Source rupture process, directivity and and Coulomb stress change of the 12 January 2010 (Port-au-Prince Haiti, Mw7.0) earthquake

The Haiti earthquake occurred on Tuesday, January 12, 2010 at 21:53:10 UTC. Its epi- center was at 18.46 degrees North, 72.53 degrees West, about 25 km WSW of Haiti’s capital, Port-au-Prince, along the tectonic boundary between Caribbean and North America plate dominated by left-lateral stri- ke slip motion and compression with 2 cm/yr of slip velocity eastward with respect to the North America plate. The earthquake was relatively shallow (about 13 km depth) with Mw 7.0 and CMT mechanism solution indica- ting left-lateral strike slip movement with a fault plane oriented toward the WNW-ESE. More than 10 aftershocks ranging from 5.0 to 5.9 in magnitude struck the area in hours following the main shock. Most of these af- tershocks have occurred to the west of the mainshock in the Mirogoane Lakes region and its distribution suggests that the length of the rupture was around 70 km. Rupture velocity and direction was constrained by using the directivity effect determined from broad-band waveforms recorded at regio- nal and teleseismic distances using DIRDOP computational code (DIRectivity DOPpler effect) [1]. The Results show that the rup- ture spread mainly from WNW to ESE with a velocity of 2.5 km/s. In order to obtain the spatiotemporal slip distribution of a fi- nite rupture model we have used teleseismic body wave and the Kikuchi and Kanamori’s method [2]. The inversion show complex source time function with a total scalar seismic moment of 2.2 x 1019Nm (Mw=6.9) a source duration of about 18 sec with a main energy relesea in the first 13 sec. Finally, we compared a map of aftershocks with the Coulomb stress changes caused by the main shock in the region [3]. [1] Kikuchi, M., and Kanamori, H., 1982, Inversion of com- plex body waves: Bull. Seismol. Soc. Am., v. 72, p. 491-506. [2] Caldeira B., Bezzeghoud M, Borges JF, 2009; DIRDOP: a directivity ap- proach to determining the seismic rupture velocity vector. J Seismology, DOI 10.1007/ s10950-009-9183-x [3] King, G. C. P., Stein, R. S. y Lin, J, 1994, Static stress changes and the triggering of earthquakes. Bull. Seismol. Soc. Am. 84,935-953. More than 10 aftershocks ranging from 5.0 to 5.9 in magnitude struck the area in hours following the main shock. Most of these aftershocks have occurred to the west of the mainshock in the Mirogoane Lakes region and its distribution suggests that the length of the rupture was around 70 km. In order to obtain the spatiotemporal slip distribution of a finite rupture model we have used teleseismic body wave and the Kikuchi and Kanamori's method [1]. Rupture velocity was constrained by using the directivity effect determined from waveforms recorded at regional and teleseismic distances [2]. The spatiotemporal slip estimated points to a unilateral rupture that propagates from WNW to ESE with a rupture velocity of 2.5 km/s. Finally, we compared a map of aftershocks with the Coulomb stress changes caused by the event in the region [3]. [1]- Kikuchi, M., and Kanamori, H., 1982, Inversion of complex body waves: Bull. Seismol. Soc. Am., v. 72, p. 491-506. [2] Caldeira B., Bezzeghoud M, Borges JF, 2009; DIRDOP: a directivity approach to determining the seismic rupture velocity vector. J Seismology, DOI 10.1007/s10950-009-9183-x [3] -King, G. C. P., Stein, R. S. y Lin, J, 1994, Static stress changes and the triggering of earthquakes. Bull. Seismol. Soc. Am. 84,935-953

