Recurrence Time Of Earthquakes Research Articles

Micropalaeontological evidence for the Holocene earthquake history of the eastern Bay of Plenty, New Zealand, and a new index for determining the land elevation record

Fossil foraminifera and diatoms are used to identify sudden, probably earthquake-related, elevational changes in three Holocene sedimentary sequences from the high-tidal fringes of Ohiwa Harbour, eastern Bay of Plenty, New Zealand. Modern analogue calibration sets of faunal and floral census data are used to estimate palaeosalinities and palaeotidal elevations that help quantify seismic-related, vertical displacements. Age models for the three vibracored sequences are built on a combination of tephrostratigraphy and radiocarbon dating. A new index, the land elevation record (LER), is introduced to graphically portray earthquake-related vertical land displacements on a time-depth diagram. Also plotted are elements used to calculate LER, such as the indicative depth estimated from microfossils, inferred sediment compaction, and the New Zealand Holocene palaeo-sea-level curve. All three Ohiwa cores, spread over 3 km of coast, contain both freshwater and intertidal sediments. A prominent erosional contact between freshwater peat or soil and overlying intertidal mud, records a major subsidence event in each core of c. 2 m, dated at ca 2600 cal years BP. The deepest core (7.4 m) indicates that this is the only substantial vertical displacement event to have occurred in the last 8 ka. A small subsidence event (ca 0.3–0.7 m) is indicated close to the top of one core, but is not present in the other two sites. This may be the result of a local land subsidence during the poorly known Taneatua Earthquake of 1866. There is no historic human record of earthquake displacements around Ohiwa, but mid-Pleistocene, interglacial, marine sediments have been uplifted 10–60 m in several identified fault blocks. Our study provides conclusive evidence of at least 2 m of earthquake-related, subsidence during the Holocene, with a recurrence time of major earthquakes of ca 5 ka.

Coseismic release of water from mountains: Evidence from the 1999 (Mw = 7.5) Chi-Chi, Taiwan, earthquake

Research Article| September 01, 2004 Coseismic release of water from mountains: Evidence from the 1999 (Mw = 7.5) Chi-Chi, Taiwan, earthquake Chi-yuen Wang; Chi-yuen Wang 1Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Chung-Ho Wang; Chung-Ho Wang 2Institute of Earth Sciences, Academia Sinica, Nankang, Taiwan Search for other works by this author on: GSW Google Scholar Michael Manga Michael Manga 3Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA Search for other works by this author on: GSW Google Scholar Geology (2004) 32 (9): 769–772. https://doi.org/10.1130/G20753.1 Article history received: 22 Apr 2004 rev-recd: 07 May 2004 accepted: 11 May 2004 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Chi-yuen Wang, Chung-Ho Wang, Michael Manga; Coseismic release of water from mountains: Evidence from the 1999 (Mw = 7.5) Chi-Chi, Taiwan, earthquake. Geology 2004;; 32 (9): 769–772. doi: https://doi.org/10.1130/G20753.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Earthquake-induced increases in streamflow, producing ∼0.7 km3 of total excess water, were documented after the 1999 (Mw = 7.5) Chi-Chi earthquake in central Taiwan. Analysis of stream gauge data and well records suggests that the excess water originated in the mountains. We propose that the extensive high-angle fractures formed during the earthquake allow rapid release of water from mountains and that mountains in tectonically active areas may be repeatedly flushed by meteoric water at time intervals comparable to the recurrence time of large earthquakes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The interplay between global tectonic processes and the seismic cycle in the Umbria-Marche seismogenic region

