Evidence for Past Subduction Earthquakes at a Plate Boundary with Widespread Upper Plate Faulting: Southern Hikurangi Margin, New Zealand

At the southern Hikurangi margin, New Zealand, we use salt marsh stratigraphy, sedimentology, micropaleontology, and radiocarbon dating to document evidence of two earthquakes producing coseismic subsidence and (in one case) a tsunami over the past 1000 yrs. The earthquake at 520-470 yrs before present (B.P.) produced 0.25 +/- 0.1 m of subsidence at Big Lagoon. The earthquake at 880-800 yrs B.P. produced 0.45 +/- 0.1 m of subsidence at Big Lagoon and was accompanied by a tsunami that inundated >= 360 m inland with a probable height of >= 3.3 m. Distinguishing the effects of upper plate faulting from plate interface earthquakes is a significant challenge at this margin. We use correlation with regional upper plate paleoearthquake chronologies and elastic dislocation modeling to determine that the most likely cause of the subsidence and tsunami events is subduction interface rupture, although the older event may have been a synchronous subduction interface and upper plate fault rupture. The southern Hikurangi margin has had no significant (M > 6.5) documented subduction interface earthquakes in historic times, and previous assumptions that this margin segment is prone to rupture in large to great earthquakes were based on seismic and geodetic evidence of strong contemporary plate coupling. This is the first geologic evidence to confirm that the southern Hikurangi margin ruptures in large earthquakes. The relatively short-time interval between the two subduction earthquakes (similar to 350 yrs) is shorter than in current seismic-hazard models.GNSEQC Biennial ProjectNew Zealand Natural Hazards Research Platform and Foundation for Research Science and TechnologyInstitute for Geophysic

Ground-Rupturing Earthquakes on the Northern Big Bend of the San Andreas Fault, California, 800 A.D. to Present

Paleoseismic data on the timing of ground-rupturing earthquakes constrain the recurrence behavior of active faults and can provide insight on the rupture history of a fault if earthquakes dated at neighboring sites overlap in age and are considered correlative. This study presents the evidence and ages for 11 earthquakes that occurred along the Big Bend section of the southern San Andreas Fault at the Frazier Mountain paleoseismic site. The most recent earthquake to rupture the site was the Mw7.7–7.9 Fort Tejon earthquake of 1857. We use over 30 trench excavations to document the structural and sedimentological evolution of a small pull-apart basin that has been repeatedly faulted and folded by ground-rupturing earthquakes. A sedimentation rate of 0.4 cm/yr and abundant organic material for radiocarbon dating contribute to a record that is considered complete since 800 A.D. and includes 10 paleoearthquakes. Earthquakes have ruptured this location on average every ~100 years over the last 1200 years, but individual intervals range from ~22 to 186 years. The coefficient of variation of the length of time between earthquakes (0.7) indicates quasiperiodic behavior, similar to other sites along the southern San Andreas Fault. Comparison with the earthquake chronology at neighboring sites along the fault indicates that only one other 1857-size earthquake could have occurred since 1350 A.D., and since 800 A.D., the Big Bend and Mojave sections have ruptured together at most 50% of the time in Mw ≥ 7.3 earthquakes

Sedimentary evidence of historical and prehistorical earthquakes along the Venta de Bravo Fault System, Acambay Graben (Central Mexico)

The Venta de Bravo normal fault is one of the longest structures in the intra-arc fault system of the Trans-Mexican Volcanic Belt. It defines, together with the Pastores Fault, the 80 km long southern margin of the Acambay Graben. We focus on the westernmost segment of the Venta de Bravo Fault and provide new paleoseismological information, evaluate its earthquake history, and assess the related seismic hazard. We analyzed five trenches, distributed at three different sites, in which Holocene surface faulting offsets interbedded volcanoclastic, fluvio-lacustrine and colluvial deposits. Despite the lack of known historical destructive earthquakes along this fault, we found evidence of at least eight earthquakes during the late Quaternary. Our results indicate that this is one of the major seismic sources of the Acambay Graben, capable of producing by itself earthquakes with magnitudes (MW) up to 6.9, with a slip rate of 0.22-0.24 mm yr− 1 and a recurrence interval between 1940 and 2390 years. In addition, a possible multi-fault rupture of the Venta de Bravo Fault together with other faults of the Acambay Graben could result in a MW > 7 earthquake. These new slip rates, earthquake recurrence rates, and estimation of slips per event help advance our understanding of the seismic hazard posed by the Venta de Bravo Fault and provide new parameters for further hazard assessment