Seismic Source Directivity from Doppler Effect Analysis, Part I: Theory

The directivity effects, a characteristic of finiteness seismic sources, are generated by the rupture in preferential directions. Those effects are manifested through different cadencies in the seismological measures from azimuthally distributed stations. The apparent durations are expressed as (e.g. Aki and Richards, 1980), (1), where L, v, c and ??are, respectively, the fault length, the rupture velocity, the wave velocity and the angle between rupture direction and ray. This time duration can be measured directly from waveform or indirectly from Relative Source Time Function (RSTF). Equation (1) is deduced from a simple source model (Haskell model) that considers unidirectional uniform rupture propagation and a homogeneous elastic isotropic media. If we consider a more general propagation model, with spherical concentric layers, we obtain (2), where p is the ray parameter and the earth radius. Similar equation can be obtained through physical considerations about a model composed by a sequence of subevents unilater- ally distributed along a line (Doppler Effect). Based on the same considerations we can do a more detailed analysis through (3), where is the time interval between 2 identified pulses in the rupture referential and j indicate the number of station. Based on this theory, we have developed a computational code DIRDOP (DIRectivity DOPpler effect) which determines the rupture direction and velocity from pulse durations observed in waveforms or RSTF. We used this code to analyse recent major seismic events including the unilateral 23 June, 1999 Arequipa (Peru, Mw=8.2) earthquake and the bilateral 21 May 2003 Boumerdes (Algeria, Mw=6.7) earthquake amongst others. The results are similar to those obtained by other methods

Slip distribution, coseismic deformation and Coulomb stress change for the 12 May 2008Wenchuan (China, Mw7.9) earthquake

The May 12, 2008 Wenchuan earthquake (Mw7.9) took place at the transition between the mountainous chain of Shan and the basin of Sichuan along the Longmen Shan Fault zone (31.1oN, 103.3oE; USGS). With a magnitude of 7.9 and a depth of ∼19 km the earthquake produced a 300-km-long fault rupture. It was the largest earthquake recorded in the region during the last centuries. It claimed more than 69,000 lives, induced widespread destruction over the region and raised concern about seismic hazard and source characterization for the Sichuan province. In the frame of our study, we selected 40 broadband waveforms (IRIS Consortium, USA) with good quality and satisfactory azimuthal coverage. Body waveforms were prepared for inversion using Kikuchi and Kanamori’s method [1] to obtain the spatiotem- poral slip distribution of a finite rupture model (length=300 km, strike=229o, dip=33o, width=60 km). The slip distribution model obtained was used to determine the coseismic deformation and the stress change distribution using the Coulomb 3.0 software [2]. Our coseismic deformation results was compared with data from GPS stations located near the fault rupture. Results show that directions of coseismic deformations are consistent with GPS observations close to the fault. Finally, we compare aftershock hypocenters that occurred during one month after the main shock with the Coulomb stress changes caused by this shock in the region. We observed that most aftershocks are located along the main fault plane without any noticeable clustering in the areas of increased stress. Our results suggest the rupture of the 2008 Wenchuan earthquake was essentially unilateral, from SW to NE (N49E), covering a 260km length and with duration about 105 sec. The strongest moment release occurred about 85km from the hypocenter, ∼30sec after the start of the rupture. Motions are dominated by thrust mechanism, but the superficial section of the second half of the rupture also shows a significant strike-slip component. [1]- Kikuchi, M., and Kanamori, H., 1982, Inversion of complex body waves: Bull. Seismol. Soc. Am., v. 72, p. 491-506. [2] -King, G. C. P., Stein, R. S. y Lin, J, 1994, Static stress changes and the triggering of earthquakes. Bull. Seismol. Soc. Am. 84,935-953

Strong ground motion in southern Portugal due to the 1755 Lisbon earthquake

The strong earthquake (M=8.8) that struck a large part of the Iberian Peninsula and Northern Morocco on November 1, 1755, was caused by the motion along a fault which localisation and spatial extent are still uncertain. According to recent numerical modelling of tsunami wave travel times, it seems that the tsunamigenic fault may be lo- cated off the southwestern coast of Portugal. Multi-channel seismic profiles in the area showed the existence of large submarine hills of tectonic origin, 100 km offshore Cabo de São Vincente, and led to the identification of active faults that may be responsible for the earthquake. E3D, a finite-difference seismic wave propagation code, is used to implement various source rupture scenarios. Based on available geophysical data and geological evidences, we propose a 3D velocity model of the upper mantle, crust, and sedimentary cover, for south Portugal and the adjacent Atlantic area. The model is constrained thanks to data available from recent instrumental earthquakes. We are able to test several possibilities, and to compare synthetic ground motion obtained onshore with historical evaluations of seismic intensity. Directivity of the source, as well as site effects, may explain the particular distribution of strong ground motion observations

Influence of model parameters on synthesized high-frequency strong-motion waveforms