SUMMARY For the central Apennines, peninsular Italy, a series of tectonic mechanisms are reproduced by means of finite-element models, in order to study the effects of active tectonics on the seismic cycle in the Umbria–Marche seismogenic zone. Continental extension and rift push effects induced by small-scale convection are modelled within 2-D viscoelastic models of the crust–lithosphere system, in vertical cross-sections perpendicular to the strike of the major tectonic structures under study, namely the Apennines and the Colfiorito fault zone, where the 1997 seismic sequence took place. With the aim of constraining the active tectonic mechanisms at the regional scale and the behaviour of the fault in the seismogenic zone at the local scale, modelled baseline rate of change are compared with newly acquired GPS data, retrieved from the two permanent GPS receivers of Camerino (CAME) and Elba (ELBA), deliberately installed along the modelled transect. These receivers are located at both edges of the continental extension in the front of the Apennines, close to the Adriatic Plate in the east, and in the rear of the chain, in the Tyrrhenian domain. The deformation pattern inferred from seismicity and from the geodetic data is consistent with small-scale convection in the Tyrrhenian domain, which reproduces extension in the rear of the Apennines and compression in the front of the chain. A convective mechanism, associated with backarc opening and doming of the asthenosphere, provides an extensional rate, along a baseline connecting two sites in the front of the chain (Camerino) and in its rear (Elba), comparable to the observed baseline rate of change. The viscosity of the lower crust plays a fundamental role in determining the style of stress in the crust–lithosphere system. Once constrained by means of the extensional baseline rate inferred from GPS, the modelled slip across the Colfiorito fault and the modelled earthquake recurrence time are consistent with the 1997 normal fault event and with palaeoseismicity, respectively.

Investigating environmental tectonics in Northern Alpine Foreland of Europe

Until now, research on neotectonics and related seismicity has mostly focused on active plate boundaries characterized by a generally high level of earthquake activity. Current seismic hazard estimates for intraplate areas are commonly based on probabilistic analyses of historical and instrumental earthquake data. The accuracy of these hazard estimates is limited by the nature of the data (e.g., ambiguous historical sources), and by the restriction of available earthquake catalogues to time scales of only a few hundred years. Both of these are geologically insignificant and unsuitable for describing tectonic processes causing earthquakes. This is especially relevant to intraplate regions, where faults show low slip rates resulting in long average recurrence times for large earthquakes (103 to 106 yrs), such as the devastating Basel earthquake of 1356, with an estimated magnitude of 6.5.

Analysis of the 23 June 2001 Mw = 8.4 Peru underthrusting earthquake and its aftershocks

On 23 June 2001, a Mw = 8.4 underthrusting earthquake occurred in the southern Peru subduction zone, followed by several large aftershocks, including 26 June (Mw = 6.7) and 7 July (Mw = 7.5). Broadband analyses of seismic data for the largest of these earthquakes show southeastward rupture of 180 km along the portion of the subduction zone previously ruptured in 1868 (Mw 8.8–9). Moment release distributions determined are consistent with aftershock location patterns. Earthquake rupture mode varies along southern Peru, from larger multi‐segment rupture in 1868 to several smaller segment ruptures in 2001, similar to rupture variation observed in northern Peru and other subduction zones. Based on models of subduction zone segment interaction, the 2001 earthquake sequence may suggest a shorter recurrence time for future earthquakes along this portion of the Peru‐Chile subduction zone.

Palaeoseismicity studies on end-Pleistocene and Holocene lake deposits around Basle, Switzerland

Summary Palaeoseismological investigations in the lakes of Seewen and Bergsee in the Basle region, Switzerland and southern Germany, revealed characteristic event horizons in an otherwise uniform background sedimentary record. Dated and correlated based on radiocarbon ages, palynostratigraphy and sedimentation rates, some of these event horizons show soft-sediment deformation features and fractures that can be interpretated as being the result of earthquake shaking. Two of the event horizons with clear indications for earthquake deformation were detected in both lakes, showing approximately the same age as indicated by radiocarbon dating. A third event horizon with fractures of apparent seismogenic origin, was detected at several drill sites in only one lake (Bergsee). Three further event horizons, two in the Bergsee and one in Lake Seewen, are of uncertain origin, though they show some characteristics that could well be caused by earthquakes. Based on the observations in both lakes, five events were detected of which three are most probably related to earthquakes which occurred between 180 BC–1160 BC, 8260 BC–9040 BC and 10 720 BC–11 200 BC, respectively. The Basle region is well known for the strongest historical earthquake north of the Alps, the AD 1356 Basle earthquake. Based on combined historical and palaeoseismological data, it has been inferred that earthquakes with size comparable to the AD 1356 Basle earthquake have occurred several times within the last 12 000 yr and that the recurrence time for such strong earthquakes are in the range of 1500–3000 yr.