Paleoearthquake history of the Spili fault

Η παλαιοσεισμική δραστηριότητα στο ρήγμα του Σπηλίου μελετήθηκε χρησιμοποιώντας μία πρωτοποριακή μέθοδο που συνδιάζει μετρήσεις Σπανίων Γαιών (REE) και κοσμογενών ισοτόπων 36Cl πάνω στην σεισμικά αποκαλυμμένη επιφάνεια του ρήγματος. Η ανάλυση των δεδομένων δείχνει ότι το ρήγμα είναι ενεργό και έχει φιλοξενήσει τουλάχιστον 5 μεγάλου-μεγέθους σεισμούς τα τελευταία 16500 χρόνια. Οι δύο πιο πρόσφατοι σεισμοί έλαβαν χώρα κατά την περίοδο 100-900 ετών πρίν από σήμερα και άθροισαν συνολικά 3.5 μέτρα σεισμικής μετατόπισης. Η χρονολογία των παλαιότερων 3 σεισμών προσδιορίσθηκε στα 7300, 16300 και 16500 χρόνια πριν από σήμερα με σεισμικές ολισθήσεις 2.5, 1.2 και 1.8 μέτρα, αντίστοιχα. Από το μέγεθος των σεισμικών ολισθήσεων συμπεραίνουμε ότι το μέγεθος των σεισμών που προκλήθηκαν από το ρήγμα του Σπηλίου κυμάνθηκε από Μ 6.3-7.3 ενώ ο μέσος ρυθμός επανάληψης τους ήταν ~4200 χρόνια. Τα παραπάνω δεδομένα αποκαλύπτουν ότι το ρήγμα του Σπηλίου είναι ένα από τα πιο ενεργά ρήγματα στην Κρήτη και οι σεισμικές παράμετροί που σχετίζονται με την δραστηριότητά του πρέπει να συμπεριληφθούν στο μοντέλο σεισμικής επικινδυνότητας της Ελλάδας.The paleoearthquake activity on the Spili Fault is examined using a novel methodology that combines measurements of Rare Earth Elements (REE) and of in situ cosmogenic 36Cl on the exhumed fault scarp. Data show that the Spili Fault is active and has generated a minimum of five large-magnitude earthquakes over the last ~16500 years. The timing and, to a lesser degree, the slip-size of the identified paleoearthquakes was highly variable. Specifically, the two most recent events occurred between 100 and 900 years BP producing a cumulative displacement of 3.5 meters. The timing of the three older paleoearthquakes is constraint at 7300, 16300 and 16500 years BP with slip sizes of 2.5, 1.2 and 1.8 meters, respectively. The magnitude of the earthquakes that produced the measured co-seismic displacements, ranges from M 6.3-7.3 while the average earthquake recurrence interval on the Spili Fault is about 4200 years. The above data suggest that the Spili is among the most active faults on Crete and its earthquake parameters may be incorporated into the National Seismic Hazard Model

Holocene earthquakes on the Zemuhe Fault in Southwestern China

The Zemuhe Fault is a prominent active fault in Southwestern China. Seven ravines along a 5 km long fault scarp indicate seven large magnitude earthquakes in the Holocene. The youngest four ravines were abandoned during four large magnitude earthquakes, the age of which are constrained by radiocarbon data: ravines 7, 6, and 4 formed in association with the earthquakes at A.D. 1850 and A.D. 814, B.C. 4477 ± 240 or older, and ravine 5 to a paleo-event between B.C. 4477 ± 240 and A.D. 814. Three trenches excavated by earlier workers together with a trench excavated and analyzed here revealed 3 or 4 earthquakes, which are consistent with those indicated by the youngest five ravines. These radiocarbon-dated earthquakes mainly occurred within two temporal clusters: the older cluster of two paleoearthquakes occurred approximately between B.C. 4250 and B.C. 6000, and the younger cluster includes two historical earthquakes of the A.D. 814 and A.D. 1850. Each cluster lasted about 1000-2000 years. A tranquil period of about 5000 years separates the two clusters, during which only one large magnitude earthquake occurred. Moreover, the average recurrence interval of large magnitude earthquake in the Holocene is about 1400-1700 years. Comparison of the maximum horizontal displacement of the A.D. 1850 earthquake, and the 85 ± 5 m cumulative lateral offset over the last 13-15 ka gives the average recurrence interval of 1000-1360 years. The different estimates may arise because moderate and small earthquakes produced a quite high cumulative lateral displacement along the Zemuhe Fault during the Holocene