Waveform modeling is an important and helpful instrument of modern seismology that may provide valuable information. However, synthesizing seismograms requires to define many parameters, which differently affect the final result. Such parameters may be: the design of the grid, the structure model, the source time functions, the source mechanism, the rupture velocity. Variations in parameters may produce significantly different seismograms. We synthesize seismograms from a hypothetical earthquake and numerically estimate the influence of some of the used parameters. Firstly, we present the results for high-frequency near-fault waveforms obtained from defined model by changing tested parameters. Secondly, we present the results of a quantitative comparison of contributions from certain parameters on synthetic waveforms by using misfit criteria. For the synthesis of waveforms we used 2D/3D elastic finite-difference wave propagation code E3D [1] based on the elastodynamic formulation of the wave equation on a staggered grid. This code gave us the opportunity to perform all needed manipulations using a computer cluster. To assess the obtained results, we use misfit criteria [2] where seismograms are compared in time-frequency and phase by applying a continuous wavelet transform to the seismic signal. [1] - Larsen, S. and C.A. Schultz (1995). ELAS3D: 2D/3D elastic finite-difference wave propagation code, Technical Report No. UCRL-MA-121792, 19 pp. [2] - Kristekova, M., Kristek, J., Moczo, P., Day, S.M., 2006. Misfit criteria for quantitative comparison of seismograms. Bul. of Seis. Soc. of Am. 96(5), 1836–1850

Earthquake Source and Seismic Strain Rate: Portugal in the Context of The Western Part of the Eurasia - Africa Plate Boundary

Fault plane solutions, stress-pattern and deformation rate along the Western part of the Eurasia-Africa Plate Boundary, particu- larly between Azores triple junction and Gibraltar are analyzed. A selection of shallow depth seismic events (1.9 = M = 8.0) occurred in the period 1900-2003 have been carefully checked and analysed. The distribution of the focal mechanisms have been analysed by means of different techniques, projections and graphic representations. Seismic moment tensors, moment rate, slip velocity and b values have been estimated. Based on these results, we propose the following: 1) Between the Azores triple junction and Terceira island predominates strike- slip motion with nodal planes trending NNW-SSE and ENW-SSE; between the Terceira island and the beginning of the of Gloria fault the normal mechanisms predominate with nodal plans in the direction of islands. Deformation rate in both regions is 7.4 and 2.4 cm/year respectively. 2) In the continuation of the plate boundary, along the Gloria Fault until the Iberian continental margin we clearly have right-lateral motion in the E-W direction with a deformation rate of 1.8 cm/year. 3) The Eastern part of the Plate boundary, in Portugal continental, is very complex, however we identify some important patterns in the following regions: western Iberian margin (strike-slip), Lisboa and Vale do Tejo (dip-slip), ...vora and vicinity (strike-slip), region of Algarve (strike-slip) and inter-plates boundary zone (inverse). These regions are affected by compression oriented and a deformation rate of 0,55 cm/year

The 2007 Azores earthquakes: A case of triggering?

On 5 April (Mw=6.3) and 7 April 2007 (Mw=6.1 ) two earthquakes occur at the Formigas Islets (Azores Islands), both with same epicenter and felt (I=V/VI MSK) in S. Miguel Island. The rupture process of these earthquakes has been studied from body wave inversion of broad band data at telesesimic distances. Results obtained shown normal faulting for both shocks, with planes oriented in NW–SE direction, with focus at shallow depth (10 km and 6 km respectively). The slip distribution over the fault plane (152/44/-88) shows for the 05-04-07 event, the rupture propagating downward and a duration of 12s for the source time function. For the 07-04-07 event, the slip distribution over the fault plane (125/52/-81) shows de rupture propagating downward and duration of 10s for the STF. From these results we have estimated the static Coulomb stress change. We find that the static stress change caused by the 5 April event is higher, about 2 bar at epicenter the location of the second event (April 7), triggering the second rupture. Locations of aftershocks do not agree well with areas of increased Coulomb failure stress, which can be explained by the complexities of the rupture process oy by uncertainties at the hypocerter locatio

Searching for Earthquake Sources in the Lower Tagus Valley (Portugal): First Results from the ATESTA Project