Interaction between shallow and subcrustal dislocations on a normal fault

We propose a model which may explain seismic sequences which are often observed in seismogenic regions, as for example in the Apenninic chain (Italy). In particular, we consider a normal fault and earthquakes taking place at different depths: a first shock in a shallower layer and a second in a deeper one. The normal fault is embedded in a viscoelastic half-space. As a consequence of the rheology, there are two different brittle layers, a shallower and a deeper one, where earthquakes can nucleate. Between these two layers, the rheological behavior is ductile. The thicknesses of the layers depend on the geothermal profile that is calculated taking into account the profile of the thermal and rheological parameters with depth. The fault plane, crossing layers with different rheological behavior, is heterogeneous in respect to the slip style: seismic in the brittle layers, aseismic in the ductile layer. Dislocations in the shallower layer are assumed to produce aseismic slip in the area of the fault belonging to the ductile layer. The stress concentrated, by the seismic and aseismic dislocations, on the fault plane section in the deeper brittle layer is evaluated. It is compared with the tectonic stress rate in order to calculate how much earlier the second earthquake would occur compared to if just the bare tectonic sstress was acting. It results that such an advance is comparable with typical recurrence times of earthquakes and so a mechanism of interaction between different seismic sources, mediated by aseismic slip, can be supposed. The strains and displacements at the Earth’s surface due to seismic and aseismic slip are calculated. They are large enough to be detected by present geodetic techniques.

Long-term slip rates and characteristic slip: keys to active fault behaviour and earthquake hazard

Over periods of thousands of years, active faults tend to slip at constant rates. Pioneer studies of large Asian faults show that cosmogenic radionuclides ( 10 Be, 26 Al) provide an unparalleled tool to date surface features, whose offsets yield the longest records of recent cumulative movement. The technique is thus uniquely suited to determine long-term (10-100 ka) slip rates. Such rates, combined with coseismic slip-amounts, can give access to recurrence times of earthquakes of similar sizes. Landform dating - morphochronology - is therefore essential to understand fault-behaviour, evaluate seismic hazard, and build physical earthquake models. It is irreplaceable because long-term slip- rates on interacting faults need not coincide with GPS-derived, interseismic rates, and can be difficult to obtain from paleo-seismological trenching.  2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS

Stress on the seismogenic and deep creep plate interface during the earthquake cycle in subduction zones

