The main scope of the InterPACIFIC (Intercomparison of methods for site parameter and velocity profile characterization) project is to assess the reliability of in-hole and surface-wave methods, used ...for estimating shear wave velocity. Three test-sites with different subsurface conditions were chosen: a soft soil, a stiff soil and a rock outcrop. This paper reports the surface-wave methods results. Specifically 14 teams of expert users analysed the same experimental surface-wave datasets, consisting of both passive and active data. Each team adopted their own strategy to retrieve the dispersion curve and the shear-wave velocity profile at each site. Despite different approaches, the dispersion curves are quite in agreement with each other. Conversely, the shear-wave velocity profiles show a certain variability that increases in correspondence of major stratigraphic interfaces. This larger variability is mainly due to non-uniqueness of the solution and lateral variability. As expected, the observed variability in VS,30 estimates is small, as solution non-uniqueness plays a limited role.
•Variability of surface wave analysis results are studied with a blind test.•Three subsoil conditions are considered: soft soil, stiff soil, rock outcrop.•Different methods are used to analyze active and passive data.•A low variability is observed on the estimates of the experimental dispersion curve.•Variability in VS profiles is due to parameterization and solution non-uniqueness.
We document the first time series of a landslide reactivation by an earthquake using continuous GPS measurements over the Maca landslide (Peru). Our survey shows a coseismic response of the landslide ...of about 2 cm, followed by a relaxation period of 5 weeks during which postseismic slip is 3 times greater than the coseismic displacement itself. Our results confirm the coseismic activation of landslides and provide the first observation of a postseismic displacement. These observations are consistent with a mechanical model where slip on the landslide basal interface is governed by rate and state friction, analogous to the mechanics of creeping tectonic faults, opening new perspectives to study the mechanics of landslides and active faults.
Key PointsWe document the first GPS time series of a landslide response to an earthquakeThe landslide response is not purely coseismic but lasts for 5 weeksWe show the mechanical analogy of slow‐moving landslides and creeping faults
The single-station microtremor horizontal-to-vertical spectral ratio (MHVSR) method was initially proposed to retrieve the site amplification function and its resonance frequencies produced by ...unconsolidated sediments overlying high-velocity bedrock. Presently, MHVSR measurements are predominantly conducted to obtain an estimate of the fundamental site frequency at sites where a strong subsurface impedance contrast exists. Of the earthquake site characterization methods presented in this special issue, the MHVSR method is the furthest behind in terms of consensus towards standardized guidelines and commercial use. The greatest challenges to an international standardization of MHVSR acquisition and analysis are (1) the
what
— the underlying composition of the microtremor wavefield is site-dependent, and thus, the appropriate theoretical (forward) model for inversion is still debated; and (2) the
how
— many factors and options are involved in the data acquisition, processing, and interpretation stages. This paper reviews briefly a historical development of the MHVSR technique and the physical basis of an MHVSR (the
what
). We then summarize recommendations for MHVSR acquisition and analysis (the
how
). Specific sections address MHVSR interpretation and uncertainty assessment.
The pattern of active deformation of frontal structures in Darjeeling Himalaya is complex with out‐of‐sequence reactivations in the chain and development of scarps associated with earthquake ruptures ...reaching the surface in the piedmont. To clarify the distribution of active deformation in this area, we analyse passive seismic records by the Horizontal‐to‐Vertical Spectral Ratio method along three NS trending profiles. We image the Siwalik sedimentary rocks / recent deposits interface under the piedmont and show folded and faulted geometries. Two of these faults are located under scarps of about 10 m affecting the 3.7 ± 0.7 ka old surface of the Tista megafan. Such features imply that about half of the convergence is expressed south of the Himalayan front while the other part occurs out‐of‐sequence in the chain, suggesting a very limited activity of the Main Frontal Thrust itself.
Single‐station H/V curves from ambient noise recordings in Boumerdes (Algeria) show smooth bumps around 1 and 3 Hz. A complementary microtremor study, based on two 34 and 134‐meter aperture arrays, ...evidences that these bumps are indeed real peaks produced by two strong VS contrasts at 37 and 118 meters depth, strongly smoothed by very high S‐wave attenuation in the two sedimentary layers. These two H/V bumps, observed over a broad area, are meaningful and reveal the importance of Q in S‐wave velocity modeling from microtremor array data processing. It also appears that Tertiary rocks should be, at least in some cases, taken into account, together with the Quaternary sediments, to explain single‐station H/V frequency peaks, and therefore that considering only the first 30 m of soil for VS amplification evaluation, as usually recommended, sometimes leads to flaky results by artificially eliminating non‐explained low‐frequency peaks from the analysis.
The morphological boundary between the Himalayas and the foreland plain is well expressed and most often corresponds to the frontal emergence of the Main Himalayan Thrust (MHT). This boundary is ...affected by surface ruptures during very large Himalayan earthquakes (Mw > 8) that regularly induce (with a recurrence of the order of 500 to 1200 years) the uplift of the foothills relative to the plain.
