As it is anywhere else, real estate is a major investment. What about shifting foundations due to the earth’s movements? Who is at fault? This is not a discussion of legal liability but rather a discussion of geology.
“Fault” usually refers to the juncture of tectonic or bedrock plates that rub against each other, which can lead to large tectonic movements and earthquakes. As these plates push against each other in either a perpendicular or parallel direction, they can interlock and become unable to move relative to each other due to friction. The pressure or load increases until, at one point, the interface suddenly releases and the two plates move. This sudden release causes a seismic event or earthquake. The greater the pressure buildup, the larger the amount of energy released during the earthquake.
In Mexico, the term “fault” has been used to describe a special soil condition that has caused damage to some homes here at Lake Chapala. The damage was generally caused by an uneven settlement of the foundations. This “special soil condition” caused the foundation to settle faster on one side, which tended to be the side closest to the lake. As one side settled faster and dropped down, it typically resulted in large cracks that formed vertically or at a 45-degree angle in walls, or horizontally across floors. The condition affects only structures that bridge or are built over this line, and does not affect adjacent structures that do not bridge the same condition. The line, or area affected, is very narrow.
In 1996, Bill Douglas — a retired geologist residing in Ajijic — took time to map the line of the damage and indications of the movement. Bill took the time to study the type of movement and differences along the line.
The movement on either side of the line was typically vertical (showing a difference in elevation on either side of the line) and not lateral (where one side travels in the opposite direction along the longitudinal axis of the line). This movement causes a structure south of the line on the lakeside to settle faster than the same structure on the north, or mountain side of the line. The subsoil strata holds water like a sponge and, when the water drains away, the soil particles can compress into the spaces where the water was.
This process is called “consolidation.” For some undetermined reason, on the south side of the line, the soil strata tend to drain much more easily than on the north side of the line. Of course, if a structure located south of this line is pushing down on the subsoil strata, the drainage occurs more quickly as the water is squeezed out.
Back in 2002 and 2003 when the water level in Lake Chapala was at a historical low, natural water levels in the subsoil strata were also lower than average. The subsoil strata drained easily and this caused it to consolidate so that the differential settlement of structures bridging this line was accentuated. Structures built over this line showed damage and, in some cases, required extensive reconstruction and reinforcement to resist further cracking and damage. In places where this line crossed the main highway, the road appeared to develop a natural speed bump or tope.
In other areas, the differential settlement cause underground water lines to shift and break. One local business owner has had to install a section of flexible tube on the street water line to avoid further breakage of the waterline. Other structures bridging this line have been built considering this special soil condition and have avoided the damage caused by differential settlement.
Now the question, where is this special soil condition line that has been called a fault? Unfortunately, it is too large to publish with this article. However, I will gladly share my copy of the map if you send an email to [email protected].