Accordingly with the United Nations (1993) the mass movements are one of the natural phenomena that cause the most financial impact and deaths in the world. The mass movements assume catastrophic proportions in urban areas causing structural damages and human losses. In Brazil rainfall is the principal natural factor that causes floods, riverside collapses, intensification of erosional processes and evidently, they decisively contribute to mass movement triggering. As a result of specifically geological risk scenarios natural processes may occur in different scales becoming disasters and catastrophes with huge material and social impact [1] . Civil Defense Preventive Plan―PPDC (in Portuguese) was elaborated in 1988, specifically to attend the frequent mass movements in Serra do Mar Mountain slope in Sao Paulo State. PPDC constitutes an important technical tool for management, monitoring and control of mass movement risk areas. Evacuation of vulnerable population living insusceptible areas, before the mass movement events occur, is a main prevention al action of the PPDC.

The main operability of PPDC involves the monitoring of the related rainfall values (hourly and accumulated 72 hours rainfall index), as well as weather forecast, field inspection and emergency situations [2] . Currently, these activities are developed in ten regions of Sao Paulo State, considering that Baixada Santista and Litoral Norteare two of them. The PPDC deals with preventive actions of mass movement associated risk in slope and talus areas inappropriate for occupation.

Therefore, the aim of this study is to present the interaction between the accumulated 6 h × 72 h rainfall correlated with the triggering time of the mass movement events in connection of the critical pluviometric thresholds [3] . The last is of extreme importance for the functional efficiency of the PPDC preventive actions. It is suggested an evaluation model of the mass movement events’ register and the rainfall database for a 4-year period (2014 to 2018) in Baixada Santista and Litoral Norte regions (Figure 1). The intention is to perform an actualization and classification of the critical thresholds. This may be considered a very important empirical tool for the landslide risk management efficiency due to the preventive action improvement and the social/material impact mitigation.

Figure 2 presents the proposed model applied in area which is situated on Sao

Paulo State north coast between Serra do Mar Mountains and the South Atlantic Ocean. This area covers 13 municipalities―Peruibe, Itanhaem, Mongagua, Praia Grande, Sao Vicente, Cubatao, Santos, Guaruja and Bertioga (Baixada Santistaregion) and SaoSebastiao, Ilhabela, Caraguatatuba and Ubatuba (Litoral Norteregion). The mass movement susceptibility varies of each COMDEC (Regional Civil Defense Municipality Units) attendance area of all these 13 municipalities, being a reflex of the physical factors and conditions (lithological and pedological substrate, relief, slope position and declivity, vegetal covering, pluviometric precipitation patterns, etc), as well as the anthropic conditions of the risk area (environmental degradation, slope cut, dumped landfills and embankments on top of the talus, pipe leakages and other).

In Figures 3-5 stands out that many mass movement events are triggered due to rainfall intensity or/and accumulated precipitation regime below the PPDC official critical threshold pattern [8] and [11] . For induced landslides in the regions Baixada Santista and Litoral Norte a new critical thresholds values of the triggering rainfall can be suggested. They could be considerably less of the PPDC proposed triggering reference lines. Then mass movement events present low critical thresholds could stay far below of the Tatizana et al. [8] triggering reference line. They could be associated with a high contribution of several triggering anthropic actions [12] .

Critical thresholds by the type of the slope and the talus rupture―the graphic suggests that the slope cut landslides could have lower triggering thresholds than the PPDC proposed (Figure 3). Dumped landfill landslides are triggered in 50 mm/72 h accumulated precipitation index, natural slope landslides occurring above of 100 mm/72 h accumulated precipitation index and debris/mud flows usually occurring above of 220 mm/72 h accumulated precipitation index.

Critical thresholds by magnitude―the graphic suggests that an accumulated rainfall above of 50 mm/72 h precipitation index could cause high magnitude landslides (Figure 4).

Critical thresholds by the impact―the graphic suggests that accumulated rainfall above of 50 mm/72 h of precipitation index could cause medium to high impact landslides (Figure 5).

Main critical threshold line―the graphic suggests that the principal critical threshold line is fixed in 100 mm (accumulated 72 h rainfall index) separating rainfall triggered mass movement events from induced events by anthropic factors (Figure 6). In the zone above the critical line, the events data presents higher level of accuracy than in the zone below this line that presents lower accuracy level.

Is important to highlight that between the first (50 mm) and the second (100

mm) critical threshold line there is clear evidence of interaction between the inducing and the natural triggering factor. However, when the critical threshold line increase, the level of data uncertainties reduce. The same happen when the critical threshold line decrease and inducing factor begins to prevail, the level of data uncertainties reduce.

The results, although preliminary, indicate some relevant trends on analysis, which point to reach a better understanding of the mass movement dynamics in the regions concerned. The COMDEC (Regional Civil Defense Municipality Units) data sources and nonevent rainfall data to be incorporated in the future will allow more consistency of the mass movement event analysis and could confirm these correlations.

This kind of data in Brazil, such as rainfall values as well as mass movement event data, is very sparse and hampers the robust analysis, making more difficult the critical threshold actualization with the necessary precision. That takes the critical threshold definition at even lower levels, which can create a disproportionately large amount of alerts, many of them without any associated mass movement event.

The authors declare no conflicts of interest regarding the publication of this paper.

Metodiev, D., de Andrade, M.R.M., Mendes, R.M., de Moraes, M.A.E., Konig, T., Bortolozo, C.A., Bernardes, T., Luiz, R.A.F. and Coelho, J.O.M. (2018) Correlation between Rainfall and Mass Movements in North Coast Region of Sao Paulo State, Brazil for 2014-2018. International Journal of Geosciences, 9, 669-679. https://doi.org/10.4236/ijg.2018.912040