The contribution of broader geological information in modern seismic hazard analyses

Transcription

1 EGU General Assembly 2006 Vienna, Austria, april 2006 The contribution of broader geological information in modern seismic hazard analyses Gianluca Valensise - Roma

2 Many people in this room are geologists (in a broad sense!) They are here because their work relates somehow to Earthquake Geology Most of them are (directly or indirectly) involved in Seismic Hazard Assessment at national or local level Geologists are indeed forcing a revolution in SHA practice. Let s keep up the good work! I will be supplying some food for thought for an ever-growing community that sometimes needs to look back and recalibrate its efforts

3 Seismic Geology Bollinger [1985] The geologic study of seismic deformation to infer the dynamics of the faulting process

4 This book reviews the disciplines that are the foundation of earthquake geology: plate tectonics, structural geology, seismic waves, geodesy, Quaternary stratigraphy and dating, and geomorphology Because active tectonics involves dynamic processes operating today, it can contribute a data set that involves a considerably thicker slice of the Earth than the study of tectonic processes which are no longer active

5 The Fathers - I The Present is the key to the Past Sir Charles Lyell Scotland 1797-London 1875 First scientist to witness abrupt modifications of the topographic surface following the 1819 Cutch, India earthquake

6 The Fathers - II Corollary 1: The Past is the key to the Present Grove Karl Gilbert mountains rise little by little a little cliff, in geologic parlance a fault scarp,, is formed Cohesion and sliding characterize this motion, and some day (the strain) will overcome the friction, lift the mountains a few feet, and re-enact the catastrophy of Owens Valley. (written after the 1872 Owens Valley earthquake, M 7.6)

7 Fusakichi Omori The Fathers - III Corollary 2: The Past and the Present are the key to the Future...the great disturbances... happen successively from different points or portions along the seismic zone. the places seismically more dangerous... are exactly those points along the seismic zone... which have not yet been visited by a very violent shock.... great earthquakes in a given region occur, not everywhere at random, but along a definite line of weakness in the earth s s crust, namely, a seismic zone. (written after the 1908 Messina Straits earthquake, M 7.2)

8 Section I Where does Earthquake Geology stand?...with respect to nearby disciplines...

9 1. Earthquake Geology Earthquake Geology is a fundamental but partial tool in any SHA application. And its is certainly NOT the first of all things to be done.

10 1. Identification of tectonically active areas 2. Earthquake Geology How would you do Earthquake Geology if you don t t even know where tectonic deformation occurs? And - frankly speaking - do we believe we know all tectonically active areas of the regions we work in?

11 1. Identification of tectonically active areas 2. Localization of tectonic strain in discrete sub-areas (faults) 3. Earthquake Geology Of course knowing that Peloponnese or the Betics or the Apennines are tectonically active is not enough. We ought to know if and where tectonic strain is localized on what we refer to as faults

12 1. Identification of tectonically active areas 2. Localization of tectonic strain in discrete sub-areas (faults) 3. Verification of stick- vs free-slip behavior (seismic/aseismic) 4. Earthquake Geology Faults that do not accumulate tectonic strain are of little or no interest for seismic hazard. In many compressional belts around the world, aseismic slip is the rule, not the exception

13 1. Identification of tectonically active areas 2. Localization of tectonic strain in discrete sub-areas (faults) 3. Verification of stick- vs free-slip behavior (seismic/aseismic) 4. Earthquake Geology Taking shortcuts from #1 or #2 to #4 may be very misleading and may result in wrong (in most cases overly pessimistic, sometimes even bizarre) conclusions...

14 Section II Outstanding problems and questions...for the Earthquake Geology et al. community Identifying active faults 2. Blind faulting 3. Seismic/aseismic slip 4. Max expected magnitude

15 1. Identifying active faults The mere first identification of an active fault is still an extremely tricky business if you rely on just one type of observations.

16 1. Identifying active faults The mere first identification of an active fault is still an extremely tricky business if you rely on just one type of observations.?

17 1. Identifying active faults Morphologic convergence may be fatal to those who are eager to find spectacular active faults in a complex and highly mountainous area Wine-glass canyon Hanging valley Facete spurs?

18 1. Identifying active faults Morphologic convergence may be fatal to those who are eager to find spectacular active faults in a complex and highly mountainous area Val d Agrid Vallo di Diano

19 1. Identifying active faults Morphologic convergence may be fatal to those who are eager to find spectacular active faults in a complex and highly mountainous area?? Val d Agrid

20 2. Blind faulting As proved by many recent EQs, the vast majority of faults that are relevant in SHA are blind. Many others are hidden due to youthfulness of tectonic regime. This cannot be ignored!