The deep creep plate interface extends from the down-dip edge of the seismogenic zone down to the base of the overlying lithosphere in subduction zones. Seismogenic/deep creep zone interaction during the earthquake cycle produces spatial and temporal variations in strains within the surrounding elastic material. Strain observations in the Nankai subduction zone show distinct deformation styles in the co-seismic, post-seismic, and inter-seismic phases associated with the 1946 great earthquake. The most widely used kinematic model to match geodetic observations has been a 2-D Savage-type model where a plate interface is placed in an elastic half-space and co-seismic slip occurs in the upper seismogenic portion of the interface, while inter-seismic deformation is modeled by a locked seismogenic zone and a constant slip velocity across the deep creep interface. Here, I use the simplest possible 2-D mechanical model with just two blocks to study the stress interaction between the seismogenic and deep creep zones. The seismogenic zone behaves as a stick-slip interface where co-seismic slip or stress drop constrain the model. A linear constitutive law for the deep creep zone connects the shear stress (σ) to the slip velocity across the plate interface (s′) with the material property of interface viscosity (ζ ) as: σ = ζ s′. The analytic solution for the steady-state two-block model produces simple formulas that connect some spatially-averaged geodetic observations to model quantities. Aside from the basic subduction zone geometry, the key observed parameter is τ, the characteristic time of the rapid post-seismic slip in the deep creep interface. Observations of τ range from about 5 years (Nankai and Alaska) to 15 years (Chile). The simple model uses these values for τ to produce estimates for ζ that range from 8.4 × 1013 Pa/m/s (in Nankai) to 6.5 × 1014 Pa/m/s (in Chile). Then, the model predicts that the shear stress acting on deep creep interface averaged over the earthquake cycle ranges from 0.1 MPa (Nankai) to 1.7 MPa (Chile). These absolute stress values for the deep creep zone are slightly smaller than the great earthquake stress drops. Since the great earthquake recurrence time ( T recur) is much larger than τ for Nankai, Alaska, and Chile, the model predicts that rapid post-seismic creep should re-load the seismogenic zone to about (1/3) of the co-seismic change; geodetically observed values range from about (1/10) to more than (1/2). Also, for the case of (Trecur/τ) ≫1, the model predicts that the slip velocity across the deep creep interface during the inter-seismic phase should be about (2/3) the plate tectonic velocity (R). Thus the deep creep velocity used in Savage-type models should be less than R. Even complex 3-D models with non-linear creep laws should make a similar prediction for inter-seismic deep creep rates. At present, it seems that geodetic observations at Nankai and other subduction zones are more consistent with a deep creep rate of R rather than (2/3) R. This discrepancy is quite puzzling and is difficult to explain in the context of a 2-D steady-state earthquake cycle model. Future observational and modeling studies should examine this apparent discrepancy to gain more understanding of the earthquake cycle in subduction zones.

Fluid pressures at hypocenters of moderate to large earthquakes

Many active faults are expected to develop fluid pressures in excess of hydrostatic pressures below 3 to 7 km depth during interseismic periods. Suprahydrostatic fluid pressures are known to reduce the stresses required for brittle failure. Stress differences that trigger moderate to large earthquakes typically range from 40 to 160 MPa, as indicated by earthquake shear stress drops and paleostress estimates obtained from mylonites. For stresses in this range (40–160 MPa), seismic fault reactivation requires in most cases pore fluid factors (fluid pressure/overburden pressure) higher than for hydrostatic fluid pressures (i.e., >0.37) along misoriented faults with fault angles ≥45°, such as many segments of the San Andreas fault system. For example, at a stress difference of 100 MPa and fault angles of 45°–55°, fault reactivation at 7 to 20 km depth requires hypocentral pore fluid factors of ≈0.8–1 for reverse faults, 0.6–0.9 for strike‐slip faults, and <0.8 for normal faults. Pore fluid factors increase with increasing cohesive strength of faults, increasing coefficient of internal friction, and increasing fault angle. Seismic reactivation of cohesive faults at stress differences of 40–160 MPa requires near the base of the seismogenic zone (≈15 km depth) suprahydrostatic fluid pressures at all possible fault angles. Pore fluid factors are ≈0.4–0.9 for normal faults, ≈0.6–1 for strike‐slip faults, and ≈0.8–1.05 for thrust or reverse faults. These constraints are potentially useful for the modelling of seismic faulting and earthquake recurrence times.

GPS Constraints on M 7-8 Earthquake Recurrence Times for the New Madrid Seismic Zone

Newman et al. (1999) estimate the time interval between the 1811–1812 earthquake sequence near New Madrid, Missouri and a future similar sequence to be at least 2,500 years, an interval significantly longer than other recently published estimates. To calculate the recurrence time, they assume that slip on a vertical half-plane at depth contributes to the current interseismic motion of GPS benchmarks. Compared to other plausible fault models, the half-plane model gives nearly the maximum rate of ground motion for the same interseismic slip rate. Alternative models with smaller interseismic fault slip area can satisfy the present GPS data by having higher slip rate and thus can have earthquake recurrence times much less than 2,500 years.

