Second LA Earthquake: A Wake-Up Call


A magnitude (M) 5.1 earthquake struck the Greater Los Angeles area in Southern California on Friday March 28 at 9:09 pm (4:09 UTC on March 29). The earthquake was located 1 mile (1 km) south of La Habre, approximately 20 miles (32 km) southeast of downtown Los Angeles and 20 miles northeast of Long Beach, CA. This event follows the M4.4 earthquake near Beverley Hills (30 miles to the northwest) on March 17. Earthquakes are frequently accompanied by a series of aftershocks and it is expected that these two events are related. Based on internal modeling, this earthquake is not expected to cause earthquake insurance losses exceeding $50 Million.

The Greater Los Angeles area is the most populous region of California and the second most populous area of the United States, and as a result, this earthquake was felt by more than 18 million people. The USGS ShakeMap (version 8) indicates a maximum shaking intensity of VII (very strong) on the Modified Mercalli Intensity (MMI) Scale, however, the USGS PAGER report states only approximately 38,000 people (13,100 households) felt this level of shaking. Approximately 927,000 people (319,700 households) were subjected to a strong (MMI VI) level of shaking, 2,601,000 people (896,900,000 households) subjected to a moderate (MMI V) level of shaking and over 14,845,000 people (5,119,000 households) subjected to a weak-to-light level of shaking (MMI II-IV).


USGS ShakeMap: La Habra, CA Shakemap
Source: US Geological Survey (USGS)

Generally, earthquakes in this magnitude range have perceived levels of ground shaking of MMI VI to VIII as was experienced in the March 28 event. Significant damage is highly unlikely, but light to moderate damage is expected and has been observed in this event. The USGS’ “Did You Feel It” shows the various levels of shaking observed from this earthquake. It is important to note the importance of the underlying soil conditions when trying to understand different levels of shaking one may experience. Soft soils, as present in the Los Angeles Basin, can amplify ground motion and result in a higher level of shaking when compared to rock or stiff soils.

The USGS suggests that the March 28 earthquake occurred on the Puente Hills thrust fault. This is a blind thrust fault similar to the source of the 1994 Northridge earthquake.

EQECAT’s USQuake™ model is guided by Uniform California Earthquake Rupture Forecast (UCERF)—the first comprehensive framework for comparing earthquake likelihoods throughout all of California. The model includes both an occurrence of an event with magnitude and location parameters of the March 28 earthquake, as well as a rare, but devastating likelihood of large rupture along this fault under the city of Los Angeles.

Drivers of Risk for Southern California

CoreLogic|EQECAT studies on the drivers of risk in Southern California indicate that the Puente Hills Fault is the fourth largest driver of risk for Southern California, following the Santa Monica, Southern San Andreas, and Sierra Madre faults.

EQECAT Proxies

EQECAT TVAR-250 Fault Drivers of Risk
Source: EQECAT, Inc.

Earthquake Aftershocks and What to Expect

Earthquakes typically occur in sequences of many earthquakes within a region. The largest earthquake is typically denoted as the main shock, with related earthquakes occurring before the main shock identified as foreshocks and earthquakes occurring afterwards identified as aftershocks. Review of historic data has provided good insights into the rate of earthquake aftershocks, and their distributions in magnitude and location. The two predominant models used in earthquake aftershock modeling are the Short-Term Earthquake Probability Model (STEP) and the Epidemic type aftershock sequence (ETAS) models. Earthquake aftershock models are used to develop probabilistic forecasts of what may occur but do not predict specific aftershock activity.

Most aftershocks do not cause as much damage and loss as the main earthquake. Notable exceptions do however occur. For example, the M6.3 earthquake in Christchurch on February 22, 2011 was an aftershock of the September, 2010 M7 further outside of town. Also, the M5.4 earthquake 3 months before the deadly 1989 Loma Prieta Earthquake in Northern California has been identified as a foreshock. The association of earthquakes as foreshocks and aftershocks requires significant analysis and sometimes lacks consensus. The USGS cites that immediately after this March 28 event there is a 5% chance of an aftershock with a magnitude higher than M5.1 (the probability of a larger aftershock has declined with time since the foreshock). Even if a larger aftershock does occur, it could occur anywhere in the general Southern California region, including the sparsely populated Mojave Desert, and may not have devastating consequences.

The March 28 earthquake is a reminder of the risk to commercial and residential properties in Southern California. Many sources note that most homeowners do not carry earthquake insurance, and those that do typically carry deductibles ranging from 10% to 15% of the replacement value of the home, and commercial insurance often carries large deductibles and strict limits on insurance coverage – the remainder of the risk which is not insured is retained by property owners and frequently, their lenders. CoreLogic regional studies have noted that a major earthquake in the Los Angeles Basin could easily produce damages to residential and commercial property exceeding $200 Billion (Source: the EQECAT Insured Loss Database, 2013). The general lack of insurance coverage and high deductibles have led to concerns over the likelihood of widespread residential mortgage defaults arising from a large basin earthquake. CoreLogic has worked with property owners, risk managers, insurers and lenders to assess this risk and to help them develop response and mitigation plans for this risk.

Proxy Events for RQE® Users

Users of the CoreLogic|EQECAT RQE model should use Event IDs 18462 and 18463 with model region 1 as suitable proxies for the March 28 M5.1 earthquake. Both of these events are located within 5km of the rupture of the event. Event 18462 is M5.15 and event 18463 is M5.05.

Science and Engineering Meets Data

CoreLogic’s deep foundation in data and analytics coupled with EQECAT’s industry-leading science and engineering, provides a combined strength that allows transformation of highly granular data into meaningful information that is easy to access, use and apply. 

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