21 2. Blind faulting Coastal Marche is an area characterized by earthquakes up to M 6 generated by faults that nobody has ever seen at the surface... Pesaro Senigallia, M 5.4, 1924 Senigallia Senigallia, M 5.9, 1930 Ancona Cagli, M 6.2, 1781 Ancona, M 5.6, 1690 Fabriano, M 6.1, 1741 Camerino, M 5.9, 1799 Sarnano,, M 5.9, 1873 Offida, M 5.9, 1943

22 2. Blind faulting anticlinal axis top of Lower Pliocene anticlinal axis seismic line well#1 Senigallia 10 km Ancona well#3 well#2 coastline base of Lower Pliocene unconformity 0 well#1 (4.5 km SE) well#2 (1.2 km SE) well#3 (1.5 km SE) Messinian SW NE TWT (s) 2 4 seismogenic source 6 key reflectors: top of Fucoidi Marls top of Evaporites top of Basement Fault 5 km

23 2. Blind faulting strike 136 dip 30 rake 90 length 12 km width 7 km depth km slip 40 cm Mw 6.0 Mondaino Anticline Senigallia + isolines of uplift generated by slip on fault at depth Off-shore Anticline Ancona - seismic line River Misa River Esino Coastal Anticline

24 2. Blind faulting Coastal Marche is a typical region whose seismogenic sources escapes the most common types of analysis. Only combined geological analyses reveal the active nature of faulting, its main characteristics and its seismogenic potential? Progetto GNDT 5.1.2, (publ. 2001)

25 3. Seismic/aseismic slip

26 Po River before VIII century b.c. Secchia River Panaro River XII-XV century A.D. Reconstruction based on Castaldini et al. (1979)

27 0.40 My A 0.65 My B 3.6 My C

28 Outline of 3.6 My horizon (C) syncline C

29 Outline of 0.65 My horizon (B) syncline B syncline C

30 Outline of 0.4 My horizon (A) syncline A syncline B syncline C

31 Present topography present depocenter syncline A syncline B syncline C The evolution inferred from subsurface data suggests that the present-day depocenter has a truly tectonic origin

32 Vertical displacement for 1 km of slip Po River Mirandola Source Length 18 km Width 14 km Min depth 6 km Max depth 12 km Strike N100 Dip 25 Rake 90 Max M Secchia River - Panaro River 20 km

33 Slip rate from dislocation modeling Topography -100 Meters m x 670 m of slip in 400 Ky on a 25 dipping fault Horizon A S 1.68 mm/y Horizon C Horizon B 0.40 My 0.65 My 3.60 My Meters N

34 3. Seismic/aseismic slip The Mirandola Anticline is one of the best investigated anticlines of the entire country. A good geological and geomorphic record constrains its slip rate to nearly 1.7 mm/y. Accounting for sediment compaction may reduce this estimate to around 1 mm/y. The fault driving the Mirandola anticline is one of the fastest slipping in the country, yet there is no historical nor instrumental evidence for activity in the area. In fact, the seismicity of the entire northern Apennines piedmont is limited and scattered. Is the Mirandola anticline a seismic gap for a M 6.5 EQ that would create serious trouble in wealthy northern Italy, or is one of many freely-slipping faults of northern Italy?

35 4. Max expected magnitude

36 Observation # Bojano Basin, M~ Messina Straits, M~7.0 Earthquake ruptures tend to mimic geological domains and large landscape features

37 Observation # Garfagnana,, M~6.5 Earthquake ruptures ends tend to coincide with singularities of the tectonic fabric

38 Observation # Umbria-Marche, Mw M 6.0 Earthquake ruptures tend to be juxtaposed but never overlap

39 seismic gap? Pesaro 10 km Fiume Foglia Fiume Metauro seismic gap? Fano Senigallia Mw 5.4, 2 January 1924 Senigallia Senigallia Mw 5.9, 30 October 1930 Ancona Fiume Cesano Fiume Misa Fiume Esino Coastal Anticline

40 4. Max expected magnitude Geologists have a strong perception of the continuity or discontinuity of geological structures. Especially in dip-slip tectonic environments, many discontinuities appear to be permanent as they are marked by sharp structural or lithological changes. In several cases the discontinuities correspond to major older lithospheric structures that are also marked by a distinct deep fluid signature. Geologists must learn how to use their geologic wisdom and have their say on maximum expected magnitude simply based on their observations and on physical limitations. Earthquakes are often smaller than they appear in the historical record... but beware that smaller EQs make a bigger hazard!

41 Section III Generations of SHA...what data are used to do what...

42 1. Traditional probabilistic (memoryless) 2. Non-traditional probabilistic (memoryless) 3. Deterministic (scenarios) 4. Time-dependent At each step the extent of geological contribution increases substantially Within each of these contexts the expression broader geological information takes on a different meaning

43 1. Traditional probabilistic It is based on EQ source zones. Each zone is considered to be homogeneous concerning distribution of seismicity (probability of occurrence is the same over the entire zone), earthquake rates and geodynamic (not faulting) style. Geological data required: 1) Geodynamic model 2) Seismotectonic model 3) A set of seismogenic zones based on 1) and 2) 4) Maximum magnitudes (normally derived from seismicity) Standard probabilistic SHA uses the smallest possible amount of geological information (and some practitioners are often tempted to use none...). Most countries worldwide are stuck at this level!

