Earth Observation for the Assessment of Earthquake Hazard, Risk and Disaster Management

Earthquakes pose a significant hazard, and due to the growth of vulnerable, exposed populations, global levels of seismic risk are increasing. In the past three decades, a dramatic improvement in the volume, quality and consistency of satellite observations of solid earth processes has occurred.

In a recent paper (https://link.springer.com/article/10.1007/s10712-020-09608-2) I review the current Earth Observing (EO) systems commonly used for measuring earthquake and crustal deformation that can help constrain the potential sources of seismic hazard. I examine the various current contributions and future potential for EO data to feed into aspects of the earthquake disaster management cycle. I discuss the implications that systematic assimilation of Earth Observation data has for the future
assessment of seismic hazard and secondary hazards, and the contributions it will make to earthquake disaster risk reduction.

**Fig. 1:** Annualised death rates from disasters resulting from natural hazards, grouped by decade from the beginning of the 20th Century (e.g., 1900 is the average annual rate of deaths for the complete years 1900–1909, coloured by hazard type). A decline is observed in the number of deaths from climatological and hydrological disasters attributed to drought and flood. However deaths due to earthquake disasters have persisted.Source: International Disaster Database, EM-DAT, CRED, UCLouvain, Brussels, Belgium. http://www.emdat .be

Whilst globally the death rate from natural disasters such as flooding and drought has declined dramatically in the past century, the fatality rates from earthquakes (Fig. 1) have remained persistent. It is a target of the Sendai Framework for Disaster Risk Reduction to reduce the global disaster mortality rate (along with those affected and economic losses) for this coming decade compared to the past decade. However, as populations continue to grow and cluster, an increasing number of people are exposed to the hazards from earthquakes, having gathered to live in urban centres and thus created megacities over recent decades (Fig. 2).

**Fig. 2:** Population exposure of capitals and major cities of the Development Assisted Countries (DAC) as a function of the strain rate determined from the Global Strain Rate Map (Kreemer et al. 2014), used as a proxy of the seismic hazard. Each city is coloured by the Gross Domestic Product (GDP) per capita for the country as a proxy for the first order control on the physical vulnerability

These population centres are often clustered along fault lines (Fig. 3) resulting from historical trade routes, water supplies, fertile basins and security that exist due to the mountains created by the same tectonic processes that cause the underlying hazard. Rapid population growth in economically developing countries, many with limited economic resources or facing other pressing near-term priorities, has led to housing structures of often limited seismic resilience, thus contributing to a heightened vulnerability of those exposed. Furthermore, entrenched and deep-seated corruption tends to by-pass any attempts made to improve this situation with seismic design codes in some countries where earthquake hazard is recognised but ignored. This duplicity results in much worse outcomes in the face of an earthquake disaster than expected, given the prevailing income level.

**Fig. 3:** Selection of global capital cities (from Fig. 2) in economically developing nations situated in regions of active tectonic strain accumulation, with major faults marked by red lines (from the Global Earthquake Model Active Fault database, Styron et al. 2010). Satellite imagery is from Sentinel 2 data and is true colour RGB composites (10 m visible bands 4, 3, 2) from cloud free images acquired in 2019.

Over the past few decades an ever-expanding fleet of Earth Observation systems have been launched in into low-earth near polar orbit, enhancing our capability to monitor solid earth hazards. This continued expansion of satellite capacity (Fig. 4) has occurred both for active radar (synthetic aperture radar—SAR) and passive optical imaging systems (multi-spectral—MS).

**Fig. 4:** Timeline showing the typical major Low Earth Orbit EO satellites used for earthquakes comprising (a) optical and (b) RADAR systems (updated from Elliott et al. (2016a) to end of 2019). Optical satellites are ordered by their resolution and RADAR ones grouped by their microwave wavelength, and both sets are colour coded by their operators/agencies (an acronym list of space agency names is provided at the beginning of the paper). Anticipated launch dates for upcoming missions are shown approximately but are often subject to delay. Arrows continuing beyond present day are only indicative, and for many existing systems this is beyond their design lifespan specification (nominally 5–7 years though often exceeded). There are many other past and present satellite systems, but the ones shown here have near global or consistent systematic coverage, or they have a greater level of availability and suitability for earthquake displacement and crustal strain deformation studies. There has been a very recent increase in the number of constellations of optical microsatellites (such as Planetscopes indicated here) and also of systems from China such as Superview-1, as well as ones in commercial SAR (ICEEYE) and video (Vivid-i) that will become increasingly important to deformation studies as exploitation of these newer datasets develops.

Earth Observation and the Disaster Risk Management Cycle

A priority for action identified within the Sendai Framework for Disaster Risk Reduction is to understand disaster risk. Earth Observation can play an important role in achieving this in terms of characterising the sources of seismic hazard as well as the exposure of persons and assets, and the physical environment of the potential disaster. Therefore, EO can contribute to both understanding the hazard element of disaster and some of the contributors to the risk calculation, namely where populations and buildings are located relative to the sources of such hazards. Furthermore, EO can drive the enhancement of disaster preparedness for earthquakes so that a more effective response and recovery can occur by highlighting the regions exposed to earthquake hazards, thus addressing a further priority of the Sendai Framework to increase resilience. Earth Observation currently feeds into all aspects of the disaster risk reduction cycle and, with upcoming advances, has the potential to contribute even further, particularly for identifying hazard in continental straining areas (Fig. 5).