Great Western Earthquake Swarm

The **Great Western earthquake swarm** was a moment magnitude 8.2 (Mw), blind thrust earthquake that occurred on March 15, 2020, at 2:39:05 a.m. PST in Los Osos, California. The quake had a duration of approximately 30-40 seconds, and its peak ground acceleration of 2.29 *g* was the highest ever instrumentally recorded in North America. Shaking from the San Andreas Fault triggered both the Cascadia Subduction Zone along the Pacific Northwest and the Wasatch Fault in northern Utah. Shaking was felt as far away as Denver, Colorado and and Chihuahua, Mexico. The peak ground velocity at the Rinaldi Receiving Station was 224 cm/s (8.0 km/h; 5.0 mph), the fastest ever recorded.

Two 6.5 Mw  aftershocks followed, the first about one minute after the initial event and the second approximately 3 hours later, the strongest of several thousand aftershocks in all. The death toll was 322,816, making it the fifth deadliest natural disaster in human history. More than 2 million people were injured. In addition, property damage was estimated to be in excess of $150 billion, making it the costliest natural disaster in U.S. history.

## Contents

* 1Contents
* 2Epicenters
* 3Damages and fatalities
* 4Cities affected
	+ 4.1Los Osos and Morro Bay
	+ 4.2San Luis Obispo
	+ 4.3Bakersfield and Fresno
	+ 4.4Greater Los Angeles area
	+ 4.5San Francisco Bay area
	+ 4.6Seattle-Tacoma area
	+ 4.7Portland, OR area
	+ 4.8Greater Salt Lake City area
	+ 4.9Sacramento
	+ 4.10East Asia and Oceania
* 5Aftermath

## Epicenters

The earthquake struck in the Los Osos Valley about 16 kilometers (10 mi) northwest of San Luis Obispo.

The United States Geological Survey placed the hypocenter's geographical coordinates at 35.319°N 120.832°W and at a depth of 14 km (8.69 mi). It occurred on a previously undiscovered section of the Hosgri Fault, which directly triggered the San Andreas Fault. Several other faults experienced minor rupture during the main shock and other ruptures occurred during large aftershocks.

## Damages and fatalities

Damage occurred up to 566 kilometers (352 mi) away, with the most damage in the San Luis Obispo (California), Willamette (Oregon), and Wasatch (Utah) Valleys, along with the Puget Sound (Washington). The William Shipsey House in San Luis Obispo was red-tagged and demolished due to structural damage, along with the majority of other buildings in the city. The Salesforce Tower in San Francisco collapsed along with the Ritz-Carlton, The official death toll was placed at 15,672; 11,984 in California, 1,604 in Washington, 1,051 in Oregon, and 1,033 in Utah. An estimated 9,403 people died immediately or within a few days from injuries sustained, and many died from indirect causes, such as stress-induced cardiac events. Nearly 100,000 people were injured, including 18,200 who required hospitalization.

The earthquake also caused damage to the Interstate system, most notably Interstates 5, 10, and 15. This caused the death of business magnate Elon Musk, who was riding eastbound on I-10 in a Tesla, which had Autopilot on. When the earthquake hit, the car was passing over an interchange with I-110 when the ramp collapsed in front of the car. The autopilot failed to notice the break in the road, and as a result, Elon fell several meters to his death. Most casualties and damage occurred because of numerous fires, caused by broken gas lines from houses shifting off their foundations or unsecured water heaters tumbling. Many underground gas and water lines were severed, resulting in some streets experiencing simultaneous fires and floods. Damage to the system resulted in water pressure dropping to zero in some areas; this predictably affected success in fighting subsequent fires. Five days later, it was estimated that between 4 and 6 million citizens were still without public water service. Unreinforced masonry buildings and houses on steep slopes suffered damage. However, school buildings (K-12), which are required by California law to be reinforced, in general survived fairly well.

## Cities affected

### Los Osos and Morro Bay

The overwhelming force of the earthquake completely leveled the Los Osos Valley, drastically changing the topography of the area. More than 99% of both towns were killed, and many bodies were not recovered in the initial rescue attempts due to the massive changes in the surrounding area. The valley expanded greatly, turning formerly mountainous landscapes into flat grasslands. One of the only survivors in the valley described the event as "a wrath greater than God himself."

### San Luis Obispo

The damage done in San Luis Obispo was only slightly less devastating than the destruction of Los Osos. The fault line tore through the center of the town, causing many of the buildings to collapse. The entire town fell in elevation by about 7 feet, furthering structural damage. Approximately 72% of the populace died, leaving 28% injured. The survivors were forced into a mass exodus, leaving San Luis Obispo abandoned to this day.

### Bakersfield and Fresno

Bakersfield and Fresno were a similar distance from the epicenter, so the effect on both areas was comparable. Many roads and buildings were badly damaged, including the historic Saint John's Cathedral in Fresno. around 47% of Bakersfield and 52% of Fresno died in the earthquake, mostly from collapsed buildings. While both towns are still populated, their numbers have drastically decreased since 2020.

### Greater Los Angeles area

In LA, the quake first struck the San Fernando Valley, causing Angelenos to initially believe that the earthquake was a localized event similar to the 1994 Northridge earthquake until internet connectivity was restored, days later. Hundreds of non-reinforced buildings collapsed, trapping occupants and damaging infrastructure. Many high-rises in the city swayed, causing windows to break and mass panic in the streets. The Ritz-Carlton Hotel in southern downtown collapsed, causing many deaths. Once the earthquake had ceased, Los Angeles had suffered moderate amounts of damage. Miraculously, the neighborhood of Highland Park suffered very little damage, even though it is located just 9.6 kilometers (5.9 mi) from downtown. In Orange County, the Disneyland Resort was forced to close for several months following the earthquake.

### San Francisco Bay area

As Los Osos was located roughly halfway between the Bay Area and LA Basin, the effects in San Francisco were similar to those of Los Angeles. Notable exceptions include numerous landslides caused by the natural hills of the region, as well as the collapse of the Salesforce Tower due to the explosion of a gas main underneath the building, killing 9,247 people. It was the only major building to collapse in San Francisco. Moderate flooding affected the low-lying portions of the peninsula, as well as parts of Oakland and Berkeley.

### Sacramento

The capital of California was only moderately affected by the earthquake. Most buildings in the city survived, but the Sacramento River caused minor flooding in downtown, necessitating evacuation for several days to avoid deaths from electrocution. Many buildings were yellow-tagged, and all normal government operations were halted. The Old Town district was significantly damaged due to the age and fragility of the buildings.

### East Asia and Oceania

Although spared from the actual earthquake itself, costal cities located in eastern Asia and northeast Australia were inundated with tsunami waves that traveled over 11,000 km (~7,000 mi) as high as 20 m (65 ft), flooding cities such as Tokyo, Osaka, Shanghai, Brisbane, and Gold Coast. These cities had time to evacuate due to the distance of the earthquake, so casualties were minimized.