44 The SESAME legacy Jimenez, Giardini and Grünthal, 2001

45 Seismic hazard in the D-A-CH countries Grünthal, Mayer-Rosa and Lenhardt 1998

46 2. Non-traditional probabilistic It is also based on EQ source zones, but each zone is described in better detail and there is an attempt at homogeneity of faulting style. Geological data required: 1) Geodynamic model 2) Seismotectonic model, including typical/mean focal mechanism of each zone 3) Depth of seismogenic layer for each zone 4) A set of seismogenic zones based on 1), 2) and 3) 5) Maximum magnitudes (derived from seismicity and from geological record) In modern probabilistic SHA broad means characterizing source zones with all available geological and geophysical information, if not using faults rather than source zones

47 Level 1: today, in Italy

48 Zonation Kinematics Focal depth Hazard map Better geologic info = Better source zones = Better EQ rates + Better hazard patterns + Better ground shaking etimates

49 Level 2: today, in New Zealand

50 Level 2: today, in New Zealand The use of numerous and carefully described source zones adds detail to the resulting SH estimates and avoids unnecessary spreading of hazard away from identified faults.

51 Level 2: tomorrow, in Italy? Association Seismogenic with areas Association between historical from DISS, SAs EQs, plus... plus... and mean focal mechanisms, plus... Thin-shell finite-element geodynamic model GPS-derived GPS strain velocities, rates in and... SAs, plus...

52 The availability of: Level 2: tomorrow, in Italy? well established and parameterized source zones (SAs) geological slip rates and strain rates for several SAs estimates of the historical earthquake rate for all SAs estimates of strain rate from GPS data for all SAs and modeling estimates of seismic vs aseismic component for all SAs... should return more accurate estimates of SH (ground shaking & EQ probabilities) than those obtained from historical and field data only. (All contributions supplied by participants to the project Assessing the seismogenic potential and the probability of strong earthquakes in Italy,, funded by Italy s s Civil Defense through INGV)

53 3. Deterministic (scenarios) It is normally based on individual potential EQ sources: For each source it normally requires: 1) Full 3D knowledge of geometry and kinematics 2) Exact location of fault & rupture ends (historical/expected) 3) Seismic moment (historical/expected) 4) Expected EQ behavior (characteristic, Poisson,...) 5) Expected rupture history (optional) In deterministic SHA broad means using any available geological (seismological, geophysical) evidence to constrain the source model and its main characteristics

54 3D knowledge of potential EQ sources is mandatory Fault projection to ground surface North Width Fault plane Top edge Rake Strike Top depth Bottom edge Dip Bottom depth Length For most analytical purposes a seismogenic fault must be represented as a rectangular plane whose geometry approximates that of a true fault and whose dimensions reflect the main properties of a true earthquake.

55 Individual seismogenic sources in DISS database

56 Generating synthetic accelerograms for scenario EQ

57 4. Time-dependent It is based on a complete set of individual potential EQ sources: For each source it normally requires: 1) Full 3D knowledge of geometry and kinematics 2) Exact location of fault & rupture ends (historical/expected) 3) Seismic moment (historical/expected) 4) Expected EQ behavior (characteristic, Poisson,...) 5) Slip-rate, recurrence interval, elapsed time Time-dependent models use the broadest possible spectrum of geological information. But the question is: how many nations/regions in the world can afford it?

58 USGS seismic s hazard map showing the probability of a M6.7 or greater earthquake occurring in the San Francisco Bay Area between 2003 and 2032

59 Summary Today we are here to discuss on the many different aspects of Earthquake Geology and the problem of Seismic Hazard Assessment based on geological data (Caputo & Pavlides, 2005). The assessment of seismic hazard is an important task that has a profound impact on Society. It is time that geologists extend their skills to set new constraints on seismogenic processes (as they see things that others will never see...). Geologists though must still work hard to be accepted as full-right participants in the SH community. But time is ripe...

60

61 European database

62 125 Geological/Geophysical Seismogenic Faults (ITA, GRE, ESP) 230 Well-constrained Historical Sources (ITA, ALB, CH, D, DAL, ESP, FRA, GRE, HUN, POR, SLK, SLO, UKF) 428 Poorly-constrained Historical Sources (ALB, ALG, AUT, BEL, BOS, BUL, CH, CRO, CZR, ESP, FRA, GRE, HUN, ITA, MAC, MON, NOR, POL, POR, ROM, SER, SLK, SLO, TUR) 2,719 References

63 ... fault data in map view, tabular view, commentaries and references