The area affected has been in a recent earthquake 'drought' with few earthquakes of significant magnitude occurring in recent years. The USGS suggest there is a potential for the recent M4.4 earthquake to be followed by another earthquake in the near future. Indeed, the entire region of Los Angeles is at risk of high magnitude earthquakes. The residents of Westwood and surrounding region are lucky the earthquake today was not of a higher magnitude and I can only hope this serves as a reminder for all those living in the region of the potential severity a bigger earthquake could cause.
There are currently no reports of landsliding in the area as a result of the earthquake. With a M4.0 the typically accepted threshold for triggering landslides, perhaps we may hear reports of landsliding coming in soon for more remote locations. The 1994 Northridge earthquake triggered tens of thousands of landslides in the surrounding region. Luckily, these landslides had minimal interaction with the population and infrastructure. However, where a landslide did affect a building, the damage from said landslide was approximately three times greater than the average damage caused by the earthquake shaking to similar buildings. Since 1994, there has been substantial development in Los Angeles, the San Fernando Velley and in the hillier areas surrounding Northridge.
Whilst I cannot go into specifics of the case study simulations I have run due to publication constraints, I can report that initial results suggest that even if the same earthquake occurred tomorrow as occurred in 1994 at Northridge, the potential impact of such an event would be greater than what was experienced in 1994. Even a smaller magnitude earthquake of, say, M6.0 could potentially cause as much damage as the M6.7 Northridge 1994 event, if it were to occur tomorrow. I will be able to give such statistics when/if the paper is accepted for publication in the near future.
The USGS PAGER estimates of losses due to secondary hazards states for today's event: "Recent earthquakes in this area have caused secondary hazards such as landslides and liquefaction that might have contributed to losses." This highlights a limitation of the PAGER loss estimates - the effect of secondary hazards are not currently accounted for in the report. This is an area the USGS is currently developing to be able to aid assessments of potential damage from the main shaking and secondary hazards for use in the future. Whilst secondary hazards such as landsliding do not always occur as a result of an earthquake, and in most cases, the majority of earthquake damages are caused by shaking, there are enough cases where the triggered landslides have caused a significant proportion of the damage to warrant concern. In some exceptional events, landsliding has caused the majority of losses as a result of an earthquake trigger. Development of a predictive tool for assessing landslide and liquefaction hazard and losses would be incredibly useful for emergency responders to get a more accurate picture of the disaster and locate areas of secondary hazard damage that may not be accounted for by earthquake shaking alone.
Whilst further research is required into the potential impact of secondary hazards, and the Southern California region provides an almost unprecedented amount of research and data to be of use in this area, I have respect and faith in the USGS and California research expertise in the area. From what I have learnt, California is one of the most regulated and highly prepared states for earthquake risk. By being located on active fault planes with high potential to experience the 'Big One' in multiple cities in the state, the officials and researchers have been forced to up their game. Whilst I hope that the region does not experience a high magnitude earthquake, the reality is that they are going to have one occur.
I hope the M4.4 experienced today has provided a timely reminder to all those living there of what they could face, and causes them to prepare and be able to respond appropriately if and when a future big earthquake comes their way.