Regional Difference in Scaling Laws for Large Earthquakes and its Tectonic Implication

We compiled 67 large earthquakes which occurred at and around plate boundaries for the last 140 yrs, and classified them into four groups; interplate strike-slip events, intraplate strike-slip events, underthrust events at island-arc subduction zones, and underthrust events at continental-margin subduction zones. For each group of earthquakes we examined relations between seismic moment M 0, fault length L, fault width W and average fault slip D, and found the following scaling laws. In the case of interplate strike-slip events, the well-known L-cubed dependence of seismic moment breaks down when L exceeds 30 km, because the extent of the seismogenic zone is limited in depth (≤ 12 km). For large events (L ≥ 30 km), D and M 0 increase with L as \( D = \overline {\Delta \tau } L/\mu \left( {\alpha L + \beta } \right) \) and \( {{M}_{0}} = \overline {\Delta \tau W} {{L}^{2}}/\left( {\alpha L + \beta } \right) \) , respectively, where the mean fault width \( \bar{W} \) is 12 km and the mean stress drop \( \overline {\Delta \tau } \) is 1.8 MPa. Here μ, α and β are structural parameters. For intraplate strike-slip events we obtained nearly the same relations, except for significantly higher stress drop (3.1 MPa). The difference in stress drop between interplate and intraplate events may be ascribed to the difference in stress accumulation rates and thus the recurrence time of earthquakes. In the case of underthrust events at island-arc subduction zones we also found the saturation of fault width \( (\bar{W} = 120km) \) and the breakaway from the L-cubed dependence of M 0 for events larger than L = 200 km. If we consider the average dip-angle of plate boundaries at island-arc subduction zones to be 20–30°, this indicates that the extent of the seismogenic zone in depth is limited to 40–60 km. In the case of continental-margin subduction zones, on the other hand, we could not find the saturation of fault width nor the breakaway from the L-cubed dependence of M 0 from the analysis of the present data set (W ≤ 200 km, L ≥ 1000 km). For sufficiently large earthquakes, in general, the downward rupture growth is limited to a certain depth due to the existence of a ductile unstressed region which extends under the brittle seismogenic zone. Since the brittle-ductile transition occurs at 300–400°C, the difference in the lower limit of the seismogenic zones between tectonically different regions may be attributed to the difference in thermal state there.

Kinetics of crack-sealing, intergranular pressure solution, and compaction around active faults

Geological evidence indicates that fluids play a key role during the seismic cycle. After an earthquake, fractures are open in the fault and in the surroundings rocks. With time, during the interseismic period, the permeability of the fault and the country rocks tends to decrease by gouge compaction and fracture healing and sealing. Dissolution along stylolite seams provides the matter that fills the fractures, whereas intergranular pressure solution is responsible for gouge compaction. If these processes are fast enough during the seismic cycle, they can modify the creep properties of the fault. Based on field observations and experimental data, we model the porosity decrease by pressure solution processes around an active fault after an earthquake. We arrive at plausible rates of fracture sealing that are comparable to the recurrence time for earthquakes. We also study the sensitivity of these rates to various parameters such as grain size, fracture spacing, and the coefficient of diffusion along grain boundaries and stylolites.

Interplate coupling in southwest Japan deduced from inversion analysis of GPS data

Recently, the Geographical Survey Institute of Japan completed the installation of a GPS continuous observation network in Japan, which has enabled us to investigate real-time crustal movements. In this study, we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest Japan, using horizontal and vertical deformation rates, which were observed at 247 GPS observation stations during the period from April 6, 1996 to March 20, 1998. For this purpose, we carried out an inversion analysis of geodetic data, incorporating Akaike's Bayesian Information Criterion (ABIC). As a result, strong interplate coupling was found off Shikoku and Kumanonada regions, which corresponds well with the fault regions of the 1946 Nankai (M 8.1) and the 1944 Tonankai (M 8.0) earthquakes, respectively. We also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the Shikoku and Kii peninsula. The recurrence time of great trench-type earthquakes was roughly estimated as 107 years, which is consistent with previous research. The direction of relative plate motion is oriented N53°W, which is close to the direction predicted from the plate motion model. On the other hand, a large forward slip was found in the Hyuganada region off southeast of Kyushu. Since the coseismic displacements associated with the two 1996 Hyuganada earthquakes (M 6.6, M 6.6) are removed from the GPS data, this suggests that after-slip occurred near the source region and/or that Kyushu moves southeastward stationarily due to other tectonic forces.

