Paleoseismology, methods and examples - behaving of seismogenic fault in geological history Paleoseismology Paleoseismology studies prehistoric earthquakes from geological record Seismologists - data measured instrumentally during earthquakes X Paleoseismologists interpret geological phenomena accompanied by individual EQs McCALPIN, J. (2009). Paleoseismology. San Diego: Academic Press. Why? Present day seismicity – plate boundaries, intraplate regions Catastrophic EQs – sometimes in areas with faults with no present day seismicity, seismic cycle – longer reccurence interval (China, New Zealand) Most areas – record of historical EQs only several hundred yrs (historical and instrumental seismicity) X some active faults expressed in morphology and geology – no historical seismicity or large EQs China and Middle-East – record thousands yrs and more, still not long enough (fault active millions yrs) The historical record of 3,000 yrs – covers only little part of faulting history Seismic hazard assessment – based on very short period of record of historical EQs, it may cause 2 problems:  overestimation of probability of future EQs based on historical large EQs, but with long recurrence interval (seismic energy is released)  underestimation – in areas with seismogenic faults but no historical record (strain accumulation) Paleoseismology extends record of EQs into the geological past Earthquakes catalogues too short Premise – EQ only larger M > 6 can create permanent deformation on the surface  topographic instability  new processes - erosion and accumulation  new landforms and structures  geological record of EQ Smaller EQ - rarely geological expression created or survives Fault type – normal faults M ≥ 6.3; strike-slips – California i – M = 6.25-6.5, Depth of seismogenic crust – deeper needs higher magnitude Loma Prieta 1989 M=6.9, 2m slip in depth 3-18km, no surface rupture Gujarat 2001 M=7.7, blind fault, 1-4m in depth 9-15km, Empirical relationships based on observation from historical EQs Relationships: fault length, amount of displacements, size of Magnitude e.g. fault 80km long can generate EQ Mw=7.5 and displacement 3m Empirical relationships – historical EQs (421), focis depth <40km, Mw > 4.5 Wells, and Coppersmith 1992 9. 4. 1968, Borego Mts, CA Average of multiple displacement measurements along the fault Seismic cycle – elastic model Earthquake deformation cycle Idealized cycle characteristic earthquake Paleoseismological study of faults  Localisation and geometry (geomorphology, geological mapping)  Slip rate – faulting velocity (= displacement/time)  Slip per event – characteristic displacement during individual EQs  Recurrence period – (repeated EQ, frequency EQ)  Elapsed time – time from the last EQ  Maximum potential magnitude  stratigraphic, structural, geomorphological, biological, archeological evidence  dating of displaced features or movement indicators  dating of multiple movements (EQs) - recurrence interval, long-term sliprate, variability of movements during EQs predict location and magnitude of future EQs Chronological reconstruction of movements  young sediments, fine grained, stratified - well recognizable displacement of layers, not thick alluvial fans, lake sediments X debris flow  datable material– chronology of movements Methods – direct observations of dislocated objects – on the surface or in trenches, outcrops Evidence of earthquakes (EQ) in geological profiles in a trench A) Difference in cumulative offset B) Buried fault scarp C) Coluvial wedge- typical for sudden movement D) Fissures filled by overlying material E) Sand dykes F) Liquefied layers Allen (1986) • Difference in cumulative offset ? How many: retrodeformation 4 events – vertical offset 2cm Oldest layer - (Qal5) all 4 events, cumulative 8cm Youngest (Qal1) has experienced only 1 event  2 cm on the layer base, but 1 cm on the surface! Surficial erosion Repated EQs Colluvial wedge Normal faulting Gravitational instabilty Fault scarp derived material - wedge • Filled fissure Aremogna-Cinquemiglia fault - Italy Suusamyr, 1992, M=7,4 Kyrgyzstan Reverse faulting 2006 Reverse faults – colluvial wedge Seismic events reconstruction Various kinematics related to different stress diection Alhama de Murcía fault (Spain) Imperial fault, 1940 M=7.6m offset, 60km Fault scarp and colluvial wedge on strike-slip fault - Collision zone Europe x Africa plate - Southern margin