there will also be numerous aftershocks, citing the 2011 Japanese earthquake when 2,500 occurred within 25 hours, and large waves for ten to 12 hours.

Of the coast’s approximately 225,000 residents, Allan said that between 24,000 and 60,000 are exposed to Tsunami risk.

Using that figure and tsunami models, DOGAMI staff have predicted that a medium sized earthquake and tsunami would cause 4,300 to 16,000 fatalities, a large event would cause 6,100 to 23,000, and an extra-large event could kill between 13,800 and 48,600. Those same calculations estimate that between 20,000 and 45,000 people will be displaced depending on the size of the event and $14 to $19 billion in damage caused.

Allan praised the work of the EVCNB in preparing north Tillamook County for a tsunami and said that such preparations on an individual and community level were critical to boosting resilience. Allan also said that DOGAMI officials continue to work on updating warning signage along Highway 101, evacuation maps and analyzing coastwide needs.