Co-seismic ruptures and deformations recorded by speleothems in the epicentral zone of the Basel earthquake

The study of growth anomalies of speleothems in a karstic environment can provide potential evidence or palaeoearthquakes. These data are used to study the recurrence times of major earthquakes in areas where evidence for historic seismicity is lacking. A study has been carried out in the epicentral area of the 1356 Basel earthquake (epicentral intensity = VII-VIII, macroseismic magnitude = 6.2). The Bättlerloch and Dieboldslöchli caves, situated in the area of greatest damage, show growth anomalies of speleothems possibly related to a seismic event (several breaks of speleothems an offsets of the axis of the regrowths). The first U/Th disequilibrum measurements by alpha spectrometry show recent ages (less than several tens of thousands of years and pro a y historic). 14C dating by AMS of carbonate laminations taken on both sides of the anomalies confirm the evidence of a seismic event around 1300 AD. More accurate datings by U/Th TIMS are carried out in order to compare the information provided by the two different dating methods. © Elsevier, Paris

Co-seismic ruptures and deformations recorded by speleothems in the epicentral zone of the Basel earthquake

The study of growth anomalies of speleothems in a karstic environment can provide potential evidence for palaeoearthquakes. These data are used to study the recurrence times of major earthquakes in areas where evidence for historic seismicity is lacking. A study has been carried out in the epicentral area of the 1356 Basel earthquake (epicentral intensity = VII–VIII, macroseismic magnitude = 6.2). The Bättlerloch and Dieboldslöchli caves, situated in the area of greatest damage, show growth anomalies of speleothems possibly related to a seismic event (several breaks of speleothems and offsets of the axis of the regrowths). The first U/Th disequilibrum measurements by alpha spectrometry show recent ages (less than several tens of thousands of years and probably historic). 14C dating by AMS of carbonate laminations taken on both sides of the anomalies confirm the evidence of a seismic event around 1300 AD. More accurate darings by U/Th TIMS are carried out in order to compare the information provided by the two different dating methods.

Historic-prehistoric earthquakes, seismic hazards, and Tertiary and Quaternary geology of the Gandoca-Manzanillo National Wildlife Refuge, Limón, Costa Rica

Thee Gandoca-Manzanillo region is part of the Limon Basin, located in southeastern Costa Rica. The de­positional environment of Testiary sedimentary rocks shows a progressive shallowing through geologic lime, from deep marine (Miocene-Pliocene) to deltaic-fuvial and coral reef environments (Quaternary). Thee most dramatic geologic effect of the Limon-I991 earthquake was coseismic uplift of the shoreline, which ranged from 0.3 to 0.6 m in the Gandoca- Manzanillo region. This event caused liquefaction of thick deposits of young, fine-grained fluvial sediments in the region, and produced a tsunami affecting the vicinity of Manzanillo and Punta Uva, but the heaviest damage was reported in northwesterm Panama. The effects of the tsunami may have been diminished in Costa Rica because of the presence of fringing reefs, and the fact that this part of the coast had been coseismically uplifted by the time tsunamis arrived at the coast. Two platform levels were dated using radiocarbon method on coral sam­ples. The platform level at 1.5·2.0 m had radiocarbon ages of 5 220±5O years B.P. and 4 540±50 years B.P., and the 8.0-10.0 m terrace level was dated at 27 14S±290 years B.P yielding a 1.8 mm/year long-term coseismic uplift rate. Variations in the heights of individual platforms reflect a very irregular coseismic uplift behavior, and proba­bly also an irregular earthquake time recurrence period. However, similarities between the 1991 and 1822 events suggest 150 to 200 year as the earthquake recurrence time.

