Innovations in the estimation of fault activity in the Negev - significant results from precariously balanced rock study

Innovations in the estimation of fault activity in the Negev - significant results from precariously balanced rock study

25.10.18

The Dead Sea and Arava Science Center invites you to a seismic hazard seminar, to be held on November 19th, 2018 at the Ben-Gurion University of the Negev. The seminar will discuss the results of an innovative research lead by Dr. Yaron Finzi, DSASC, Mitzpe Ramon Branch.

On November 19th, 2018, the Ben-Gurion University of the Negev will host a seminar on seismic hazard. The seminar will present the conclusions of an innovative research lead by Dr. Yaron Finzi, DSASC, Mitzpe Ramon Branch.

Seismic hazards directly and indirectly affect our lives - from building standard protocols which impact housing prices, to tangible risks to national infrastructure and delayed development of nuclear power plants. Seismic hazard assessment is crucial for economics and industry, and to improve earthquake preparedness at the personal and national levels. This research employs an innovative method for fault activity analysis, based on the stability analysis of Precariously Balanced Rocks (PBR). These semi-stable formations have been standing in seismically active regions for thousands of years and indicate the maximum seismic ground motions that have occurred throughout their lifetime. The objective of the study is to adopt and adapt the PBR method to the unique morphology and lithology of the Negev and to use it in order to better constrain regional, long-term seismic activity along the Negev and Arava.

 Methodology

The research included extensive field surveys in the Negev to locate PBRs. These can be rocks that have disconnected from a cliff, toppled and halted in an unstable position, or slender rock pillars that are disconnected from the cliff by wide cracks but attached at their base to the underlying layers. Mapping surveys included locating PBRs and photographing them to characterize their geometry.  Each rock's geometry and resting angle enable determination of the critical ground acceleration (PGA) required to topple it. Dust from beneath/behind each PBR was sampled and dated to evaluate the time elapsed since the rock entered an unstable state.

Evaluating the stability of rock pillars posed a unique challenge during the study. The basal-attachment of in-situ rock pillars may enable amplification of seismic excitation during an earthquake. Therefore, slender rock pillars (10-40 m high and a few meters wide) may experience resonance, as their natural frequency is within the relevant range of seismic frequencies (1Hz-10Hz). In order to validate the analytic calculations of the natural frequency, the research team installed a tri-axial seismometer on top of a rock pillar located at the northern cliff of the Ramon Crater National Park. The measurements revealed that the ambient vibrations of the rock pillar are amplified relative to the vibrations of the cliff, making than extremely sensitive to ground motions from strong, distant earthquakes (along the Dead Sea and Arava transform).


Figure 1 – deploying the accelerometer on a rock pillar to measure amplification of motion at the natural frequency of the pillar.

Results

Dozens (~80) of rock pillars and unstable boulders were assessed during the field surveys; about a 15 of them were dated to be ancient and therefore significant to determine past earthquake intensities. For example, several PBRs, dated to be over 1300 years old and unstable at relatively low earthquake intensities, indicate that since the seventh century CE, earthquakes of magnitude 7.5 most probably have not occurred in the central Arava Valley. By extending the research area to the south and north, we will be able to determine whether historic earthquakes in the Arava were of lower magnitude than previously estimated, and perhaps to justify modified seismic hazard assessment and construction standards throughout the region.

The stability and ages of PBRs were analyzed to determine the maximal magnitude of the Negev faults in a deterministic approach and to produce maximal ground motion maps. The data indicate that the maximum magnitude earthquake that occurred along specific segments of the Negev and Arava faults are significantly lower than stated in the Israel Building Code (magnitude 4 compared to 5.5, and 6.5 compared 7.5, respectively).

To facilitate further research based on the PBR approach in Israel and to assimilate its use, we established an online PBR dataset including access to all PBR data, stability analyses and spatial risk implication tools.


Figure 2 – Maximum PGA calculated for the Negev and Arava – left: based on maximum magnitude stated in the national building code (SI 413) and, right: based on constrains derived from PBRs that are at least 7000 years old (assuming an amplification of X3 at two slender pillars). The maximum magnitude along the central Arava fault is estimated to be M6.2 (rather than the previously assumed <7.5).