However, a thrust-fold system is hidden beneath the plain and is displayed by the seismic profiles of oil companies in east/central Nepal and by H/V passive geophysical techniques in Darjeeling. Its long-term kinematic evolution is slow, with a tectonic uplift of the hanging wall that is lower than the subsidence rate of the foreland basin, that is, less than approximately half a millimetre per year. During phases of low sedimentation controlled by climatic fluctuations, the morphological surfaces of the piedmont are incised by large rivers for several tens of metres; therefore, structures hidden under the sediments emerge slightly in the plain.
The evolution of the hidden structures corresponds to an embryonic thrust belt mainly affected by a long-term shortening rate of 1.4 +2.5/−1.2 mm·yr−1, that is, 2–20% of the shortening rate of the entire Himalayan thrust system. Nonetheless, the details of the deformation associated with the embryonic thrust belt are still poorly understood. Several deformation components could affect the central Himalayan and Darjeeling piedmonts. i) Any slow steady-state deformation, such as layer parallel shortening (LPS) is not detected by Global Navigation Satellite System (GNSS) data, and such deformation would therefore absorb less than 0.5 mm·yr−1. The geodetic data that suggest the aseismic growth of some of the structures are highly controversial. ii) For the rest of the deformation of the embryonic thrust wedge, it is yet to be proven whether deformation occurs during rare great earthquakes affecting the piedmont during medium earthquakes and/or during post-seismic deformation related to great earthquakes. The amplitude of this long-term low deformation is too limited to significantly reduce the seismic hazard in the seismic gaps of the Himalayan belt. iii) In some portions of the Himalayan front, such as Darjeeling (India), the thrust deformation related to great earthquakes propagates several tens of kilometres south of the morphological front in the zone previously affected by the long-term low deformation. It induces multi-metre surface ruptures in the piedmont and a mean shortening of 8.5 ± 6.2 mm·yr−1. iiii) Pre-existing faults in the bedrock of the Indian craton, often oblique to the Himalayan structures, are locally reactivated beneath the foreland plain with low deformation rates.
We present three geomorphologic and geological phenomena that have occurred in Algeria in recent years: (i) the Bab El Oued mudflow on 11 November 2001, which claimed several hundred lives, (ii) a ...soil collapse induced by sand liquefaction triggered by the Boumerdes earthquake (
M
w
= 6.8) on 21 May 2003, and (iii) landslides that are threatening Constantine city, for which a hazard map is presented using a qualitative approach. We briefly describe and analyze these natural disasters, and in the first two cases propose the application of geophysical techniques such as ambient noise recordings and electrical imagery to help evaluate their extent and potential threat. Finally a landslide hazard map of Constantine is proposed.
Analysis of the spatial distribution of seismicity beneath central Ecuador from a temporary network gives new insights on two main points of the Ecuadorian tectonics. Major structures in the ...Ecuadorian Andes are East‐dipping Late Jurassic to Paleogene sutures reactivated by present‐day compression. The oceanic plate is plunging continuously down to a depth of about 200 km with a dip of 25°–35°. We also show that the coastal plain acts as a buttress transmitting stresses to the Andes, beneath which deformation is concentrated.
The phreatic activity and the subsequent dacitic dome growth in 1998–1999 at Guagua Pichincha volcano, Ecuador, were associated with two seismic swarms: one located in the northern part of Quito ...(population: 1,500,000) and another one, just below the active volcano, about 15–20 km SW from the first one. Quito swarm tectonic events have high frequencies (from 1 to 10–15 Hz). We registered more than 3200 events (among which 2354 events of 1.4≤
M
L≤4.2) between June 1998 and December 1999 at the −2- and −17-km depth. The volcanic events below the Guagua Pichincha caldera have high (from 1 to 10–15 Hz) and low (less than 3 Hz) frequencies. Approximately, 130,000 events were registered between September 1998 and December 1999 at the +2.4- and −3.5-km depth. Here, we study the stress tensors of these two swarms deduced from the polarities of P first motions and compare them to the regional stress tensor deduced from CMT Harvard focal mechanisms. The Quito swarm stress tensor is relatively close to the regional stress tensor (the
σ
1 axis was oriented N117°E close to the N102°E direction of the plate motion found by the GPS measurement, and
σ
3 is nearly vertical). The difference may be due to the action of the closely active Guagua Pichincha volcano. The Guagua Pichincha stress tensor is very different from the regional tectonic one. The
σ
1 axis of the volcano is oriented N214°E, almost perpendicular to the
σ
1 of the swarm of Quito and
σ
3 is almost horizontal. Even if these two tensors are different, they can be explained in a more general tectonic scheme. The almost horizontal direction of
σ
3 just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow
extrados part of a compressional region and consistent with an opening of the top of the Guagua Pichincha volcano. The movement of the fluids (magma, gas and/or groundwater) produced by the closely active Guagua Pichincha volcano seems to have an influence in the acceleration of the generation of seismic events.
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