GPS measurements of present-day convergence across the Nepal Himalaya

The high elevations of the Himalaya and Tibet result from the continuing collision between India and Asia, which started more than 60 million years ago1–4. From geological and seismic studies of the slip rate of faults in Asia5, it is believed that approximately one-third of the present-day convergence rate between India and Asia (58 ± 4mmyr−1) is responsible for the shortening, uplift and moderate seismicity of the Himalaya. Great earthquakes also occur infrequently in this region, releasing in minutes the elastic strain accumulated near the boundary zone over several centuries, and accounting for most of the advance of the Himalaya over the plains of India. The recurrence time for these great earthquakes is determined by the rate of slip of India beneath Tibet, which has hitherto been estimated indirectly from global plate motions6, from the slip rates of faults in Asia7,8, from seismic productivity9, and from the advance of sediments on the northern Ganges plain10. Here we report geodetic measurements, using the Global Positioning System (GPS), of the rate of contraction across the Himalaya, which we find to be 17.52 ± 2 mm yr −1. From the form of the deformation field, we estimate the rate of slip of India beneath Tibet to be 20.5 ± 2 mmyr–1. Strain sufficient to drive one or more great Himalayan earthquakes, with slip similar to that accompanying the magnitude 8.1 Bihar/Nepal 1934 earthquake, may currently be available in western Nepal.

Fault model for preseismic deformation at Parkfield, California

We construct an earthquake instability model to estimate precursory faulting and ground deformation before the next moderate (M 5.5–6) earthquake on the San Andreas fault near Parkfield, California. The quasi‐static model simulates fault slip, fault shear stress, and ground deformation for all stages of repeated earthquake cycles. Unstable slip, which is the analog of a moderate Parkfield mainshock, is caused by overall failure of a postulated strong area or patch of the fault zone. The brittle patch and surrounding weaker viscous fault are defined by the sign of a spatially varying coefficient of a slip rate dependent fault law. Stable failure of the patch leading up to mainshock failure causes slip rate to increase at depth. Amplitudes and timescales of the resulting preseismic deformation anomalies are found for the current earthquake cycle at Parkfield, starting just after the 1966 mainshock, in two ways. In the first way we vary parameter values in a sequence of simulations so as to find the simulation giving best agreement with locations, moments, and recurrence times of past moderate earthquakes and also with fault creep and trilateration line length changes since the 1966 mainshock. In the second way we assign values of most parameters from laboratory friction experiments and a temperature‐depth profile inferred from heat flow data. Overall agreement between model results and observations is comparable for the two methods. In all simulations, preseismic deformation anomalies arise from slip rate increase near the model earthquake focus and start several years or less before instability. Preseismic anomalies in surface fault creep and line length are always too small to detect in current measurements. However, for some simulations constrained mainly by field data the predicted anomalies in borehole dilatation are large enough to detect.

How Old is the New Madrid Seismic Zone?

Abstract Current seismicity levels on the New Madrid seismic zone should produce about 0.11 cm/year of horizontal slip which, when compared with uplift of 42 m in the subsurface strata below the Lake County uplift and assuming a 31° reverse fault model, indicates that the present seismicity levels could not have been present for more than about 64,000 years. If seismicity in the region has persisted for a much longer period of time, then (1) the seismicity has moved spatially between several deformed zones (Crowley’s Ridge and the Crittenden County fault zone); (2) the seismicity is episodic in nature, and active periods similar to the present occur between long quiescent times; or (3) there have been far fewer large earthquakes than predicted by extrapolation of the Gutenberg-Richter relation to higher magnitudes. Any of these scenarios indicates that assessing the hazard from large earthquakes is more complicated than conventional analyses have assumed because either the seismicity locations or rates change or analysis techniques relying on the Gutenberg-Richter relation are invalid for estimating the recurrence times of large earthquakes in the New Madrid area.