The area of Lisbon has been struck by destructive earthquakes in the past and with very intense consequences. As of today, two main areas host active faults with concern for the region: offshore with the still unclear source of the famous and catastrophic 1755 earthquake and inland with the Lower Tagus Valley where unknown fault(s) have produced the 1909 and 1531 events with estimated magnitudes ranging from 6 to 7. Those latter events are of particular importance due to their location within an area that is now densely populated. The repetition of such a shock today would have a barely imaginable impact on the population and economy of Portugal. An apparent paradox is that in spite of the high stake and expected impact on the Greater Lisbon area, little is known about the source fault(s) of the 1531 and 1909 earthquakes in terms of location, dimensions, maximum magnitude, slip rate and recurrence period. The ATESTA Project aims at answering those questions by deploying an integrated paleoseismological approach to the Lower Tagus Valley. By combining detailed geomorphological mapping using high-resolution digital eleva- tion models with shallow geophysical imaging (reflection seismics, electrical tomography and ground-penetrating radar), our goal is to identify the continuation of crustal faults at the surface. Paleoseismic trenching is conse- quently used to characterize surface rupture in terms of large recent events. Preliminary results suggest the presence of several fault trace in the Lower Tagus Valley outlined by uplifted ter- races and offset streams and visible in satellite images and the national 10-m-resolution digital elevation model. Those fault traces correspond to structures at depth, as identified by geophysical imaging

Tomographic three-dimensional seismic velocity structure of the SW Ibero-Maghrebian region

The present tomographic study focuses on SW Ibero-Maghrebian region. To locate the seismic events and find the local velocity structure of epicentral area, the P and S arrivals at 42 stations located at north of Morocco, south of Portugal and Spain are used. The arrival times data used, in this study, were obtained by the “Instituto de Meteorologia” (IM, Lisbon, Portugal), the National Institute of Geophysics (CNRST, Rabat, Morocco) and the “Instituto Geografico Nacional” (IGN, Madrid, Spain) (between 12/1988 and 30/2008). The preliminary estimate of origin times and hypocentral coordinates are determined by the hypocenter 3.2 program. In this study we use a linearized inversion procedure comprising two steps: 1) finding the minimal 1-D model and simultaneous reloca- tion of hypocenters and 2) determination of local velocity structure assuming a continuous velocity field. The earth structure is represented in three dimensions by velocity at discrete points, and velocity at any intervening point is determined by linear interpolation among the surrounding eight grid points. The resolutions tests results indicate that the calculated images give near true structure for the studied region at 15, 30, 45 and 60 km depth. At 5km depth it gives near true structure in the continental region of Portugal, Spain, and Morocco. This study shows that the total crustal thickness varies from 30 to 35 km and contains low-velocity anomalies. A prominent low velocity anomaly that shows a maximum decrease in P-wave velocity of approximately 6 per cent in the Gibraltar region is observed extending down to a depth of approximately 30 km. This low velocity demarcates a small bloc located between Iberia and Nubia plates. The resulting tomographic image has a prominent high velocity anomaly that shows a maximum increase in P-wave velocity of approximately 6 per cent between 45 to 60 km depth beneath South of Portugal and the Golf of Cadiz. High-velocity anomalies could be associated with the location of deep active faults in the uplift and upper crust of South of Portugal. In the Golf of Cadiz, these anomalies could be associated with the seismogenic zone and probably more at the south with the Iberia-Nubia plate boundary

Study of the fracture process of Al Hoceima earthquake (24/02/2004, Mw=6.2) from regional and teleseismic data.

We studied the source time function (STF) and rupture process of the 2004 Alhoceima, Morocco earthquake of Mw = 6.2 using teleseismic and regional broad-band data. From regional broad-band data, STF function was determined using three large after- shocks as empirical Green functions. We inverted of body wave forms at teleseismic distances using an extended source model with rupture velocity between 2.5-3.0 km and using as preliminary orientation the fault plane solution derived from 126 P-wave polarities. Results show a complex bilateral rupture formed by four shallow subevents (2-8 km) with a maximum seismic moment release during the first seconds (more than 80% of a total of 1.8x1018 Nm) and time duration of 8-10 s. The focal mechanism shows a strike slip motion with a normal component. Nodal planes strike on NNE- SSW and WNW-ESE direction with horizontal pressure axes in NNW-SSE direction. The rupture propagated mainly towards the North. This propagation is in agreement with the damages caused in the epicentral region. The larger aftershocks have been relocated using a master event method. Comparisson of these results with those ob- tained for the 1994 earthquake shown similar behaviour: complex rupture process and, apparently, no relation of the 1994 nor the 2004 shocks with the Nekor fault, the most important geological feature in the area. The stress pattern derived from focal mech- anisms of 1994 and 2004 are in agreement with the regional stress pattern, horizontal compression in NNW-SSE and horizontal extension in E-W direction in the Alboran Sea