2020 Local Hazard Mitigation Plan

City of Irvine

by determining the acceleration of the earthquake’s motion relative to the force of gravity. The acceleration of gravity is 980 centimeters per second, so if, for example, an earthquake’s acceleration is measured at 765 centimeters per second, the shaking is modeled as 765/980, or .781 g (78.1% g). Figure 3-11 shows the predicted intensity of seismic shaking in Irvine using percent g. Darker shaded areas depicted on this map are anticipated to feel earthquakes more intensely than lighter areas.

Table 3-12: Modified Mercalli Intensity Scale 84

Intensit y

Description

Instrumental

Felt only by a very few people, under especially favorable conditions

Feeble

Felt only by a few people at rest, especially on the upper floors of buildings

Noticeable by people indoors, especially on upper floors, but not always recognized as an earthquake Felt by many indoors, and by some outdoors. Sleeping people may be awakened. Dishes, windows, and doors are disturbed Felt by nearly everyone, and many sleeping people are awakened. Some dishes and windows broken, and unstable objects overturned

III

Slight

Moderate

Slightly strong

Strong

Felt by everyone. Some heavy furniture is moved, and there is slight damage

Negligible damage in well-built buildings, slight to moderate damage in ordinary buildings, and considerable damage in poorly built buildings Slight damage in well-built buildings, considerable damage and partial collapse in ordinary buildings, and great damage in poorly built buildings Considerable damage in specially designed structures. Great damage and partial collapse in substantial buildings, and buildings are shifted off foundations Most foundations and buildings with masonry or frames are destroyed, along with some well-built wood structures. Rail lines are bent Most or all masonry structures are destroyed, along with bridges. Rail lines are greatly bent Damage is total. The lines of sight are distorted, and objects are thrown into the air

VII

Very strong

VIII

Destructive

Ruinous

Disastrous

Very disastrous

XII

Catastrophic

Past Events

Fault Rupture The largest recent fault rupture near Irvine was the 1994 Northridge earthquake, a 6.7 Mw event approximately 56 miles from Irvine, and the most destructive earthquake in the United States in nearly 100 years. This event killed sixty people, injured more than 7,000, left 20,000 people homeless and damaged more than 40,000 buildings in Los Angeles, Ventura, Orange, and San Bernardino Counties. Damage to Angel Stadium occurred as a result of this event. 85 More recently and closer to the City, a 5.1 Mw earthquake occurred in La Habra in 2014. This event caused fault rupturing adjacent to but not directly on the Puente Hills and Whittier faults. 86 Most recently, a significant swarm of earthquakes in the Ridgecrest area occurred on July 4 th and 5 th , 2019. Three tremors ranging from 5.4 to 7.1 Mw occurred within the Eastern California shear zone, a region of distributed faulting associated with motion across the Pacific-North America plate boundary, and an area of high seismic hazard. 87 Liquefaction There is limited information available on the occurrence of past liquefaction events in Irvine. Since these events occur in conjunction with strong earthquakes, the nearest and most recent liquefaction event would have occurred near the mouth of the San Gabriel River at Alamitos Bay as a result of the Long Beach

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