The use of Terrestrial Laser Scanning in characterizing active tectonic processes from postseismic slip to the long term growth of normal faults

This thesis investigates two main hypotheses regarding uncertainty in the measurement of paleoseismic offsets used to estimate fault activity and paleoearthquake magnitudes on normal faults: (1) That variations in fault geometry have a significant effect on throw-rates and fault offsets; and (2) that postseismic deformation can be a significant component of the total fault slip for moderate magnitude earthquakes. These hypotheses are tested using high resolution terrestrial laser scan datasets of normal fault topographic offsets and surface ruptures. The first hypothesis is addressed by studying the crustal scale Campo Felice active normal fault in the Central Apennines, Italy. Variation in throw-rate along strike since the last glacial maximum (15 ka ±3) is measured from an offset periglacial surface at two hundred and fifty sites using cross sectional data derived from a high resolution terrestrial laser scan (TLS) dataset. The measurements are used to create a detailed throw-rate profile. Field measurements of fault geometry (strike, dip and kinematic slip direction) are also gathered. Variation in fault throw-rate is found to correlate with fault strike. A study of weathered band thickness on the exposed Miocene limestone bedrock fault scarp, thought to have been created by single past slip events on the fault also appears to correlate with fault strike. A strain-rate profile is calculated using the throw-rate profile and the field measurements of kinematic slip. In contrast to throw-rate, strainrate is independant of changes in fault strike and dip. It is suggested that strain-rate in comparison to throw-rate provides a more robust measure of fault activity as it is unaffected by changes in fault geometry. The outcome of this study is that paleoseismic studies on active faults should take into account fault geometry before choosing sites which may have anomalously high or low paleoseismic offsets. Fault geometry introduces significant uncertainty into the estimation of inferred paleoearthquake magnitudes from paleoseismic offsets and hence seismic hazard analysis. The second hypothesis is addressed through the study of near-field postseismic deformation (surface rupture afterslip) following the 6th April 2009 6.3 Mw L’Aquila earthquake, created by slip on the Paganica normal fault in the Central Italian Apennines. A novel use of TLS technology allowed the postseismic deformation at four sites along the L’Aquila surface rupture to be measured between 8 – 126 days after the earthquake. Complimentary measurements of postseismic deformation at a fifth site using a robotic total station were combined with the TLS datasets to describe the along strike variation in postseismic deformation. The near-field postseismic deformation measured occurred mostly in the immediate hangingwall of the surface rupture and increased with decreasing rate over time. The postseismic deformation measured is comparable to theoretical and empirical models which have been used to describe afterslip for previous earthquakes. The magnitude of near-field postseismic deformation was up to 60% that of the coseismic offset in the near-field and suggests that postseismic deformation can form a significant component of paleoseismic offsets of moderate magnitude. Postseismic deformation was also found to be greatest above regions of the fault zone where a high coseismic slip gradient existed, suggesting that postseismic deformation occurs at the periphery of the coseismic slip patch within the fault zone. Regression relationships which relate surface offset to moment magnitude are populated by field observations of surface offsets where earthquake magnitude is known. These regression relationships are then used to infer paleoearthquake magnitudes from paleoseismic offsets. The field studies used to populate regression relationships do not routinely take into account the potential effects of fault geometry and significant postseismic slip. As a result paleoearthquake magnitudes inferred from such regression relationships are maybe over estimated. It is suggested that future regression relationships of surface offset and moment magnitude should factor in the effects of fault geometry and postseismic deformation in order to produce a relationship in which surface offset (both coseismic and postseismic) is described for a range of magnitudes and, where possible, any local effects of fault geometry are removed from the input dataset. The production of such a relationship will allow paleoseismologists to measure combined coseismic and postseismic offsets from field studies and to infer paleoearthquake magnitude with decreased uncertainty