of Alpine orogen - Part of Bettic Cordillera Carboneras – outer zone (nappe from Mesozoic to Tertiary rocks) paleo-margin of Iberian plate - inner zone– metamorphic complex + Neogene to Quaternary sediments – intramontane basins bounded by faults NE-SW Carboneras fault zone – Spain Masana E. et al. Case study Carboneras - formed in the last period of collision of inner and outer zone of Betic cordillera in early Miocene  Miocene to Quaternary – stress field rotation - normal faults - mid-Miocene – part of rifting (volcanism) - reverse faults – early Pleistocene (formation of small mountains e.g. La Serrata) - strike-slips – left-lateral (up to present-day) Almería Nijar Carboneras Almería  seismicity – SE margin of Iberian peninsula – permanent shallow earthquakes M < 5.5 (transversal faults now without seismicity – Carboneras)  last 2.000 years – at least 50 larger earthquakes Previous studies in 9Oth of 20th century Movements in late Quaternary- relatively slow, mainly vertical, horizontal movements of 80-100m offset channels in La Serrata – older than 100.000 years 1) Study of evidence of left-lateral movements dated by radiometric dating of marine terraces and their recent uplift 2) Measuring and dating of left-lateral movements based on offset channels Methods of study of Carnoboneras fault on the sea  Bathymetry  Sidescan sonography  High resolution seismic reflection  Marinne sediments samples analysis  Dating of the sediments Bathymetry Sonography – high resolution Seismic profilling Results: Carboneras fault zone – 5-10 km wide, 100 km long, subvertical faults, Drainage network on the inland margin – deflected, Morphology formed by horizontal movements – pressure ridges, water gaps, late Holocene sediments, landslides. Methods of fault study on the land - offshore  Photointerpretation – air photos  Geomorphological mapping of dislocated landforms  Structural mapping (faults)  Sedimentology (identification of generations of alluvial fans)  Microtopography (total station)  Geophysics (georadar, electrotomography – fault tracing and goundwater level)  Paleoseismic trenching  Dating of materials cut by the fault El Hacho 2005 La Serrata Pliocene Quaternary 3 generations of alluvial fans – Mid and Late Pleistocene/Holocene - 3 various generations of fault movements (erosion - accumulation)  all 3 alluvial fan generations (chronology)  visible morphological scarps (0.7m) Paleoseismic trenches Geophysics Georadar Antenna (MHz): 25 50 100 200 - resolution + + reach - Electrotomography - fault position and characterictsics of the material at the depth November 2005 Cleaning the wall, grid Identification of sedimentary layers and dislocations/faults - Complex structure -flower structure -transpressive regime -Horizontal movements - strike-slips with vertical component- Repeated movements S-C structures photomosaic 1 m 1 m 1 m 1 m SE-NW depth(m)v=0.07m/s distance (m) situation in situ versus geophysics SE NW Dating – material cut by the seismic event  14C radiocarbon dating method  organic material and carbonatic shells (reach 40 thousands years) – charcoal, gastropodus, organic material, wood, etc. 14C – in living organism, added from the environment, it decays, after death of organism, ratio of 14C/stabile 12C changes  Optically stimulated luminiscence OSL – electrones trapped in crystal lattice of sand grains - released by light activation or stimulation (reset – zero signal). After finishing of sedimentation – signal increases due to radioactive decay. Luminiscence relased by light activation in the lab is proportional to sediments age (the time until which the electrones were accummulated – until next reset (reach 250-300 thousands yrs)  Thermoluminiscence TL  fine-grained sediments (100 thousands yrs)  U/Th  carbonatic material (reach 300 thousands years) – laminar caliche  laboratory results of dating chronology of tectonic activity on the Carboneras fault  interpretation of trench logs, assessment of type and amount of movements reconstruction of deformation (retrodeformation) Results: Trench analysis and dating on Carboneras fault:  B1, B2 – colluvial wedge were recognized (surface degradation after suddent event) – earthquake  Minimum 4 earthquakes during last 50 thousands years  Recurrence period – min. 14 thousands yrs  Last event – minim 1310 years ago  Empiric relationship Magnitude X Displacement for 2 events – minim. M= 6.59 and 6.97